EINSTEIN COLLEGE OF ENGINEERING               DEPT OF CIVIL ENGG            EINSTEIN   COLLEGE OF ENGINEERING Sir.C.V.Rama...
EINSTEIN COLLEGE OF ENGINEERING                                           DEPT OF CIVIL ENGGUNIT - IIntroduction to Course...
EINSTEIN COLLEGE OF ENGINEERING                                       DEPT OF CIVIL ENGGImportant point:The Earth has evol...
EINSTEIN COLLEGE OF ENGINEERING                                         DEPT OF CIVIL ENGGChemical cementation of sand gra...
EINSTEIN COLLEGE OF ENGINEERING                                           DEPT OF CIVIL ENGGInternal ProcessesProcesses th...
EINSTEIN COLLEGE OF ENGINEERING                                        DEPT OF CIVIL ENGGAtomic Energy -- Energy stored or...
EINSTEIN COLLEGE OF ENGINEERING                                       DEPT OF CIVIL ENGGLayers     of     Differing     Ph...
EINSTEIN COLLEGE OF ENGINEERING                                         DEPT OF CIVIL ENGGPlates: are lithospheric plates ...
EINSTEIN COLLEGE OF ENGINEERING                                        DEPT OF CIVIL ENGG�The surface below which all rock...
EINSTEIN COLLEGE OF ENGINEERING                                        DEPT OF CIVIL ENGGreaches the surface at streamsand...
EINSTEIN COLLEGE OF ENGINEERING                                       DEPT OF CIVIL ENGGthe Earth, where pressure is highe...
EINSTEIN COLLEGE OF ENGINEERING                                        DEPT OF CIVIL ENGGusually have higher porosity than...
EINSTEIN COLLEGE OF ENGINEERING                                       DEPT OF CIVIL ENGGto the unsatisfied ionic charge on...
EINSTEIN COLLEGE OF ENGINEERING                                        DEPT OF CIVIL ENGGbetween two points on thewater ta...
EINSTEIN COLLEGE OF ENGINEERING                                        DEPT OF CIVIL ENGGoccur. Most common is lowering of...
EINSTEIN COLLEGE OF ENGINEERING                                         DEPT OF CIVIL ENGG�Caves and Caverns - Iflarge are...
EINSTEIN COLLEGE OF ENGINEERING                                       DEPT OF CIVIL ENGGIn fact, the term epicentre is imp...
EINSTEIN COLLEGE OF ENGINEERING                                             DEPT OF CIVIL ENGGthe severe earthquakes of th...
EINSTEIN COLLEGE OF ENGINEERING                                         DEPT OF CIVIL ENGGThis theory, thus, explains to s...
EINSTEIN COLLEGE OF ENGINEERING                                           DEPT OF CIVIL ENGG        The P-waves are the fa...
EINSTEIN COLLEGE OF ENGINEERING                                         DEPT OF CIVIL ENGGvalues of the epicentral distanc...
EINSTEIN COLLEGE OF ENGINEERING                                       DEPT OF CIVIL ENGGslightly tilted, the pendulum will...
EINSTEIN COLLEGE OF ENGINEERING                                         DEPT OF CIVIL ENGG         Seismographs will recor...
EINSTEIN COLLEGE OF ENGINEERING                                       DEPT OF CIVIL ENGG      Table 6.1. Intensity Scale F...
EINSTEIN COLLEGE OF ENGINEERING                                          DEPT OF CIVIL ENGG XI         9800*          Very...
EINSTEIN COLLEGE OF ENGINEERING                                       DEPT OF CIVIL ENGG        The angle ө is thus known;...
EINSTEIN COLLEGE OF ENGINEERING                                         DEPT OF CIVIL ENGGcharacterised by exceptional vio...
EINSTEIN COLLEGE OF ENGINEERING                                         DEPT OF CIVIL ENGGmore damage to life and property...
EINSTEIN COLLEGE OF ENGINEERING                                         DEPT OF CIVIL ENGGneighbouring areas, the extra-pe...
EINSTEIN COLLEGE OF ENGINEERING                                           DEPT OF CIVIL ENGGquake (s), that may occur duri...
EINSTEIN COLLEGE OF ENGINEERING                                         DEPT OF CIVIL ENGG        The ground acceleration ...
EINSTEIN COLLEGE OF ENGINEERING                                       DEPT OF CIVIL ENGGwhere 𝛼h as the ground acceleratio...
EINSTEIN COLLEGE OF ENGINEERING                                        DEPT OF CIVIL ENGGreservoir water, thus, causing di...
EINSTEIN COLLEGE OF ENGINEERING                                       DEPT OF CIVIL ENGG100,000 times more at pH 6 than at...
EINSTEIN COLLEGE OF ENGINEERING                                          DEPT OF CIVIL ENGGremoved the pressure on underly...
EINSTEIN COLLEGE OF ENGINEERING                                       DEPT OF CIVIL ENGG       chemical reactions, of resp...
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
Ag33 applied geology notes
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Ag33 applied geology notes

  1. 1. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG EINSTEIN COLLEGE OF ENGINEERING Sir.C.V.Raman Nagar, Tirunelveli-12 Department of Civil Engineering AG 33- APPLIED GEOLOGY Prepared by A.Ponniah RajuEINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG
  2. 2. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGGUNIT - IIntroduction to CourseGeo – derived from Greek word means – Earth , Logy study of .The earth has evolved (changed) throughout its history, and will continue to evolve.Earth – 4.6 billion years 9715469641 old, human beings have been around for only thepast 2 million years.Why study the earthWe’re part of it. Dust to Dust. Humans have the capability to make rapid changes. Allconstruction from houses to roads to dams are effected by the earth and thus require somegeologic knowledge. All life depends on the earth for food and nourishment. The earth isthere everyday of our lives.Energy and mineral resources – depend on for our lifestyle come from the earthGeologic Hazards - Earthquakes, Volcanic eruptions , hurricanes, Landslides, - affect usany time. A better understanding of the earth is necessary to prepare these eventualities.Minerals – Element – substance that cannot be separated in to simpler forms of matter byordinary chemical meansTwo or more elements – CompoundPhysical properties of Minerals Depends on – degree of aggregation , degree of Cohesion , senses, light. Magnetism,heat , electricity.Properties – External appearance and internal structure, Cleavage fracture, hardness,Sp.gr , Tenacity, Colour, Streak, Lusture, Transparency, Fluorescence, Phosphorescence External appearance – definite geometric shapes bounded –smooth planes – well definedsolids – Crystals.Crystallography –study of crystals Geology, What is it?Geology is the study of the Earth. It includes not only the surface process which haveshaped the earths surface, but the study of the ocean floors, and the interior of the Earth.It is not only the study of the Earth as we see it today, but the history of the Earth as it hasevolved to its present condition.EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG
  3. 3. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGGImportant point:The Earth has evolved (changed) throughout its history, and will continue to evolve.The Earth is about 4.6 billion years old, human beings have been around for only thepast 2 million years.Thus, mankind has been witness to only 0.043% of Earth history.The first multi-celled organisms appeared about 700 million years ago. Thus, organismshave only been witness to about 15% of Earths history.Thus, for us to have an understanding of the earth upon which we live, we must look atprocesses and structures that occur today, and interpret what must have happened in thepast.One of the major difficulties we have is with the time scale. Try to imagine 1 millionyears-- Thats 50,000 times longer than most of you have lived. It seems like a long timedoesnt it?Yet, to geologists, 1 million years is a relatively short period of time. But one thing wehave to remember when studying the earth is that things that seem like they take a longtime to us, may take only a short time to earth.Examples:A river deposits about 1mm of sediment (mud) each year. How thick is the mud after100 years? -- 10 cm hardly noticeable over your lifetime.What if the river keeps depositing that same 1 mm/yr for 10 million years? Answer10,000 meters (6.2 miles). Things can change drastically!Earth Materials and ProcessesThe materials that make up the Earth are mainly rocks (including soil, sand, silt, dust).Rocks in turn are composed of minerals. Minerals are composed of atoms, Processesrange from those that occur rapidly to those that occur slowly Examples of slowprocessesFormation of rocksChemical breakdown of rock to form soil (weathering)EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG
  4. 4. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGGChemical cementation of sand grains together to form rock (diagenesis)Recrystallization to rock to form a different rock (metamorphism)Construction of mountain ranges (tectonism)Erosion of mountain rangesExamples of faster processesBeach erosion during a storm.Construction of a volcanic coneLandslides (avalanches)Dust StormsMudflowsProcesses such as these are constantly acting upon and within the Earth to change it.Many of these processes are cyclical in nature.Hydrologic CycleRain comes from clouds - falls on surface, picks up sand, silt and clay, carriesparticles to river and into ocean. Water then evaporates to become clouds, whichmove over continents to rain again.Rock CycleMost surface rocks started out as igneous rocks- rocks produced by crystallization from aliquid. When igneous rocks are exposed at the surface they are subject to weathering(chemical and mechanical processes that reduce rocks to particles). Erosion movesparticles into rivers and oceans where they are deposited to become sedimentary rocks.Sedimentary rocks can be buried or pushed to deeper levels in the Earth, where changesin pressure and temperature cause them to become metamorphic rocks. At hightemperatures metamorphic rocks may melt to become magmas. Magmas rise to thesurface, crystallize to become igneous rocks and the processes starts over.External ProcessesErosion- rocks are broken down (weathered) into small fragments which are then carriedby wind, water, ice and gravity. External because erosion operates at the Earths surface.The energy source for this process is solar and gravitational.EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG
  5. 5. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGGInternal ProcessesProcesses that produce magmas, volcanoes, earthquakes and build mountain ranges.Energy comes from the interior of the Earth, Most from radioactive decay - nuclearenergy.Principle of UniformitarianismProcesses that are operating during the present are the same processes that have operatedin the past. i.e. the present is the key to the past. If we look at processes that occur today,we can infer that the same processes operated in the past.Problems:Rates -- rates of processes may change over time for example a river might deposit 1 mmof sediment /yr if we look at it today. but, a storm could produce higher runoff and carrymore sediment tomorrow. Another example: the internal heat of the Earth may have beengreater in the past than in the present -- rates of processes that depend on the amount ofheat available may have changed through time.Observations -- we may not have observed in human history all possible processes.Examples: Mt. St. Helens, Size of earthquakes.Perhaps a better way of stating the Principle of Uniform itarianism is that the laws ofnature have not changed through time. Thus, if we understand the physical and chemicallaws of nature, these should govern all processes that have taken place in the past, aretaking place in the present, and will take place in the future.EnergyAll processes that act on or within the Earth require energy. Energy can exist in manydifferent forms:Gravitational Energy -- Energy released when an object falls from higher elevations tolower elevations.Heat Energy -- Energy exhibited by moving atoms, the more heat energy an object has,the higher its temperature.Chemical Energy -- Energy released by breaking or forming chemical bonds.Radiant Energy -- Energy carried by electromagnetic waves (light). Most of the Sunsenergy reaches the Earth in this form.EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG
