4.8 Stress Paths in Excavations

As already mentioned in section 2.5.1, it is possible to take the specimen in a triaxial cell to failure either in axial compression or axial extension. For ready reference Fig. 2.17 is reproduced here as Fig. 4.40. The specimen can be compressed axially either by increasing the axial compressive stress (path OA in Fig. 4.40) or decreasing the lateral stress (path OB) or varying the stresses in both direction (path OC). Similarly, axial extension can be achieved either by decreasing the vertical stress (path OD) or increasing the lateral stress (path OE) or varying the stresses in both direction (path OF). Path OA is typical to the state of stress under the axis of an embankment or foundation footing, path OD is typical to the state of stress under the centre of excavation, path OB is typical to the state of stress behind a retaining structure, and path OE is typical to the state of stress in front of the retaining structure and below the excavation level (passive case), where the change of vertical stress due to vertical stress relief is assumed very small. The relief of vertical stress due to excavation and increase of horizontal stress due to the movement of the wall on the passive side may be represented by the path OF, provided that the stress increments in both directions remains the same. Similarly, the increase of vertical stress due to external load at the surface and decrease of the horizontal stress...