Sm Chapter I


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Sm Chapter I

  1. 1. Chapter I INTRODUCTION For the purpose of understanding soil and Specific Objectives characterize it soils are classified. There exist To get a broad out line of various classifications of soil based on topics of study in soil Mechanics –I. particular purpose. Agricultural scientists To understand importance of classified soil based on fertility; road engineers soil mechanics in Civil Engineering. have classified soil on criteria of suitability of To identify typical problems soil for pavement construction and so on. From encountered in soil mechanics. engineering point of view soil have been To know the development of classified on the basis of plasticity soil mechanics over ages. characteristics and grain size distribution. To acknowledge the It has been very well said that most of the importance of learning from failures in problematic behaviour of soil are not on its soil mechanics. own, but due to water present in it. In soil To pay honour to those mechanics, we study behaviour of soil with worked for the development of the various moisture content, movement of water subject. through soil, its hydraulic conductivity / permeability, seepage forces on soil masses, The subject / course Soil mechanics piping failure, quick conditions etc. discuss over the properties of soil, formation of Stresses on soil mass due to its own soil, its behaviour under loads, movement of weight and due to externally applied load are water through it, pressure it exerts on interesting. Due to presence of water and low structures retaining it, its compressibility, its permeability of some soils these stresses are stability in its natural forms etc. quite different from stresses in homogeneous To understand soil and its properties and material. The particulate nature of soil adds causes of its behaviour it is necessary to know some complexity to the problem. These how a particular soil is formed, a soil deposit is concepts are deliberated in soil mechanics. formed. Whether a soil deposit was formed due A special characteristic of soil is to undergo to weathering of Igneous rocks or sedimentary large compression deformation without rocks or metamorphic rocks, whether it was undergoing failure and simultaneous increase formed by a process of mechanical weathering in its strength will be learnt in soil mechanics. or chemical weathering, whether it was And another unique phenomenon of gradual deposited at a place by water or wind or by transfer of load from entrapped water to solid gravity does have a bearing on the behaviour particles of soil, consolidation, reveals itself in of soil. soil mechanics. To learn and anticipate behaviour of soil Soil being particulate mass derives its from the point of view of engineering / resistance to external load / pressure etc from technology number of properties have been its shear strength. We will study basic defined, by knowing them for a particular soil properties contributing to its shear strength. its behaviour can be anticipated. These We also study the effect of water in voids and properties are index properties, consistency disturbance from its natural state on shear indices and grain size distribution. Soil strength. comprises of solid particles, water and air and It also enriches our knowledge as to the as such a three phase model has been magnitude of forces a soil exerts on its suggested for it. These simple properties of soil retaining structures, excavated cliffs etc. It enable us to understand it from engineering also deals with stability of slopes of soil under perspective. various conditions.
  2. 2. HAWASSA UNIVERSITY FACULTY OF TECHNOLOGY CIVIL ENGINEERING DEPARTMENT It deals with load carrying capacity of soil • Are natural or proposed earth or bearing capacity of soils, when it is slopes stable? If not what must we do homogeneous, layered, at edge of slope etc. to stabilize them? and when subjected to axial load along with • What type of foundations is moment and inclined load etc. necessary to support structures; how Apart from the topics listed so far it also we can design them? addresses various fields of knowledge • If project requires retaining associated with soil and necessary for walls, what kind would be best and engineering community like behaviour of soil how should we design them? under dynamic loads along with • How will the site respond to reinforcement, soil improvement etc. potential earthquakes? It is a well developed subject, with theories A safe, economical and satisfactory and concepts well defined, addressing host of solution to above issues can be obtained with situations, for civil engineers, in their projects / the help of soil mechanics. As we saw earlier construction. nearly all Civil engineering projects must be supported by ground and thus require at least Role of Soil Mechanics some Geotechnical Engineering. Soil is more or less taken for granted by an Soil engineers/ geotechnical engineers average person. It makes up the ground on usually begin by assessing the underground which we live, it is