Soil Conditioning for EPB Shield Tunneling in Coastal Silty Clay Strata: Laboratory Research and Field Application

Earth pressure balance (EPB) shield tunneling in coastal silty clay strata often faces the problems of clogging on the screw conveyor or the belt conveyor due to lumps of clay soils formed because of the large cohesion of clay particles. Soil conditioning using common foaming agents is not enough to alleviate the problem. Therefore, the novel dispersed foaming agent was studied in this work using performance and orthogonal compound tests. The foam microstructure was observed using an electron microscope to analyze the evolution mechanism of bubbles for different additives. Both laboratory and in situ tests were carried out to assess the effectiveness of the novel dispersed foaming agents. The testing results showed that 50 vol.% is an optimum foaming injection ratio to improve rheological properties and undrained shear strength of silty clay to avoid the unnecessary waste of conditioning materials. The half-life time (T1/2) and foam expansion ratio (FER) of novel foaming agents using the formation ionic solution as the solvent increased due to existing hydrophilic polar groups based on undisturbed soil samples taken from a Xiamen Metro construction site. It was shown that acids, alkalis, and salt ions had little effect on the FER and T1/2 of foaming agents (Foams A and B) using the macromolecular dispersant. The variations in the plasticity index were similar to those of the liquid limit for the muck conditioned using dispersed foaming agents with sodium citrate, sodium bicarbonate, and sodium chloride at a concentration of 0.1 wt%. Sodium bicarbonate had the most significant impact on the foaming agents' anticlay effect. Compared with commercial and house-made foaming agents of the Fuzhou Metro project, the muck was effectively conditioned by Foams A and B based on the slump and temperature values. In addition, the average cutter-head torque was reduced by about 250 kN center dot m. The amount of foaming agents used was reduced by about 18.6% and 12.9% on average in the two testing sites, respectively.