Experimental Study On Soil’s Improvement During The Construction Of EPB Rectangular Pipe Jacking In Sand And Gravel Stratum

When the method of EPB rectangular pipe jacking is applied to build middle and shortlength tunnel of city in sand and gravel stratum, due to the soil’s flow plasticity of this typeof stratum is poor, the cutter will engender such phenomenons,"caking, occlusion " and soon, so that it is difficult to maintain the dynamic equilibrium between the soil ballast andthe water and soil pressure of the excavation face, even lead to serious accident happen,such as instability or collapse of the excavation face. Therefore, how to take the muck ofsoil cabin improving to plastic flow state that "good liquidity","low friction","highcompression","low permeability" is the important premise to ensure the smoothconduction of pipe jacking.Based on the background of Inner Mongolia University of Science and Technologyunderground passage that bulit by the method of EPB rectangular pipe jacking, this articleused several research methods, such as monitoring and measurement, mixing test, directshear test, slump test and so on, to study the soil’s improvement methods. The maincontents and conclusions are summarized as follows:Through field monitoring data and indoor test, the soil’s improvement method andimprovement effect of PAM+Silt mud used in this project were studied. The study resultsshow that: the improvement effect of the muck in this project was not ideal and the cutter’swear and occlusion phenomenon was serious, the surface subsidence overrun many timesand the actual top speed was only1.5mm/min, the soil’s improvement methods still hadroom for improvement.Based on the deficiency of soil’s improvement effect of this project, theimprovement method of PAM+bentonite mud was putted forward, and the influence ofdifferent additives’ ratio on the properties of the modified soil were studied. The studyresults show that: the optimal concentration of PAM was2g/L, the optimal concentration range of bentonite mud was9%～11%; with the PAM’s injection ratio increased, theslump or compression coefficient of the soil increased gradually and the stirring torqueincreased fristly and then decreased gradually and the internal friction angle of the soildecreased gradually; according to the properties of the soil that had improved, the finalselection of the slurry injection ratio was10%～14%and the PAM injection ratio was17%～18%.Through the methods of stirring and roche bubble test, two best fomaing systems werecompounded, including: FW2whose foaming rate was5.75and half-life was330s, FW3whose foaming rate was5.5and half-life was420s. On this basis, the influence of differentadditives’ ratio on the properties of the modified soil were studied, the study results showthat: with the foam’s injection ratio increased, the slump of the soil increased gradually andthe stirring torque or internal friction angle of the soil decreased gradually; the optimalsoil’s improvement plan that the slurry injection ratio was8%～10%and the foaminjection ratio was15%～20%was determined.By the comprehensive evaluation and analysis，the results show that: when the slurryinjection ratio was more than8%, the soil’s the improvement effects of bubble was betterthan PAM during the EPB pipe jacking construction in sand and gravel stratum; accordingto the results of the experimental and the construction experience, the constructiontechnology of the two kinds of soil’s improvement methods was summarized.