for growing crops and it conditions and the engineering properties of makes us dirty. Most people are not overly the various strata; process called site concerned with soil beyond these observations. exploration and Characterization. However a Civil Engineer, Geologist, The next step is to perform engineering Constructors, Hydrologists, Agronomists are analysis based on the information gained from deeply concerned over soil. site exploration. The analytical tools we use to “Virtually every structure is supported by perform these analyses are collectively known soil or rock. Those that aren’t either fly, float or as soil mechanics and rock mechanics. Thus fall over” said Richard L Handy. Most Soil Mechanics and Rock Mechanics are to structures of all types rest either directly or Geotechnical Engineering what Structural indirectly upon soil, and proper analysis of the Mechanics is to Structural Engineering. In both soil and design of structure’s foundation is fields; “mechanics” refers to the analytical necessary to ensure a safe structure free of tools, while “engineering” is a broader term that undue settling and/or collapse. A also includes the rest of the design and comprehensive knowledge of soil in a specific construction process. location is also important in many other contexts. Thus, study of soils should be an Some Typical Soil Problems important component in the education of Civil In design and construction of various Civil engineers. engineering projects we come across different For a Civil engineer Soil mechanics/ Soil problems related to soil. Some of them are engineering/ Ground engineering/ listed and diagnosed briefly below to Geotechnical engineering addresses host of underscore the role of Soil Mechanics for Civil issues few of which are listed below. Engineers. • Can the soils and rocks A typical problem we come across beneath a construction site safely frequently is foundation problem. One will support the proposed project? speculate as to a particular soil sustaining the • What will be the impact of any load coming from a column, wall or machine planned excavation, grading or filling? foundation. We want to know whether the soil in site able to withstand a particular footing ----------------------------------------------------------------------------------------------------------------Page 2 of 8 SOIL MECHANICS – I CHAPTER-I INTRODUCTION 2009-10 M U JAGADEESHA
  3. 3. HAWASSA UNIVERSITY FACULTY OF TECHNOLOGY CIVIL ENGINEERING DEPARTMENT pressure without failure and excessive settlement. When a vertical surface is required or a steep slope of soil, at which it can not stay without failure, is required we support the soil by a retaining wall or sheet pile wall or bracing. To design them safe and economically we need to know the soil pressure on them. What happens to its magnitude when rain water enters in to soil in the back fill? When an earthen dam is being designed we need to select slopes which are safe against failure ( sliding) also does not allow any considerable amount of water to pass through or seep through its body or through foundation soil. We need to examine water tightness and erodability of soil. For a roadway or railway we need to cut soil to a particular slope so that it won’t slide or fail. We speculate about angle of slope to ensure no failure occurs or methods of avoiding failure with required slope i.e. stabilizing measures. ----------------------------------------------------------------------------------------------------------------Page 3 of 8 SOIL MECHANICS – I CHAPTER-I INTRODUCTION 2009-10 M U JAGADEESHA
  4. 4. HAWASSA UNIVERSITY FACULTY OF TECHNOLOGY CIVIL ENGINEERING DEPARTMENT While designing a foundation for machine ground gets in to motion”. These design we need to have idea about behaviour of methods were usually satisfactory so long as foundation soil under cyclic loads. Also the the construction projects were modest in settlement under these loads is of interest for scope, similar to previous projects (and thus the designer. tied to experience) and built away from The above soil problems are very typical obviously poor sites. Using these methods, the and we come across them very frequently in ancient builders sometimes accomplished Civil Engineering projects. Apart from these amazing feats of construction, some of which there are numerous other issues related to soil still exist. Some dams in India were in service such as differential settlement, time bound for more than 2000 years. Pyramids built in settlement of saturated fine grained soils, pharos time (300BC) still stand tall resisting heave of soil on availability of moisture, stability natural forces. of excavation trenches, dewatering of site for During middle ages, builders built larger construction, underground contamination, uplift and more sophisticated structures such as pressures, seepage through crevices etc. cathedrals etc. These projects pressed builders beyond the limits of experience, so the old Historical development rules of thumb did not always apply, and Even though we are interested in concepts, unfortunate failures