First the stability is to be checked at the excavation level as follows:
 = c.  is the force that resists the heave which is brought about by the weight of soil of magnitude magnitude  H .
The stability factor is calculated as S =  H /  .
If, S< 6 ------ stable , where  is taken as 6.
S > 6 ------- unstable.
S = 6 ------- limiting condition.
If unstability criteria occurs the idea is to increase the depth of the diaphragm wall in order to take advantage of the layers of higher strength lying below. The failure plane as shown cannot penetrate through the hard stratum and is tangent to the same. This is shown in the fig. 6.7.3.
F.O.S should be more than 2 for bottom heave.
The depth  is calculated as follows: |
= D, since failure surface cannot penetrate the hard stratum. 
= 0.7B , which is obtained from bearing capacity analysis. 
is taken as least of the above two values. 
