Conditioning experiment on sand and cobble soil for shield tunneling

Sand and cobble soils have a large internal friction angle and poor fluidity. Many problems have been encountered during shield tunneling in sandy cobble stratum, such as cutter head wearing, difficult soil discharge, and excessive thrust force. In this study, foam or foam and bentonite slurry were tested to improve the soil properties. First, the optimum modifier percentages, namely, those of the foaming solution concentration and the bentonite slurry soil–water ratio, were determined; the optimum values were 3% and 1:10, respectively. Second, various percentages of cobble contents (CCs, i.e., 20%, 40%, 60%, and 80%) were added in the sand and then subjected to large-scale indoor shear test and slump test by incorporating foam. Experimental results indicate that as the foam injection ratio (FIR) increased, the foam soil content wrapped around the sand and cobble soil also increased, the separation of cobble from sand weakened gradually, the shear strength nonlinearly decreased, and the slump gradually increased. To satisfy the shear strength and fluidity requirements in shield tunneling, the FIR of sand and cobble soil with CCs of 20%, 40%, and 60% was determined. The sand and cobble soil with a CC of 80% was further improved by mixing foam and bentonite slurry, thereby obtaining the best modifier ratio suitable for shield construction. Finally, the results of the indoor test were further verified through the monitoring and analysis of the tunneling parameters and earth pressure before and after the conditioning of the sand and cobble soil during the shield tunneling. These results can be of great importance in solving of the problems encountered in shield tunneling in sandy cobble stratum.