  6. 6. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGGAtomic Energy -- Energy stored or released in binding atoms together. Most of the energygenerated within the Earth comes from this source.Heat TransferHeat Moves through material by the following modes:Conduction - atoms vibrate against each other and these vibrations move from hightemperature areas (rapid vibrations) to low temperature areas (slower vibrations).- Heatfrom Earths interior moves through the solid crust by this mode of heat transfer.Convection - Heat moves with the material, thus the material must be able to move. Themantle of the Earth appears to transfer heat by this method, and heat is transferred in theatmosphere by this mode.Radiation - Heat moves with electromagnetic radiation (light) Heat from the Sun or froma fire is transferred by this modeGeothermal GradientTemperature and pressure increase with depth in the Earth. Near the surface of the Earththe rate of increase in temperature (called the Geothermal Gradient) ranges from 15 to35oC per kilometer. Temperature at the center of the Earth is about 4500oCThe Earth -- What is it?The Earth has a radius of about 6371 km, although it is about 22 km larger at equatorthan at poles.Internal Structure of the Earth:Density, (mass/volume), Temperature, and Pressure increase with depth in the Earth.Compositional LayeringCrust - variable thickness and compositionContinental 10 - 50 km thickOceanic 8 - 10 km thickMantle - 3488 km thick, made up of a rock called peridotiteCore - 2883 km radius, made up of Iron (Fe) and small amount of Nickel (Ni)EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG
  7. 7. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGGLayers of Differing PhysicalPropertiesLithosphere - about 100 km thick(deeper beneath continents)Asthenosphere - about 250 km thick todepth of 350 km - solid rock, but softandflows easily.Mesosphere - about 2500 km thick, solid rock, but still capable of flowing.Outer Core - 2250 km thick, Fe and Ni, liquidInner core - 1230 km radius, Fe and Ni, solidAll of the above is known from the way seismic (earthquake waves) pass through theEarth as we will discuss later in the course.Surface Features of the EarthOceans cover 71 % of Earths surface -- average depth 3.7 km. Land covers remainingsurface with average of 0.8 km above sea levelOcean BasinsContinental Shelf, Slope, and riseAbyssal PlainsOceanic ridgesOceanic TrenchesPlate TectonicsTectonics = movement and deformation of the crust, incorporates older theory ofcontinental drift.EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG
  8. 8. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGGPlates: are lithospheric plates - about 100 km thick, which move around on top of theasthenosphere.Plate BoundariesDivergent Boundaries occur at Oceanic Ridges, where new Oceanic lithosphere isformed and moves away from the ridge in opposite directions Continental rifting maycreate a new divergent margin and evolve into an oceanic ridge, such as is occurring inEast Africa and between the African Plate and the Arabian Plate.Convergent Boundaries occur where oceanic lithosphere is pushed back into the mantle,marked by oceanic trenches and subduction zones. Two types are possible -When two plates of oceanic lithosphere converge oceanic lithosphere is subductedbeneath oceanic lithosphere.When ocean lithosphere runs into a plate with continental lithosphere, the oceaniclithosphere is subducted beneath the continental lithosphere.Continental Collisions: may occur at a convergent boundary when plates of continentallithosphere collide to join two plates together, such as has occurred recently where theIndian Plate has collided with the Eurasian Plate to form the Himalaya Mountains.Transform Boundaries occur where two plates slide past one another horizontally. TheSan Andreas Fault, in California is a transform fault.Plate tectonics explains why earthquakes occur where they do, why volcanoes occurwhere they do, how mountain ranges form, as well as many other aspects of the Earth. Itis such an important theory in understanding how the Earth works that we cover it brieflyhere, but will return for a better understanding of later in the course.GroundwaterGroundwater is water that exists in the pore spaces and fractures in rock and sedimentbeneaththe Earths surface. It originates as rainfall or snow, and then moves through the soil intothegroundwater system, where it eventually makes its way back to surface streams, lakes, oroceans.�Groundwater makes up about 1% of the water on Earth (most water is in oceans).�But, groundwater makes up about 35 times the amount of water in lakes and streams.�Groundwater occurs everywhere beneath the Earths surface, but is usually restricted todepths less that about 750 meters.�The volume of groundwater is a equivalent to a 55 meter thick layer spread out overtheentire surface of the Earth.EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG
  9. 9. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG�The surface below which all rocks are saturated with groundwater is the water table.The Water TableRain that falls on the surfaceseeps down through the soiland into a zone called the zoneof aeration or unsaturatedzone where most of the porespaces are filled with air. As itpenetrates deeper it eventuallyenters a zone where all porespaces and fractures are filledwith water. This zone is calledthe saturated zone. The surfacebelow which all openings inthe rock are filled with water(the top of the saturated zone)is called the water table.The water table occurs everywhere beneath the Earths surface. In desert regions it isalwayspresent, but rarely intersects the surface.In more humid regions itEINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG
  10. 10. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGGreaches the surface at streamsand lakes, and generally tendsto follow surface topography.The depth to the water tablemay change, however, as theamount of water flowing intoand out of the saturated zonechanges. During dry seasons,the depth to the water tableincreases. During wet seasons,the depth to the water tabledecreases.Movement of GroundwaterGroundwater is in constant motion, although the rate at which it moves is generallyslower thanit would move in a stream because it must pass through the intricate passagewaysbetween freespace in the rock. First the groundwater moves downward due to the pull of gravity. Butit canalso move upward because it will flow from higher pressure areas to lower pressureareas, ascan be seen by a simple experiment illustrated below. Imagine that we have a "U"-shapedtubefilled with water. If we put pressure on one side of the tube, the water level on the othersiderises, thus the water moves from high pressure zones to low pressure zones.The same thing happens beneath the surface ofEINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG
  11. 11. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGGthe Earth, where pressure is higher beneath thehills and lower beneath the valleysGroundwatThe rate of groundwater flow is controlled by two properties of the rock: porosity andpermeability.�Porosity is the percentage of the volume of the rock that is open space (pore space).Thisdetermines the amount of water that a rock can contain.�In sediments or sedimentary rocks the porosity depends on grain size, the shapes ofthe grains, and the degree of sorting, and the degree of cementation.�Well-rounded coarsegrainedsedimentsEINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG
  12. 12. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGGusually have higher porosity than finegrained ,because the grains donot fit together well.�Poorly sorted sediments usually have lowerporosity because the fine-grained fragments tendto fill in the open space.�Since cements tend to fill in the porespace, highly cemented sedimentaryrocks have lower porosity.�In igneous and metamorphic rocks porosity is usuallylow because the minerals tend to be intergrown,leaving little free space. Highly fractured igneous andmetamorphic rocks, however, could have highporosity�Permeability is a measure of the degree to which the pore spaces are interconnected,andthe size of the interconnections. Low porosity usually results in low permeability, butGroundwaterhigh porosity does not necessarily imply high permeability. It is possible to have a highlyporous rock with little or no interconnections between pores. A good example of a rockwith high porosity and low permeability is a vesicular volcanic rock, where the bubblesthat once contained gas give the rock a high porosity, but since these holes are notconnected to one another the rock has low permeability.A thin layer of water will alwaysbe attracted to mineral grains dueEINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG
  13. 13. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGGto the unsatisfied ionic charge onthe surface. This is called the forceof molecular attraction. If the sizeof interconnections is not as largeas the zone of molecular attraction,the water cant move. Thus, coarse-grainedrocks are usually morePermeable than fine-grained rocks,and sands are more permeable thanclays.Movement in the Zone of AerationRainwater soaks into the soil where some of it is evaporated, some of it adheres to grainsin thesoil by molecular attraction, some is absorbed by plant roots, and some seeps downinto the saturated zone. During long periods without rain the zone of aeration may remaindry.Movement in the Saturated ZoneIn the saturated zone (below the water table) water percolates through the interconnectedpore spaces, moving downward by the force of gravity, and upward toward zones oflower pressure.Where the water table intersects the surface, such as at a surface stream,lake, or swamp, the groundwater returns to the surface.Recharge Areas and Discharge AreasThe Earths surface can be divided into areaswhere some of the water falling on the surfaceseeps into the saturated zone and other areaswhere water flows out of the saturated zoneonto the surface. Areas where water enters thesaturated zone are called recharge areas,because the saturated zone is recharged withgroundwater beneath these areas. Areas wheregroundwater reaches the surface (lakes,streams, swamps, & springs) are calleddischarge areas, because the water isdischarged from the saturated zone. Generally,recharge areas are greater than dischargeareas.GroundwaterPage 4 of 8 10/20/2003Discharge and VelocityThe rate at which groundwatermoves through the saturatedzone depends on thepermeability of the rock andthe hydraulic gradient. Thehydraulic gradient is definedas the difference in elevationdivided by the distanceEINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG
