An approach to evaluate the efficiency of compensation grouting

Compensation grouting is often employed as a mitigation technique to reduce the settlements induced by tunnelling. The efficiency of compensation grouting, defined as the ratio of the volume of heave induced at ground surface to the volume of injected grout, is strongly dependent on grout properties, injection characteristics and soil properties. In clayey soils, site observations and laboratory tests show high values of compensation efficiency immediately after injection. However the efficiency can decrease with time if positive excess pore water pressures develop in the soil during the injection process. Conversely, in sandy soils, a loss of volume occurs due to pressure filtration of the grout during and soon after the injection process, which reduces the compensation efficiency. This paper describes an analytical model that can be used to evaluate the volume loss produced by pressure filtration of cement-bentonite grouts as a function of soil, grout and injection parameters. The magnitude of pressure filtration is evaluated as the grout efficiency, which is given by the ratio of the volumes of the final grout body to the injected grout. Results show that the grout efficiency increases with decreasing soil permeability and with increasing grout bentonite content. The result of a grout injection test on a silty soil is back-analysed using this approach in which the grout efficiency is first evaluated through the proposed filtration model and then used to calibrate a numerical simulation of the test. The good agreement between experimental data and computed results shows that prediction of the efficiency of compensation grouting can be improved using the proposed approach.