Selection criteria for foaming agent and mechanical performance evaluation of conditioned soil for EPB shield tunneling in water-rich sand strata

The selection of a suitable foaming agent for soil conditioning in the Earth Pressure Balance (EPB) shield tunnelling through water-rich sand strata currently lacks a standardized test method and mechanical indices. This paper presents systematic investigations on several key factors influencing the soil conditioning of EPB shield tunnelling regarding foam. The mechanical properties of the foaming agent solution and foam have been assessed for eight different foaming agents through a series of specifically devised laboratory tests. The efficacy of the foaming agent solution has been validated through laboratory tests on conditioned water-rich sandy soil for three types of foaming agent solutions. Furthermore, a standardized method has been proposed for selecting foaming agent and a mechanical characterisation scheme for conditioned soil of EPB tunnelling in water-rich sand strata. Experimental results indicate that the optimum foaming concentration should be controlled to be at 3% for EPB shield tunnelling in water-rich sand strata. At an optimal foaming concentration of 3%, the investigation test results indicate the following mechanical indices of foaming agent should be satisfied: 1) surface tension < 40 mN/m at the critical micelle concentration; 2) the foam volume > 150 mL after 15 min with the Roche foam meter; 3) the foam expansion ratio under the atmospheric pressure > 12; and 4) the half-life time > 400 s. Furthermore, the concave decay type of foam (class II) is preferred over the linear decay type (Class I), as confirmed by the tests on the mechanical performance of foam conditioned sand. It has also found that the conditioned soils having indices with the slump of 150–200 mm, permeability coefficient k < 10–5 m/s, and undrained shear strength of 3–7 kPa, work well for EPB shield TBM tunnelling through sandy soils in Nanchang.