  14. 14. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGGbetween two points on thewater table.Velocity, V, is then:V = K(h2 - h1)/Lwhere K is the coefficient of permeability.If we multiply this expression by the area, A, through which the water is moving, then wegetthe discharge, Q.Q = AK(h2 - h1)/L,which is Darcys Law.Springs and Wells�A spring is an area on the surface of the Earth where the water table intersects thesurfaceand water flows out of the ground. Springs occur when an impermeable rock (called anaquiclude) intersects an permeable rock that contains groundwater (an aquifer). Suchjuxtaposition between permeable and impermeable rock can occur along geologicalcontacts (surfaces separating two bodies of rock), and fault zones.�A well is human-made hole that is dug or drilled deep enough to intersect the watertable.Wells are usually used as a source for groundwater. If the well is dug beneath the watertable, water will fill the open space to the level of the water table, and can be drawn outby a bucket or by pumping. Fracture systems and perched water bodies can often make itGroundwaterPage 5 of 8 10/20/2003difficult to locate the best site for a well.AquifersAn aquifer is a large body of permeable material where groundwater is present in thesaturatedzone. Good aquifers are those with high permeability such as poorly cemented sands,gravels,and sandstones or highly fractured rock. Large aquifers can be excellent sources of waterforhuman usage such as the High Plains Aquifer (in sands and gravels) or the FloridianAquifer(in porous limestones) as outlined in your text. Aquifers can be of two types:�Unconfined Aquifers - the most common type of aquifer, where the water table isexposed to the Earths atmosphere through the zone of aeration. Most of the aquifersdepicted in the drawings so far have been unconfined aquifers.�Confined Aquifers - these are less common, but occur when an aquifer is confinedbetween layers of impermeable strata. A special kind of confined aquifer is an artesiansystem, shown below. Artesian systems are desirable because they result in free flowingartesian springs and artesian wells.Changes in the Groundwater SystemWhen discharge of groundwater exceeds recharge of the system, several adverse effectscanEINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG
  15. 15. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGGoccur. Most common is lowering of the water table, resulting in springs drying up andwellshaving to be dug to deeper levels. If water is pumped out of an aquifer, pore pressure canbereduced in the aquifer that could result in compaction of the now dry aquifer and result inlandsubsidence. In some cases withdrawal of groundwater exceeds recharge by naturalprocesses,and thus groundwater should be considered a non-renewable natural resource.GroundwaterPage 6 of 8 10/20/2003Water Quality and Groundwater ContaminationWater quality refers to such things as the temperature of the water, the amount ofdissolvedsolids, and lack of toxic and biological pollutants. Water that contains a high amount ofdissolved material through the action of chemical weathering can have a bitter taste, andiscommonly referred to as hard water. Hot water can occur if water comes from a deepsource orencounters a cooling magma body on its traverse through the groundwatersystem. Such hot water may desirable for bath houses or geothermal energy, but is notusually desirable for human consumption or agricultural purposes. Most pollution ofgroundwater is the result of biological activity, much of it human. Among the sources ofcontamination are:  �Sewers and septic tanks  �Waste dumps (both industrial and residential)  �Gasoline Tanks (like occur beneath all service stations)  �Biological waste products - Biological contaminants can be removed from the  groundwater by natural processes if the aquifer has interconnections between pores that  are smaller than the microbes. For example a sandy aquifer may act as a filter for  biological contaminants.  �Agricultural pollutants such as fertilizers and pesticides.  �Salt water contamination - results from excessive discharge of fresh groundwater in  coastal areas.  GroundwaterGeologic Activity of Groundwater�Dissolution - Recall that water is the main agent of chemical weathering. Groundwaterisan active weathering agent and can leach ions from rock, and, in the case of carbonaterocks like limestone, can completely dissolve the rock.�Chemical Cementation and Replacement - Water is also the main agent actingduringdiagenesis. It carries in dissolved ions which can precipitate to form chemical cementsthat hold sedimentary rocks together. Groundwater can also replace other molecules inmatter on a molecule by molecule basis, often preserving the original structure such as infossilization or petrified wood.EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG
  16. 16. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG�Caves and Caverns - Iflarge areas of limestoneunderground are dissolvedby the action ofgroundwater these cavitiescan become caves orcaverns (caves with manyinterconnected chambers)once the water table islowered. Once a cave forms, it is open to the atmosphere and water percolating in canprecipitate new material such as the common cave decorations like stalagtites (hang fromthe ceiling), stalagmites (grow from the floor upward), and dripstones, and flowstones.�Sinkholes - If the roof of a cave or caverncollapses, this results in a sinkhole.Sinkholes, likes caves, are common in areasunderlain by limestones. For example, inFlorida, which is underlain by limestones, anew sinkhole forms about once each year,gobbling up cars and houses in process.�Karst Topography - In an area where the main type of weathering is dissolution (likeinlimestone terrains), the formation of caves and sinkholes, and their collapse andcoalescence may result in a highly irregular topography called karst topography (seepages 404 - 406 in your text).GroundwaterEARTHQUAKES AND THEIR STUDY Most of us must have personally experienced earthquakes, and are, therefore,aware of them. Earthquake is something which causes the shaking of the Earth ; and assuch, all our buildings and structures erected on the Earths surface start trembling, as andwhen a quake comes. An earthquake, is therefore, defined as a natural vibration of theground (or the Earths crust) produced by forces, called earthquake forces or seismicforces. Many the these vibrations are very feeble, and may not even be felt to anyappreciable extent, by human beings. Some other vibrations, however, may be verysevere, and may cause the collapse and rupture of buildings and other structures, bringinglarge scale destruction and disaster in its wake. Before we discuss the various possible causes of earthquakes in our next article,we shall like to define two very important technical terms that are associated withearthquakes. These terms are focus and epicentre. The focus is the place beneath the Earths surface from where an earthquakeoriginates, and the point or line on the Earths surface immediately above the focus iscalled the epicentre or epicentral line (Refer Fig. 6.6). The focus is also sometimestermed as seismic centre. The point which is diametrically opposite to the epicentre iscalled anticentre.EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG
  17. 17. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGGIn fact, the term epicentre is important as it represents the point on the Earth, where theearthquake waves reach for the first time, after they are generated from the focus. Thearea around the epicentre will be subjected to earthquake vibrations, and is generallyindicated as epicentral area. Earthquake foci are generally distributed in three general depth ranges. Shallowearthquakes originate within about 60 kilometres of the surface ; Intermediateearthquakes have foci between 60 to 300 kilometres down; and the Deep seatedearthquakes originate at depths below 300 kilometres, or so. The deepest focus everrecorded was about 700 kilometres. In the case of shallow focus, the area affected issmaller compared to that in a deeper focus ; this is because, in the latter case, theearthquake waves assume a wider dispersion. The deep focus earthquakes are generallyvery rare: and most of the million earthquakes occurring in a year are, generally shallow.Nevertheless, it is generally difficult to know the focus quite accurately, and therefore,most frequently, only epicentral area is indicated. The shallow earthquakes whichoriginate at depths up to about 35 km are generally more damaging than the others.Causes of Earthquakes and Their Types: Our ancestors used to believe that the earthquakes were the manifestations ofGods wrath. The first nearly scientific approach to find out a suitable cause for anearthquake was made by our philospher Aristotle (384—322 B.C.), the Einstein of hisdays, who explained that earthquakes were the result of entrapped air escaping from theEarths interior. Modern earthquake theories are, however, based on factual data andstudy of actual earthquakes of the world.Depending upon the possible cause of an earthquake, earthquakes are now-a-daysgenerally classified into two categories, i.e. (1) Tectonic earthquakes ; and (2) Non-tectonic earthquakes.The Tectonic Earthquakes: The tectonic earthquakes are perhaps caused by the slippage or movement of therock masses along a rupture or break called a fault. These are generally very severe, andthe area affected is often very large. The non-tectonic type of earthquakes includeearthquakes caused by a number of easily understandable processes, such as ;volcanic.eruptions ; superficial movements like landslides ; subsidence of the groundbelow the surface, etc. All such processes may introduce vibrations into the ground byjerks, etc. There is, thus, not much controversy on the possible causes of non-tectonicearthquakes. However, nothing can be said with absolute certainty regarding the origin ofthe tectonic earthquakes. Most probably, as pointed out above, these tectonic earthquakesare caused by the occasional movements of the crustal blocks along the fractured planes,called the faults. Faulting is a phenomenon which has been found associated with most ofEINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG
