为使隧道施工中开挖进尺的选择更加合理，提出了掌子面超前核心土对数螺旋破坏模式，该模式由拱顶围岩与超前核心土两部分组成; 针对拱顶围岩，基于Janssen 筒仓
理论和Mohr-Coulomb 强度准则建立了竖向力计算公式，而针对掌子面超前核心土，基于对数螺旋破坏模式推导了循环进尺计算公式，建立了循环进尺与内摩擦角、黏聚力、支护压力、开挖高度等参数之间的相互关系． 研究结果表明: 在施工阶段考虑隧道掌子面稳定性时，必须考虑循环进尺的影响，掌子面附近围岩状态的短期改变会使掌子面超前核心土所需的支护力比未考虑循环进尺的情形急剧增长; 黏聚力与内摩擦角的改善对循环进尺与掌子面区域稳定性具有重要的意义，而支护力对该区域的影响具有临界性，不可盲目增大．

In order to determine the round length reasonably in the process of tunnel excavation,the logarithmic spiral failure mode of advanced core soil on the tunnel face is proposed,which consists of two parts respectively considering vaults and advanced core soil in the surrounding rock.Then,in light of Janssen's silo theory and Mohr- Coulomb strength criterion,a calculation formula of vertical force loading on vaults is established,and the formula to calculate the round length in advanced core soil on the tunnel face,which is governed by such relevant parameters as internal friction angle,cohesive force,support pressure and excavation height,is set up on the basis of logarithmic spiral failure mode.The results show that ( 1) the round length produces very important impact on the stability of tunnel face,especially in the construction stage; ( 2) the support pressure on tunnel face considering round length may increase rapidly; and ( 3) the reinforcement of cohesive force and internal friction angle is of great significance for the improvement of round length and tunnel face stability,while the support pressure is of a critical value in terms of affecting tunnel face.