Experimental investigation on viscoplastic parameters of conditioned sands in earth pressure balance shield tunneling

A device was developed to determine the viscoplastic parameters of conditioned soil, which works on the principle of torsional shear applied to a standard vane with controlled strain rate. Rheological measurements were taken for three sand samples with the same bentonite slurry injection ratio (14.3%), but with different foam injection ratios (19%, 23%, and 27%), by changing the gasbag pressure levels (i.e., 0, 100, 200, 300, and 400 kPa) and vane speeds (i.e., 1/3.4, 1/4.1, 1/5, 1/6.4, 1/9, 1/15, and 1/30 RPM) to determine the effects of confining pressure and foam injection ratio on the viscoplastic parameters. The results show that the values of viscosity coefficient and yield stress for conditioned sands are in the range of 9.1–53.5 kPa·s and 1.5–10.2 kPa, respectively. The viscoplastic parameters increase with the increase of the confining pressure, and decrease with the increase of the foam injection ratio for all tested samples.