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Sm Chapter II Sm Chapter II Document Transcript

  • Chapter II SOIL FORMATION frame work known as geologic cycle. This Specific Objectives includes many processes acting To identify the sources of soil. simultaneously. The most significant of these To identify the rock forming being with molten magma from within the earth minerals and types of rocks. forming into rock, then continue with the rocks To understand the geology behind being broken down into soil, and soil being rock weathering. converted back in to rock (fig.no.1). To understand process of Soils are the products of weathering of weathering and soil formation. rocks. Soil grains are disintegrated rock To co relate the agency of crystals. As such the behaviour of soils is weathering and properties of soil formed. dependent on the rock from which they To co relate the properties of soil to originated and also on the process of type of rock from which it is got. weathering and agency responsible for deposition as we see later in this discussion. Rocks–The Sources of Soils Earth materials are divided into rock and soil. Normally, the soil particles are the result of weathering of rocks and decay of vegetation. Some soil particles may, over a period of time, become consolidated under the weight of overlying material and become rock. For civil engineers rock is a “hard, durable material that cannot be excavated without blasting”. The following are the important differences between these two materials. Rocks are generally cemented; soils are rarely cemented. Rocks usually have much lower porosity than soils. Rock masses are often discontinuous; soil masses usually can be represented as continuous. Rocks have more complex and generally unknowable stress histories. In many rock masses, the minor principal stress is Fig.no.1. Rock–Soil formation geological cycle. vertical; in most soils the major principal stress is vertical. So an understanding of rocks is necessary The geologic processes acting on earth’s to understand the behaviour of soils. Also crust are extremely slow by human time scale, some of engineering structures are constructed and as such no direct observation is possible. directly on or in rock. For instance gravity dams Theories have been developed on the basis of are usually directly rested on rock because of observations of the earth as it exists now. large masses it transfers and high bearing Geologic theories are organized around a capacities it demands from the supporting
  • HAWASSA UNIVERSITY FACULTY OF TECHNOLOGY CIVIL ENGINEERING DEPARTMENT media. Hence an understanding of rocks and precipitates out of solution. It also precipitates discontinuities associated with it and the in soil acting as cementing agent. formation of rock is also necessary for a civil Mica – Translucent thin sheets or engineer. flakes. Muscovite has silvery flakes, while Biotite is dark grey or black, which causes Rock Forming Minerals shear failures in certain rocks, such as Schist. Minerals are naturally formed compounds with specific structures and chemical Types of Rocks compositions. As the basic constituents of Rocks are classified according to their rocks, minerals control much of rock behaviour. place in the geologic cycle. The three major Some minerals are very strong and resistant to categories are Igneous, Sedimentary and deterioration, and produce rocks with similar Metamorphic rocks as discussed below. properties, while others are much softer and Igeneous Rocks: Igneous rocks form produce weaker rock. More than 2000 different when molten magma, from deep inside the minerals are present in the earth crust. Most earth, moves upward toward ground surface common minerals in majority of rocks are and gets cooled. Intrusives or Plutonic rocks discussed below. form below ground surface, where they cool Feldspar – This is the most abundant slowly and are coarse grained. Extrusives or mineral. Orthoclase feldspar contains Volcanic rocks arrive at ground surface in potassium (KaAlSi3O8) and usually range from molten state through volcano and cool rapidly white to pink. Plagioclase feldspar contains acquiring a fine grained structure. However, a sodium (NaAlSi3O8), Calcium (CaAl2Si2O8), or minority of igneous rocks are formed by both, and range from white to grey to black. volcanism. Most of them however have been They have moderate hardness. formed by the cooling of liquid crust of earth Quartz – Also very common and major during Precambrian age. Some extrusive ingredient in rocks. It is a silicate (SiO2), and material, such as volcanic ash, bypasses the usually has a translucent to milky white color. rock stage and forms directly into sediment. Quartz is harder than most minerals and resistant to weathering. Some common igneous rocks are Ferro magnesium minerals – A class - Granite: An intrusive and of minerals, which