The use of soils and rocks in construction is older than history.?Soils and rocks are still one of the most important construction materials used either in their natural state in foundations or excavations or recompacted in dams and embankments.

Most people have some direct personal experience of soil mechanics and geotechnical engineering. Children at the beach digging holes in the sand, making sand-castles and building dams across streams, or in the country losing their boots in the mud are learning about soil mechanics and geotechnical engineering. Soils behave in a variety of different ways. For example, you can pour dry sand like water and you can pour saturated sand under water in the same way, yet you can make sand-castles from partly saturated sand that will support loads like a concrete cylinder. Clay behaves more like plasticine or butter. If the clay has a high water content it squashes like warm butter, but if it has a low water content it is brittle like cold butter and it will fracture and crack if it is compressed.

The essential features of soil behavior are as follows:

(1) External loads and water pressures interact with each other to produce a stress that is effective in controlling soil behavior.

(2) Soil is compressible; volume changes occur as the grains rearrange themselves and the void space changes.

(3) Soil shearing is basically frictional so that strength increases with normal stress, and with depth in the ground.

(4) Combining these basic features of soil behavior leads to the?observation that soil strength and stiffness decrease with increasing water pressure and with increasing water content.

(5) Soil compression and distortion are generally not fully recoverable on unloading, so soil is essentially inelastic.

The four basic types of geotechnical structure are illustrated in?Figure 38.1; most other cases are variations or combinations of these.?Foundations (Figure 38. 1a) transmit loads to the ground and the basic criterion for design is that the settlements should be relatively small. The variables of a foundation are the load F, the size of the base B and the depth D. Foundations may support loads that are relatively small, such as car wheels, or relatively large, such as a power station. Slopes (Figure 38. 1b) may be formed naturally by erosion or built by excavation or filling. The basic variables are the slope angle i and the depth H, and the design requirement is that the slope should not fail by landsliding. Slopes that are too deep and too steep to stand unsupported can be supported by a retaining wall (Figure 38. 1c) The basic variables are the height of the wall H and its depth of burial D, together with the strength and stiffness of the wall and the forces in any anchors or props. The basic requirements for design are complex and involve overall stability, restriction of ground movements and the bending and shearing resistance of the wall. In any structure where there are different levels of water, such as in a dam (Figure 38. 1d) or around a pumped well, there will be seepage of water.

Within civil engineering the major divisions are structural (bridges and buildings) , hydraulic ( moving water) and geotechnical (foundations and excavations). These are all broadly similar in the sense that a material, such as steel, water or soil, in a structure, such as a bridge, river or foundation, is loaded and moves about. The?fundamental principles of structural, hydraulic and geotechnical engineering are also broadly similar and follow the same fundamental laws of mechanics. It is a pity that these subjects are often taught separately so that the essential links between them are lost.

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In each case materials are used to make systems or structures and engineers use theories and do calculations that demonstrate that these will work properly; bridges must not fall down, slopes or foundations must not fail and nor must they move very much. These theories must say something about the strength, stiffness and flow of the materials and the way the whole structure works. They will deal with ultimate states to demonstrate that the structure does not fall down and they will deal also with working states to show that the movements are acceptable.

All structural and geotechnical analyses contain uncertainties of one kind or another. These may involve uncertainties in prediction of?maximum loads (particularly live loads due to wind or earthquakes) approximations in the theories adopted for material behavior and structural analysis, and uncertainties in the determination of strength and stiffness parameters. To take account of these approximations and uncertainties it is usual to apply a factor of safety in the design.

Use of natural soil and rock makes geotechnical engineering different from many other branches of engineering and more interesting. The distinction is that most engineers can select and specify the materials they use, but geotechnical engineers must use the materials that exist in the ground and they have only very limited possibilities for improving their properties.?This means that?an?essential part of geotechnical engineering is a ground investigation to determine what materials are present and what their properties are.?Since soils and rocks were formed by natural geological processes, knowledge of geology is essential for geotechnical engineering.