大跨度软岩公路隧道仰拱承载性能及安全性研究

Research on Bearing Performance and Safety of Invert of Large-span Soft-rock Highway Tunnel

以陕西宝汉高速公路连城山隧道(双洞六车道)绿泥石片岩段为例,分析了大跨度软岩公路隧道仰拱病害原因,建立了隧道仰拱的弹性地基曲梁模型,推导了仰拱结构内力、仰拱地基反力等计算公式,分析了原设计仰拱二次衬砌极限承载力和受力规律,评价了原设计仰拱结构安全性;在此基础上,探讨了各仰拱参数对仰拱极限承载力的影响规律及敏感度,计算了参数变更后仰拱二次衬砌的极限承载力,并结合仰拱受力测试,进一步考察了参数变更后仰拱结构安全性。结果表明:隧道墙脚以沉降变形为主,导致仰拱两端承受很大的竖向荷载,而原设计仰拱本身承载力较弱,加上仰拱地基软弱并且受地下水的软化效应和高应力下的蠕变效应影响,是连城山隧道仰拱开裂破坏的主要原因;仰拱最危险截面距离仰拱端部约为半幅仰拱相应圆心角的1/5~1/4处,即位于墙脚附近,与现场观察到的墙脚附近仰拱回填开裂、仰拱与仰拱回填脱离等破坏现象一致;增大仰拱厚度、减小仰拱半径、增大仰拱钢筋直径和减小仰拱钢筋间距均能显著提高仰拱极限承载力,其中减小仰拱钢筋间距的效果相对最为显著;而由于仰拱最危险截面的受压区高度很小,提高混凝土强度等级对于改善仰拱整体安全性并不显著;参数变更后的仰拱二次衬砌采用C35钢筋混凝土,厚度为1 m,半径约为13.4 m,钢筋直径为28 mm,钢筋间距为20 cm,极限承载力可达原设计的3.6倍以上,结构安全性大幅提高;为提高材料利用率,建议仰拱混凝土强度等级采用C30。

Taking the chlorite schist section of Lianchengshan Tunnel(two-hole six-lane) of Baoji-Hanzhong Expressway in Shaanxi as an example, the causes of tunnel invert disease of long-span soft-rock highway tunnel were analyzed. The elastic foundation curved beam model of tunnel invert was established. The expressions of the internal forces and foundation reaction force of the inverted arch were derived. The ultimate bearing capacity and stress law of the secondary lining of the originally designed invert were analyzed, and its safety was evaluated. On this basis, the influence law and sensitivity of various invert parameters on the ultimate bearing capacity of the invert were discussed. The ultimate bearing capacity of the secondary lining of the invert after the parameter change was calculated, and the tunnel invert safety after the parameter change was examined according to the stress test results of the tunnel invert. The results show that the deformation of the tunnel feet is mainly vertical settlement, which causes the two ends of the invert to bear a large vertical load. The bearing capacity of the originally designed tunnel invert is relatively weak, while the tunnel invert foundation is weak and is suffered the influence of the softening effect of groundwater and the creep effect of high stress, which are the main reasons for the cracking and damage of the Lianchengshan Tunnel invert. The angle of the most dangerous section of the tunnel invert from its the end is about 1/5~1/4 times of the corresponding central angle of the half tunnel invert, that is, the most dangerous section is located near the tunnel foot, which is consistent with the observed damage phenomena such as the concrete cracking of the invert backfill near the tunnel feet, the tunnel invert separated from the invert backfill. The ultimate bearing capacity of the tunnel invert can be significantly improved by increasing the thickness of the tunnel invert, reducing the radius of the tunnel invert, increasing the diameter of the steel bars of the tunnel invert, and reducing the spacing of the steel bars of the tunnel invert. Among them, the effect of reducing the spacing of the steel bars is relatively the most significant. While because the height of the compression zone of the most dangerous section of the tunnel invert is very small, increasing the concrete strength grade is not significant for improving the overall safety of the tunnel invert. The secondary lining of the tunnel invert after the parameter change was made of C35 reinforced concrete, with a thickness of 1 m, a radius of about 13.4 m, the steel bar diameter of 28 mm, and the steel bar spacing of 20 cm. Its ultimate bearing capacity is more than 3.6 times that of the original design, and the structural safety is greatly improved. In order to improve the utilization rate of concrete materials, it is recommended that the strength grade of tunnel invert concrete should be C30.