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软弱围岩一般强度不高、完整性不好、结构相对松散，围岩力学性质较差，自稳能力差[1]。我国隧道工程施工过程中，不可避免遇到大量的软弱围岩。目前三台阶七步法在隧道施工中应用较为广泛[2]，围岩条件合适时具有施工速度快、机械作业空间充足、初期支护操作便捷、开挖面稳定性好等优点[3-4]。然而，在大量的工程实践表明存在初期支护变形大、地表沉降大、对突发事件的应变能力较差等特点。在软弱地层中，三台阶临时仰拱法亦是较为常见的开挖工法，其结合支护措施增强了围岩的稳定性，能较好的控制隧道开挖支护过程中塑性区的范围[5]。
需要指出的是三台阶七步法在软弱地层中大量采用，其应用效果有待于进一步量化评价。基于此，依托地质条件基本一致且相邻的广东赤竹隧道和高栋隧道，分别采用三台阶七步法和三台阶临时仰拱法进行对比研究，结合现场存在拱顶沉降、地表沉降较大的问题，从工程应用效果以及数值仿真模拟两个方面对三台阶七步法和三台阶临时仰拱法进行系统分析对比，并分析其产生的原因。通过对比表明：在该地层中三台阶临时仰拱法优于三台阶七步法，并提出类似软弱围岩地层大断面隧道施工方法的建议。
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赤竹隧道、高栋隧道位于广东省河源市，其中赤竹隧道全长947.74 m，隧道进口段，表层为
${\rm Q}_{4}^{\rm el+dl} $ 粉质黏土，厚度为1 m~1.6 m。洞身主要位于泥盆系中下统桂头群(D1-2gt)全风化石英片岩地层中，全风化，黄褐色，结构构造均被破坏，岩体破碎，主要岩层产状为359°∠40°，隧道进口段围岩等级为Ⅴ级[6]；高栋隧道全长944 m，隧道进口段，表层为${\rm Q}_{4}^{\rm el+dl} $ 粉质黏土，硬塑，黄褐色，厚约1 m~2 m，洞身主要位于泥盆系D1-2gt石英片岩地层中，全风化，黄褐色，岩体风化较完全，呈砂土状，层厚约5 m~8 m，岩层产状为8°∠34°，隧道进口段围岩等级为Ⅴ级[6]。两隧道进口段纵断面图如图1、图2所示。两隧道采用复合式衬砌，隧道主要支护参数如图3所示，其中，赤竹隧道钢拱架为HW175钢架，高栋隧道钢拱架为I22a钢架，其余支护参数相同。两隧道进口段结构断面图如图3所示。
图 1 赤竹隧道进口段纵断面图[6]
Figure 1. Profile of entrance section of chizhu tunnel
图 2 高栋隧道进口段纵断面图[6]
Figure 2. Profile of entrance section of gaodong tunnel
图 3 隧道进口段结构断面图 /cm
Figure 3. Structural section of tunnel entrance section
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赤竹隧道进口段采用三台阶七步法施工，上台阶采用弧形开挖并预留核心土，中、下台阶左右错开开挖，左右错开3 m，台阶长度为5 m。中、下台阶预留核心土长度5 m，开挖后及时施做初支，仰拱按照施工要求及时施做。三台阶七步法施工工序图如图4所示。
高栋隧道进口段采用三台阶临时仰拱法施工，上、中台阶开挖，台阶长度为5 m，开挖后及时施做初支及临时仰拱。下台阶开挖，台阶长度为5 m，开挖后及时施做初支，仰拱按照施工要求及时施做。三台阶临时仰拱法施工工序图如图5所示。
图 4 三台阶七步法施工工序图
Figure 4. Three-step seven-step construction procedure drawing
图 5 三台阶临时仰拱法施工工序图
Figure 5. Construction procedure drawing of three-step temporary invert method
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赤竹隧道进口段，隧道开挖面积约为160.46 m2，洞径为15.1 m。上台阶每日开挖进尺1.2 m，台阶长度达5 m时，开挖中右、中左、下右、下左台阶，每日开挖进尺1.2 m，仰拱紧跟下左台阶，距离下左台阶5 m。核心土每2.4 m开挖一次。
高栋隧道进口段采用三台阶临时仰拱法施工，隧道开挖面积约为160.46 m2，洞径为15.1 m。上台阶每日开挖进尺0.6 m，台阶长度达5 m时，开挖中台阶；中台阶每日进尺1.2 m，台阶长度达5 m时，开挖下台阶；下台阶每日进尺1.2 m，仰拱紧跟下台阶，距离下台阶5 m。两隧道进尺情况如图6、图7所示。
图 6 三台阶七步法开挖进尺情况
Figure 6. Three-step seven-step excavation footage
COMPARATIVE STUDY ON ADAPTABILITY ANALYSIS OF THREE BENCHING SEVEN STEPS AND TEMPORARY INVERT METHOD FOR LARGE-SECTION TUNNEL IN SOFT STRATUM
