Experimental Study on the Macro and Micro Characteristics of Sand-Grout Mixture Formed by Pressure Cement Grouting in Soils

In recent years, soil grouting techniques have been widely used in maintenance of operating tunnels. By injecting cement-base grout materials in the surrounding ground, the deformation performance of the existing tunnel lining structure is intended to be controlled and recovered to some degree. However, due to the random diffusion patterns and complex phase change process during the grouting process, the grouting conduction on site and the strengthening performance is hard to control. In this paper, a series of model tests on grouting in sand have been performed with different grouting pressure values and soil stress conditions, where a group of sand-grout mixture samples have been obtained. The macroscopic characteristics of sand-grout mixture were captured, and influence of random grout diffusion and solidification on geometric patterns were analyzed. Subsequently, the microscopic characteristic of sand-grout mixture was investigated using scanning electron microscope (SEM). Small sand-grout mixture samples with a scale of 1 cm were extracted at different locations away from the grouting pipe nozzle, which were scanned by SEM with 1000 times magnification. Based on the analysis on the micro-structure images of the sand-grout mixture samples, it has been found that the density of the sand-grout mixture decreased as the grout diffused away from the pipe nozzle. In the same horizontal cross section, the grouting pressure dissipated along with the grouting diffusion. As the distance between the soil and the grouting pipe increased, the disturbance of the grouting to the soil and the grouting accumulation decreased.