Shear Strength Of Sands

Special tests

In practice, there are verry few problems in which a state of axial sym metry exists as in the triaxial test. In practica l states of stress the intermediate principal stress is not usually equal  to  the  minor  prrincipal stress  and  the  principal  stress  directions  can undergo rotation as the failure co ndition is approached. A common condition is that of plane strain in which the strain in the direction of the intermediate principal stress is zero due to restraint imposed by vi rtue of the length of the structure in question. In the triaxial test, consolidation proceed s   under equal   all-round pressure   (i.e. isotropic consolidation)whereas   in-situ   consolidation   takes   place   under   anisotropic   stress conditions.

Tests of a more comple x nature, generally employing adaptions o f triaxial equipment, have been devised to simulate the more complex states of stress en countered in practice but  these  are used  principally in  research.  The plane strain test uses a prismatic specimen in which strain in one direction (that of the intermediate principal stress) is maintained at zero throughout the test by means of two rigid side plates tied together. The all-round  pressure  is the  minor  principal stress  and  the  sum of  the applied  axial stress and  the  all-round  pressure  the  major  principal stress. A more sophisticated test, also  using  a  prismatic  specimen, enables  the values of all three principal stresses to be controlled independently, two side pressure bags or jacks being used  to apply  the intermediate   principal  stress.  Independent control of the three principal stresses can also be achieved by means of tests on soil specimens in the form of hollow cylinders in which different values of external and internal fluid pressure can be applied in addition to axial stress. Torsion applied to the hollow cylinders results in the rotation of the principal stress directions. Because of its relative simplicity it seems likely that the triaxial test will continue to be the main test for the determination of shear strength characteristics. If considered necessary, corrections can be applied to the results of triaxial tests to obtain the characteristics under more complex states of stress.

SHEAR STRENGTH OF SANDS

The  shear  strength  characteristics of a  sand can  be determined  from  the  results of either  direct  shear  tests or drained  triaxial tests,  only the drained  strength  of a sand normally being relevant in practice. The characteristics of dry and saturated sands are the same, provided there is zero excess pore water pressure in the case of saturated sands. Typical curves relating shear stress and shear strain for initially dense and loose sand specimens  in direct shear tests are shown in Figure. Similar curves are obtained  relating  principal  stress  difference  and  axial  strain  in drained    triaxial compression tests.

In a dense sand there is a considerable degree of interlocking between particles. Before shear  failure  can take place, this interlocking  must  be overcome  in addition to the frictional resistance at the   points of contact. In general,   the degree of interlocking is greatest in  the  case  of  very dense,  well-graded sands  consisting  of angular particles.  The characteristic  stress-strain  curve for  an initially dense  sand shows a  peak  stress  at a  relatively low strain  and thereafter,  as interlocking is Progressively overcome, the stress decreases with increasing strain. The reduction  in the degree  of interlocking produces an  increase  in  the  volume  of the specimen during shearing as   characterized by the relationship, shown in Figure , between volumetric strain  and shear strain in the direct shear test.