explain

Limit equilibrium method: The limit equilibrium method combines features of upper and lower bound methods. The geometry of slip surfaces must form a mechanism that will allow collapse to occur, but they may be any shape so they need not meet all the requirements of compatibility. The overall conditions of equilibrium of forces on blocks within the mechanism must be satisfied, but the local state of stress within the blocks are not investigated. There is no formal proof that limit equilibrium method leads to correct solutions, experience has shown that the method usually gives solutions that agree quite well with observations of the collapse of real structures