Permeation Law And Long-Term Solidification Stability Evaluation In Loose Coal With Silica Sol Grout

A large number of millimeter-micron-nanometer multiscale fractures and pores are distributed in loose coal.Cement slurry used in coal mines cannot seal and consolidate micropores and the infiltration capacity of cement slurry is very small.Loose coal is difficult to be reformed.Nanomaterial grouting is an effective method to solve this problem.Permeation law of the injection period and long-term solidification stability after injection are one of the key issues in the reformation of loose coal.In this paper,the comprehensive research methods of literature review,laboratory testing and analysis,theoretical analysis,engineering geological survey of typical loose coal seam and in-situ cement slurry grouting test were adopted.Pore structure of raw coal and basic characteristics of the silica sol grouting were characterized.A multi-parameter fine online monitoring platform of grouting permeation was developed and a matching method of sample pretreatment has been explored.Permeation law and basic mechanism in loose coal with silica sol grouting was revealed using the platform.Long-term deformation and strength parameters time-varying law of silica sol consolidation body was analysed.Time-varying mechanism of mechanical behavior of silica gel was obtained.Long-term solidification stability of silica sol grouting with coal was evaluated.The main conclusions are listed as below.(1)No.8 coal in Huaibei mining area was a typical loose coal body.Coal sample with a particle size of less than 2 mm was nearly 75%,and the pulverization phenomenon was serious.The total porosity of raw coal met an exponential decay function with the relation of ground pressure.Permeability of raw coal was 6.76 mD and raw coal was a low permeability medium.Pore diameter distribution law was “most pores belonging to < 20 nm or > 1 ?m with less pores belonging to the middle”.Compared with other commonly used grout materials,the injectability of silica sol was the best.After catalyst was added into silica sol,the polycondensation reaction was giving.Early gelation had little effect on the injectability and fluidity of grout.Later gelation made the injectability and fluidity deteriorate and the deterioration point of the injectability could be noted earlier than the fluidity.Before the densification of the grout,the injection operation should be completed ahead of time.Time-varying curves of viscosity of silica sol grout were in good agreement with the compound function.The amount of catalyst and gelation time were well subject to the exponential negative correlation.The static contact angles between silica sol,catalyst and raw coal were all less than 90°.The affinity of silica sol and catalyst to raw coal was lower than that of deionized water to raw coal.Thus imbibition potential of grout was quite significant,but saturated environment would impede the imbibition effect.(2)Multi-parameter fine online monitoring platform of grouting permeation was developed,including pump and storage system,grouting platform and monitoring system.It could be used to collect the parameters such as pump pressure,fluid characteristics of the inlet grout and outlet grout and fluid pressure of different positions in the process of grouting.There was a basic periodical law of “displacement flow-Darcy flow-percolation flow” in the permeation mechanism of silica sol grout in loose coal.Coal sample fragment had a significant influence on the time-varying curves of pump pressure in the Darcy flow stage and the percolation flow stage.The larger the fragment was,the shorter the time of Darcy flow stage was and the more significant the effect of colloid retention and blockage was.The coal sample with small-fragment(< 2 mm)had an approximately equal time of Darcy flow stage.Tests with suppress load below 20 MPa all obviously showed “displacement flow-Darcy flow-percolation flow” stage characteristic,Darcy flow stage time accounted for the largest proportion of the whole process of grouting,and steady pressure in Darcy flow increases with the increase of suppress load.Permeation mechanism of test with suppress load of 20 MPa mainly was percolation flow.The increase of flow rate would indece the change of grouting permeation mechanism to the percolation flow.Pump pressure rised rapidly,and grouting time was reduced with the incerese of flow rate.Therefore,a slow injection method for high stress and low permeability stratum could be used to reduce the percolation apparent intensity and to prolong the penetration radius of grout.(3)In the 500-day curing period,mass,volume,diameter and height curves of silica gel could be divided into a shrinkage stage,a stable stage,and a second shrinkage stage.Uniaxial compressive stress-strain curves for silica gel were elastoplastic.The deformation can be divided into four phases,a compaction phase,an elastic phase,a plastic phase,and a post-peak stress phase.The curves for uniaxial compression strength,secant modulus and ultrasonic wave velocity can be divided into an ascending stage,a descending stage,and a second ascending stage.Plastic strength-time curves for samples with different proportions of catalyst exhibit allometric scaling.Influence of curing environment on the mechanical behavior of silica gel was mainly reflected in free water and physical adsorption water.When silica sol was injected into the coal sample involving big-fragment(5-10 mm),the long-term strength stability of samples cannot be achieved.When silica sol was injected into the coal sample not involving big-fragment(5-10 mm),200 days was the start time that strength reached stable state.When silica sol was grouted into the coal sample involving small-fragment(<2 mm),the long-term stiffness of samples can be achieved.The above research results can provide theoretics support for application of silica sol grout in the field of engineering anti-seepage and surrounding rock reinforcement,such as construction,stability of surrounding rock and security control of rich water or high humidity tunnel.