Stabilizing Fine-Grained Soils by Phosphate Electrogrouting

Abstract

The results of a laboratory study demonstrate the significant potential of electrogrouting, a new method for stabilizing soft soils with relatively low hydraulic conductivity. The experiments reported show that migration of stabilizing ions through soil pore fluid under applied direct current (dc) electric fields can be substantial. The method can achieve a more stable soil (i.e., less compressibility and higher shear strength, among other property improvements) without causing significant volume changes. In the experiments reported, the effect of stabilizing Boston Blue Clay (BBC) samples using phosphate ions was investigated. Initially, batch tests were conducted by mixing BBC samples with specific volumes of phosphoric acid solutions prepared at different concentrations. An increase of as much as 280% over the initial strength was achieved at the highest concentration of the acid used (10% of concentrated phosphoric acid solution). From the success of these batch tests, electrogrouting experiments were conducted by placing the soil in special treatment cells and applying a constant voltage gradient for 2 weeks. Electrogrouting the BBC under 1 V/cm dc is 15 times faster than is injection under a unit hydraulic gradient and produced increases in shear strength across the soil, with the greatest strength gain (160% of the initial shear strength) occurring in the soil section near the cathode (negatively charged probe). Additional results show the effects of electrogrouting on soil compressibility. A detailed review is given of the considerations necessary to implement the electrogrouting method in the field.