The undrained response of sands with additions of particles of various shapes and sizes

As a natural deposit, sand is rarely found clean—that is, without fractions of silt or clay. The engineering behaviour of two different host sand gradations mixed with additions of particles of various shapes and sizes is compared with that of clean sand. Anisotropically consolidated specimens are sheared in triaxial compression and extension at their ‘loosest’ initial state resulting from the same depositional method, namely air pluviation. Additive materials at contents less than 2·5% by weight can dramatically change the undrained behaviour of the host sand. The addition of rotund particles (e.g. silt) should be distinguished from that of flat or platy particles of different size (e.g. kaolin, silt or sand-size mica). The importance of shape and location of additives in modifying the sand structure is not reflected in measures such as void ratio. Mixtures of both sands with silt-size mica exhibit the most unstable response, although these had the lowest values of void or granular void ratio. The undrained response of each sand mixed with different amounts of sand-size mica is also presented. An increase in mica content usually leads to more stable behaviour at contents above a certain threshold, depending on the grading of the sand. The shear response of the sands and most mixtures is much weaker and more contractive in triaxial extension than in triaxial compression. High anisotropy is also indicated by the stiffness measurements at small strains in both loading directions.