240 varnes. LANDSLIDES [Ch. 13 



denseness of the deposit and the ability of water to migrate as stress 

 is applied. If the density of the material is so low that shear stresses 

 from additional loading tend to decrease the volume of the material, 

 and if the load is applied so fast that the water content cannot change, 

 part or most of the vertical load is transferred to the water, the ef- 

 fective intergrain pressure and friction are lowered, and failure may 

 result. If the sand is dense, the volume will increase on shearing and 

 the effective intergranular pressure will remain. Recent work by 

 Geuze (1948) and others indicates that, if the original void ratio of 

 a sand is above a critical value characteristic of that deposit, the sand 

 will compact on shearing and, if drainage is not immediate, high pore- 

 water pressures can develop. 



Clays consolidate slowly, and the difference between shear strengths 

 of saturated clays determined experimentally by quick or slow tests 

 is frequently quite large. Some of the difference is due to the building 

 up of pore pressures in the quick test, but the problem is still complex 

 and subject to much current research. It has been found that, for soft 

 clays, a satisfactory approximation to the shear strength of the ma- 

 terial in place is given by: 



s — \ unconfined compression strength 



The presence of gas bubbles within a fine-grained deposit decreases 

 its permeability somewhat and, hence, the rate of drainage, but it 

 accelerates the rate of consolidation. Also, if clay contains air, the 

 shearing strength derived from a quick test increases with increasing 

 normal stress (Terzaghi and Peck, 1948, p. 89). 



It is not the intention of the author to go further into the soil me- 

 chanics of slopes than is necessary to indicate that the distribution 

 of stress among the three phases of a natural deposit is critical to the 

 stability of a slope. From the above discussion it is apparent that 

 shear strength of earth materials is a complex function of the relative 

 density of the deposit, its water content, its permeability, and the 

 strength of the individual particles. For natural sands, the relative 

 density depends largely on the conditions and rate at which it was 

 deposited; the relative density of clays depends chiefly on the loads 

 that the soils have carried and, in some instances, the rate at which 

 the loads were applied. 



The permeability of a deposit is, of course, a function of many vari- 

 ables of the material, including particle size, size distribution, miner- 

 alogy, and structure. The development of pore pressures and the rate 

 of consolidation depend in large degree on the permeability. 



