Question

Explain why the UU triaxial shear test would be the most appropriate test for simulating the strength of clay below an embankment at the end-of-construction.

Answer 1

Determining the mechanical properties of soils is a very important step to design foundations, embankments and other soil structures. Building constructions, excavations, tunnelling and similar applications have several effects on the subsoil structures. These effects are successfully simulated with Triaxial Tests where the stress-strain relation of undisturbed soil specimen are investigated by subjecting the soil sample to different stress levels and drainage conditions. Triaxial test facilities need to possess considerable flexibility to simulate stress and deformation conditions likely to be encountered in various practical problems.

For the UU test , the saturated specimens are subjected to a confining fluid pressure in a triaxial chamber. Once the specimen is inside the triaxial cell, the cell pressure is increased to a predetermined value by rotating the knob of the constant pressure unit, and the specimen is brought to failure by increasing the vertical stress by applying a constant rate of axial strain. Since saturation and consolidation do not exist in this method, original structure and water content of sample is untouched. Pore and back pressures are not measured during this test and therefore the results can only be interpreted in terms of total stress over a confinement pressure (stress).

The unconsolidated undrained strength is applicable to critical design situations where the loading is too rapid that there is no time for the excess pore pressures mobilised to dissipate. Hence UU triaxial test is very vital in case of construction of embankments and dams and foundations on soft soils like clay. Often the critical design condition is immediately after the load application. This condition is simulated in UU test. Once the consolidation begins, the water content decreases with a consequent increase in strength leading to a higher safety factors with time.