  18. 18. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGGthe severe earthquakes of the world. As such, it can generally be considered as theimmediate cause of many tectonic earthquakes. Whether "faulting is due to earthquakes"or "earthquakes are due to faulting" is infact among the most complicated geologicalproblems that still await perfect solution. The modern well known Elastic rebound theory explains as to how faulting takesplace, and how it leads to earthquakes. However, this theory does not account for theforce which produced faulting, but it explains only the manner in which the rocks yield tothese forces. Ultimately, it may be added that these unknown tectonic forces which causefaulting are the same as those which produce fold mountains and other structural featuresof the Earths crust. The elastic rebound theory is explained below : The Elastic Rebound Theory. According to this theory, the rocks of the Earthscrust like any other elastic solid, would undergo elastic deformation* when subjected tounequal frictional forces or stresses (either compression or tension). But this deformationis possible only up to a certain limit, i.e. till the breaking point or elastic limit* is reached.As and when the stress exceeds the frictional resistance of the rock block, it will break,producing rupture in the rock. This rupture takes the form of faulting, when the rupture isproduced by a stress which was building up rapidly ; and then there occurs a relativemovement on either side of the plane of rupture. Such movements always involve suddenrelease of enormous amount of elastic energy (which was stored in the folded rocks)making it possible for the rock block to acquire new positions of least strain. where allthe three stages have been explained quite clearly). The elastic energy so released mayproduce powerful seismic waves, which travel in all directions from the place of faulting,and which induce shaking movements in material through which they travel, thusproducing earthquake shocks. *When a solid like rubber, wooden Stick, rock blocks, etc. is squeezed (bycompression) or stretched (by tension) it deforms according to certain physical lawswhich depend on the inherent properties of the solid itself. The squeezing or stretchingforce on a unit is called a stress, and the deformation of the solid yielding to the stress iscalled strain. Elastic materials are those in which stress is directly proportional to thestrain. Thus, a rubber band if stretched to twice its original length (L) required a pullequal to say P, then a pull equal to 2P and 3P respectively will be required to produce astretching equal to twice or thrice of its original length. But the band cannot be stretchedindefinitely because eventually it will break. This limiting point is called elastic limit,and the earlier * deformation is called elastic deformation. When the force or stressexceeds the elastic limit, the solid continues to deform without any additional stress. Suchdeformation is called plastic deformation, The stress value where deformation changesfrom elastic to plastic, is called yield point. Ultimately with plastic deformation, the solidruptures or breaks. Moreover, it has been found by experiments that in some solids, likerocks, if the stress is applied very slowly, the material will deform plastically at stressvalues far below the "normal yield point". But when the stress in built up rapidly, thematerial ruptures or breaks shortly after the yield point is reached. This explains as whyrocks will be folded under certain conditions and faulted under others.EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG
  19. 19. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGGThis theory, thus, explains to some good enough extent, as to how tectonic earthquakesdo occur. Tectonic earthquakes are quite common; and here in India, all the earthquakesare generally of this nature.Earthquake Waves, Their Recording and Types: The energy released during faulting, produces seismic waves, which can bedetected by sensitive and delicate instruments, called seismographs, installed at speciallydesigned seismographic stations. The record of seismic waves is called seismogram. By the study of a lot of such data collected on various actual earthquakes of theworld, it has now become possible to differentiate between different types of waves thatare generated in an earthquake. Basically, two classes of waves are produced during an earthquake. One group,called the body waves (consisting of P-waves and S-waves), travels downward into theEarth; whereas, the other group, called the surface waves (popularly called L-waves),travels along the surface of the Earths crust. Since it is only the L waves that passthrough the Earths surface, the entire destruction] caused during an earthquake will becaused by these L-waves only. The other kinds of waves (i. e. P and S waves) are helpfulin detecting the origin and epicentre of an earthquake, and for estimating the interior ofthe earth*. All these three types of waves obey the laws of reflection and refraction, as theypass through, the Earths materials of varying density. The P and S waves, though movetowards the interior of the Earth, yet some of the wave energy is reflected upward to thesurface by certain underlying layers, and thus recorded by the seismographs—located atdifferent distances from the shot point (i.e. the focus). A brief description of these waves is given below:P-Waves (i.e. Primary Waves). The P-waves are com-pressional in nature, and travellike sound waves. The particles thus vibrate in longitudinal direction (i.e. in the directionof propagation of the waves) with a push and pull* effect, and these waves are, therefore,also called as longitudinal waves or compressipnal waves or push and pull waves. Thevelocity of such a wave depends upon the resistance of a medium to compression, andhence they travel with greater velocities in rocks which are rigid, compact and dense.EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG
  20. 20. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG The P-waves are the fastest of all the three types ; and are, therefore, first to bereached at the seismograph stations (Refer Fig, 6.8). Also they are capable of passingthrough solids, liquids as Well as gases.S-waves (i.e. Secondary Waves). These waves are transverse or distortional like those oflight waves. The particles, therefore, travel in a direction at right angles (i.e. transverse)to the direction of propagation of the wave. The velocity of S-wave is controlled by theresistance of a medium to shear. Due to this reason, these waves, though capable ofpassing through solids, yet cannot pass through liquids, as liquids donot have anydistortional elasticity. These waves travel slower than the P-waves, and are second to berecorded at the seismographic stations (Refer Fig, 6.8).L-waves (Le. Long Waves). These waves travel alongthe Earths surface, following acircumferential path. They are also called surface waves, because their journey isconfined only to the surface layers of the Earth. In other Words, they do not traveltowards the interior of the Earth from the point of origin of disturbance. The behaviour of these waves is similar to that of sea waves. One type of L-wavesinvolves both vertical and horizontal motions ; whereas another type involves onlyhorizontal motions. These waves are the slowest to travel, and therefore, reach in the lastat the seismographic stations (Refer Fig. 6.8). These waves are responsible for causing allthe damage done on the surface by an earthquake.Travel Time Curves and Locating the Epicentre The distance from a seismograph station to the place of origin of an earthquakecan be determined by the time interval between the arrivals of the first P and S waves.The more distant the earthquakes focus, the longer is the S.P. interval. This is somewhatsimilar to a type of problem in which two cars start together and move down the sameroad at constant rates, of say 40 km/hr and 30 km/hr, respectively. The faster car will,therefore, arrive at a station first, and if it arrives say 6 hrs ahead of the slower car, theneventually one can easily calculate that they have travelled a distance of 720 km.* It, thus, follows that the distance of focus from the recording station depends uponthe S.P interval. Moreover, since the epicentre of an earthquake is the immediate verticalreflection of its focus, the S.P. interval will also, in the same way, depend upon thedistance of the station from the epicentre (i.e. the epicentral distance). The greater is theS-P interval, the larger is this distance. By a study of the records of a number of past earthquakes in a given region,scientists have been able to plot certain curves called the travel-time curves, such asshown in Fig. 6.9. Such travel time curves can be used to analyse a future earthquake inthe given region, and thus to determine its epicentre and depth of focus, etc.Locating the Epicentre by Three Circle Method. In this method, standard tables or travel time curves, relating the epicentraldistance (i.e. the distance of the epicentre from the seismograph station) with the S.P.interval are used. The S-P interval at a station being known, the epicentral distance isknown. However, by analysing the record of a single seismograph station, although wecan know the epicentral distance, but we cannot ascertain its location as the direction ofthis is not known. This job of locating the epicentre can be completed easily, providedsuch records are available from atleast three seismological stations. These stationsshould, of course, be conveniently located. By knowing the S-P intervals for the sameearthquake at these three different stations, we can find out the three correspondingEINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG
  21. 21. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGGvalues of the epicentral distance. With each of the three stations marked on a map or aglobe, three circles, can be drawn with radii equal to the epicentral distance of each, andthe point of inter-section will represent the epicentre (Refer Fig. 6.10). To illustrate, let us suppose that a particular earthquake was recorded at threewidely separated seismograph stations A, B and C, as marked on a map or globe. The S-Pinterval recorded at these three stations was, say, 15 seconds, 20 seconds and 30 seconds,respectively. The respective corresponding epicentral distances from these stations arenow worked out from standard tables or curves, as say 130 km, 160 km and 240 km.Now, with A, B and C as centres and the respective distances as radii (i.e. 130, 160 and240 km), circles are drawn on the map to the given scale. The epicentre is finally locatedat the point of inter-section of the three circles, say at point E (Refer Fig. 6.10).Seismographs or Seismometers As stated earlier, a seismograph is an instrument which receives and records theearthquakes waves. Different designs have been prepared for seismographs. Aseismograph may be designed to record only the horizontal motion of the ground, or onlythe vertical motion of the ground, or both. Moreover, the seismographs may be providedwith ordinary paper and pencil device for recording the waves, or may be provided withphotographic papers which may record the waves under dark room conditions by using athin beam of light reflected through a mirror ; or may be electronised or computerised inthe advance stages. Ordinarily, a good seismological station would generally have two horizontalseismographs mounted at right angles (usually one to record the North-South horizontalmovements, and the other to record the East-West horizontal movements), and onevertical instrument to record the vertical movements of the ground. These threeseismographs can give a complete picture of the ground motions in three directions. Even today, simple pendulum type seismographs are used quite frequently,although a lot of improved electronised and computerised systems have been developed.The essential part of such a seismograph is a pendulum (or heavy weight), whichnormally remains at rest, but starts swinging either horizontally (to and fro) or vertically(up and down) during an earthquake. Moreover, the horizontal motion of the ground maybe recorded either by a pendulum which swings in a vertical plane, or by a pendulumwhich swings in a horizontal plane. A simplest type of a seismometer is shown in Fig. 6.11. It uses a heavy mass (m)as a pendulum suspended from the top, as shown. The mass of the pendulum tends tostand still, as the supporting frame moves during the horizontal motion of the ground,caused by an earthquake. As the ground and frame moves to one side, gravity pulls onsuch a pendulum, and the pendulum tends to follow the motion of the ground, and tocontinue to swing after the ground comes to rest. This horizontal motion of the groundcan, thus, be recorded by observing the relative position of a pointer on the mass and ascale attached to the ground, as shown. This particular arrangement (Fig. 6.11) will record the horizontal motion of theground by a pendulum which swings horizontally (i.e. to and fro), but in a vertical plane.A better system for earthquake observations is obtained by swinging the pendulum in ahorizontal or nearly horizontal plane instead of a vertical plane, as shown in Fig. 6.12 (a). When the pendulum swings in a horizontal plane and about a vertical axis [Fig.6.12 (a)], it has no tendency to return to any particular position. But, if the plane isEINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG
