管幕冻结隧道“顶管−冻土”复合结构力学特性试验研究

Experimental research on mechanical properties of “jacked pipe-frozen soil” composite structure in freeze-sealing pipe roof tunnel

  • 摘要: 拱北隧道暗挖段作为港珠澳大桥珠海连接线的重点工程,首次运用管幕冻结法进行施工。该法综合管幕法和人工地层冻结法的优势,可在隧道断面形成“顶管−冻土帷幕”复合支护体系,有效实现“承载”与“顶管间止水”的双重目标,确保隧道开挖时的稳定与安全。为获得“顶管−冻土”复合结构的温度、变形与力学特性,基于相似理论自主研发构建一套相似模型试验系统并开展试验研究,同时利用有限元软件COMSOL Multiphysics建立数值计算模型进行模拟验证。结果表明:复合结构的冻结温度场因空、实顶管及其内部冻结器的布置形式呈现不均匀分布特征,冻土形成速率在冻结后期明显变缓;土体竖向冻胀变形在60~160 min内急剧增大,且冻胀量随深度增加而增大,整体规律与温度场分布密切相关;土体冻结产生的冻胀力对顶管水平受力影响较大,空顶管相对刚度较小而产生较大水平变形;在加载阶段,顶管受力与变形均以竖向为主。因空、实顶管刚度差异和冻土厚度不均匀的共同影响,空顶管竖向变形包含了“弯曲”与“压扁”并具有非线性特征,其跨中截面底部竖向位移峰值约为实顶管的1.6倍;加载至0.28 MPa时,管间冻土首先发生破坏,进而导致顶管间封水功能失效,实际施工中应重点监测空顶管的变形规律、管间冻土帷幕的温度变化及其完整性。研究成果可为管幕冻结法的施工与监测提供参考,也可为热力耦合数值计算模型提供验证依据。

     

    Abstract: As the key project of Zhuhai Connecting Line of Hong Kong‒Zhuhai‒Macao Bridge, the underground excavation section of Gongbei Tunnel has applied the freeze-sealing pipe roof method for the first time. Combining the advantages of the pipe roof method and the artificial ground freezing method, this method could form a “jacked pipe-frozen soil curtain” composite support system on the tunnel section, so as to effectively achieve the dual goals of “bearing” and “water-stopping between pipes”, thereby ensuring that stability and safety during tunnel excavation. To obtain the temperature, deformation and mechanical properties of the “jacked pipe-frozen soil” composite structure, a set of scaled model test system was independently developed based on the similarity theory and the experimental research was carried out. At the same time, a numerical calculation model was established using the finite element software COMSOL Multiphysics for simulation verification. According to the results, the freezing temperature field of the composite structure is characterized by uneven distribution due to the arrangement of hollow pipe, solid pipe and their internal freezers, and the formation rate of frozen soil slows down significantly in the later stage of freezing. Besides, the vertical frost heaving deformation increases sharply within 60‒160 min, the frost heaving amount increases with the depth, and the overall law of frost heaving is closely related to the distribution of the freezing temperature field. The frost heaving force caused by soil freezing has a great influence on the horizontal force of the pipe, so that the hollow pipe with relative low stiffness may be subjected to larger horizontal deformation. In the loading stage, the jacked pipe is mainly applied with force and deformed in the vertical direction. Due to the joint influence of the stiffness difference between the two pipes and the uneven thickness of frozen soil, the vertical deformation of the hollow pipe, including “bending” and “flattening”, has nonlinear characteristics, and the peak value of the vertical displacement at the bottom of the midspan section is about 1.6 times that of the solid pipe. When the load reaches 0.28 MPa, the frozen soil between pipes will be damaged at first, which leads to the failure of water sealing between pipes. In actual construction, the deformation law of hollow pipe, as well as the temperature change and integrity of the frozen soil curtain between adjacent pipes, is the key for monitoring. The research results can provide reference for the construction and monitoring of the freeze-sealing pipe roof method and verification basis for the thermodynamic coupling numerical calculation model.

     

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