1134 Subsurface Geologic Methods 



when subjected to water in an unconfined state may be competent when 

 properly confined. 



Shearing resistance, which is the property of a material to withstand 

 loading without objectionable lateral deformation, is, for the engineer, 

 a most important consideration. The original basic assumptions on sta- 

 bility may be stated as follows: 



1. A force applied to a granular mass is transmitted throughout the 

 mass by contact pressure between the individual particles. 



2. The loading will cause the particles to rearrange, and as they 

 rearrange they will tend to slide with respect to each other. 



3. This sliding is resisted by frictional resistance, which is a func- 

 tion of the coefficient of friction between the particle surfaces and the 

 pressure between the surfaces. 



4. The sliding is also resisted by mechanical interlocking of the 

 aggregation of particles and by cohesion between the particles. 



These basic assumptions would serve for the evaluation of shear tests 

 if all materials were free-draining. However, when materials of relatively 

 low permeability are tested or when drainage restrictions are imposed, 

 the test results vary with such conditions as those of testing, sample thick- 

 ness, drainage restrictions, density, and moisture content. The principal 

 cause of these variations is variations in induced pore pressures within 

 the test specimens. For this reason pore-pressure measurements are taken 

 and the laboratory-test data are reduced to zero-pore-pressure conditions 

 to aid in the analysis. 



All geologic formations, with the exception of a few, are composed 

 of material having a definite particle size and shape. Void spaces occur 

 between the particles or grains. These voids may be filled with air, water, 

 or a combination of both. This void filling is called pore fluid. 



When a load is applied, the initial effect is an increase in contact 

 pressure between grains and a subsequent reduction in volume by re- 

 adjustment. If the material is 100 percent saturated with water, a slight 

 load will produce a fluid condition where all the load is carried by the 

 water. Usually the void spaces are filled with a mixture of air and water 

 so that the loading will cause a reduction in void space and will compress 

 the air, building up a pressure within the material. This pressure is called 

 "pore pressure." Pore pressure opposes the applied loading and thus 

 reduces the contact pressure between the soil particles. That part of the 

 load carried by the fluid space does not increase the shearing resistance 

 of the mass because the fluid has no resistance to change in shape. Because 

 all soils are more or less permeable, pore pressure once produced will 

 decrease with time as the pore fluid is extruded. One of the important 

 factors in pore-pressure determinations, therefore, becomes the rate at 

 which this pore pressure is dissipated. If the load is applied faster than 

 the pressure is dissipated, failure through flow may result. 