  22. 22. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGGslightly tilted, the pendulum will be acted upon by a small component of gravity, and ifdeflected, it will return slowly to its original position Damping is easily provided by aplate attached to the mass and extending into a cup of Oil on the ground. Such ahorizontal pendulum is the basic principle of most earthquake seismometers, whetherrecording horizontal ground motion, or vertical ground motion. In order to make the seismometer indicate the ground motion accurately, it isnecessary that the rate at which the pendulum returns to its rest position, is very slow,which means that the natural period of oscillation of the pendulum must be long. Theseismometer indicates clearly only those ground oscillations that have periods shorterthan the natural pendulum period. If the period of the ground motion is longer than thatof the pendulum, the inertial mass tends to move the ground, and motion is not accuratelyindicated. Vertical motion of the ground can be detected by swinging the pendulum in ahorizontal plane about a horizontal axis (i.e. the pendulum moving up and down about thehorizontal axis), as shown in Fig. 6.12 (b). The pendulum, in such a case, is supportedagainst gravity by a spring. A long weak spring can be used to keep the natural period ofthe pendulum long, or the spring can be adjusted so that it tends to increase, wheneverrelative movement of the ground and inertial mass occurs. A diagonal spring, such asshown in Fig. 6.12 (b), is the commonest type. In this case, when the ground rises, theangle 9 decreases, and althought the spring is stretched, the restoring moment about theaxis of rotation of pendulum is increased very little. Similarly, if the ground drops,although the length of the spring and hence its tension decreases, 0 increases, preventinga large decrease in the torque of the spring. A small restoring moment, thus, results in along period of oscillation, and high sensitivity to the ground motion. In order to obtain a permanent record of the ground motion with time, etc., itis,.necessary to attach some sort of recording arrangement with the pendulums of thetypes shown in Fig. 6.12 (a) and (6). This can be done by attaching a pen or stylus to thependulum arm and letting it write a record on a rotating drum covered with a paper. Aclock is also provided, which starts running as soon as the first shock is experienced, andthe pendulum is thrown into motion. restrain to the relative movement of the ground andpendulum. This makes it difficult to record small ground movements, Moreover, sincethe ground motion is usually very small, it is always desirable to amplify it, and theamplification provided by such a mechanical recorder is only in the ratio of the distanceof the recording stylus top from the axis of motion (L2) to the distance of the centre ofoscillation of the pendulum arm from the axis of rotation {L1). These shortcomings can be removed (i.e. greater amplification obtained withoutthe disadvantage of friction between pen nib and paper) by using photographic recording.In such a recording, a mirror is mounted on the seismometer, and a beam of light isreflected from it on the recording drum, which is wrapped round by a photographic paper.The recording drum can be placed at a large distance from the seismometer, thus makingthe length of the recording arm much greater than the length of the pendulum, and hencegiving the desired amplification for recording small motions. Moreover, in such a case,since the light beam is deflected through twice the angle the pendulum rotates, it givesanother amplification factor of 2.EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG
  23. 23. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG Seismographs will record all the three types of waves during an earthquake.Firstly, the P-waves will be received ; followed by S-waves after a small interval of time ;and L-waves are recorded in the end. . A seismograph is generally suitable only as long as the vibrations are not of avery high intensity—which may throw the instrument out of balance. Hence, forrecording high intensity earthquakes in highly seismic regions, special strong motioninstruments are to be installed.Intensity and Magnitude of an Earthquake:Intensity of an Earthquake. Intensity of an earthquake may be defined as the rating ofan earthquake based on the actual effects produced by the quake on the Earth. Theseobserved effects may eventually range from simple harmless vibrations to mild jerkscapable of disturbing movable things and causing some damage to structures, to completeoverturning and collapse of buildings and subsidence of crustal segments. All theseeffects will no doubt depend partly upon the number of jerks and tremors, but will mainlydepend upon the maximum rate of change of the movements of the ground, i.e. by itsmaximum acceleration. Hence, now a days, it is customary to express the intensity of aquake by the maximum acceleration of the ground. This value can be estimated fromseismograph records. Initially, a scale of earthquake intensity with ten divisions was given by Rossi andForel, which was based entirely on the sensation of the people and the damage caused.However, it was modified by Mercalli and later by Wood and Neumann. The present dayintensity scale which takes into account the range of maximum acceleration of the groundis given as follows.EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG
  24. 24. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG Table 6.1. Intensity Scale For Earthquakes with Approximately Corresponding MagnitudesInten- Maximum Name of the shock Effects observed Magnitude sity Acceleration of (M) corres- class the ground in ponding to 2 mm/sec highest inten- sity reached (1) (2) (3) (4) (5) I 10 Imperceptible Recorded only by sensitive 3.5 to seismographs II 25 Feeble Recorded by all seismographs, and may be felt by some sensitive persons at rest. III 50 Slight Commonly felt by all people 4.3 at rest, especially on upper floors.- IV 100 Moderate Commonly felt by all people 4.3 to either at rest or in motion ; 4.9 nocking of loose objects in- cluding standing vehicles V 250 Fairly strong Generally felt ; most sleeping persons are awakened ; ringing of bells VI 500 Strong Trees sway and all suspended 4.9 to 5.5 objects swing ; fall of weak plasters : general panic ; damage by overturning and falling of loose objects. VII 1000 Very strong Damages to buildings 5.5 to 6.2 producing cracks in walls, etc., fall of chimneys ; general alarm and panic.VIII 2500 Destructive Car drivers seriously disturbed 6.2 to ; masonry fissured ; poorly 7.0 constructed buildings damaged. IX 5000 Ruinous Some houses collapse where ground begins to crack ; pipes break open. X 7500 Disastrous Ground cracks badly; many 7 to 7.3 buildings destroyed ; railway lines bent ; landslides occur on steep slopes.EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG
  25. 25. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG XI 9800* Very Dis- Few buildings remain 7.4 to 8.1 astrous standing; bridges destroyed ; and services like railways, pipes, cables, etc. getting out of action ; great landslides and floods. XII Over 9800* Catastrophic Total destruction ; objects >8.1 thrown into air, ground rises (maximum and falls in waves known 8.9)*The value equalling the acceleration due to gravity, i.e. 9800 mm/sec2.Magnitude (M) of an Earthquake. Magnitude of a tectonic earthquake may be definedas the rating of an earthquake based on the total amount of energy released when theover-strained • rocks suddenly rebound, causing the earthquake. In fact, during anearthquake, the released energy travels in the form of earthquake waves; and though theentire released energy (E) never reaches the recording station, yet it can be made as thebasis for defining the magnitude (M) if it could be linked to ground acceleration (a).Many equations have been put forward to relate "M, E and a. One of such relationship between the magnitude (M) and the total amount ofenergy released (E) is given by the simple equation Log10 E = 4.4 + 2.14 M - 0.054 M2 ...(6.1)The total amount of energy released is in ergs, and can be evaluated by the equation 𝐸 = 𝐶 . 𝑎 ℎ (𝐷2 + 𝐻2 ) where a is the ground acceleration; h is the depth of focus inkm; and D is the distance of the station under consideration from the the epicentre in km; and C is a constant = 0.625Knowing the value of E from Eq. (6.2), the value of M from Eq. (6.1) can be evaluated. It may be noted that although intensity varies from place to place during anearthquake, the magnitude remains the same ; because the magnitude corresponds tomaximum intensity reached or the total energy released in an earthquake. Theapproximate magnitude values, on the scale of intensity, are shown in col. (5) of Table6.1.Isoseismic Lines and Depth of Focus (h) Whenever an earthquake occurs, its intensity, which is maximum at the epicentre,decreases outwards. The decrease in intensity (expressed in terms of acceleration ofcourse) is inversely proportion-al to the square of the distance from the focus. If now, inan earthquake hit area, points of equal intensity are marked, and lines are drawn throughthe points of equal intensities, then these line are known as isoseismal lines. The areaenclosed by an isoseismal line is roughly circular if the focus of the earthquake is a point,and elliptical if it is an elongate zone or line. (For determining the depth of focus (h) of anearthquake, Oldham has devised a method. According to which, ifa is the intensit y(i.e.ground acceleration) at the epicentre point (say A) and & represents the intensity atanother station (say B) and if the distance between If ө is known, h can be evaluated. 0 can be found from using the fact thattheoretically the intensityfrom focus decreases as the square of the distance which is alsoequal to sin2 ө. 𝑏/𝑎 = 𝜋𝑟 2 = 𝜋𝑟 2 = sin8 өEINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG
