2.1 INTRODUCTION

As has been indicated in Chapter 1, a complete progressive failure analysis of stress and strain in a soil mechanics problem as the load is increased from zero to failure is almost always impracticable. It is often satisfactory, however, to know the load at which the soil mass will collapse or deform excessively. For this purpose, the slipline field analysis and limit equilibrium method have been the methods of solving various soil stability problems. However, the slipline field analysis described in the preceding chapter is often considered to be too esoteric and complex to be a useful tool for engineers. In fact, so much depends on the skill and intuition of the engineer that solving a mixed (displacement and stress) boundary-value problem appears to be an art rather than an engineering method. On the other hand, the success of the relatively crude approach of the limit equilibrium method requires a blend of theoretical deduction, engineering intuition and experimental confirmation. Under these circumstances, it is hardly surprising that often the engineer employs well-known texts, such as Terzaghi (1943) or Sokolovskii (1965), or the more recent technical literature as a magic handbook and tries to fit his problem to the particular solutions he finds. Intuition and innovation seem discouraged by unfamiliarity and apparent complexity.

The basic theorems of limit analysis which have been stated in Chapter 1, give upper and lower bounds on the collapse load. Close approximation to the collapse load with known limits of error, as well as...