Question 5 Swelling and shrinkage of soils are caused by a change in water conte

Question

Question 5 Swelling and shrinkage of soils are caused by a change in water content. Explain why swelling and shrinkage are only relevant to fine grained soils but not coarse grained soils? (a) (3 marks) (b) Why are some fine grained soils (e.g. clay derived from a volcanic basalt origin) more "reactive" (i.e. shrinking and swelling more) than others? (3 marks) The table below shows the properties of three different types of soil collected from 3 sites, namely A, B and C as shown in column 1 (c) Site Plastic Limit Liquid Coefficient of | | Permeability (m/s) 10 12 18 Limit (96) 15 32 5.5x 10 1.5 x 10 3.0 x 10 A builder is to construct a house comprised of a light structure supported on shallow footings. Which site (i.e. A, B or C) should the builder avoid in order to minimise the risk of building damage caused by potential ground instability? You need to provide justification for your answer (4 marks)

Explanation / Answer

(a) Fine-grained clay–rich soils can absorb large quantities of water after rainfall, becoming sticky and heavy. Conversely, they can also become very hard when dry, resulting in shrinking and cracking of the ground. This hardening and softening, with associated volume change, is known as shrink-swell.

(b) The genral term for the montmorillonitic clay is derived from volcanic ash is bentonite. Because of its expanded lattice, montmorillonite is one of the most reactive types of clay and exhibits a considerable propensity for base exchange and ion adsorption.

(c) The liquid limit is adresses the amount of moisture content at which soil begins to behave as a liquid material and begins to flow while the plastic limit is the moisture content at which soil begins to behave as a plastic material. The property of a plastic material can be molded into a shape and the material will retain that shape. As the question states soil B is the most suitable because the coefficient of permeability(describes how easily a liquid will move through a soil) of this soil is low and its liquid limit is highest giving it the most resistance to behave as a liquid and its plastic limit is also mediocre hence it is the most suitable type of soil.

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