软岩隧道超前导洞应力释放力学机制及适用性

为研究软岩隧道超前导洞的适用性，基于经典弹塑性理论，建立深埋软岩隧道超前导洞法开挖应力释放的力学模型，采用2阶段的方法推导考虑超前导洞应力释放的隧道开挖弹塑性解。定义围岩应力释放比来反映应力释放的效果，并研究不同地应力、围岩弹性模量、强度等条件下超前导洞开挖半径对应力释放效果的影响。结果表明： 通过施作超前导洞可以降低作用在支护结构上的围岩压力，尤其是在高地应力环境或围岩较软弱的条件下，采用超前导洞法进行应力释放效果更加明显。但导洞半径并非越大越好，现场试验表明： 对于3车道大断面软岩隧道，导洞断面太大对隧道围岩的稳定不利。隧道施工中，应在保障围岩稳定的前提下进行应力释放，做到初期支护尽早封闭成环，实现隧道安全快速施工。

Mechanical Mechanism and Applicability of Stress Release for Pilot Heading in Soft Rock Tunnel

To investigate the applicability of stress release for pilot heading in soft rock tunnel, the mechanical model of stress release for soft rock tunnel excavation by pilot heading method is established based on the classical elastic plastic theory. The elastic plastic solution of tunnel excavation considering the stress release of pilot tunnel is derived by a two stage method. The stress release ratio of surrounding rock is defined to reflect the effect of stress release, and the influences of pilot tunnel radius on stress release effect under different in situ stresses, elastic modulus and strengths of surrounding rock are studied. Results show that the ground pressure applied on the support can be reduced by the excavation of a pilot tunnel, especially in high geostress environment or weak rock, the stress release effect is much more obvious by pilot heading method. However, the pilot tunnel radius is not as larger as possible. The field tests show that for a three lane large cross section soft rock tunnel, the large section of the pilot tunnel is unfavorable to the stability of the surrounding rock. Stress release measure should be carried out under the premise of ensuring the stability of surrounding rock in tunnel construction. Thus, the primary support should be installed as soon as possible to realize the safe and rapid construction.