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Non-uniform slab support can also originate from subgrade movements especially where subsoil such as active clay and collapsing sand are traversed. Furthermore non-uniform settlement as a result of poorly compacted soils is also a source of unevenness and thus voids under the slab. All these are very difficult to predict and the designer is best advised to reduce the risk of unevenness below the slab by suitable specifications and site control.

 

It is strongly recommended that the designer make use of the guidelines as documented in TRH9(7).

 

The following equation may be used to give an indication of the length of a void as a result of swelling clay (eventual saturation of the subgrade is assumed):

 

 

where

 

Su      standard deviation of the weighted liquid limit,

LL      mean of the weighted liquid limit.

 

 

Sp   standard deviation of overburden pressure

p     mean of overburden pressure

S     standard deviation of initial moisture content

w     mean of initial moisture content