Analysis On Engineering Characteristics And Grouting Reinforcement Of Thick Sand Layer In Shijiazhuang Metro

In the sand construction of underground engineering,because of its special engineering nature that engineering accidents such as instability and collapse of surrounding rock can be easily caused during the construction process.Based on the background of the Shijiazhuang subway construction project and the information about the main sand layers involved in the project,some research about the regional distribution law,engineering physical and mechanical properties and grouting reinforcement effect had been literally finished,through data research,regional statistics,field sampling,indoor test,microscopic analysis and numerical simulation.The main research contents and conclusions are as follows:(1)Through investigation and analysis of existing data,the engineering geological and hydrological characteristics of Shijiazhuang city are summarized.(2)Through on-site investigation and data research,the statistical analysis of sand distribution in Shijiazhuang city was conducted.It was found that the distribution of sand layers in Shijiazhuang urban area is mainly dominated in the north,and the main sand layers include fine silt layer,fine sand layer,medium sand layer and medium coarse sand layer.Then the sand layers were divided into the northwest,northeast,southeast,and southwest regions,in which the types and thickness of sand were of great differences.For instance,the thickness of fine silt layer is 1~12.4m in a general way and the depth of burial 4.2~12m;the thickness of fine sand layer is 0.3~10.2m and the depth of burial 7.3~16.4m;the thickness of medium sand is 1.4~8.6m and the depth of burial 6.8~10m;the thickness of fine sand layer is 1.1~11.9m and the depth of burial 9.7~26.3m.(3)Based on the indoor tests for the physical and mechanical properties as well as statistical analysis of the data collected in the survey reports of the Shijiazhuang subway and nearby areas,the engineering characteristics of coarse sand,medium sand,fine sand and silt were acquired in combination with the SEM microscopic test results and statistical analysis of a total of 451 sand samples.It showed that the natural density and natural moisture content of sand in Shijiazhuang urban area are small and the gradation poor;the content of fine particles in the coarse sand is the smallest,the particles are mainly hexahedral or similar to the ellipsoid,and the roundness of the soil particles is also good;the contact between soil particles is mainly point contact;the compaction characteristics of different sands are also different;sand strength is low and susceptible to shear failure;the pore ratio of the sample taken is between 0.79 and 0.88,the porosity is Large,all belong to the medium compressive soil;sand permeability is better,with the increase of particle size,the permeability coefficient of soil samples also gradually increased.(4)Based on the obtained physical mechanics property indexes and permeability grouting theory,the injectability of the sand layer was analyzed and discussed.With the finite element numerical analysis,the feasibility of using the step method to construct the subway interval in the thick sand layer was discussed.Comparing the stability of the tunnel before the reinforcement with after,there are some valuable information provided for reinforcement effect of the thick sand layer of the Shijiazhuang subway project.Besides,the settlement of the vault was reduced by 61.8%,the convergence value of the tunnel level by 48.2% and the surface settlement by 68%,compared with that before reinforcement.All in all,grouting has significant effect on soil reinforcement.