Effect of Setting Method on the Behaviour of Clays in Triaxial Compression from Saturation to Undrained Shear

Abstract

In the conventional method of setting a saturated clay specimen for triaxial testing, water saturated porous stones, wet filter paper and drainage lines filled with water are used (the wet method). Since this method introduces free water to the surfaces of the specimen under zero total stress, under suction pressure, the specimen takes in water and swells. It is shown that this swelling and its effects on the subsequent specimen behaviour can be reduced by setting specimens of clay with dry porous stones and dry filter paper and applying an effective pressure before allowing the surfaces of the specimen to come into contact with free water (the dry setting method). It is further shown that the consequent difficulty associated with achieving a high degree of saturation in the dry setting method can be overcome by a combination of vacuuming, flushing with de-aired water, and back-pressurizing giving degrees of saturation comparable to those obtained by the properly executed conventional wet setting method. The responses of reconstituted specimens of kaolin and an in-situ clay mounted by the dry method are also compared with those mounted by the conventional wet setting method over the whole stress range from saturation to failure in triaxial compression. It was observed that at low stress levels the wet method gives softer specimen response which can result in inaccurate determination of the pre-consolidation stress. The results further suggest that unless the effect of setting method is severe, consolidation to higher stress levels may be enough to erase the effect of setting procedure.