In vacuum preloading, the drainage boundary of clay is isolated from the atmosphere by a membrane. A partial vacuum (e.g. suction of 80kPa) is applied within the membrane to reduce the water pressure so as to speed up consolidation.
The rate of consolidation can be increased by surcharge preloading in which the excess pore water pressure in clay is temporarily increased. Alternatively, the rate of consolidation can also be increased by vacuum preloading by a decrease in water pressure.
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Vacuum preloading is generally faster in operation than surcharge preloading which requires timely delivery of fill on top of clay. Moreover, it is unnecessary to consider the stability criterion which surcharge preloading should require.
This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.
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