Soil Mechanics: Effects of water on soil

• Water can be chemically combined within soil particles ( chemically or electrically)

• Water can be easily absorbed in a thin layer of soil.

• It can freely flow within the pores affecting the overall strength of the soil.

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EFFECTS of water on soil

Consolidation

Soil changes volume gradually in response to a change in pressure due to pore water pressure.

Clay: has a very low coefficient of permeability.

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definition of the coefficient of permeability: The measure of the capacity of the soil with which the water can easily flow through it.

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Tower of Pisa

Consolidation takes a long time and they have a non-uniform settlement happens (e.g. tower of Pisa)

The effects of consolidation are most conspicuous where a building sits over a layer of soil with low stiffness and low permeability, such as clay, leading to a large settlement over many years.

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Types of construction project where consolidation often poses technical risk include the construction of embankments, tunnel and basement excavation in clay

Capillary Action

Capillary action is the ability of a liquid to flow in narrow spaces without the assistance of external forces such as gravity.

With the combination of the surface tension caused by cohesion within the liquid and adhesive forces between the liquid and container wall act to propel the liquid.

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Cohesion: property of like molecules sticking together

The surface tension of the liquid: The property of the surface to resist an external force, due to the cohesive nature of its molecules.

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Pore water pressure (u) = h*γw

γw = unit weight of water (10 kN/m3)

A History lesson

The first recorded observation of capillary action was by Leonardo da Vinci. A former student of Galileo, Niccolò Aggiunti, was said to have investigated capillary action.

In 1660, capillary action was still a novelty to the Irish chemist Robert Boyle, when he reported that "some inquisitive French Men" had observed that when a capillary tube was dipped into water, the water would ascend to "some height in the Pipe".

Boyle then reported an experiment in which he dipped a capillary tube into red wine and then subjected the tube to a partial vacuum. He found that the vacuum had no observable influence on the height of the liquid in the capillary.

Some OTHER effects of water on soil

• Shrinkage/Swelling: due to loss or gain of water.

• Collapse compression: loose of strength.

• Liquefication: bonds break due to water pressure.

• Piping/erosion: formation of voids within a soil causes the removal of material by seepage displacement.

• Blow out/ Heave: displacement resulting from moisture absorption.

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