INTRODUCTION AND PURPOSE 



The infinite slope theory is based upon the assumption that a conjugate relation- 

 ship exists between the vertical pressures and the lateral pressures acting on vertical 

 planes, and that the stress conditions at any two points of equal depth are identical. 



"Thus the stresses at various depths on any vertical plane must be 

 the same as those at corresponding depths on any other vertical plane." 

 (Taylor 1948) 



The infinite slope theory is often used to assess the stability of long, natural 

 slopes wherein the soil properties are constant at a given depth below the surface of 

 the slope. It appears to be most useful for situations wherein the soils are rela- 

 tively uniform in depth and underlain by a stronger medium, such as bedrock. 



Because no slopes are infinitely long, the applicability of this theory is a 

 matter of judgment. It is recognized that, if failure is incipient, the lateral 

 pressure has an active, or minimum, value at the upper end of the failing mass and 

 a passive, or maximum, value at the lower end. In practice, however, the difference 

 in end conditions is usually ignored, or else a simple correction factor is intro- 

 duced to account for the difference. 



Usually the criterion used to aid in judging applicability of the infinite slope 

 theory is the ratio of length of slope to critical depth. A ratio of 20:1 commonly 

 is used. 



The purpose of this paper is to describe the failure surfaces in slopes that 

 comply with the infinite slope theory and thereby, to aid the judgment of the user 

 of this theory in assessing its applicability. 



