Question 5 In the context of soil mechanics, soil water can be described as \"fr

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Question 5 In the context of soil mechanics, soil water can be described as "free" water and "adsorbed" water. Explain briefly the difference between the two types of water that appear in soil. (a) (3 marks) (b) Explain why a sand castle is most stable when the sand is kept moist but would collapse when the sand is dried under the sun or when the sand is saturated by rain. (3 marks) (c) The table below shows the properties of three different types of soil, i.e. SM, CH and CL They have been labelled as A, B and C as shown in column 1. Suggest with brief explanation which soil type that A, B and C each represents. Soil Plastic Limit Liquid Limit Shrinkage Limit Coefficient of Permeability (m/s 5.5x 10 1.5 x 1 3.0 x 10 16 4 10 65 32 18 Note: USCS denotations -S (sand), M (silt), C (clay), H (high plasticity) and, L (low plasticity) (4 marks)

Explanation / Answer

a) Soil water can be described as the water present in the voids of soil. This soil water is divided into two types i.e. free water and held water. Held water is again sub-divided into three types- structural water, adsorbed water and capillary water.

The difference between free water and adsorbed water depends on the movement of water through soil. Free water is the water that freely moves through a soil under the influence of gravity. Adsorbed water is freely adsorb on a soil particle by adhesion forces.

b) The stability of sand castle depends on two factors- i. individual grain properties and ii. the presence of water between them. More angular grains result in better locking and more transported grains form rounded grains. Now, finer grains can hold more water due to tensional forces. Film of water between individual grains gives the strength to the castle. Therefore, sand castle becomes most stable when it is kept moist. But when it dries under sun or saturated by rain, the grains loose their strength and it gets collapse easily.

c) On the basis of the given plastic limit, liquid limit, shrinkage limit and coefficient of permeability, soil A, B and C can be classified as follows:

A : This is CL type or inorganic clays of low to medium plasticity , gravelly clays, sandy clays, silty clays or lean clays. Here liquid limit is 50% or less.

B: This type is MH soil or inorganic silts, micaceous or diatomaceous fine sandy or silty soils, elastic silts. Liquid limit is greater than 50%.

C : This type also falls under CL type with more or less similar characteristics.

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