contain both iron and most common and familiar rock. It contains magnesium. This class includes Pyroxene, primarily orthoclase feldspar and Quartz, Amphibole, Hornblende, and Olivine. These with some Biotite and Amphibole. minerals have dark colour and moderate - Basalt: A dark, dense rock, hardness. most abundant extrusive rock. Very hard, Iron oxides – Another class of however possesses joints due to rapid minerals which essentially contains iron cooling. (Fe2O3), includes Limonite and Magnetite. - Diorite: Similar to granite, with Though less common give a distinctive rusty Plagioclase feldspar instead of Orthoclase colour to some rocks and soils and act as with little or no Quartz. cementing agents. - Andesite: A very hard Calcite – A mineral made of Calcium extrusive. carbonate (CaCO3); usually white, pink or grey. - Rhyolite: Extrusive equivalent Insoluble in water; and thus can be transported of Granite. by ground water into cracks in rock where it ----------------------------------------------------------------------------------------------------------------------------- 10 SOIL MECHANICS – I CHAPTER II SOIL FORMATION M U JAGADEESHA
  • HAWASSA UNIVERSITY FACULTY OF TECHNOLOGY CIVIL ENGINEERING DEPARTMENT - Gabbaro: The intrusive organisms on the bottom of the ocean, and equivalent of Basalt. Darker in colour than usually extend over large areas. Some of Granite or Diorite. these deposits were later uplifted by Unweathered igneous rocks generally have tectonic forces of the earth and now exist excellent engineering properties and are good below land areas. materials to build on. Intrusive rocks are Chalk- these are similar to limestone especially good. However fractures form but much softer and porous. planes of weakness. Dolomite- similar in grain structure Sedimentory Rocks: These form the and color to limestone and are in fact, lime second major category of rocks. They are stones in which the calcite (CaCO3) formed due to induration or lithification of soil interbonded with magnesium. Hence, the deposits. These are of two types, viz.,Clastic principal ingredient of dolomite is calcium and Carbonate. magnesium carbonate [CaMg(CO3)2] • Clastic rocks – Clastic rocks are formed when deep soil deposits become hardened Metamorphic Rocks: Metamorphic as a result of pressure from overlying rocks are much less common at the earth’s strata and cementation through surface than sedimentary rocks are. They are precipitation of water soluble minerals such produced when Igneous and Sedimentary as calcium carbonate or iron oxide. rocks literally change their texture and structure Because of their mode of deposition, many as well as mineral and chemical composition, clastic rocks are layered or stratified, which as a result of heat, pressure and shear. make them quite different from massive Some metamorphic rocks are foliated, formations. which means they have oriented grains similar Most Conglomerate, Breccias, Sand to bedding planes in sedimentary rocks. These stone and Arkose rocks generally have foliations are important because the shear favorable engineering properties. Those strength is less along these foliations. cemented with silica or iron oxide are Common foliated rocks: especially durable and are difficult to Slate- derived from shale, dense. excavate. Some fine and very fine grained Schist-with large mica content, foliation is Clastic rocks are subject to slaking, which called schistosity. is a deterioration after excavation and Gneiss- derived from granite, coarse exposure to the atmosphere and wetting grained banded rock. and drying cycles. Rocks that exhibit Common non foliated rocks: strong slaking will rapidly degenerate to Quartzite-composed mainly or entirely of soil, and thus can create problems for quartz, derived from limestone, very strong engineering structures built on them. and hard. • Carbonate rocks – A different type of Marble-derived from limestone or dolomite, sedimentary rock forms when organic used for decorative purposes and for materials accumulate and become statues. indurated. Because of their organic origin, they are called carbonates. Common Structural Geology carbonate rocks are, Structural geology is the study of the Lime stone- common type of configuration and orientation of rock carbonate rock, is composed primarily of formations. This is an important part of calcite (CaCO3). Most lime stones are engineering geology because it gives important formed from the accumulation of marine insights on how a rock mass will behave. ----------------------------------------------------------------------------------------------------------------------------- 11 SOIL MECHANICS – I CHAPTER II SOIL FORMATION M U JAGADEESHA
