Strength and Compressibility of Soil-Bentonite Mixtures for Cutoff Walls

The strength and compressibility of two soil-bentonite backfill mixtures were characterized through a laboratory program that included 1-D consolidation tests and triaxial compression tests. The triaxial compression tests were performed on normally consolidated and overconsolidated specimens, and they included isotropically consolidated undrained and drained test conditions. Procedures for preparing and testing soft soil-bentonite mixtures are discussed, including procedures for forming specimens and appropriate use of area and membrane corrections. The test results are presented in this paper, and they are compared with other published values, where available. The results include values of coefficient of consolidation as a function of effective stress, compression and recompression ratios, pore pressure response during undrained loading, the ratio of undrained shear strength to consolidation pressure, effective stress friction angle, and at-rest lateral earth pressure coefficient. Index property values for the soil-bentonite mixtures are provided so that the results presented here can be compared with those for other soil-bentonite mixtures. Findings for soil-bentonite mixtures include the following: (1) the values of compression ratio are smaller than for most natural clays, (2) excess pore water pressure generation during undrained compression is much greater than for most natural clays, (3) the ratio of undrained shear strength to consolidation pressure is much lower than for most natural clays, (4) the effective stress friction angle often falls in the range of 31 to 33 degrees, and (5) the measured value of at-rest lateral earth pressure coefficient was 0.57.