砂土地层泥水盾构开挖面孔隙变化特征理论研究

为保证开挖面的稳定，泥水盾构压力舱中的泥浆必须及时在开挖面土体表层形成泥膜，泥浆压力才能有效平衡地层中的水土压力。在考虑泥浆渗滤效应的基础上，建立了砂土地层泥水盾构开挖面泥浆恒压渗透模型，分析了渗透时间、泥浆浓度、泥浆压力和地层初始孔隙率对土体孔隙率及泥浆成膜的影响。结合刀具周期性切削作用，总结了泥浆反复渗透引起的开挖面表层土体孔隙率变化规律。基于刀盘、刀具布置形式和盾构掘进参数，分析了盾构掘进期间开挖面支护机制。结果表明：在无刀具切削作用时，泥浆渗透规律主要受到泥浆和地层特性影响；在刀具周期性切削作用下，在相同的切削深度下，刀具切削周期越短，泥膜在开挖面上越难形成；刀具较小的纵向切入速度有利于开挖面上更快形成泥膜；适当减少刀盘上同一轨迹刀具布置数量，降低盾构掘进速度和刀盘转动速度，有利于盾构掘进期间开挖面上泥膜存在面积的增大，对开挖面稳定起到积极作用。

A theoretical study of porosity characteristics on the excavation face of slurry shield in sand stratum

To maintain the stability of excavation face in slurry shield, a filter cake must be timely formed during the excavation work. With the filter cake, the slurry pressure can counteract the earth pressure and water pressure in the excavation stratum. In this study, a mathematical model of slurry penetration under constant pressure in sand stratum is established by considering the filtration effect. By means of this model, the influence of penetration time, slurry concentration, slurry pressure and initial stratum porosity on soil porosity and filter cake formation is analyzed. Combining the periodic disturbance of cutting-tools, the variation of soil porosity on the excavation face caused by multiple slurry penetration is summarized. Finally, the supporting mechanism of the excavation face is discussed during the shield tunneling based on the design of cutting wheel and tunneling parameters. The study results show that the slurry penetration behaviour is mainly affected by the characteristics of slurry stratum without disturbance of cutting-tools. In contrast, with the periodic disturbance of cutting-tools, the decreasing cycle of cutting will increase the difficulty of filter cake formation under the same cutting depth. In addition, the filter cake formation and the stability of excavation face can be significantly improved if the number of cutting-tools on same cutting track is reduced and the advance speed of shield and rotate speed of cutting wheel are decreased appropriately.