occurred. logic reasoning and application of them for the The Leaning tower of Pisa is the most betterment of man kind through safe and famous example of soil related problems from economical engineering achievement, historical this era. Its construction began in AD 1173 and vignettes are necessary to gain an appreciation continued off-and-on nearly 200 years. The for our heritage as engineers. It also helps us tower began to tilt during construction, so the understand how technical advances have builders attempted to compensate by providing occurred in the past, because this guides us in a slight taper to the upper stories. The developing future advancements. movement continued after construction, and by People have been building structures, 1982 the top of the 58.4m (192 ft) tall structure dams, roadways, aqua ducts and other projects was 5.6m (18.4 ft) off plumb. for thousands of years. However until recently Investigations of the sub surface conditions these projects did not include any rational of leaning tower of Pisa revealed a weak clay engineering assessment of the underlying soil stratum about 11m below the ground surface. or rock. Early construction was based on The clay is very compressible, and has settled common sense, experience, intuition and rules- under the concentrated weight of the tower. of thumb, and builders passed this collective South side has settled more than the north, wisdom orally from generation to generation which has caused the tower to tilt. through trade guilds. Early scientists were Leonardo da Vinci (1452 – 1519), the concerned with more lofty matters and artist/ scientist; was one of the few to briefly generally considered the study of soil and rock study the behaviour of soils. He observed the beneath their dignity. angle of repose in sands, proposed test Builders used crude methods / tests to methods to determine the bearing capacity of assess the soil conditions. Italian architect soils and speculated on the processes of Palladio (1508 – 1580) wrote that firm ground granular hydrology. could be confirmed “if the ground does not Engineers and scientists began to address resound or tremble if something heavy is the engineering behaviour of soil more dropped. In order to ascertain this, one can seriously during the 17th and 18th centuries. observe whether some drum skins placed on Most of this work focused on the analyses and the ground vibrate and give off a weak sound design of retaining walls. It was generally or whether the water in a vessel placed on the dictated by military needs and mostly done by ----------------------------------------------------------------------------------------------------------------Page 4 of 8 SOIL MECHANICS – I CHAPTER-I INTRODUCTION 2009-10 M U JAGADEESHA
  5. 5. HAWASSA UNIVERSITY FACULTY OF TECHNOLOGY CIVIL ENGINEERING DEPARTMENT individuals associated with army, especially in One particularly disastrous failure occurred France. Henry Gautier, B.F.Belidor, Charles in 1913 when 185m of track slipped in to Lake Augustin Coulomb and others developed Aspin. This event prompted the formation of methods of predicting forces imparted by soil Geotekniska kommission (geotechnical on to retaining walls. Coulomb’s work, which he Commission) of the Swedish state Railways to published in 1776, is often considered the first study the problem and develop solutions. example of rational soil mechanics. Wolmer Fellinius (1876 – 1957) who had Unfortunately, much of this work extended become familiar with soil problems when he well beyond the 18th century abilities to served as the port engineer in Göteburg was measure relevant engineering properties in made chair of the commission. soil, and thus was difficult to apply to practical They developed lab test equipment, problems. developed methods of obtaining undisturbed The last half of nineteenth century was a samples of soils, studied the behaviour of soils period of rapid industrialization, which and produced new methods of analysis and produced tremendous growth in both the design. They investigated more than 300 sites, amount and scope of construction projects. collected 20,000 soil samples. Their soil These projects drove advances in structural mechanics laboratory established in 1914 engineering, Hydraulic engineering and other appears to have been the first of its kind in the fields. Some of the advancements become world. useful to Geotechnical Engineers. Henry The commission’s final report completed in Darcy’s research on flow through sand filters 1922 was the world’s first comprehensive helped in analysis of ground water flow. Otto Geotechnical report. However the rest of the Mohr’s research on stresses helped in world had little or no knowledge of this work analyzing shear strength of soils. until much later. The task of promoting When working with problematic soils some Geotechnical Engineering on a widespread engineers drove steel rods in to soil to roughly international level required more people to assess soil conditions. These tests were called spread the message, and one of them became soundings. recognized as a leader in