Study On Propagation And Reinforcement Mechanic Of Fracture Grouting In Soil And Its Engineering Application

Fracture grouting is an effective way to improve the strength and impermeability of soft soil,and has been widely used to prevent related disasters and risks.However,the propagation and reinforcement mechanismt of fracture grouting lag far behind the engineering practice,because of its concealment and the lack of research means.It has become a key factor restricting the development of fracture grouting towards scientific and controllable.In this paper,the propagation and reinforcement mechanism of fracture grouting were systematically studied by means of laboratory tests,numerical simulation and theoretical analysis.The main research contents and results are as follows:(1)A set of self-designed fracture grouting test device was used to reproduce the propagation process of the fracture grouting in soil with a weak interface.The stepping mode of the crack propagation,the spatial morphology of the crack,and the variation of the propagation speed and crack width were obtained.The test results show that the stepping mode of crack propagation is "expansion?stagnation?re-expansion?re-stagnation...";When fracturing grouting in clay,oblique micro cracks first appear in front of the crack tip before the formation of the fracture channel.(2)A self-developed numerical method coupling finite element metod(FEM)and volume of fluid(VOF)technique was used to simulate the fracture grouting process in different strata.The simulation results consist well with the field test and laboratory test results,which proves that it is applicable.Then,the FEM/VOF method was used to analyze the influence of different soil parameters on the crack propagation in soil with weak interface.The results show that the soil cohesion is the main factor affecting the fracturing pressure;the soil modulus is the main factor affecting the crack width;and the permeability coefficient is the main factor affecting the diffusion form.With the increase of soil cohesion and decrease of soil modulus,the shape of the crack tip changes from a long and pointed "wedge head" to a short and blunt "blunt head."(3)The FEM/VOF method was used to simulate the process of single-hole fracture grouting in homogeneous soil.The whole dynamic process of fracture grouting from fracture initiation and propagation to grout vein formation was reproduced.The variation process of stress field and displacement field in each stage were analyzed.The effects of grouting depth,soil modulus,penneability coefficient and grouting pressure on the morphology of grout vein were obtained.The results show that the vertical displacement increases rapidly with the generation and expansion of the oblique vein branch after the secondary fracturing;The distribution of the grout veins in different grouting depths and different soils is different,but the morphology of the grout veins can be controlled by adjusting the size and the application mode of grouting pressure.(4)A multi-hole fracture grouting model was established to obtain the propagation law of fracture grouting.Firstly,the diffusion and uplift laws as well as the mechanism of mutual interference between grout holes were analyzed in the mode of double-hole simultaneous grouting.Then,the double-hole sequential and multiple-hole simultaneous fracture grouting diffusion laws were studied.It is found that when the two holes are simultaneously grouted,the grout veins are horizontally deflected,and the smaller the grouting hole spacing is,the larger the deflection angle is;The deflection angle of the grout veins is affected by the difference between the horizontal stress and the vertical stress;The uplifting of the double-hole simultaneous fracture grouting is greater than the sum of value when the holes are separately grouted;As the spacing of the grouting holes increases,the uplift curve transitions from "single peak" to "double peak".(5)A grouted composite soil model based on the grout vein morphology of single-hole fracture grouting was established to analyze the reinforcement mechanism and compressive modulus.The influence weight of the effect of the skeleton and the compaction on the grouting reinforcement effect is analyzed.The relationship between the compressive modulus of the composite soil and the grouting pressure,grouting depth,initial modulus of the soil and skeletal modulus was obtained.(6)Based on the research results of the fracture grouting propagation and the reinforcement mechanism,the "multi-step grouting pressure and rate control" method are proposed,and it is successfully applied in the curtain grouting project of the Beijing New Airport Integrated Pipeline Crossing Project.The research in this paper enriches the theoretical system of fracture grouting in soft stratum,and provides a theoretical reference for the design.The FEM/VOF method proposed in this paper provides a new idea for simulation of fracture grouting.