Causes of secondary lining cracking in tunnels are complex and it is important to identify them so that reasonable treatment measures can be proposed to ensure safety during tunnel construction and operation. Using a tunnel that passes through carbon-based phylite as an example, and in light of the cracking characteristics of the secondary lining at the exit section, the measured data at the site and the geological conditions, it is determined that the main reasons for cracking in the secondary lining are poor quality of the surrounding rock, low levels of surround-ing rock and early implementation of the secondary lining. Early installation of the secondary lining will result in large tensile stress at the inner side of the secondary lining near the sidewall and haunch, and the earlier the installa-tion of the secondary lining, the larger the tensile stress, which will result in cracking of the secondary lining. It is appropriate to install the secondary lining when the deformation of the primary support is 65%~70%of the final con-vergence. It is recommended to put more effort into monitoring and measuring and to strictly control the installation schedule of the secondary lining.%隧道二次衬砌开裂成因复杂,只有分清隧道二次衬砌开裂原因,才能对其提出合理的处治对策,以保证隧道施工、运营安全.文章以某高速公路(炭质)千枚岩隧道为例,基于隧道出口段二次衬砌开裂特征,结合现场监控量测以及工程地质条件,分析该隧道二次衬砌开裂主要原因是围岩地质条件差、围岩级别低且二次衬砌施作过早.然后进一步采用数值计算分析不同二次衬砌施作时机下的应力特征,结果表明:二次衬砌施作过早导致边墙和拱腰附近二次衬砌内侧产生较大的拉应力,且二次衬砌施作越早,拉应力越大,这将导致二次衬砌张拉开裂;在初期支护变形量为隧道最终变形收敛值的65%~70%之后施作二次衬砌较适宜.建议在类似隧道施工中加强监控量测,严格控制二次衬砌支护时机.
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