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摘要: 软弱围岩力学性质较差、强度低、自稳能力差。目前在软弱围岩隧道施工中常采用的三台阶七步法经大量工程实践表明存在初期支护变形大、地表沉降大、难以及时封闭、对突发事件的应变能力较差等特点。针对广东赤竹、高栋两座高速铁路隧道洞口段软弱围岩具体条件，进行三台阶七步法和三台阶临时仰拱法两种施工工法的对比试验，在隧道拱顶沉降、周边收敛和地表沉降测试基础上，从工程应用效果以及数值仿真模拟等方面进行了系统的工法适应性研究。结果表明：在该类软弱围岩地层大断面隧道施工中，三台阶临时仰拱法在控制拱顶沉降、地表沉降、围岩应力、塑性区扩展面积及深度方面要优于三台阶七步法，所得结论对其他类似工程具有借鉴意义。Abstract: Soft rock has poor mechanical properties, low strength and stability. At present, three benching seven steps commonly used in the construction of a soft rock tunnel have the characteristics of large initial support deformation, large surface subsidence, difficulty in timely closure, and low strain capacity to emergencies, etc. In view of the specific conditions of weak surrounding rock at the entrance of two high-speed railway tunnels in Chizhu and Gaodong of Guangdong province, the comparative tests of three-step seven-step method and three-step temporary inverted arch method are carried out. Based on the tests of tunnel vault settlement, peripheral convergence and surface settlement, a systematic study on the adaptability of construction method is carried out from the aspects of engineering application effect and numerical simulation. The results show that: the three-step temporary invert method is superior to three-benching seven-steps in controlling arch settlement, surface settlement, surrounding rock stress, plastic zone expansion area and depth in large-section tunnel construction of this kind of soft rock stratum. The conclusions can be used for reference to other similar projects.
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图 5 三台阶临时仰拱法施工工序图
Figure 5. Construction procedure drawing of three-step temporary invert method
图 9 三台阶七步法各施工步序沉降图 /mm
Figure 9. Settlement diagram of each construction step sequence with three steps and seven steps
图 10 三台阶临时仰拱法各施工步序沉降图 /mm
Figure 10. Settlement diagram of each construction step of three-step temporary invert method
图 11 三台阶七步法数值模拟网格划分
Figure 11. Three-step seven-step numerical simulation of mesh generation
图 12 三台阶临时仰拱法数值模拟网格划分
Figure 12. The three-step temporary invert method numerical simulation of mesh generation
图 14 两隧道开挖前大、小主应力云图 /Pa
Figure 14. Large and small main stress nephogram before excavation of two tunnels
图 15 三台阶七步法开挖后大、小主应力云图 /Pa
Figure 15. Large and small main stress nephogram after three-step seven-step excavation
图 16 三台阶临时仰拱法开挖后大、小主应力云图 /Pa
Figure 16. Large and small principal stress cloud maps after three-step temporary invert excavation
表 1 数值模拟计算参数
Table 1. Numerical simulation calculation parameters
材料名称 弹性模量/
MPa重度/
(kN/m3)泊松比 粘聚力/
kPa内摩擦角/
(°)粉质黏土 4 20.5 0.30 6 28 全风化石英片岩 30 24.0 0.28 29 29 C25喷射混凝土 2.58×104 23.5 0.21 — — 
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