GEOTECHNICAL ENGINEERING Geotechnical Engineering A specialty of civil engineering Deals with properties, behaviour and use of earth materials in engineering works Geotechnical engineering requires the integrated knowledge from: a. geology – is an earth science concerned with the solid Earth, the rocks of which it is composed, and the processes by which they change over time. b. material science and testing c. mechanics and hydraulics d. environmental science and engineering Geotechnical engineering includes several aspects: a. Soil Mechanics – used to analyse the deformations of and flow of fluids within natural and man-made structures that are supported on or made of soil, or structures that are buried in soils. b. Rock Mechanics – is a theoretical and applied science of the mechanical behaviour of rock and rock masses. c. Engineering Geology – application of the geology to engineering study for the purpose of assuring that the geological factors regarding the, location, design, construction, operation and maintenance of engineering works are recognized and accounted for. d. Soil Engineering – is a specialized engineering discipline that helps in understanding the behaviour of soil. e. Rock Engineering – the discipline of designing and supporting stable excavations in rock. f. Foundation Engineering – the application of soil mechanics and rock mechanics in the design of foundation elements of structures. h. Geo-Environmental Engineering – is concerned with engineering solutions relating to environmental impacts of contaminants within soils, and includes such aspects as understanding the migration, interactions and fate of contaminants, the protection of uncontaminated regions, the remediation or clean-up of contaminated sites. Geotechnical engineering is a universal set of related subjects. Subject Concerns with Soil Mechanics Engineering Mechanics Properties of soil Rock Mechanics Engineering Mechanics+Properties of rock Engineering Geology Formation and Features of earth crust+earthquake Soil Engineering Geology+soil mechanics+structural engineering Rock Engineering Geology+rock engineering+structural engineering Foundation Engineering Soil Engineering+Rock Engineering Geo-environmental Engineering Environmental effects (earthquake, rainfall and ground water) Scopes of Soil Mechanics (I) Subject Scope of soil mechanics Foundation of Structure Type, dimension and details of foundation will depend on the strength and deformation characteristic of supporting soil Underground and earth retaining structure loading on structure such as tunnels, earth retaining structures, sheet piles, etc, Subject Embankment, Excavation and Dam Pavement Subject Special Problems Scope of soil mechanics Requires the knowledge of slope stability (involving soil behaviour). For dams, consider seepage problem. Design of rigid or flexible problems depends on the behaviour of subgrade soil (related to settlement, swelling, repetitive loading, frost action, etc, Scope of Soil Mechanics Collapse, shrinkages welling, soil subsidence, soil heave, frost action, soil erosion. Father of Modern Soil Mechanics- Karl Von Terzaghi Soil The term derived from the latin word ‘solium’ Solium: upper layer of earth crust that may be dug or ploughed Soil is use by geologist (unconsolidated sediments overlying solid bed rocks) Agronomist, Agriculturist, Soil Scientist (thin layer of loose surface materials of earth crust) Civil Engineers (soil defined by agriculturist is known as top soil) Engineering use soil 1. Support the foundation of structures 2. Construction materials/underground and Earth –retaining Structures (Dam) 3. Pavement Design Definition of Soil, according to civil engineers soil includes all naturally occurring loose of soft deposit overlying the solid bed rock and soil is formed due to disintegration and decomposition of rocks by the process known as weathering. Soils Three phase system: 1. Solid phase 2. Liquid phase 3. Gas Phase Formation of soil: 1. Disintegration of rock 2. Decomposition of organic matter Soil Forming rocks a. Rock: Outer rocky shell or crust of the earth b. Igneous Rock: cooled from molten state c. Sedimentary Rock: deposit from a fluid medium d. Metamorphic Rock: form from a pre-existing rock by the action of heat and pressure. Soil Forming Minerals A naturally occurring crystalline material formed by inorganic process with a definite chemical composition with an ordered internal agreement of atoms having a definite crystal structure. Weathering The process of disintegration and decay of rock Result from exposure to the atmospheric agent (e.g., pressure and temperature) and also for their influences. Controlling factors: A. Climatic Condition of the area B. Temperature and