Mechanics of soil and Foundations

Mechanics of soil and Foundations

The use of engineering soils and rocks in construction is older than history and no other
materials, except timber, were used until about 200 years ago when an iron bridge was
built by Abraham Darby in Coalbrookdale. 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.
Engineering soils are mostly just broken up rock, which is sometimes decomposed
into clay, so they are simply collections of particles. Dry sand will pour like water but it
will form a cone, and you can make a sandcastle and measure its compressive strength
as you would 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. The mechanics that
govern the stability of a small excavation or a small slope and the bearing capacity of
boots in soft mud are exactly the same as for large excavations and foundations.

In the ground soils are usually saturated so the void spaces between the grains are
filled with water. Rocks are really strongly cemented soils but they are often cracked
and jointed so they are like soil in which the grains fit very closely together. Natural
soils and rocks appear in other disciplines such as agriculture and mining, but in these
cases their biological and chemical properties are more important than their mechanical
properties. Soils are granular materials and principles of soil mechanics are relevant to
storage and transportation of other granular materials such as mineral ores and grain.
Figure 1.1 illustrates a range of geotechnical structures. Except for the foundations,
the retaining walls and the tunnel lining all are made from natural geological materials.


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