Geo-Technical Engineering-I Part-A Unit-01 Introduction
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Compiled by: Prof.B.S.Chawhan M.Tech(Geo-Tech Engg), Asst.Professor,CED,Government.Engineering College,Haveri-581110(12/4/2011-Till date) 1 GEO-TECHNICAL ENGINEERING-I PART-A UNIT-01 INTRODUCTION History of Soil Mechanics: The weathered rocky materials found on the earth’s surface, such as sand, clay, silt, gravel, peat, etc., are broadly called as soils; and the science which analysises and defines their actions, properties and behaviors, is called Soil-Mechanics, or Soil- Engineering or Geotechnical Engineering. The science of soil-mechanics is of a recent origin, say as far back as 1925 or so. Prior to that, engineering structures were generally designed and constructed on different kinds of soils, or by different kinds of soils, without considering the peculiarities of the given foundation soil or construction soil, respectively. But, however, nowadays, this science has developed to a large extent. Almost all the engineering structures, founded on the earth (or soil), or structures constructed with the soil (such as earth dams, etc), are thus nowadays designed quite rationally, considering the properties and possible behavior of the existing available soil. Only a few years back, we get these days due to their rational scientific and assured designs. The failures of structures have, thus become very rare. We, therefore, owe a great deal of thanks to the pioneer research workers of this science-the important of whom include W.Fellinius and A.Atterberg-two Swedish scientists, Dr.Karl Terzaghis (who published the very first book on this science in 1925), Dr.Arthur Casagrande, Dr.Glennon Gilboy, Late Donald W.Taylor, Ralph B.Peck, A.W.Skempton, etc., etc. Dr.Terzaghi among them is known as the father of soil mechanics. Karl Terzaghi writing in 1951, on The Influence of Modern Soil Studies on the Design and Construction of Foundations, commented on foundations as follows; Foundation can appropriately be described as a necessary evil. If a building is to be constructed on an outcrop of sound rock, no foundation is required. Hence, in contrast to the building itself which satisfies specific needs, appeals to the aesthetic sense, and fills its matters with pride, the foundations merely serve as a remedy for the deficiencies of whatever whimsical nature has provided for the support of the structure at the site which has been selected. On account of the fact that there is no glory attached to the foundations, and that the sources of success or failure are hidden deep in the ground, building foundations have always been treated as step children; and their acts of revenge for the lack of attention can be very embarrassing. Definition of Soil: The term “soil” has different meanings in different scientific fields. It has originated from the Latin word Solum. According to agricultural scientist “the loose material on the earth’s crust consisting of disintegrated rock with an admixture of organic matter, which supports plant life”. According to geologist “the disintegrated rock material which has not been transported from the place of origin”. According to Civil Engineer “the loose unconsolidated inorganic material on the earth’s crust produced by the disintegration of rocks, overlying hard rock with or without organic matter”. Origin of soil: Soil may remain at the place of its origin or it may be transported by various natural agencies. It is said to be residual in the earlier situation and transported in the latter. Rocks or soils are generally classified into three groups according to their mode of origin- 1.Igneous rocks 2.Sedimentary rocks 3.Metamorphic rocks All rocks are derived from the material of igneous rocks. Infact, 95% of the volume of the outermost 10 miles of the globe is composed of rocks of igneous origin. Sedimentary rocks are formed of hardened, loose sediments and are composed of plant and animal remains and of end product of chemical and physical disintegration of igneous, sedimentary or metamorphic rocks. About 75% of the rocks exposed on the earth’s surface are sedimentary rocks or metamorphic rocks derived from them. Metamorphic rocks are produced when a rock changes from its primary form to a new form by internal process like pressure, heat and chemically active fluids, with the limitation that during the transportation the rock remained essentially solid. Formation of soils: Soils is formed by the process of ‘Weathering’ of rocks, that is, disintegration and decomposition of rocks and minerals at or near the earth’s surface through the actions of natural or mechanical and chemical agents into smaller and smaller grains. The factors of weathering may be atmospheric, such as changes in temperature and pressure; erosion and transportation by wind, water and