  • HAWASSA UNIVERSITY FACULTY OF TECHNOLOGY CIVIL ENGINEERING DEPARTMENT Bedding planes and schistocity: mass, so they form potential failure surfaces. Sedimentary rocks are formed in horizontal or There are three types of fractures. near horizontal layers and these layers reflect - Joints: are fractures that have not successive layers of deposition. This process experienced any shear movement. They may produces parallel bedding planes. When these be due to cooling (in case of igneous rocks) rocks are uplifted by tectonic forces in the tensile tectonic stresses etc. Joints usually earth, the bedding planes usually were rotated occur at fairly regular spacing and a group of to a different angle, as shown in fig.no.2. such joints is called a set. - Shear Jones: are fractures that have experienced a small shear displacement, of the order of few cms. Serve as water conduits. - Faults: similar to shear zones, except they have experienced much greater shear displacements (>1m). These displacements are normally associated with earth quakes. Faults are classified according to their geometry and direction of movement, If overhanging block is moving downward – normal fault, if it is moving upward – reverse fault. A fault with small dip angle is called thrust fault. In the above Dip slip fault movement is primarily along dip. In strike slip fault movement is primarily along strike. The fault trace is the intersection of the fault and the ground surface. The term discontinuity is often used to include bedding planes, Fig.No.2. Orientation of bedding planes. schistocity, joints, shear zones, faults and all other defects in rock. A slope as suggested in fig.no.2.(a) is more likely to fail than that in fig.no.2.(b) as it undermines the bedding planes. Many land slides have occurred on slopes with unfavourable bedding orientations. Folds: Tectonic forces also distort rock masses. When horizontal compressive forces are present, the rock distorts into a wavy pattern called folds. Sometimes these are gradual, other times very abrupt. When folds are oriented concave downward they are called anticlines; when concave upward they are called synclines. Fractures: Fractures are cracks in rock mass. Their orientation is very important because the shear strength along these fractures is less than that of the intact rock ----------------------------------------------------------------------------------------------------------------------------- 12 SOIL MECHANICS – I CHAPTER II SOIL FORMATION M U JAGADEESHA
  • HAWASSA UNIVERSITY FACULTY OF TECHNOLOGY CIVIL ENGINEERING DEPARTMENT Strike and Dip: When developing geologic maps, we are interested in both the presence of certain geologic structures and their orientation in space. A rock mass may be Fig.no.3. Strike and dip unstable if it has joints oriented in one The Dip (fig.no.3) is the angle between the direction, but much more stable if they are geologic surface and the horizontal, and is oriented in other direction, refer fig. no.2. For measured in a vertical plane oriented similar reasons we are interested in their perpendicular to the strike. The Dip also needs presence and orientation. Many of these a direction, when expressed together; this data structures are roughly planar, at least for short is called an altitude. distances, and therefore may be described by Sometimes we also need to know the dip defining the orientation of this plane in space. as it would appear in a vertical plane other than The strike (fig.no.3) is the compass one perpendicular to strike. This Dip is called direction of the intersection of the plane and apparent dip and computed as (refer fig.no.4) the horizontal, and is expressed as a bearing tan δa = tan δ sinα from true north. Fig.no.4. Apparent Dip ----------------------------------------------------------------------------------------------------------------------------- 13 SOIL MECHANICS – I CHAPTER II SOIL FORMATION M U JAGADEESHA
  • HAWASSA UNIVERSITY FACULTY OF TECHNOLOGY CIVIL ENGINEERING DEPARTMENT Rock Weathering and Soil produce, transport and deposit will facilitate the important function of engineering judgement. Formation Residual soils: when rock weathering Rocks exposed to the atmosphere are process is faster than the transport processes immediately subjected to physical, chemical, resulting soil remains in place. It is known as and biological break down through weathering. residual soil. It retains many characteristics of There are many weathering processes parent rock. The transition with depth from soil including, to weathered rock to intact rock is typically Erosive action of water, ice and wind. gradual with no distinct boundaries. Chemical reactions induced by In tropical regions, residual soil layers can exposure to O2, H2O and other chemicals. be very thick, sometimes extending hundreds Loosening through the growth of of meters before reaching unweathered rock. plants. Cooler and more arid regions normally have Growth of minerals in cracks thinner layers and often no residual soil at all. Thermal expansion and contraction. Land