Note for Geotechnical Engineering - GTE By shweta sharma

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FCE 311 – GEOTECHNICAL ENGINEERING I 2 2.1 OSN - Lecture Notes INTRODUCTION TO SOIL MECHANICS DEFINITION OF SOIL Soil is the relatively loose mass of mineral and organic materials and sediments found above the bedrock, which can be relatively easily broken down into its constituent mineral or organic particles. Fig. 2-1: Soil layers Soil consists of layers of minerals constituents of variable thickness, which differ from the parent materials in the morphological, physical, chemical and mineralogical characteristics, as shown in Fig. 2-1. It is thus a natural product of weathering of rocks and decomposition of organic matter. It is an accumulation of individual particles that are bonded together by mechanical or attractive means, the strength of the bonds being a small fraction of the mineral particles. The particles may range from colloidal size to small boulders. Soil can also be referred to as regolith, or loose rock material. 2.2 SOIL MECHANICS AND GEOTECHNICAL ENGINEERING Soil mechanics is a branch of engineering mechanics that describes the behaviour of soils. Soil mechanics provide the theoretical basis for analysis in geotechnical engineering. UNIVERSITY OF NAIROBI Page 2

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FCE 311 – GEOTECHNICAL ENGINEERING I OSN - Lecture Notes Geotechnical Engineering is the branch of civil engineering concerned with the engineering behaviour of earth materials. It uses principles of soil mechanics, rock mechanics and engineering geology to investigate subsurface conditions and materials, determine the relevant physical/mechanical and chemical properties of the materials, evaluate stability of natural slopes and man-made soil deposits, access risks posed by site conditions, design earthworks and structure foundations and monitor site conditions, earthwork and foundation construction. A typical geotechnical engineering project begins with a review of project needs to define the required material properties. Then follows a site investigation of soil, rock, fault distribution and bedrock properties on and below an area of interest to determine their engineering properties. Site investigations are needed to gain an understanding of the area in or on which the engineering will take place. Investigations can include the assessment of the risk to humans, property and the environment from natural hazards such as earthquakes, landslides, soil liquefaction, debris flows and rock falls. A geotechnical engineer then determines and designs the type of foundations, earthworks and pavement subgrades required for the intended man-made structures to be built. Foundations are designed and constructed for structures of various sizes such as high-rise buildings, bridges, medium to large commercial buildings, and smaller structures where the soil conditions do not allow code-based design. Foundations built for above-ground structures include shallow and deep foundations. Retaining structures include earth-filled dams and retaining walls. Earthworks include embankments, tunnels and sanitary landfills. Geotechnical engineering is also related to coastal and ocean engineering. Coastal engineering can involve the design and construction of wharves (structures on the shore of harbour where ships may dock to load and unload cargo or passengers) and jetties (structures that projects into a body of water to influence the current or tide or to protect a harbour or shoreline from storms or erosion). UNIVERSITY OF NAIROBI Page 3

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FCE 311 – GEOTECHNICAL ENGINEERING I 3 OSN - Lecture Notes SOIL FORMATION 3.1 DEFINITION Soil formation is the process by which soil is created. The formation of soil happens over a very long period of time. Soil is formed from the weathering of rocks and minerals. 3.2 WEATHERING 3.2.1 Introduction Weathering is the process of breaking down rocks. Weathering occurs in situ or “with no movement”, and thus should not be confused with erosion, which involves the movement of rocks and minerals by agents such as water, ice, wind, and gravity. Two important classifications of weathering processes exist – Physical and Chemical Weathering 3.2.2 Physical weathering Involves the breakdown of rocks and soils through direct contact with atmospheric conditions, such as heat, water, ice and pressure, without any change in chemical condition. The soil formed due to physical weathering will be cohesionless (sand and gravel). In summary, the physical agencies causing mechanical weathering of rocks are; (i) Daily and seasonal temperature changes. (ii) Flowing water, glaciers and wind, which produce impact and abrasive action on rock. (iii) Splitting action of ice. (iv) Growth of roots of plants in rock fissures and to a minor degree burrowing activities of small animals like earthworms. 3.2.3 Chemical weathering Chemical weathering changes the composition of rocks by decomposing the parent minerals, transforming them into new compounds such as clay silica particles, carbonates and iron oxides. The (i) (ii) (iii) (iv) decomposition of rock is the result of the following reactions; Oxidation Carbonation Hydration Leaching i) Oxidation Within the weathering environment, oxidation of a variety of metals occurs. The most commonly observed is the oxidation of Fe2+ (iron) and combination with oxygen and water to form Fe3+ hydroxides and oxides such as goethite, limonite and hematite. This gives the affected rocks a reddish-brown coloration on the UNIVERSITY OF NAIROBI Page 4

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FCE 311 – GEOTECHNICAL ENGINEERING I surface which crumbles easily and weakens the rock. known as ‘rusting’. OSN - Lecture Notes This process is better ii) Carbonation Carbonation of rock material is caused by carbon dioxide in the presence of water. Limestones are very much affected by carbonation. iii) Hydration Mineral hydration is a form of chemical weathering that involves the rigid attachment of H+ and OH- ions to the atoms and molecules of a mineral. When rock minerals take up water, the increased volume creates physical stresses within the rock. For example iron oxides are converted to iron hydroxides and the hydration of anhydrite forms gypsum. Another example of hydration is the chemical decomposition of mineral fieldspar in granite to form kaolite. iv) Leaching Leaching is the process in which percolating water washes out water-soluble salts from the soil. Soil produced by chemical weathering of rocks will be cohesive (silt and clay). 3.3 RESIDUAL AND ALLUVIAL SOILS 3.3.1 Residual soils Residual soils are those which have remained over the parent rock from which they have been formed. They are relatively shallow in depth. They are characterized by a gradual transition from soil through partially weathered rocks, fractured and fissured rock, to bedrock. 3.3.2 Alluvial soils Alluvial soils are the soils which have been transported and subsequently deposited by flowing water. An alluvial fan is formed when the velocity of a soilladen stream suddenly deceases due to abrupt decrease in gradient. Floodplains are formed on the sides of a stream due to overflowing of flood water. A delta is formed just before a stream reaches the standing water of the sea. Alluvial soil deposits are usually stratified because of fluctuations in velocity of flowing water. The average particle size of alluvial deposits decreases with increasing distance from the source of stream. The delta soils are soil deposits farthest from the source of a stream and usually consist of silt and clay. Marine deposits are formed when fine-grained soils are carried beyond deltas into the sea. Lacustrine soils are soils deposited at the bed of lakes. UNIVERSITY OF NAIROBI Page 5