  26. 26. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG The angle ө is thus known; and using this value of 9 in Eq. (6.3), h can beestimated.Distribution of Earthquakes Although, earthquakes occur in different parts of the world, but they occur morefrequently and severely along certain zones of marked instability. Two well definedearthquake belts, which have been recognised are: (i) the drcum-Pacific belt; and (ii) the Mediterranean belt. The Circum-Pacific belt borders the pacific ocean and covers North America,most of Asia, and Europe. It passes through North America, Alaska, New Zealand, NewGuinea, Chile, Indonesia, and Japan. About 68% of the earthquakes occur in this region. The Mediterranean belt extends from the Mediterranean towards the east, andcovers India, Africa, Arabia, South America, Italy, Sicily, and Australia. About 21% ofthe earthquakes occur in this region.Certain Important Indian Earthquakes Of the various great Indian earthquakes recorded in history, the best known are :of Delhi in 1720, of Calcutta in 1737, of Eastern Bengal and the Arkan coast in 1762, ofKutch in 1819, of Kashmere in 1885, of Bengal in 1885, of Assam in 1897, of Kangrd in1905, of North Bihar in 1934, of Quetta or Baluchistan in 1935, of N.E.Assam in 1950,etc. All these earthquakes have occurred in the areas which are unstable and arecompartively young geological formations, such as in Himalayan ranges and the Indo-gangetic plains. The peninsular India, which is quite old and inactive, is generally freefrom earthquakes. Starting from the great Assam quake of 1897, the various major earthquakes arebriefly described below :The Assam Earthquake (1897). This earthquake occurred in Assam on 12th June, 1897with a roar of extra-ordinary loudness ; and it proved to be one of the most catastropicearthquakes of the world. The total area where the shock was felt was about 4.14 millionsq. km. But shillong along with surrounding country of about 3,80,500 sq. km were worstaffected, and laid waste in less than one minute. All communications were .destroyed ;the ground fissured with eruption of ground water and large scale flooding due toobstructed drainage ; buildings collapsed with displacement of land masses and heavyloss of life. The ground acceleration perhaps exceeded the acceleration due to gravity, and themaximum amplitude of horizontal vibrations was as much as 17.5 cm with only onesecond period. The main shock was succeeded by hundreds of after-shocks for about amonth, and were felt all over the shaken area. For this earthquake, Oldham has calculated the velocity of the earthwaves asabout 3 km/sec, and the depth of the focus as only 8 km or even less. The main cause for this earthquake was traced to the movement along animportant fault-scrap running parallel to the Childrang river for a length of about 20 km,with a vertical throw varying from 30 cm to 11 metres. .The Kangra Earthquake (1905). This earthquake occurred on the early morning of 4thApril, 1905. The shock, which was felt over whole of India north of the Tapti valley, wasEINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG
  27. 27. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGGcharacterised by exceptional violence and destruction along two linear tracts betweenKangra and Kulu, and between Mussoorie and Dehra Dun. The main shock was of sever intensity, and mostly sudden with only a fewpremonitory warnings ; but the after shocks continued for about 2 years, with 10 to 30shocks per month. Slowly, their number decreased, till they gradually disappeared. The geological effects of this earthqua.se were markedly different from those ofthe Assam earthquake. The effects observed were : usual disturbances of streams, springsand canals ; collapse of buildings ; landslides and rockslides ; and a few slight alterationsin the level of some stations and hill tops. No true fissure or dislocations were, however,seen. There, however, occurred a loss of human life worth about 20,000 persons. The cause of this earthquake was again the tectonic one, with movement takingplace along a fault plane. The magnitude of the quake was calculated to be about 8.6, anddepth of focus about 34 to 64 km or so.The Bihar Earthquake (1934). This earthquake occurred on the afternoon of 15thJanuary, 1934, North Bihar and Nepal were shaken by this earthquake of very highintensity of magnitude about 8.4. Within three minutes, the cities of Monghyr andBhatgaon (Nepal) were in ruins, and towns so far apart as Kathmandu, Patna, andDarjeeling were strewn with debris of many public and private buildings. Houses inPurnea and Sitamarhi got tilted and sunk under the ground, and sand and water wereerupted from countless fissures in the ground, opened on either side of the Ganges. About12,000 persons were estimated to have been lost in this earthquake. The period and amplitude of vibrations and the maximum acceleration caused bythis quake was however, not as marked as in Assam quake of 1897. As far as the question of its origin is concerned, there is some agreement that thisearthquake was not caused by displacements along the Himalayan Boundary Faults, butthat a more probable source of disturbance lay in the folded and fractured zone of thecrust underneath the Gangetic basin. The movement along such highly inclined fracture(s) was the probable cause of this earthquake.The Quetta Earthquake in Baluchistan (1935). This earthquake occurred on the night of31st May, 1935. It was more or less a locally confined (less widely spread) quake,bringing unusual destruction of life and property on the town of Quetta. In a few minutes,this large military station was converted into a grave-yard with death toll mounting toabout 20,000. The epicentral tract was calculated to be only about 110 km long and 26 km widebetween Quetta and Kalat, away from which the intensity of damage rapidly decreased.The fact that the intensity of damage fell off rapidly away from the epicentre, makes itquite evident that the focus of this earthquake could not be very deep-seated.Assam Earthquake (1950). On the evening of 15th August, 1950, north-east Assam wasshaken by an earthquake of high intensity, comparable in some respects to the 1897quake. The area suffering extensive damage was about 39,000 sq. km. including thedistricts of Lakhimpur, Sibsagar, and Sadiya. This earthquake was accompained by the usual surface effects, such as ; hugeground fissures discharging sand and water ; sub* sidence of ground at various places ;altering the level of the lands, and thus, affecting the drainage of the country, andbringing extensive floods. Landslides causing temporary obstructions in rivers, led tomore serious flooding for months after the quake. The peculiarity of this quake was thatEINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG
  28. 28. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGGmore damage to life and property was caused by these indirect river floods than directlyby the earthquakes. The epicentre of this earthquake, as recorded by seismographs in India and othercountries, was found to be about 320 km north of Sadiya in mountainous country on thenorth east border of Assam.Seismic Zones of IndiaBy our knowledge of Geography, most of us may be aware of the fact that India, andmore precisely the Indian sub-continent (which includes India, Burma, Pakistan andBangla Desh) can be divided into three zones. These divisions are : (i) Extra-Peninsula; (ii) Indo-Gangetic plain ; and (iii) Peninsula. These three divisions are shown in Fig. 6.16. In fact, the India has been geologically divided into these three distinctive zones,not on the basis of any topography or relief, but on the basis of their geological history,structure, and physiographic. And it is these, that impart to each of these divisions, itsown individuality and distinctive character. The Extra-Peninsula is the most unstable mountainous region ; the Indo-Gangeticplain is moderately unstable plain area; and the Peninsula is an old formation which isquite stable and geologically inactive. Since the occurrence of earthquakes (precisely speaking tectonic earthquakes) isvery much connected to the geological stability of a region, it can be easily stated that theearthquakes will occur most severely in the Extra-Peninsula, less severely in the Indo-gangetic plain, while the third stable region of Peninsular India would almost be freefrom them. However, when seismic shocks are to be felt in the first two zones, naturallythe third zone will also get some shock (through of lower intensity) in addition to someminor shocks of its own origin. Depending on such reasonings, and for all practical purposes, India can be dividedinto three seismic zones. These three seismic zones are ; (1) Highly seismic zone or the zone of maximum intensity ; (2) Moderately seismic zone or the zone of intermediate intensity ; and (3) Poorly seismic zone or the zone of minimum intensity.These three zones respectively represent Extra-peninsula, Indo-gangetic plain, andpeninsula, regions of India ; and are briefly described below:Highly Seismic Zone. The Himalayan ranges are the areas which are still passing througha state of instability. They are comparatively recent formations, being formed by orogenicuplifting over the seas and oceans. Their stratified rocks are very much folded andfaulted. Some of these faults are thrust faults. One of these, very well known in thegeology of India, is the Main-Boundary-Fault which runs all along the foot hills ofHimalayas, and demarcates the junction between the older (Trikuta and Murres) rocksand the younger Siwalik rocks. It is a reverse fault produced during the last phase of theHimalayan upheaval; and beingyoung, it is not difficult to locate it in the field. This faultruns all along the foothills of Himalayas from Punjab to Assam, and thereby makingthese areas susceptible to occasional earthquakes. Similarly, the areas where theHimalayan ranges make sudden and sharp bends southwards, such as the one near Gilgitand the other near Sadya (as shown in Fig. 6.17), are highly seismic. On account of suchreasons and due to the existence of other weak structures in Himalayan mountains and itsEINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG
  29. 29. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGGneighbouring areas, the extra-peninsula is a highly seismic zone, and is susceptible torapid earthquake disturbances. Some of the important earthquakes that have occurred inthis region are those of Assam in 1897, of Kangra in 1905, of Bihar in 1934, of Quetta in1935, and again of Assam in 1950, as described in the previous article.Moderately Seismic Zone. The Indo gangetic plain has been, formed by the deposition ofalluvial sediments brought by Himalayan rivers over a passage of time. There exist verythick alluvium deposits between the two rocky portions {i.e. extra-peninsula andpeninsula), which shows that the alluvium must have been deposited in a great and deeplongitudinal depression that must have existed in front of Himalayas. It has not beenpossible so far to explore the basin below these alluvium deposits, because of their greatdepths. In any case, it is believed that this basin as well as the alluvium deposits have notattained stability, and they are passing through an intermediate stage towards attainingstability. The folded and fractured unstable rocks existing in this region may, thus, atmany times, give way, producing earthquakes-although of moderate intensity. This Indo-gangetic plain area is therefore, included in this moderately seismic zone (Refer Fig.6.16).Poorly Seismic Zone. The entire triangular plateau* like region of the south, called thepeninsula or the Deccan plateau, which is a region of remarkable stability, is included inthis zone. This part of India is geologically inactive, as besides being very old, none ofthese excepting the Aravallis (mountain ranges) is the result of erogenic or mountainbuilding forces. On the other hand, this region represents mountains of circum-denudation, the remnants of hard rocks that have strangely escaped denundation, standingout as outliers. Due to its inherent stability, earthquakes are very rare in this zone. However, themarginal areas (i.e. areas along the boundaries with seas) are still somewhat prone toearthquakes, which proves that they have not yet reached complete state of equilibrium ;and some minor earthquakes are possible in this zone also. The main purpose of dividing the country into these three seismic zones is to takesuitable necessary precautions, while designing engineering structures and otherconstructions in different areas, so as to make them proof against quakes. In order to takecare of these additional forces of earthquakes, generally additional safety factors are usedwhile designing structures in seismic areas, depending upon the possible intensity of anexpected earthquake. Normally, additional seismic force factor of 0.1 to 0.15g is takenfor the first zone, 0.05 g for the second zone, and 0.01 g for the third zone. Such seismicconsiderations, which are made while designing modern engineering structures, arebriefly discussed below :Engineering Considerations in Seismic Areas During the recent times, the earthquake engineering has made considerableprogress, and most of the countries have been able to demarcate their different areas,indicating the severity or intensity of the possible earthquakes. Attempts are also beingmade to predict the quakes in advance. Rather, a major earthquake in Assam—due forsometimes in the years 1977-80 had been predicted by certain seismologists. But theprediction seems to have gone wrong. In any case, whether we are able to predict them ornot in advance, the job of a civil engineer who has to design and construct engineeringstructures in the seismic areas, is in no way lessened. He will have to design andconstruct these structures, in such a way that they remain intact even during the worstEINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG
