The Mechanism And Experimental Study Of Penetration Grouting In Water-rich Sand Layer Considering Filtration Effect

With the increasing number of domestic and foreign metro tunnel construction projects and the continuous improvement of the metro network,the control of undesirable geological disasters during metro construction has become an important research topic.More and more subway tunnels need to go through the water-rich sand layer,which has the significant characteristics of rich water content,low cement strength and poor self-stabilization ability,and is easily induced by construction disturbance and groundwater under the action of tunnel collapse,surge routing sand and other engineering disaster accidents.In order to ensure the safety of tunnel construction,the project widely uses infiltration grouting to pretreat the water-rich sand layer.However,there is a big difference between water-rich sand layer and conventional stratum grouting,and it is difficult to obtain ideal grouting effect by using existing conventional grouting technology.This paper takes urban water-rich sand stratum as the object of research,takes the percolation effect in water-rich sand stratum as the core,and comprehensively adopts various research methods such as literature research,theoretical analysis,numerical simulation and indoor test to reveal the permeation mechanism and diffusion mechanism of cement slurry in sand stratum based on percolation effect,proposes a new standard for permeability of water-rich sand stratum,and reveals a new standard for permeability of cement slurry in water-rich sand stratum considering initiation.It has developed a new material for controlled grouting of water-rich sand layer tunnel,and successfully applied the research results to the grouting reinforcement project of Nanchang Honggu Tunnel through water-rich sand layer.The main contents and conclusions of this paper are as follows.(1)Based on mass conservation law,momentum conservation law and other theories,and taking into account the effect of percolation effect,a porous media suspension flow model was established and a macroscopic control equation for suspension permeation based on percolation effect was deduced.The company developed a water-rich sand layer percolation test device,and carried out deep penetration grouting test of cement slurry to explore the influence of sand column porosity,initial particle volume fraction of cement slurry and grouting pressure on the percolation effect.The test results show that:the lower the sand column porosity,the more significant the percolation effect;the higher the initial particle concentration of the slurry,the more significant the percolation effect;the strength of the percolation effect and the grouting pressure has no obvious law.(2)Summarized the existing sand layer groutability indexes and analyzed the reasons for the failure of these indexes.Developed a set of visualized indoor grouting test device,carried out infiltration grouting test research of water-rich sand layer based on a new sand grade preparation method,and proposed a grouting index based on‘corrected inhomogeneity coefficient'C_u5 and‘corrected groutability ratio'N_m according to the groutability test results.On the basis of which the groutability prediction model based on the novel groutability metric is proposed with an accuracy of nearly 80%,which is at least10%higher than all existing groutability criteria.(3)Based on the basic theory of percolation mechanics,the dimensionless percolation mathematical model of porous media percolation of Bingham fluid considering the starting pressure gradient is established,which reveals the moving law of dynamic boundary of cement slurry liquid rich sand layer permeation and grouting diffusion range.The exact analytical solution of the porous media percolation model of Bingham's fluid under constant injection pressure at the inner boundary is obtained by the similar variable transformation method.Based on the spatial coordinate transformation method,the numerical solution of porous media infiltration model of Bingham fluid under constant grouting rate at the inner boundary is obtained.This part of the study provides a theoretical basis for determining the diffusion range of cement grout in sandy stratum.(4)Based on the basic equation of fluid dynamics and discrete phase equation,the solid-liquid two-phase flow model of cement slurry was established by Euler's method,and the CFD software FLUENT was used to realize the simulation of the grouting and diffusion process of small conduit in water-rich sand tunnel.The results show that during the grouting process,the slurry spreads to the surrounding area in a fusiform shape,and the diffusion pattern of cement slurry in the water-rich sand layer is influenced by grouting pressure,slurry water-cement ratio,grouting time,percolation coefficient and other parameters,and finally tends to be stable.In addition,based on the water-rich sand layer infiltration grouting special requirements,the new slurry formula of high-flowing and high-stability(HFSG)with ultrafine cement,ultrafine fly ash and bentonite as the main materials and sodium pyrophosphate and sodium citrate as the additives is determined.The orthogonal test was carried out to study the macroscopic fluidity,viscosity-rheology,water precipitation stability and initial and final setting performance of the new slurry,and to study the influence of different raw material components,different admixtures,different water-cement ratio and other influencing factors on the performance of the slurry,which was used as the basis for the optimization of HFSG slurry performance.0.15,slurry liquid-solid ratio 1:1,content of sodium pyrophosphate2%,content of sodium citrate 0.6%.The penetration grouting test between HFSG slurry and ordinary cement slurry was carried out,and the test results showed that the fluidity and stability of HFSG slurry were greatly improved compared with that of ordinary cement slurry,and the microscopic morphology of HFSG slurry stone body also showed higher density under SEM electron microscope.