There are three different failure mechanisms of multi-bench retained foundation pit with different widths of the bench

i.e. overall overturning failure

mutual-effect failure and separate failure. The separate failure mechanism is studied with shear strength reduction method; and the influences of three main factors

i.e. soil strength; the length of first retaining structure L1 and the length of second retaining structure L2

to the critical bench width of separate failure Bs

are studied. The results show that the relative stability of the two level retaining structures of the multi-bench system changes with different soil strength. The parameter “Rfos” is introduced to reflect the influence of the soil strength

which is the ratio of the factor of safeties of the two level retaining structures when they are separated; Bs decreases with the increase of Rfos. A simplified analysis method is proposed to find Bs when Rfos is small; Bs will be constant when Rfos is large enough; Bs increases with the increase of L1; while it increases with the increase of L2 when Rfos is small

it decreases with the increase of L2 when Rfos is large.