this effort: Karl The increasing size of projects, especially Terzaghi. after 1880, raised more concerns about the Karl Terzaghi (1883 – 1965) has often consequences of failure. The time was ripe for been called “father of soil mechanics”. His Geotechnical Engineering to emerge as clearly contribution to soil mechanics is immense even defined discipline with in Civil engineering for though several others have contributed for its inventing better techniques of assessing soil shaping. and rock conditions, and for developing sound Terzaghi born in Prague, then part of methods of integrating them into Civil Austria earned an undergraduate degree in Engineering. mechanical engineering. However he found The first large scale attempts at that it was not his liking and so started his Geotechnical Engineering occurred in Sweden career with a civil engineering firm in Vienna during early decades of 20th century. They that specialized in RCC. He later earned a introduced the word “Geotechnical” (in Swedish doctorate based on his work in RCC design. ‘Geotekniska’). During this period, Terzaghi became “Necessity is the mother of all inventions”. interested in the ignorance of Civil Engineers in Extremely poor soil conditions underlie much of matters relating to earth work and foundation Sweden. Soft, weak clays are present beneath design. He felt this topic needed scientific the most populated areas, and they are the approach, and decided to focus his attention on source of many problems, including excessive developing rational design methods. settlement and catastrophic land slides. ----------------------------------------------------------------------------------------------------------------Page 5 of 8 SOIL MECHANICS – I CHAPTER-I INTRODUCTION 2009-10 M U JAGADEESHA
  6. 6. HAWASSA UNIVERSITY FACULTY OF TECHNOLOGY CIVIL ENGINEERING DEPARTMENT In 1916 he joined at Imperial School of previously been a maze of incoherent facts and Engineers at Istanbul. Later he moved to observations. Robert College in Istanbul. He researched in to Other prominent individuals who made the behaviour of soils, including studies of contributions to the fledging profession of piping failures in sands beneath and settlement geotechnical engineering are in clays. The work on clays eventually led to his • Willium Housel - University theory of consolidation. It has been considered of Michigen - Soil sampling, analysis one of the most significant mile stones in Civil and design. Taught first university soil Engineering. mechanics course at Michigen in 1927. • Gregory Tschebotorioff -- German - served at Princeton University – worked on earth retaining structures. • Arthur Casagrande – Harvard University – disciple of Terzaghi – analysis of soft clays, soil composition and classification, seepage, earth dams etc. • Fred Converse – professor at California Institute of technology. Cofounder of one of first geotechnical consultancy firm. • Daniel Moran – Foundation’s engineer and builder – constructed major bridges across country. • R R Proctor - assessment of compacted fills during construction. In 1936 first international Conference on If we wish to define a certain time as the Soil Mechanics and Foundation Engineering ‘birth’ of Geotechnical Engineering as a widely met in Cambridge, Massachusetts. recognized discipline, it would be the year In spite of these advancements, Terzaghi 1925, in which year Terzaghi published the first was presenting lectures with titles like “Soil comprehensive book on subject titled Mechanics – a new chapter in Engineering “Erdbaumechanik auf Bodenphysikalischer Science” as late as 1939. Fifties and sixties Grundlage” ( germane for The mechanics of finally established Geotechnical Engineering as earth construction based on soil physics) was a routine part of nearly all significant Civil published in Vienna. In 1925 he started to give engineering projects. lectures at MIT USA as visiting lecturer. In Modern geotechnical engineering has 1929 he returned to Vienna to hold a developed a matured ‘personality’ that is Professor’s post at Technical University and slightly different from other civil engineering continued research, speaking and consulting. disciplines. In 1939 he returned to USA accepting We work with soil and rock, which are professorship at Harvard University and it natural materials. As such their properties are became his home for rest of his life. more complex and difficult to characterize than Terzaghi had a remarkable ability to those of manufactured materials such as steel. develop rational and practical solutions to real For site exploration and characterization engineering problems from jumble of what had geotechnical engineers and their staff spend a great deal of time in field and laboratory. ----------------------------------------------------------------------------------------------------------------Page 6 of 8 SOIL MECHANICS – I CHAPTER-I INTRODUCTION 2009-10 M U JAGADEESHA
