Heaving 
 
Clay may flow beneath the retaining structure into the excavation side if sufficient soil is removed.  The overburden pressure loss from the soil removal results in a base instability.

The vertical pressure (Pv) on the horizontal plane at the depth H is formulated by the summing of the vertical forces.
 

         where Ff = cuH
Hence            Pv = H(+qs) - 0.707cu
For deep excavation H/B>1, Bjerrum and Eide analyzed that the ultimate bearing pressue is
 
qult = cuNc

When H/B>1 the Factor of safety (FS) against heaving can be obtained as the ratio of qult to Pv by neglecting the shear resistance Ff on the block.

 
Back