glaciers; chemical action such as crystal growth, oxidation, hydration, carbonation and leaching by water, especially rainwater with time. Obviously, soils formed by mechanical weathering (that is, disintegration of rocks by the action of wind, water and glaciers) bear a similarity in certain properties to the minerals in the parent rock, since chemical changes which could destroy their identity do not take place. Compiled by: Prof.B.S.Chawhan M.Tech(Geo-Tech Engg), Asst.Professor,CED,Government.Engineering College,Haveri-581110(12/4/2011-Till date) 2 It is to be noted that 95% of the earth’s crust consists of igneous rocks, and only the remaining 5% consist of sedimentary and metamorphic rocks. However, sedimentary rocks are present on 80% of the earth’s surface area. Felsphers are the minerals, abundantly present (60%) in igneous rocks. Amphiboles and pyroxenes, quartz and micas come next in that order. ‘Leaching’ is the process where by water-soluble parts in the soil such as Calcium Carbonates are dissolved and washed out from the soil by rainfall or percolating subsurface water. ‘Laterite’ soil, in which certain areas of kerala abound, is formed by leaching. Harder minerals will be resistant to weathering action, for example, Quartz presenting igneous rocks. But, prolonged chemical action may affect even such relatively stable minerals, resulting in the formation of secondary products of weathering, such as clay minerals-illite, kaolinite and montmorillonite. The following figure shows a typical soil profile. A-horizon 30 to 50cm B-horizon 60 to 100cm C1-horizon 3 to 4m C2-hirizon below 4 to 5m. The above figure shows a typical soil profile. Fig.1. A typical soil profile Where, A-Light brown loam; leached, B-Dark brown clay, leached C1-Light brown silty clay oxidized and unleached. C2-Light brown silty clay unoxidized and unleached. SOME COMMONLY USED SOIL DESIGNATIONS: The following are some commonly used soil designations, their definition and basic properties; 1. Bentonite: Decomposed volcanic ash containing a high percentage of clay mineral montmorillonite. It exhibits high degree of shrinkage and swelling. 2. Boulder clay: Glacial clay containing all sizes of rock fragments from boulders down to finely pulverized clay minerals. It is also known as ‘Glacial till’. 3. Black cotton soil: Black cotton soil containing a high percentage of montmorillonite and colloidal material; exhibits high degree of shrinkage and swelling. The name is derived from the fact that cotton grows well in the black soil. 4. Caliche: Soil conglomerate of gravel, sand and clay cemented by calcium carbonate. 5. Hard pan: Densely cemented soil which remains hard when wet. Boulder clays or glacial tills may also be called hard pan- very difficult to penetrate or excavate. 6. Laterite: deep brown soil of cellular structure, easy to excavate but gets hardened on exposure to air owing to the formation of hydrated iron oxides. 7. Loam: Mixture of sand, silt and clay size particles approximately in equal proportions; sometimes contains organic matter. 8. Loess: Uniform wind-blown yellowish brown silt clay; exhibits cohesion in the dry condition, which is lost on wetting. Near vertical cuts can be made in the dry condition. 9. Marl: Mixtures of calcareous sands or clays or loam; clay content not more than 75% and lime content not less than 15%. 10. Moorum: Gravel mixed with red clay. 11. Top-soil: Surface material which supports plant life. 12. Verved Clay: Clay and silt of glacial origin, essentially a lacustrine deposit; varve is a term of Swedish origin meaning thin layer. Thicker silt varies of summer alternate with thin clay varves of winter. Compiled by: Prof.B.S.Chawhan M.Tech(Geo-Tech Engg), Asst.Professor,CED,Government.Engineering College,Haveri-581110(12/4/2011-Till date) 3 Sizes of soils: Gravel 80mm to 4.75mm Sand 4.75mm to 0.075mm Silt 0.075mm to 0.002mm Clay less than 0.002mm. PHASE DIAGRAM: Va Air Wa Vv V Water Wv Vw Ww W Ws Solid 1=Vs (Soil) Fig:(2) Soil- three phase diagram representing a soil mass (Partially saturated),(Volumes and weights of phases). Va=volume of air Wa=Weight of air Vw=Volume of water Ww=Weight of water Vv=Volume of voids Wv=Weight of material occupying void space Vs=Volume of solids Ws=Weight of solids V = total volume of soil mass W =Total weight of soil mass ,Wv=Ww. Porosity (n): Porosity of a soil mass is the ratio of the volume of voids to the volume of the soil mass. i.e., n=Vv X100 (1) V Where,Vv =Va+Vw; V =Va+Vw+Vs Void ratio (e): Void ratio of a soil mass is defined as the ratio of the volume of voids to the volume of solids in the soil mass. It is denoting by the letter symbol e and is generally expressed as a decimal fraction; i.e., e = Vv (2) Vs Where, Vv= Va+Vw Void ratio is used more than porosity in soil mechanics to characterize the natural state of soil.