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Engineering Geology

Engineering Geology - geomuseu

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Chapter 9<br />

Figure 9.29<br />

Settlement of a building on clay, Goteborg, Sweden. Note the window frames are twisted somewhat and the lower ones boarded up;<br />

the shearing in the brickwork, especially beneath the second floor windows; and the slope on the down-comer.<br />

Settlement can present a problem in clayey soils, so that the amount that is likely to take<br />

place when they are loaded needs to be determined. Settlement invariably continues after the<br />

construction period, often for several years. Immediate or elastic settlement is that which<br />

occurs under constant-volume (undrained) conditions when clay deforms to accommodate<br />

the imposed shear stresses. Primary consolidation in clay takes place due to the void space<br />

being gradually reduced as the pore water and/or air are expelled therefrom on loading.<br />

The rate at which this occurs depends on the rate at which the excess pore water pressure,<br />

induced by a structural load, is dissipated, thereby allowing the structure to be supported<br />

entirely by the soil skeleton. Consequently, the permeability of the clay is all important.<br />

After sufficient time has elapsed, excess pore water pressures approach zero, but a deposit<br />

of clay may continue to decrease in volume. This is referred to as secondary consolidation<br />

and involves compression of the soil fabric.<br />

Settlement is rarely a limiting condition in foundations on most fresh rocks. Consequently,<br />

it does not entail special study except in the case of special structures where settlements<br />

545

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