Module 1 Geotechnical Engineering PDF

CE 314 2022 CHAPTER 1 SOIL FORMATION 1.1. Soil and its Constituents The word soil h...

CE 314 2022 CHAPTER 1 SOIL FORMATION 1.1. Soil and its Constituents The word soil has different meanings for other professions. In Geotechnical Engineering, soil refers to a particle that consists of three phase solid, liquid (water) and gas (air). This phase can be separated through mechanical means. The term soil, as used by civil engineers, refers to a collection of solid particles, of varying sizes, with voids in between which may contain water. The solid particles are either organic or inorganic, depending on their origin. Soil is formed by different particles such as gravel, rock, sand, silt, clay, loam and humus. Geotechnical engineering is the sub discipline of civil engineering that involves natural materials found close to the surface of the earth. It includes the application of the principles of soil mechanics and rock mechanics to the design of foundations, retaining structures, and earth structures. The behavior of a structure depends upon the properties of the soil materials on which the structure rests. The properties of the soil materials depend upon the properties of the rocks from which they are derived. A brief discussion of the parent rocks is, therefore, quite essential in order to understand the properties of soil materials. [p5, Geotechnical Engineering Principles and Practices of Soil Mechanics and Foundation Engineering V.N.S. Murthy, CRC Press, 2002]. 1.2. Formation of Soils Soil is defined as a natural aggregate of mineral grains, with or without organic constituents that can be separated by gentle mechanical means such as agitation in water. By contrast rock is considered to be a natural aggregate of mineral grains connected by strong and permanent cohesive forces. The process of weathering of the rock decreases the cohesive forces binding the mineral grains and leads to the disintegration of bigger masses to smaller and smaller particles. Soils are formed by the process of weathering of the parent rock. The weathering of the rocks might be by mechanical disintegration, and/or chemical decomposition. [p7, Geotechnical Engineering Principles and Practices of Soil Mechanics and Foundation Engineering V.N.S. Murthy, CRC Press, 2002]. 1.2.1. Mechanical Weathering Mechanical weathering of rocks to smaller particles is due to the action of such agents as the expansive forces of freezing water in fissures, due to sudden changes of temperature or due to the abrasion of rock by moving water or glaciers. Temperature changes of sufficient amplitude and frequency bring about changes in the volume of the rocks in the superficial layers of the earth's crust in terms of expansion and contraction. Such a volume change sets up tensile and shear stresses in the rock ultimately leading to the fracture of even large rocks. This type of rock weathering takes place in a very significant manner in arid climates where free, extreme atmospheric radiation brings about considerable variation in temperature at sunrise and sunset. [p7, Geotechnical Engineering Principles and Practices of Soil Mechanics and Foundation Engineering V.N.S. Murthy, CRC Press, 2002]. 1.2.2. Chemical Weathering Chemical weathering (decomposition) can transform hard rock minerals into soft, easily erodible matter. The principal types of decomposition are hydration, oxidation, carbonation, desilication and leaching. Oxygen and carbon dioxide which are always present in the air readily combine with the elements of rock in the presence of water. [p7, Geotechnical Engineering Principles and Practices of Soil Mechanics and Foundation Engineering V.N.S. Murthy, CRC Press, 2002]. 1.3. General Types of Soils In general, there are three basic types of soils; sand, silt and clay. However, most of the soils are a combination of the other types. We know that soil is formed by the process of mechanical and chemical weathering; it means that all weathered constituents of parent rock can’t be termed as soil. Soil is classified according to its grain sizes. Grains having diameters in the range of 4.75 to 76.2 mm are called gravel. If the grains are visible to the naked eye, but are less than about 4.75 mm in size the soil is described as sand. The lower limit of visibility of grains for the naked eyes is about 0.075 mm. Soil grains ranging from 0.075 to 0.002 mm are termed as silt and those that are finer than 0.002 mm as clay. GEOTECHNICAL ENGINEERING SOIL MECHANICS 1  CE 314 2022 1.3.1. Organic Soils Organic soils are mixtures in which a significant part of the solids is derived from the growth and decay of plant life and/or from the accumulation of skeletons or shells of small organisms. Peat is a good example of an organic soil, consisting largely of decaying wood and other plant remains. 1.3.2. Inorganic Soils Inorganic soils are formed as a result of the breaking down of rocks into smaller fragments. This process is referred to as rock weathering, and is either a purely mechanical process called physical weathering (disintegration) or a chemical alteration of the parent material referred to as chemical decomposition. 1.3.3. Residual Soils Residual soil is the material resulting from the in situ weathering of the parent rock. Residual soils are distributed throughout many regions of the world, such as Africa, South Asia, Australia, Southeastern North America, Central and South America, and considerable regions of Europe. The largest areas and thickness of these soils occur normally in humid tropical regions, such as Brazil, Nigeria, South India, Singapore, and the Philippines. [Encyclopedia of Engineering Geology, 2018 Edition, Peter T. Bobrowsky, Brain Marker, Springer link] 1.3.4. Transported Soils Transported soils are soils that are found at locations far removed from their place of formation. The transporting agencies of such soils are glaciers, wind and water. The soils are named according to the mode of transportation. Alluvial Soils - transported by running water and deposited along streams and so-called alluvial plains Aeolian Soils - transported and deposited by wind. Examples: Sand dunes, air transported volcanic deposits, Loess. Colluvial Soils - formed by movement of soils from its original place by gravity, such as during landslides. Lacustrine Soils - formed by deposition in quiet lakes. Glacial Soils - formed by transportation and deposition of glaciers. Marine Soils - formed by deposition in the seas. 1.4. Composition of Clay Minerals The word 'clay' is generally understood to refer to a material composed of a mass of small mineral particles which, in association with certain quantities of water, exhibits the property of plasticity. According to the clay mineral concept, clay materials are essentially composed of extremely small crystalline particles of one or more members of a small group of minerals that are commonly known as clay minerals. These minerals are essentially hydrous aluminum silicates, with magnesium or iron replacing wholly or in part for the aluminum, in some minerals. Clays can be divided into three general groups on the basis of their crystalline arrangement and it is observed that roughly similar engineering properties are connected with all the clay minerals belonging to the same group. An initial study of the crystal structure of clay minerals leads to a better understanding of the behavior of clays under different conditions of loading. [p11, Geotechnical Engineering Principles and Practices of Soil Mechanics and Foundation Engineering V.N.S. Murthy, CRC Press, 2002]. Activity; A soil particle is assumed to be a perfect sphere in shape with a diameter of 100 μm. This soil is put into a cylindrical shape container with a radius of 100 μm. Five soil particles are to be put into the cylindrical container and to be arranged randomly. Determine the least height of arrangement that the water will submerge all the particles in the container. Determine the volume of water that needed to submerge all particles. Reference: ✓ Geotechnical Engineering Principles and Practices of Soil Mechanics and Foundation Engineering V.N.S. Murthy, CRC Press, 2002 ✓ Terzaghi, Karl et. al. 1995. Soil Mechanics, For Engineering Practice. John Wiley and Sons, Inc. GEOTECHNICAL ENGINEERING SOIL MECHANICS 1