MOST CRITICAL FAILURE SURFACE:

In homogeneous soils relatively unaffected by faults or bedding, deep seated shear failure surfaces tend to form in a circular, rotational manner.

We aim to find the most dangerous, ie.the most critical surface, and using the assumption above, we can find this surface using "trial circles".

The method is as follows:-

• Consider a series of slip circles of different radii but the same centre of rotation. Plot the Factor of Safety (FoS) for each of these circles against radius, and find the minimum FoS.
• This should be repeated for several circles, each investigated from an array of centres. The simplest way to do this is to form a rectangular grid from the centres:-
• Each centre will have a minimum FoS, and the overall lowest FoS from all the centres shows tha FoS for the whole slope. This assumes that enough circles, with a large spread of radii , and a large grid of centres have been investigated.
• We then have an overall failure, surface, with smaller individual ones which should not be ignored.

BEWARE:-

A series of circles in a granular fill neglect the possibility of a deep seated failure in the clay, unless investigated further.