Laboratory Experimental Study On Bentonite For Shield Tunneling To Improve Sandy Soil

In recent years,EPB shield has been widely used in the construction of urban metro tunnels due to its advantages of fast construction speed,little environmental impact and economic construction.Adjusting the soil excavated from the cutter head to"plastic flow state"in the pressure chamber is the key for EPB shield to smoothly complete a series of construction processes,such as excavating the soil layer,maintaining the pressure of the pressure chamber and discharging the dregs.However,because of the high permeability,high friction and poor fluidity of sandy formations,it is difficult to form a"plastic flow state",so bentonite is usually used to improve sandy soil.In the process of shield tunneling on site,bentonite has a single function,a large area of expansion tank and a long expansion time,which often results in the bentonite viscosity not reaching the standard due to insufficient expansion time,thus affecting the tunneling efficiency of shield tunneling and causing great waste and hidden dangers to engineering safety.In order to improve the defects existing in the application of bentonite in shield tunneling,this paper,by means of theoretical analysis and laboratory experiments,optimizes the bentonite proportion scheme with good performance according to the engineering characteristics of sandy strata,and determines its practicability in the engineering field.The main research results are as follows:?1?Understanding the structure of bentonite and the micro-mechanism of bentonite improving sandy soil to provide theoretical basis for selecting appropriate bentonite additives.?2?Considering the characteristics of plastic fluidity of shield tunneling soil,sodium bentonite is added with additives such as NaOH,Na₂CO₃ and sodium carboxymethyl cellulose.Through a series of physical and chemical experiments,the parameters of bentonite are analyzed,and the additives and their dosage with the best improving properties are determined.The optimal bentonite formulation scheme under different additives conditions is obtained.It has achieved the effect of reducing the amount of bentonite and shortening the expansion time.On this basis,the chemical composition changes of bentonite before and after adding different additives were analyzed by polycrystalline X-ray diffraction pattern,which verified the feasibility and superiority of the formulation scheme of bentonite.?3?In order to verify the improvement effect of bentonite,combined with sandy strata with different particle sizes,the performance of improved sandy soil under different bentonite formulation schemes was comprehensively evaluated by orthogonal test and direct shear test,and the influence rule of the improvement method of bentonite under different strata on the shear strength parameters of improved sandy soil was obtained.The optimal formulation scheme of sandy soil was given and verified.It has remarkable effect on the improvement of pure sand and gravel soil.Sandy soil has reached a better"plastic flow state".The shear strength of sandy soil decreases,the cohesion increases obviously,and the internal friction angle decreases.It is an ideal scheme for improving sandy strata because of its high performance-price ratio.Combining theory with experiment,this paper optimizes the proportion of bentonite additives to enhance the expansion effect and reduce the expansion time of bentonite,and improves the field construction efficiency while ensuring the improvement effect of soil.It provides a reference and basis for the future research on the improvement of sandy soil.