  30. 30. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGGquake (s), that may occur during the life time of the structure. This job is a difficult job inthe precise sense, firstly because too much provision cannot be made for earthquakeswhich may not occur at all during the life of the structure; and secondly because theforces produced by earthquakes are quite complex and not fully amenable to perfectmathematical solutions. During an earthquake, the ground on which the structure is erected, suddenlyacquires very strong motions in directions which can not be known before hand, and fora duration which is also indeterminable. Not only the direction of motion but also thenature of the motion of the ground is highly complex and unpredictable. It might behorizontal acceleration, or vertical acceleration, or rotation, or a combination of some orall of these, simultaneously The problem becomes still more complicated as it is notknown whether the structure has to withstand one or two shocks or repeated shocks,during the life time of the structure. Moreover, the stresses that are likely to develop in astructure during an earthquake are entirely of dynamic nature, which are different fromthose determined for static conditions, as is the practice with engineering structures..These dynamic stresses are of a magnitude normally beyond the scope of theoretical andpractical analysis. Similarly, the periods of seismic vibrations, which are still beyondprediction, are more dangerous. From the above discussion, it becomes evident that the design of ah earthquakeproof structure is quite a difficult task, and rather its perfect design is impossible with thepresent day knowledge, Still, however, based on a lot of research work carried out in thisfield, engineers have evolvedgeneralguidelines and codes for earthquake-resistantconstructions, for various countries. The Indian BIS code for such constructions isnumbered as I.S. 1893—1979, and may be referred to in special needs. These codes and guidelines are, infact, highly simplified solutions of thisextremely complicated problem ; and although they are not fully satisfactory, but shouldbe followed strictly, so as to increase the chances of minimising the probable lossesduring a future shock in the given seismic region. A brief summary of some such generalguidelines, which are to be followed in designing buildings and dams in seismic areas, isgiven below.Safety Measures to be adopted for Buildings to be Constructed in Seismic Areas. Asargued earlier, during an earthquake, the to and fro, or up and down motion of the groundshakes the structures. A compact and a sturdy structure built on rigid foundations mayjust oscillate along with the ground vibrations, and survive even a strong shock. Poorlybuilt structures, on weak and soft foundations, may however, get badly destroyed. While constructing structures or buildings in seismic regions, therefore, specialcare must be taken to choose an excellent foundation ground, and to use excellentbuilding materials. Moreover, additional safety factor must be considered, whiledesigning buildings or other structures in such areas. This additional safety factor means the consideration of base shear force whichtends to topple the buildings, etc. This inertia force can be calculated by the seismiccoefficient which is, defined as the ratio of the ground acceleration to the accelerationdue to gravity, i.e.(α/g)EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG
  31. 31. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG The ground acceleration (a) for a given seismic area can be computed for the max.magnitude (M) of a past earthquake by using empirical formulas. However, for allearthquake prone areas, the values ofa have mostely been standarised in terms of g. ForIndia, it generally varies between 0.15 g (for highly seismic zone) to O.Olg (for poorlyseismic zone). The base shear force to be accounted for, as the additional force formaking earthquake resistant buildings, is then given by: F=(α/g).W ...(6.3)where F = additional shear forceW = weight of the building or structure.The overturning moment can also now be calculated very easily by using M = FY ..(6.4)where Y is the vertical distance of the e.g. of the structure above the horizontal sectionunder consideration. The building or the structure should be designed to take care of these additionalforces and moments.In case of multi-storeyed buildings, allowance may be made for the increased flexibilityof the structure, by calculating the base shear force on the formula 4.5 𝛼 F = 𝑁+4.5 𝑔 . 𝑊 where N is the number of storeyes above the surface under consideration. As stated earlier, an earthquake-resistant building must be strong and sturdy.Hence, besides incorportating these additional safety factors in the design of suchbuildings, the following other points must be given due attention to : (i) Good quality materials, strictly according to the specifications, should be used. (ii) The foundation should not be on soft ground, and rather it should preferablybe on the solid rocks. The depth of foundation should also be uniform. (iii) The walls should be continuous in nature, i.e. the long walls and cross wallsbe erected simultaneously without any joints. (iv) Doors and windows should be minimum ; and should not be in vertical rows,and should preferably be along the diagonals. (v) R.C.C. may be preferred to brick work. (vi) A.C. sheet roofing should be avoided, and more feferably R.C.C. flat roofsshould be laid. (vii) Height of the building should be kept uniform, as it adds to its stability. (viii) All parts of the building, particularly its edges and corners should be welltied, so that it moves as a single unit during an earthquake vibration. (ix) Construction of cantilevers, chimneys, domes, arches and other extraprojections should be avoided.Safety Measures to be Adopted for Dams Located in Seismic Areas. When a dam is tobe located in earthquake prone areas, then besides taking general precautions of usingexcellent building materials and choosing excellent strong foundations and abutmentrocks, we have to give due consideration to seismic effects, while designing the dam. In case of a gravity dam, the horizontal acceleration due to an earthquake causestwo types of forces, i.e. (α) One force is the ordinary inertia force or base shear force which is given by 𝛼 F = Force due to inertia (D) = 𝑔 . 𝑊EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG
  32. 32. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGGwhere 𝛼h as the ground acceleration resolved in horizontal direction. (b) Another force which is especially caused in dams is that which causesmomentary increase in the horizontal water pressure exerted by the reservoir, and iscalled the hydrodyanamic pressure. This extra force is caused during an earthquake, asthe dam accelerates towards the reservoir, and the water resists the movement owing toits inertia. The hydrodynamic force can generally be evaluated by Von-Karman equation,given by Pe = Force due to hydrodynamic pressure = 0.555(αh/g).γw H2 ...(6.6) 4𝐻and its acts at a height of 3𝜋 above the base. where H is the height of the dam and γw is the unit wt. of water.The effect of vertical acceleration is only to reduce the effective weight of the dam, thenet effective weight being given by 𝑊 W = W- 𝑔 au au = W(1- ) ...(6.7) 𝑔 where au represents the ground acceleration in vertical direction. The effects of earthquakes prove much more serious in case of earth dams, ascompared to that in the case of solid masonry or concrete dams. This is because of thefact that the materials used in earthen dams are generally loose and weak in nature.Hence, for earthen dams, to be constructed in seismic areas, it is absolutely essential toprovide a clay core within the structure, which makes it impermeable, strong and stable.Moreover, the free-board to be provided in such areas should be about 50% more thanthat is reqd. in normal areas, this is because when earthquake comes, the water of theoscillating reservoir rises up and tends to overtop the dam.Earthquakes Triggered off by Dam Reservoirs, and Preventive Measures. During therecent times, a very strange phenomenon has been observed in several dams of the world.What actually has happened is that the reservoir basins or areas which wereseismologically inactive, started showing seismic activities as the reservoir was filled upwith water. The magnitude of these earthquakes, increasing with the filling of thereservoir, thus giving severe shocks when the reservoir was full. In all such cases, theepicentre is always seen to be located within the resevoir or along its border. An important Indian example of such earthquakes has been in the case of Koyhadam (Maharastra), which is situated in the seismologically inactive zone in peninsularIndia. However, when the dam got ready and water started collecting (1962) in the reser-voir, the earthquakes also started occurring (1963). The frequency and intensity of theseearthquakes had been increasing as more and more water went on collecting in thereservoir. On 10th December, 1967, the severest shock occurred with a magnitude ofabout 6.5, followed by three more shocks of decreasing magnitudes. The epicentres of allthese shocks were traced to be within the reservoir area. Some other examples of such earthquakes outside India are of Lake Mead(U.S.A.) ; Grand Val lake (France) ; Vorgorno lake (Switzerland); etc. This phenomenon had been studied by various seismologists, and it has beensuggested by some that in most such cases, there must have existed some inactive faultsin the reservoir basin area, which became active again due to the extra-ordinary load ofEINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG
