Development of numerical model for grout injection and its application to the real in-situ injection tests

摘要

For the construction of underground facilities and dams, the control and sealing groundwater is one of the key issues. Grouting (grout injection) is one of the commonly used methods to seal groundwater during the constructions. Grout is the mixture of water and cement with a certain mixing rate and is injected into the rock fractures from the injection boreholes drilled into rock masses in order to improve hydraulic conductivity fields of target rock masses. Recently, underground facilities have been built under various geological conditions with limited budget, and for grouting system, more effective and economical injection method has been required. For grout injection, so far the injection method which changes the water/cement mixing ratio (W/C) stepwise from high to low has been commonly used in order to inject cement into the fractured rock masses effectively. Unlike this conventional method, in the newly developed method, the W/C is changed from high to low continuously. This method is more effective injection method and starts being applied to the construction of various underground facilities. In this study, to investigate the grout injection mechanism and processes, the numerical simulations were carried out by solving unsteady seepage flow and advection-dispersion equations using 2D finite element method (FEM) assuming the grout is Newtonian fluid. In the numerical model, the time dependent physical properties of grout (increase of viscosity) and the reduction of porosity in the porous media were considered to express the reduction of hydraulic conductivity during grout injection (improving hydraulic conductivity field). In the numerical simulations, two different grout injection methods mentioned above were simulated using the in-situ grout injection data including the change of the grout injection pressure and grout density. The simulation results were compared with the ones obtained from the in-situ grout injection tests. The simulation results clearly showed that the change of the grout viscosity as well as the decrease of porosity in time played important roles for the grout injection mechanism. The simulation results also agreed well with the ones obtained from the in-situ grout injection tests. Figure 1 shows the comparison of hydraulic conductivity field after 18, 30 and 180 minutes between the two injection methods. The newly developed injection method was more effective and was able to inject more cement in shorter time comparing with the conventional injection method.