precipitation TYPES OF WEATHEING a. Physical or mechanical weathering – term use in science that refers to the geological process of rocks breaking apart without changing their chemical composition. b. Chemical weathering – the erosion or disintegration of rocks, building materials caused by chemical reactions rather than mechanical processes c. Biological weathering – is the weakening and subsequent disintegration of rock by plants, animals and microbes. Residual soils – soils which are formed by weathering of rocks may remain in position at the place of region. Transported Soils – soils may get transported from the place of origin by various agencies such as wind, water, ice, gravity, etc. Transported soils may be further subdivided, depending upon the transporting agency and the place of deposition a. b. c. d. e. Alluvial Soils – Soils transported by rivers and streams : sedimentary clays Aeolian soils – soils transported by wind : loess Glacial Soils – Soils transported by glaciers : Glacial till Lacustrine soils – soils deposited in lake beds : lacustrine silts and lacustrine clays Marine soils – soils deposited in sea beds : marine silts and marine clays Broad classification of Soils 1. Coarse-grained soils - with average grain size greater than 0.075 mm: gravels and sand 2. Fine- grained Soils – with average grain size less than 0.075mm: silts and clay SOME COMMONLY USED SOIL DESIGNATIONS a. Bentonite- decomposed volcanic ash containing a high percentage of clay mineralmontmorillonite. It exhibits high degree of shrinkage and swelling. b. Black Cotton Soil – black 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. c. Boulder Clay – Glacial Clay containing all sizes of rock fragments from boulders down to finely pulverised clay materials. It is also known as “Glacial Tiil”. d. Caliche – Soil conglomerate of gravel, sand and clay cemented by calcium carbonate. e. Hard pan – densely cemented soil which remains hard when wet. Boulder clays or glacial tiils may also be called hard-pan-very difficult to penetrate or excavate. f. 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. g. Loam – Mixture of sand, silt and clay size particles approximately in equal proportions; sometimes contains organic matter. h. Loess – Uniform wind-blown yellowish brown silt or silty-clay; exhibits cohesion in the dry condition, which is lost on wetting. Near vertical cuts can be made in the dry condition. i. Marl – mixtures of calcareous, sands or clays or loam, clay content not more than 75% and lime content not less than 15%. j. Moorum – Gravel mixed with red clay. k. Top-soil – Surface material which supports plant life. l. Varved Clay – clay and silt of glacial origin, essentially a lacustrine deposit a term of Swedish origin meaning thin layer. Thicker silt varves of summer alternate with thinner clay varves of winter. SRUCTURE OF SOILS 1. Single-grained structure – is characteristic of coarse grained soils, with a particle size greater than 0.02mm. Gravitational forces predominate the surface forces and hence grain contact results. The deposition may occur in a loose state, with large voids or in a sense state, with less of voids. 2. Honey-comb Structure – this structure can occur only in fine-grained soils especially in silt and rock flour. Due to the relatively smaller size of grains, besides gravitational forces, inter-particle surface forces also play an important role in the process of settling down. Miniature arches are formed, which bridge over relatively large void spaces. These results in the formation of honey-comb being made up of numerous individual soil grains. The structure has a large void space and may carry high loads without a significant volume change. The structure can be broken down by external disturbances. 3. Flocculent Structure- this structure is characteristics of fine-grained soils such as clays. Inter-particle forces play a predominant role in the deposition. Mutual repulsion of the particles may be eliminated by means of an appropriate chemical; this will result in grains coming close together to form a ‘floc’. Formation of flocs is ‘flocculation’. But the flocs tend to settle in a honey comb structure, in which in place of each grain, a floc occurs. Thus, grains grouping around void spaces larger than the grain size are flocs and flocs grouping around void spaces larger than even the flocs result in the formation of a ‘flocculent’ structure. Texture of Soils – the term ‘Texture’ refers to the appearance of the surface of a material such as fabric. It is used in a similar sense with regard to soils. Texture of soil is reflected largely by the particle size, shape and gradation. The concept of texture of a soil has found some use in the classification of soils.
