Study On Soil Improving Techniques In Rich-water Content Sand Stratum For Earth Pressure Balance Shield

In urban subway tunnel construction,Earth-Pressure-Balance shield（EPB）has become the main method of urban subway excavation because of its advantages of safe construction environment,low interference with ground traffic and high boring efficiency in tunneling.When crossing the water-rich sandy strata,due to poor particle gradation of slag soil,strong water content,high permeability,easy to appear poorly discharged slag as well as spurt and other construction problems.To solve such problems,it is often necessary to improve the slag soil,so that the slag soil has good fluidity,suitable plasticity,low shear strength and small permeability coefficient,so as to prevent the occurrence of"spewing"phenomenon and poor slag discharge and other problems.Based on the background of Nanchang Metro Line 4 Qili Station-Minyuan Road West Station project,this paper studies the improvement technology of shield tunneling in water-rich sand layer from two aspects of improver and factors influencing the improvement of residual soil.Obtained the optimizing parameters of improver;analyzed the influence law of different factors on the nature of improved residue,studied the influencing factors of spray surge through pressurized infiltration device,and obtained the critical spray surge permeability coefficient,and obtained and optimized the best improvement plan according to the indoor test,theoretical formula,field parameters comparison.The main conclusions are obtained as follows:（1）The optimal parameters of foam are 3%solution concentration,4 bar foaming air pressure,optimal foaming multiplicity and half-life of 26 and 16 min,respectively,and FER of 9-18 when considering the soil chamber pressure.optimal bentonite slurry parameters are:1:10 soil to water mass ratio,16-24 h expansion time,plastic viscosity of 23 mPa.s,martensite funnel viscosity of 50-100 s,slurry specific gravity of 1.06-1.08 g/cm³.（2）As the foam and bentonite slurry injection ratio increases,the slump value of the slag increases,the permeability coefficient decreases,and the shear strength value decreases;while the polymer is making the slag cohesive and the flow plasticity decreases.（3）The optimum moisture content interval for slag improvement is 15%-20%;the optimum FIR for gravel content 30%-70%slag is 5%-15%,and the optimum cross-plate shear strength is 0.9kPa-2.2kPa.（4）When the water pressure is greater than 2 bar,the foam and bentonite mud cannot reduce the permeability of the slag;the higher the gravel content,the worse its impermeability.The critical permeability coefficient of the sandy stratum of the shield interval between Qili Station and Minyuan Road West Station of Nanchang Metro Line 4 is 4.849×10^-6m/s and6.962×10^-6m/s for slight and severe spurting respectively.（5）The gravel content is increased from 30%to 70%,FIR is reduced from 15%to 0%,and BIR is increased from 0 to 10%.When the gravel content is greater than 50%,CMC polymer needs to be added and PIR is increased from 0 to 5%.For sandy strata containing muddy siltstone,water needs to be added to improve the fluidity,and the best water content is17.5%-22.5%.The injection ratio of improver can be calculated by the formula(IR=(e_max-βwG_S)（1-n）·100%);the amount of raw foam per ring of shield can be calculated by combining the indoor slag improvement scheme with the shield construction parameters,which can avoid the shield slag improvement relying solely on experience and reduce the blindness.