超大跨度隧道上台阶CD法中隔壁力学计算模型及施工力学行为研究

为研究超大跨度隧道分部开挖法施工中隔壁结构的施工力学行为,以山东滨莱高速公路改扩建工程双向八车道乐疃隧道为依托,基于初期支护钢架与中隔壁钢架之间的内力传递、变形协调及拱脚变位,将支护体系等效为支座可移动的三次超静定无铰拱-梁固接结构,建立了上台阶先导初期支护钢架-中隔壁钢架共同承载变位力学计算模型,采用理论分析、现场测试和力学模型计算相结合的方法,对超大跨度隧道上台阶CD法施工时中隔壁的力学行为进行分析。研究结果表明:拱顶沉降和周边收敛主要经历急剧增长、缓慢变形和趋于稳定3个阶段,且各变形值均小于设计预留变形量150 mm;受施工工序和结构约束条件变化的影响,钢架内外侧应力整体呈现出先急剧变化后逐渐趋于稳定的规律,各测点应力小于型钢屈服强度235 MPa;力学模型计算结果和现场实测数据的平均相对误差为12.6%,且规律基本一致;钢架轴力在上台阶施工过程中始终为受压,且最大值均在钢架拱脚处,受后导开挖影响,中隔壁钢架轴力增大,初期支护钢架轴力减小;先导开挖时钢架弯矩大部分部位为正,拱顶部位为负,受后导开挖影响,中隔壁钢架正弯矩值及正弯矩区域减小,同时初期支护钢架正弯矩区域减小,钢架拱脚附近弯矩出现负值;钢架结构整体处于偏心受压状态,受后导开挖影响,中隔壁钢架和初期支护钢架小偏心受压区域均发生移动,且两者钢架小偏心受压长度占比增大。

Mechanical Calculation Model and Research on Construction Mechanical Behavior of Middle Diaphragm in Upper Bench CD Method for Super-large Span Tunnel

To study the construction mechanical behavior of the middle diaphragm structure during the construction of a super-large span tunnel by sequential excavation method, this study was based on the Letuan Tunnel of the Shandong Binlai Expressway reconstruction and expansion project and established a mechanical calculation model of the steel frame of primary support and middle diaphragm with bearing load and deformation. The support system is equivalent to the three times statically indeterminate hinge free arch-beam fixed structure with movable support with considering the internal force transfer, deformation coordination, and arch foot displacement between the side wall steel frame and the middle diaphragm. In addition, the mechanical behavior of the middle diaphragm in the construction of a super-large span tunnel by the upper bench CD method was analyzed by a combination of theoretical analysis, field test, and mechanical model calculation. The results show that the settlement of the arch crown and surrounding convergence at the monitoring section experienced three main stages:rapid growth, slow deformation, tending to stability, and finally a relatively stable state. Each deformation value was less than 150 mm, which was the limit established in the design. The stress of the steel frame is mainly affected by the construction process and the change in the constraint conditions of the support structure, and the stress of each measuring point is below 235 MPa. The relative error between the calculated results of the mechanical model and measured data is 12.6%. The axial force of the steel frame is always under pressure during the construction of the upper bench, and the maximum value occurs at the arch foot of the steel rib. Under the influence of back-guide hole excavation, the axial force of the middle diaphragm steel rib increases, and the axial force of the primary support steel frame decreases. The bending moment of a majority of the steel rib is positive, and that of the vault part is negative. Affected by the excavation of the back-guide hole, the positive bending moment and the positive bending moment area of the middle diaphragm steel rib are reduced. Simultaneously, the positive bending moment area of the primary support steel rib is reduced, and the bending moment near the arch foot of the steel rib is negative. The entire steel frame structure is in the state of eccentric compression. Affected by the back-guide hole excavation, the small eccentric compression area of the middle diaphragm steel rib and primary support steel rib move, and the proportion of small eccentric compression length of the two steel ribs increases.