Experimental Study On Air Pre-fracturing Grouting For Low Permeability Cohesive Soil

Air pre-fracturing technology is a potential technical means that can effectively enhance the effect of fracture grouting.The research on air pre-fracturing grouting technology is currently at the model test and theoretical level.Because the model test cannot accurately monitor the internal particle displacement and the change of soil porosity,and consider the fluid-solid coupling between air and soil.At this stage,it is reasonable and necessary to use discrete element numerical simulation and model test to study the mechanism of air pre-fracturing grouting.Based on the hydraulic fracturing mechanism,this paper analyzes the initiation and failure mechanism of air fracturing,the mechanism of crack propagation,and the method of crack monitoring,and a theoretical model is formed.The specific work is as follows: 1.Take the sieved clay as the research object,establish a soil model in the numerical simulation software PFC,monitor the soil displacement and the change of soil porosity during the air fracturing simulation,and analyze the effect of air fracturing on the soil.The perturbation effect confirms the feasibility of air fracturing the soil.2.Design air fracturing model experiments to monitor soil surface pressure and air injection hole pressure,analyze the mechanical characteristics of air fracturing,and perform threshold binarization processing on the crack image to calculate the crack area.While analyzing the mechanism of air cleavage,compare the advantages and disadvantages of instantaneous loading and gradual loading,and finally the gradual loading method is preferred.3.Test the viscosity and setting time of the grouting slurry,analyze the rheological characteristics of the slurry,and prepare a suitable grouting slurry.Design the air pre-fracturing grouting model experiment,use the electronic displacement meter to monitor the displacement of the overlying soil and draw the displacement cloud diagram,and analyze the influence of the air pre-fracturing grouting on the soil.After the end of the experiment,the morphology of the grout consolidation body was analyzed and the quality of the grout consolidation body was measured,so as to analyze the difficulty of grouting and the generation and expansion law of the grout veins.In this paper,through theoretical analysis,numerical simulation,model test and other research methods,the occurrence and diffusion mechanism of cracks during air fracturing and the interaction between grout and soil during air pre-fracturing and grouting are studied,and the following laws are obtained: 1.The fracturing effect of the soil is related to the air pressure and the air loading time.When the air pressure is greater than the initiation pressure,the soil forms through cracks.2.During the air pre-fracturing grouting,whether it is the air-fracturing stage or the fracture grouting stage,the pressure in the soil body will increase and shear failure will occur,and the local soil body will move while the rest remain in place.3.The grout will spread in the soil along the pre-fracturing cracks.When spreading,part of the pre-fracturing cracks will expand and develop into a vein,and some of the cracks will be compacted and closed again with the injection of grout.4.Compared with ordinary fracture grouting,under the same grouting pressure,air pre-fracturing can increase the grouting volume and reduce the waste caused by the overflow of grout from the bottom end.