Soils generally consist of three phases: solid, liquid, and gas. The mechanical properties of soil depend directly on the interaction of these phases with each other and on the applied potential (e.g. stress, hydraulic head, electrical potential and temperature difference).The solid phase of soils contains various amounts of crystalline clay and non-clay minerals, non-crystalline clay material, organic matter and precipitated salts. These minerals are commonly formed by atoms of elements such as oxygen, silicon, hydrogen and aluminium, organized in various crystalline forms. These elements along with calcium, sodium, potassium, magnesium and carbon comprise over 99 per cent of the solid mass of soils. Although the amount of non-clay material is greater than that of clay and organic material, clay and organic material have a greater influence on the behaviour of soils. Solid particles are classified by size as clay, silt, sand, gravel, cobbles or boulders.
The liquid phase in soils is commonly composed of water containing various types and amounts of dissolved electrolytes. Organic compounds, both soluble and immiscible, are present in soils from chemical spills, leaking wastes and contaminated groundwater. The gas phase, in partially saturated soils, is usually air, although organic gases may be present in zones of high biological activity or in chemically contaminated soils. Soil mineralogy controls the size, shape and physical and chemical properties of soil particles and, thus, its load-carrying ability and compressibility.
The structure of a soil is the combined effects of fabric (particle association, geometrical arrangement of particles, particle groups and pore spaces in a soil), composition and interparticle forces. The structure of soils is also used to account for differences between the properties of natural (structured) and remoulded soils (destructured). The structure of a soil reflects all facets of the soil composition, history, present state and environment. Initial conditions dominate the structure of young deposits at high porosity or freshly compacted soils, whereas older soils at lower porosity reflect the post-depositional changes more.
Soil, like any other engineering material, distorts when placed under a load. This distortion is of two kinds – shearing or sliding distortion and compression. In general, soils cannot withstand tension. In some situations, the particles can be cemented together and a small amount of tension may be withstood, but not for long periods.
Particles of sands and many gravels consist overwhelmingly of silica. They can be rounded due to abrasion while being transported by wind or water, or sharp-cornered, or anything in between, and are roughly equidimensional. Clay particles arise from weathering of rock crystals like feldspar and commonly consist of aluminosilicate minerals. They are generally flake shaped with a large surface area compared with their mass. As their mass is extremely small, their behaviour is governed by forces of electrostatic attraction and repulsion on their surfaces. These forces attract and adsorb water to their surfaces, with the thickness of the layer being affected by dissolved salts in the water.
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