  7. 7. HAWASSA UNIVERSITY FACULTY OF TECHNOLOGY CIVIL ENGINEERING DEPARTMENT Practical economic constraints limit As city grew and water demands increased number of exploratory borings. As a result we many wells were installed through this clay in have direct knowledge of very small portion of to deeper water bearing sand layers. Pumping soil or rock beneath a project site. This of water increased effective stress and caused introduces many potential sources of error. large settlements. Between 1898 and 1966 Because of potentially large errors in our parts of the city settled 6m to 7m. At times rate site characterization programme, we use large of settlement has been as great as 1mm/ day. measure of ‘engineering judgement’ when Fortunately Mexican Geotechnical engineers using lab and field data in to our analysis. As a (Dr. Nabor Curillo) recognized the association result we use large factors of safety and more of settlement with ground water extraction and conservative designs. convinced government to prohibit pumping in We rely heavily on engineering judgement central city area. – combination of experience, subjectivity, Apart from widespread settlement, local reliance on precedent etc. settlements also occurred beneath heavy We have more extensive involvement structures and monuments. Palacio de las during construction, and frequently revise our Bellas Artes ( palace of fine arts) built between design recommendations when conditions 1904 and 1934 experienced large settlements encountered during construction are different even before completion. By 1950 the palace from those anticipated. and immediately surrounding ground were 3m Accuracy in geotechnical engineering lower than adjacent streets. Stair cases were analysis is as good as accuracy of data. One of built from streets down to building area. the most common mistakes among students Settlement problems are not limited to studying Geotechnical engineering and even buildings. For a highway bridge underlain by among practicing engineers, is to overestimate soft clay deposit pile foundations were used the accuracy of geotechnical analysis. This through the clay into harder soils below and the often leads to overconfidence and ultimately bridge was built on the piles. These may result in construction failures. foundations protect it from large settlements. It also was necessary to place fill adjacent Soil failures to the bridge abutments so the roadway could Some of most dramatic examples of soil reach the bridge deck. These fills are very settlement are found in Mexico city. Parts of heavy and their weight increased effective the city are underlain by one of the most stress causing underlying clay to settle. After troublesome soils in any urban area of the twelve years fill had settled 1m in comparison world, a very soft lacustrine clay that was to bridge deck causing great inconvenience to deposited in the former lake Texcoco. road traffic. It was an extraordinary soil with water Individual slope failures can be very content varying from an average of 281% to disastrous and expensive. For example Thistle max 500; liquid limit from average of 289% to debris slide near Thistle, Utah in 1983 created max 500% and plastic limit from average of a huge dam across a canyon, forming a new 85% to max 150% and void ratio of average lake. It caused $ 200 million in direct damages, 6.9. including the destruction of two major highways and main line of the Denver and Rio Grande western Rail road. This new lake had no out let, so it would have eventually overtopped the side and quickly eroded it causing a massive flood. Therefore lake was drained, first with pumps and later with a permanent tunnel. ----------------------------------------------------------------------------------------------------------------Page 7 of 8 SOIL MECHANICS – I CHAPTER-I INTRODUCTION 2009-10 M U JAGADEESHA
  8. 8. HAWASSA UNIVERSITY FACULTY OF TECHNOLOGY CIVIL ENGINEERING DEPARTMENT In 1786 in China’s Sichuan province similar land slide happened. It dammed a river and river soon overtopped the dam and rapidly eroded it causing extensive floods downstream, suddenly, drowning 1,00,000 people. The soils below apartment buildings in Niigata, Japan liquefied during 1964 earthquake, which produced bearing capacity failures. The failure reportedly occurred very slowly and buildings were very strong and rigid, so they remained virtually intact. There was very little damage to interior, doors and windows functioned after failure. Inhabitants Teton dam was a 93m tall earth dam on were able to evacuate by walking down the the Teton River in South eastern Idaho. It failed exterior wall. on June5, 1976; only eight months after the first filling of reservoir began. The failure began as a cloudy seep on the face of the dam, and rapidly progressed to a piping failure through the embankment. These failures teach us about the failures of concepts and /or unforeseen actions and we can learn a lot from these failures. ***Hand outs are not meant to be exhaustive; students are advised to refer various books on the subject. ----------------------------------------------------------------------------------------------------------------Page 8 of 8 SOIL MECHANICS – I CHAPTER-I INTRODUCTION 2009-10 M U JAGADEESHA