合理选择注浆压力是确保盾构隧道壁后注浆效果良好的前提。假定在黏土地层中，壁后注浆先对周围土体产生压密效应，当注浆压力超过一定值以后，浆液开始劈裂土体。为得到最优注浆压力，基于弹塑性理论，推导了考虑浆体无限扩张时的注浆压力上临界值计算式；将接头螺栓的抗剪效应与注浆对管片产生的压力结合起来，推导了考虑螺栓剪切破坏的注浆压力上临界值计算式；基于主、被动土压力公式，提出了保持地层稳定的注浆压力上、下临界值计算式。在此基础上，提出了最优注浆压力计算方法。通过工程实例，分析了土体的弹性模量、黏聚力、内摩擦角、初始地下水压，及隧道埋深对临界注浆压力的影响。结果表明：临界注浆压力与土体弹性模量、黏聚力、内摩擦角、初始地下水压，管片结构性能以及隧道埋深等因素有关；上临界值随着土体弹性模量、黏聚力、内摩擦角、初始地下水压及隧道埋深的增大而增大；下临界值亦随隧道埋深的增大而增大。

Proper selection of grouting pressure is required to ensure the good performance of back-fill grouting. It is suggested that that the back-fill grouting compacts the surrounding soil first, and then fracture the soil when the grouting pressure exceeds a certain value. To determine the optimal grouting pressure, a formulation for calculating the upper critical value of the grouting pressure is developed based on the elastio-plastic theory with considering the unlimited expansion of the grouts. By combining the shear resistance of bolts and the grouting pressure acting on the segments, the upper critical value of the grouting pressure is determined with considering the shear failure of the bolts. Based on active and passive earth pressures, the formulations of upper and lower critical value for the grouting pressure, which meet the requirement for soil strata stability, are developed. The calculating method of the optimal grouting pressure is also presented. A practical engineering case is analyzed, illustrating the effects of elastic modulus, cohesion, internal friction angle of soil, initial underground water pressure, and tunnel depth buried on the critical grouting pressure. It is found that the critical value of grouting pressure is influenced by many factors such as the elastic modulus, cohesion, internal friction angle of soil, initial pressure of ground water, segmental structure performance, and tunnel depth. The upper pressure limit increases with the increase of the elastic modulus, cohesion, internal friction angle of soil, initial pressure of ground water, and tunnel depth, while the lower pressure limit increases with the increase of tunnel depth as well.