102 



kaye. PRINCIPLES OF SOIL MECHANICS 



[Ch. 5 



- Kaolin-type clays — 

 -(Vera, Wash. and 

 South Carolina) 

 ~^-(Mica powder) | 



■ Organic silt and clay 

 (Flushing Meadows, LI.) 

 Organic clay (New London, Conn.) 

 'Organic silt and clay (Panama) 



°Kaolin (Mica Wash.) 

 Micaceous sandy silt (Cartersville, Ga.) 



Fig. 1. 



50 60 70 80 90 100 110 

 Liquid limit 



Relation of liquid limit to plasticity index. (After Casagrande, 1947, 

 p. 803.) 



Slope-Stability Analysis 



When a deep road cut is designed, how does the engineer know that 

 the slope will be stable? Or, for that matter, how can the geologist 

 determine the quantitative conditions for landslides and the formation 

 of land forms developed from mass displacements of soil? To answer 

 this type of question a number of methods for the analysis of slope 

 stability have been devised in soil mechanics. The following exposi- 

 tion is only a brief account of the basic principles; the standard 

 textbooks should be referred to for more detailed explanation. 



To determine the stability of a soil slope against sliding, the dis- 

 tribution of soil types behind the slope and in the toe of the slope 

 must be known, as well as the position of the water table. Undisturbed 

 samples of each soil type are obtained, and laboratory tests are made 

 to determine their unit weights (weight per unit volume) , cohesion, and 

 angle of internal friction. 



In general, it can be said that a slope will slide if there exists some 

 surface within the embankment along which the resultant of all shear- 

 ing stresses exceeds the total shearing resistance along that surface. 

 To determine whether this is a possible condition for a given slope, a 

 graphical method is widely employed. On a geological cross section 

 (Fig. 2) taken at right angles to the slope, a possible sliding surface 

 in the form of a circular arc is drawn more or less at random. In 

 Fig. 2 this circle has its center at O. The circular-arc sliding sur- 



