泥水盾构支护压力的上、下限值是确定合理支护压力的关键依据。文章通过对泥水盾构施工过程中的掘削面稳定机制进行分析后发现，在浅覆土砂层地段中，地层产生击穿破坏必须具备三个条件：(1)泥浆失水严重或者直接向地层中渗透，引起地层中超净孔隙水压的急剧升高；(2)较高的支护压力使得泥水具有良好的定向性，并与地层中的静止水压之间形成足够大的压力差；(3)覆土地层具有足够小的渗透系数，不至于使泥水直接从地层中渗流扩散，能在土颗粒的接触部位形成极高的水压力梯度。其实质就是由于高压泥浆直接向地层中入渗，引起覆土地层中超净孔隙水压升高，在地层中形成较大的水头差从而产生渗透破坏。据此提出了一种物理意义明确、操作简单的最大支护压力实用计算方法，该方法所需参数较少并能方便、快捷地根据常规土工试验获得。结合某过江隧道的工程实践，对最大支护压力进行了计算并与实际工程进行了对比，验证了这一计算方法的合理性与有效性。%The upper and lower limits of support pressure are key for determining the reasonable support pressure of slurry shields. Based on an analysis of the stability mechanism of the working face during slurry shield operation, it is determined that there are three necessary conditions for ground breakdown failure in shallow-buried sand strata: 1) severe slurry dehydration or slurry penetration directly into the ground, causing a drastic increase of the extra-static pore water pressure; 2) good directionality of the slurry with a great pressure difference compared with that of static water pressure in the ground due to higher support pressure; and 3) a permeability coefficient of the overburden that is too small to cause slurry directly seeps and diffuses into the ground, creating an extremely high water pressure gradient at the contact site of the soil particles, which means that the extra-static pore water pressure increases along with the high-pressure slurry permeating directly into the ground, and seepage failure occurs thereafter due to large water head difference. Accordingly, a practical calculation method of maximum support pressure, which is convenient and requires fewer parameters, is proposed. The maximum support pressure is calculated and a comparison with a practice case is carried out based on a certain river-crossing tunnel project, verifying the rationality and effectiveness of the method.
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