SHEAR-STRENGTH OF VERY SOFT CLAY-SAND MIXTURES

To treat a deposit of clay slurry in situ, sand is often added with the aim of trapping the sand in the upper portion of the slurry. The presence of the trapped sand may directly improve the shear strength of the slurry or contribute indirectly to the improvement by accelerating the consolidation. For this treatment to be effective, an important consideration is the improvement in the shear strength of the slurry as sand is added. A laboratory study of this is reported here. To rapidly determine the shear strength of the mixture, the fall cone method and a penetration test that was developed to measure very low shear strength were used. For the clay slurries studied, the added sand grains seemed to ''float'' in the clay without increasing the shear strength if they were sufficiently far apart. Treating the clay slurry as a fluid filling the ''voids'' in the sand, the void ratio of the sand for which a marked increase in shear strength is observed is about 5. The liquid limit of the mixture is also found to follow approximately a linear mixture law unless the amount of sand added is enough to ensure that the void ratio of the sand is less than 5, which is in agreement with the earlier result.