(3) Consolidated-Drained triaxial test, commonly known as 

 a CD-Test or S-Test. 



The Q, R, and S designations are standard use in Corps literature. The 

 descriptions are indicative of the conditions under which the tests are run. 

 From the results of these tests the stress-strain characteristics are estab- 

 lished under the various loading conditions noted, and of equal importance, 

 the conditions of failure for the soil are established. The strength of a 

 soil is usually defined in terms of the stress developed at the peak of the 

 stress-strain curve and is presented in the form of Mohr circles and a Mohr 

 failure envelope. The strength is then expressed in terms of cohesion and 

 the angle of internal friction. 



b. Compressibility . The simplest compressibility or consolidation test 

 is the one-dimensional, laterally confined compression test (often referred 

 to as oedometer test). In this test the soil sample is placed within a 

 restraining ring and loaded with special types of plates on either top or 

 bottom or both. The change in sample height is measured by a deflection 

 gage and is used to calculate the change in void ratio (e) at different 

 normal pressures (P) . If the soil is saturated, the sample is placed 

 between two porous disks that permit the water to drain away during com- 

 pression. This in turn leads to information which permits plotting of the 

 so-called e-log P relationship. From such plots for either sands, silts or 

 clays, or mixtures of them, normally moduli are estimate d which can then be 

 used for consolidation and settlement estimates. It should be emphasized 

 that in such tests the lateral expansion is restrained. In real situations 

 this is only approximated by the loading of relatively thin layers of com- 

 pressible soil through load distribution over a large area. 



(1) Compressibility of Sands . The most important property of the 

 sand, which governs the stiffness of the sand, is relative density. The 

 relative density of the sand is usually determined in the field by means of 

 standard penetration tests or Dutch cone penetration tests. 



(2) Compressibility of Clays and Silts . The predictions of static 

 settlement of silts and clays are usually made on the basis of consolidation 

 or oedometer tests. The rate of settlement and the time for essential com- 

 pletion of primary consolidation can be predicted on the basis of this test. 

 Typically, silts are less compressible than clays. 



c. Permeability. Permeability is the soil property that indicates the 

 relative ease with which a fluid will flow through the soil. The coefficient 

 of permeability (k) of a soil is defined as the average percolation velocity 

 (v) divided by the hydraulic gradient (i) in the soil at that particular 

 point. It is seen then that the coefficient of permeability has units of 

 velocity, commonly centimeter per second or foot per minute. Permeability 

 depends on the characteristics of both the pore fluid and the soil. Viscos- 

 ity, unit weight, and polarity are the major pore fluid characteristics. 

 Particle size, void ratio, composition, fabric, and degree of saturation are 

 the major soil characteristics. In general a qualitative approximation of 

 the permeability of the materials can be made on the basis of grain size. 

 For example, clean gravels will have permeabilities ranging from 10 + to 

 10 +2 centimeters per second. Clean medium to coarse sands will have perme- 

 abilities ranging from 10" 2 to 1 centimeter per second. Very fine sand 



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