  33. 33. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGGreservoir water, thus, causing displacements along these faults, and consequentlyresulting in earthquakes. Some other seismologists have suggested that these earthquakesare caused due to increased pore pressure in the adjoining rocks, which lowers theirshearing strength, resulting in the release of tectonic strain. Based on these explanations, the various preventive methods suggested forpreventing or reducing such earthquakes, include : (i) filling of the reservoir to a limitedsafe level ; (ii) reducing pore pressure by draining out water from weaker adjoining rocks; (Hi) to actively explore the dam site for the absence of inactive faults before selectingthe same. Such earthquakes, however, show a decreasing tendency with time. Thisphenomenon is indeed very complex and interesting, and still needs further research.WEATHERINGThe physical and chemical conditions of rocks are altered when they are exposed to theatmosphere. Such an altered product is known as weathered material and the processinvolved is called weathering. When weathering is accompanied by erosion it results inthe denudation of an area.GEOLOGY AND WEATHERINGAll exposed surface of the Earth are susceptible to weathering. Only the degree of in-tensity of weathering may vary from place to place, because weathering depends onseveral factors like location on the surface of the Earth with reference to latitude andlongitude, elevation, climate etc. Everlasting mountains are not everlasting in reality. Allare subject to endless change.The crust of the Earth is composed of rocks. Rocks themselves are composed of minerals.These are both in solid form and in loose form. The physical and chemical proper-.ties of minerals control weathering. For exam; mineral possessing cleavage may be easilywe If a mineral is composed of easily soluble then it is weathered.AGENTS OF WEATHERINGThe agents of weathering are atmospheric gases, temperature, water, lightning andorganisms. Atmospheric gases like CO2 mixes with rain water and forms carbonic acidand affects exposed surfaces. Temperature fluctuation causes contraction and expansionwhich weakens a rock. Water is by far, the chief agent of weathering. Obviously, water isa good solvent. It dissolves materials as it flows. In cold climates freezing and thawinggreatly effects disintegration of rocks. Lightning dissociates atmospheric gases into ionswhich are more reactive. Considerably, heat energy is also released. Growth of organismsdisintegrates and decays rocks. When plants die they turn into humus and on combiningwith water humic acid is produced.RATE OF WEATHERING3. MOISTURE: Moisture plays an Important rote in weathering. Freezing and thawingof moisture results In disintegration of rocks. Chemical weathering is by solution activityof water. Water, by supporting organic growth, aids weathering by organism.Mechanical stress caused by the change in volume of water resufts in the disintegration ofrocks. Chemical reactions such as solution, carbonation, hydration and hydrdysfe occur inthe presence of water. Hydrogen ion concentration (pt-fy controls these reactiona The pHvalue ranges from 1 for add to 10 for alkali. 7 is the neutral and is shown by river and fabwater. The pH value determines the solubility of a substance. For example iron dissolvesEINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG
  34. 34. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG100,000 times more at pH 6 than at pH 8.5. Thus dissolved iron in river water getspredpftateti at sea In desert regions a small amount of water performs weathering withtemperature changes. Seasonal water supply will influence weathering a lot.4. ALTITUDE: The temperature and the mGisturg content of atmosphere varies withaltitude. Hence the rate of weathering of rocks depends on their altitude.Rate of weathenng depends on the followingfactors.1. Mineral or chemical composition2. Temperature 3. Moisture4. Altitude .5. Tectonism and6. Organisms1. MINERAL OR CHEMICAL COMPOSITION:Minerals differ in their durability according to their chemical composition. Rocksconsisting of less durable minerals are easily weathered. Removal of cer-"Jafe mineralsrenders a rock porous. Weathering may be rapid in a porous rofck.2. TEMPERATURE: Rate of weathering increases with the increase of temperature.Temperature changes involve a freezing and thawing of water which amounts to thedestruction of rocks. Chemical reactions involved in weathering are faster whentemperature is high. Increasing temperature removes organic matter and dissolves silica5. TECTONISM: Areas affected by earth movements (tectonism) are more prone toweathering. These deformations develop joints, fractures, fault planes, folds andcleavages in rocks. These structural ruptures permit flow of water and weathering results.A smaH crack may be widened through temperature fluctuations.6. ORGANISMS: Plants and animals take part In weathering directly or indirectly.Plants release carbon dioxide which is an essential partner in weathering. By growth theydisintegrate rocks. Animals from worms to, man, take a little share in the rockweathering.PROCESSES OF WEATHERING Weathering occurs in three ways. Rocks break due to stress developed in ti ,em, mechanically. Rocks decay due to chemical reactions on them. Organisms by way of growth and movement alter the rocks physico-chemical conditions, although these three ways of weathering seem to be separate entities they are interconnected. And further .one process induces the other process. As such, these three ways are termed: 1. Physical or Mechanical weathering, %. Chemical weathering and 3. Biotic weathering.PHYSICAL WEATHERINGRocks and minerals are disintegrated into smaller and smaller particles. The processeswhich bring about the disintegration without any chemical reaction as a response to thechange in conditions of environment are collectively known as mechanical or physicalweathering. The chief processes that result in mechanical weathering are Frost Action.Exfoliation etc. The agents are water and temperature. When overlying rocks areEINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG
  35. 35. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGGremoved the pressure on underlying rocks is released. It causes sheeting and spatting ofrocks.FROST ACTION (FROST RIVING)In high altitudes and high latitudes frost is the chief agent of mechanical weathering. Theprocess of freezing and melting of water due to temperature change and its effects on therocks is known as frost action or frost riving. Frost action is best seen in areas wherefreezing and thawing occur many times a year. In permanently frozen areas it is lesseffective. Rock debris in glaciated region is mainly due to frost action. There are twotypes of frost action, namely frost wedging and frost heaving.FROST WEDGING: Frost wedging refers to the lateral sepStttton of blocks along avertical joint. This hap-$»fts as foHows. Water fills the gap, even the capillary fissures.During winter water freezes to ice. Normally water expands in volume by 10.9 per centwhen it freezes. This expansion exerts some pressure on the walls of fissures or cracks.The pressure is equal to 110kg/cm2. Over a period of time, repeated freezing pushes rockblocks apart and finally disintegrates.Water, openings in rocks and temperature collectively create a suitable environment forthe occurrence of frost wedging.FROST HEAVING: Frost heaving occurs when wajter beneath the surface freezes duringcold season. Micro layers of ice are formed. These layers exert a vertical pressure and therocks disintegrate.SALT ACTIONIt is analogous to frost action but is less significant. Repeated solution and crystallizationof salts in the water filling the crevices cause disintegration of rocks.On the other hand, materials once like silica, hyd rated oxides of iron, manganesealuminium do not redissoive. These substances ment the cracks and bind the blocks.SHEETINGRocks in depth are under a great confining | sure due to the overburden of rocks or water,tion removes overburden. It effects the stress. This produces fractures parallel to thesurface] a regular manner. These fractures render rocks sheets and the process is termedsheeting. Sheeting i more commonly associated with the igneous particularly of membersof granite family. As depth I creases the thickness of sheets increases. This an increase inthe space between joints. In quar operations sheeting is felt as explosions. When a face"is opened afresh it bursts releasing the confining pressure.. Rockfails occurring along canyon walls are attributed to this process.Basalts of ocean floor when brought out of waterMISCELLANEOUS PROCESSESMechanical weathering may include disintegration of rocks due to the effect of insolationand forest fire which cause the surface to cleave and crack. Further the effect of abrasion,wetting and drying, cavita-tion and collapse due to undercutting may be added. Impactand/or the friction of boulder crumbles another, rock. Alternate wetting and drying alongthe shore platforms affect largely. The exploding bubbles of turbulent water effectscavitation.CHEMICAL WEATHERING The transformation of rocks into new substances is known as chemical weathering. Natural chemical agents alter the chemical composition, the structure and the physical, appearance of the original. It is a complex process, involvingEINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG
  36. 36. EINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG chemical reactions, of responses of rocks to water and gases of atmosphetiM at or near the surface of the Earths crust. It leads to decomposition.AGENTS OF CHEMICAL. WEATHERINGThe chief agent is water. Water transforms into carbonic acid when combined withcarbon dioxide. Decayed vegetation combining with water transforms into huttiic acid.These acids and water dissolve or react with rocks they contact and give rise to newmaterials. Thus the constituents of atmosphere, namely water, carbon dioxide, oxygenand vegetation form an agency of chemical weathering.RATE OF CHEMICAL WEATHERINGThe rate of chemical weathering depends on the mineral-composition, size of thematerials and the environment.1. MINERAL COMPOSITION: The stability of minerals is not uniform. They aredifferently decomposed according to their chemical constituents. Basalt is easilyweathered rather than granite.REACTIONS OF CHEMICAL WEATHERINGSolution, oxidation/reduction, carbonation, hydration, hydrolysis arid chelation are thereactions through which chemical weathering takes place.SOLUTION: Water, being a good solvent, dissolves A number of substances. Ifsaturated, precipitation occurs elsewhere. The removal of soluble matter from regolith orbed rock by water is termed leaching and the residue, iechates. Water soluble rockexamples are rock salt, gypsum and limestone in the Order of decreasing solubility. Rocksalt survives solution in the moist arid regions. Outcrops of gypsum are not seen in humidregions since surface waters dissolve it. Limestone dissolves in water and forms valleysin humid regions and cliffs in arid regions. A comparative chemical analyses of rainwater and stream water reveal how effectively water dissolves mineral matter.2. SIZE OF THE MATERIALS:Larger the block lesser is the area available to chemical action. Smaller the fragmentgreater is the area and in turn faster the ntte of chemical weathering.3. THE ENVIRONMENT: The environment of a rock determines the rate. Warm moistclimate, gentle slopes and abundant vegetation are most suitable environment forchemical weathering.OXIDATION/REDUCTION: Rain water behaves as mild acid because of dissolved CO2and Oxygen with pH value 6-7 and Eh value +0.3 to 0.4 when ft lates through decayingmatter (humus) its acidity Increased forming hufflic acidv With this nature it oxidisessubstances to form oxides. The process of combination of oxygen with another element isknown as oxidation. Oxidation is speeded up in the presence of water. Oxygen hasaffinity for irOR. iron is a common constituent element in rock forming minerals such asbiotite, augite and hornblende. Ferrous iron transforms to ferric iron on oxidation. Inalkaline en-vironment ferrous iron converts to ferric hydroxide.3MgFeSiO4 (-t Olivine + Water2H2O)-» H4MgSi2Og + SiO2 + 3FeO -^^Serpentine + Silica + Ferrous Oxide4FeO ( + O2) ^ Ferrous Oxide + Oxygen Hematite + Water -* UrnoniteEINSTEIN COLLEGE OF ENGINEERING DEPT OF CIVIL ENGG

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