slides and rock falls. Abrasion from down hill movement of nearby rock and soil. Rock passes through various stages of weathering, eventually being broken down into small particles, soil. Weathering processes continue even after the rock becomes a soil. As soil become older, they change due to continued weathering. The rate of change depends on many factors including The general climate, especially precipitation and temperature. The physical and chemical make up of soil. The elevation and slope of ground surface. The depth of ground water table. saprolite The type and extent of flora and fauna. The presence of micro organisms. Decomposed granite (DG) is a sandy The drainage characteristics of the soil. residual soil. Shales, consisting largely clay minerals Soil Formation, Transport and weather to form clayey residual soils. Deposition Saprolite, used to refer for residual soils that are not extensively weathered and still Geotechnical engineers need to focus on retain much of the structure of the parent rock. soil than rock for two reasons. Laterite, residual soil formed in tropical I. More civil engineering projects are built regions. Soil is cemented with iron oxides, on soil. which gives it a high dry strength. II. Soil being generally weaker and more compressible than rock, is more often a source of problems. Glacial soils: Much of the earth’s Along with engineering properties of soils northern region was covered with huge masses and understanding of geologic cycle that of ice called Glaciers. They were extending up ----------------------------------------------------------------------------------------------------------------------------- 14 SOIL MECHANICS – I CHAPTER II SOIL FORMATION M U JAGADEESHA
  • HAWASSA UNIVERSITY FACULTY OF TECHNOLOGY CIVIL ENGINEERING DEPARTMENT to Ohio River in N. America and Germany in Europe. Now these areas are heavily populated and number of civil engineering activities happening in this area necessitates study of soil deposited by Glaciers. Glaciers grind down the rock and soil, and transport these materials over hundreds of Kms. As such resulting deposits often contain a mixture from many different sources. These deposits can have a wide range of hardness and particle size and are among the most Glacial deposit complex and heterogeneous soils. Till is soil deposited directly by Glacier, soil particles vary from clay to gravel. Soil deposited in ridges or mounds is called ablation till. These ridges and mounds are called moraines and are loose and easy to excavate. Soil caught beneath the Glacier, called lodgement till, has been consolidated heavily under the weight of ice. It has a very high unit weight and often is nearly as strong as concrete. They are sometimes also called as hard pan. Varved clays When Glaciers melted, they generate large Alluvial soils: Alluvial soils or fluvial soils quantities of runoff. This water erodes much of or alluvium are those transported to their the till and deposits it downstream forming present position by the rivers and streams. glaciofluvial soils (outwash) (fig.no.5). They are These soils are very common. more uniform and sources of sand and gravel. When river or stream is flowing rapidly silts The fine grained portion of till often remain and clays remain in suspension and carried suspended in run off water until reaching a lake downstream, while sands, gravels and or ocean, where it finally settles to the bottom. boulders are deposited. When they loose their These are called glacio-lacustrine soils and velocity more of the finer soils also are glaciomarine soils. Sometimes silts and clays deposited. Because of rapid flow in heavy were deposited in alternating layers according rainfall time and slow flow in draught alluvial to the seasons, thus forming a banded soil soils often contain alternating horizontal layers called varved clay. These soils are soft and of different soil types. compressible and pose problems. When a stream reaches the foot of a canyon, looses its velocity considerably and deposit much of its soil load. This process forms alluvial fans, most obvious type of alluvium. Fig.no.5. Glacial soils ----------------------------------------------------------------------------------------------------------------------------- 15 SOIL MECHANICS – I CHAPTER II SOIL FORMATION M U JAGADEESHA
