Foundation, Concrete and Earthquake Engineering

Effect of Rainfall on Residual Soil

It should keep in mind that water/ moisture is the main enemy to soil and geotechnical engineering. While try to simulate a soil sample to represent actual filed/foundation condition, put at least to two conditions; they are:

a. overburden pressure

b. moisture content

Though in some apparatus we can control moisture content, usually, we test sample under saturated condition. It is assumed that fully saturated soil is the worst condition it can face in field.

Triaxial test setup with sample soil moisture control facilities
In our upcoming post we will learn how shear strength of a partially saturated soil is greater than fully saturated soil. However, come to the point rainfall:

Two factor controls extent of internal poor material

-extent and degree of weathering

-amount of regional rainfall

This extent may be




The interface where residual soil and its parent rock meets, water can accumulate to saturate (or almost saturate) soil as rocks are usually impermeable and water cannot penetrate and trapped into there.

Such soils are usually extended up to a thickness from 0 to around 20m. Thickness depends on types of rocks and climatic condition throughout a geologic period.

The upper or top layers of such deposits are found seldom saturated but as discussed above soil-rock interface may have 100% saturation. Though it is believed that residual soils are competent but many experiences were also observed.

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