than the total stresses. Also effective stresses serve as a bridge be- 

 tween short-term (undrained) and long-term (drained) behavior as will 

 be discussed later. 



A triaxial test includes two distinct phases. During the first, 

 the effective stresses of a certain location in the ground are applied, 

 and the sample is allowed to consolidate (drain) until an equilibrium is 

 reached. A soil specimen that would be taken from that location in the 

 ground is approximately simulated in this manner. A problem again 

 arises concerning the influence of time: the soil in-situ has taken 

 millions of years to consolidate under the overburden while in the lab- 

 oratory consolidation is accomplished in a few days. As discussed in 

 the section on one-dimensional consolidation, the problem of time effects 

 will not be approached in this report. It will be assumed that labora- 

 tory consolidation simulated field consolidation. Conclusions derived 

 from this assumption will be modified if future research shows this 

 assumption to be incorrect. 



The second phase of a triaxial test involves closing the drainage 

 lines to the sample and then gradually increasing the axial force until 

 a certain strain (usually 20 percent) is reached. The test results are 

 formulated in terms of effective stresses, a failure criterion is 

 adopted, and the shear strength is estimated. 



A concise way of expressing much of the information obtained from 

 a triaxial test is through a stress path (Lambe and Whitman, 1969, p. 

 112). A stress path is a plot of one effective stress parameter versus 

 another for the_entire undrained shear portion_of the triaxial test. 

 The parameter (oy-Oo) /2 is plotted versus (a;j+03)/2, where a-j_ and 03 

 are the major and minor principal effective stresses, respectively. In 

 the NCEL tests the major principal stress was always equal to the ver- 

 tical stress, and the minor principal stress, to the horizontal stress. 

 The stress paths are defined in such a way that each point on the path 

 represents the top point of a Mohr circle. A stress path is thereby a 

 way of representing the infinite number of Mohr circles which correspond 

 to the changing stresses occurring during a shear test. 



The stress paths for the five pelagic clay triaxial tests are shown 

 in Figure 8. Four of those were isotropically consolidated prior to 

 shear, that is, the horizontal and vertical stresses were equal during 

 consolidation. One of the tests (Number 5) involved anisotropic (K Q ) 

 consolidation prior to shear. The stresses were gradually increased in 

 such a way that the sample average cross sectional area remained con- 

 stant (no lateral displacement) . Anisotropic (K ) consolidation corres- 

 ponds to the way soils compress in the field during sedimentation. It 

 represents a state of shear, and the ratio of horizontal to vertical 

 stress is identified as K Q , the coefficient of lateral earth pressure 

 at rest. 



The stress paths of Figure 8 appear to define relatively well a 

 failure envelope. That is, the stress paths approach and then follow a 

 particular line. Since the paths do not cross the line, it may be taken 

 as a limiting stress condition or failure. The failure envelope is 



16 