  • HAWASSA UNIVERSITY FACULTY OF TECHNOLOGY CIVIL ENGINEERING DEPARTMENT Alluvial fan Braided streams are high gradient rapidly flowing streams. They are highly erosive and carry large range and quantity of sediment. Minor changes in velocity cause deposition of sediments. The deposits from braided streams are very irregular in stratification and have wide range of grain sizes. The grain sizes usually vary from gravel to silt. (fig.no.6) Clay size Braided stream deposits particles are generally not found. Soil in a given The valley floor in which a river meanders pocket or lense is uniform. Void ratio and unit is referred to as meander belt. As water moves weight may vary over a wide range at given through a channel bend, velocity along the depth within a lateral distance of few mts. inside edge decreases, while that along the outer edge increases. Therefore soil at outer edge is eroded and carried further while its sediment is deposited along inner edge called point bar deposits. This action, over a period of time, may increase the bend significantly, eventually leading the river to cut across a large bend to form Oxbow lakes (fig.no.7). Fig.no.6.Alluvim of braided streams. ----------------------------------------------------------------------------------------------------------------------------- 16 SOIL MECHANICS – I CHAPTER II SOIL FORMATION M U JAGADEESHA
  • HAWASSA UNIVERSITY FACULTY OF TECHNOLOGY CIVIL ENGINEERING DEPARTMENT formed due to action of water. They have respectively similar properties. Aeolian soils: Soils deposited by wind are known as Aeolian soils. This mode of transport generally produces very poorly graded soils because of the strong sorting power of wind. These soils are generally very loose and thus have only fair engineering properties. Wind causes soil transportation in the following ways. Suspension – wind lifts individual silt and clay particles to high altitudes and transports them great distances. Fig.no.7. Oxbow lakes Fig.no.8.Flood plains During floods, rivers overflow low lying areas. The sand and silt size particles carried by the river are deposited along the banks to form ridges known as natural levees. Finer soil Fig.no.9.Suspension, saltation and creep. particles are carried beyond levees on to flood plains (fig.no.8). These particles settle at Saltation – (saltacio – to different rates to form back swamp deposits. dance) soil particles become temporarily Oxbow lake may be filled during floods and the airborne, and then fall back to earth. Upon clayey particles settle to the bottom to form landing, the particle either bounces or highly plastic and compressible layers. dislodges another particle, initiating Lacustine and marine soils similar to another flight. Particles rise to 1m altitude glaciolacustrine and glaciomarine soils may be and move horizontal distances around 4m. ----------------------------------------------------------------------------------------------------------------------------- 17 SOIL MECHANICS – I CHAPTER II SOIL FORMATION M U JAGADEESHA
  • HAWASSA UNIVERSITY FACULTY OF TECHNOLOGY CIVIL ENGINEERING DEPARTMENT Creep: medium to coarse sands roll and slide along the ground surface. Aeolian sands can form irregular hills called sand dunes which migrate in wind direction between 2-3m per year. Aeolian silts often form deep deposits called loess. Because of its deposition mode, loess typically has a very high porosity. It is fairly strong when dry but collapses if wetted. These soils are very much prone to erosion and often have deep gullies. . ----------------------------------------------------- Note: ASSIGNMENT Give detailed information about a major soil failure from engineering history which is not mentioned in the handout. Colluvial soils: a colluvial soil is one Give two examples for different type of transported down slope by gravity. The cause rocks and soils other than those mentioned of movement may be creep or landslide. Creep in the handout. occurs due to gravity induced down slope Answer neatly, legibly and within three to shear stresses (fig.no10) four pages and submit it within one week from the day of completing the discussion on this chapter. Note: Hand outs are not exhaustive. Students are advised to refer numerous Fig.no.10.Colluvial soils references available in the library and suggested in the course outline. Creep may extend to depth 0.3 to 3m, maximum being at surface. Rapid down slope movement may be due to landslide and mud flow. ----------------------------------------------------------------------------------------------------------------------------- 18 SOIL MECHANICS – I CHAPTER II SOIL FORMATION M U JAGADEESHA
  • HAWASSA UNIVERSITY FACULTY OF TECHNOLOGY CIVIL ENGINEERING DEPARTMENT ----------------------------------------------------------------------------------------------------------------------------- 19 SOIL MECHANICS – I CHAPTER II SOIL FORMATION M U JAGADEESHA
  • HAWASSA UNIVERSITY FACULTY OF TECHNOLOGY CIVIL ENGINEERING DEPARTMENT ----------------------------------------------------------------------------------------------------------------------------- 20 SOIL MECHANICS – I CHAPTER II SOIL FORMATION M U JAGADEESHA
  • HAWASSA UNIVERSITY FACULTY OF TECHNOLOGY CIVIL ENGINEERING DEPARTMENT ------------------------------------------------------------------------------------------------------------------------- 21 ---- SOIL MECHANICS – I CHAPTER II SOIL FORMATION M U JAGADEESHA