砂土地层中不同支护模式下盾构隧道开挖面稳定性研究

防止盾构隧道开挖面失稳的关键是合理设置不同盾构支护平衡模式下的支护压应力。在改进的筒仓楔形体模型计算方法得出的开挖面松动土体对刀盘压力呈近似呈抛物线分布的基础上，研究了气压支护模式、泥水支护模式和土压支护模式下，盾构隧道开挖面分别在地下水位以上和地下水位以下时开挖面的稳定性，研究结果表明:有效支护应力均匀分布时，除粘土开挖面下部失稳外，其余土体均为开挖面中下部失稳；有效支护压应力呈上小下大的梯形分布时，除软粘土开挖面下部失稳外，其余土体均为开挖面上部失稳；有效支护应力呈上大下小的梯形分布时，所有土体开挖面均为下部失稳；在气压、泥水和土压平衡支护模式下，开挖面在未到达筒仓楔形体模型所假设的开挖面整体失稳前，开挖面已经发生了局部失稳，采用筒仓楔形体模型确定的极限稳定支护力是不安全的。最后给出了开挖面松动土体对刀盘压应力公式中计算参数的无量纲化图，以方便实际工程运用。 

FACE STABILITY OF SHIELD TUNNEL WITH DIFFERENT SUPPORT MODELS IN SAND

Setting reasonable support pressure for different support model shield tunnel machine is extremely important for the face stability of the shield tunnel. The face stability is discussed respectively when the face is above and under water for the typical distribution of face support in tunneling by compressed air shield, slurry shield and EPB shield. The research results show that, When the effective support stress is uniformly distributed, the soil mass is unstable in the middle and the bottom of the face except for the instability in the lower part of the clay face; when the effective support stress is distributed in trapezoid with a small top and big bottom, the soil mass is unstable in the top of the face except for the instability in the lower part of the soft clay face; on the contrary, the lower half part of the face is instable for all kinds of soil mass when the effective stress distribution is in trapezoid with a big top and small bottom. In the balanced support mode of compressed air shield, slurry shield and EPB shield, the face has already gone through local instability before the overall instability of face assumed by the wedge-shaped model; therefore, traditional wedge model based on overall stability is unsafe. Furthermore, in order to facilitate the calculation of active earth pressure, active pressure coefficient and others design parameters are also provided as function of the soil friction angle and the friction between cutter and the soil.