上下台阶开挖下城门形隧洞纵向变形规律分析

Analysis of longitudinal deformation profile in city-gate shaped tunnel using top heading and bench excavation method

隧洞纵向变形规律曲线能有效地反映洞壁围岩变形随掌子面推进的演化过程,然而,原有适用于圆形隧洞全断面开挖的纵向变形规律公式是否适用于城门形隧洞分部开挖,尚需进行进一步的验证。本文借助于FLAC3D软件对城门形隧洞上下台阶分部开挖下的纵向变形规律进行了研究,并对不同初始地应力条件、不同围岩类型下该规律的变化进行了深入分析,提出了确定该规律的经验公式,并与紫坪铺工程导流隧洞现场监测数据进行了对比,研究发现:上下台阶开挖下的纵向变形规律曲线与全断面开挖具有相似的形状,但是掌子面处的位移比ur(0)/ur∞存在较大差异;当侧压力系数大于1.0时,变化水平应力对边墙测点的纵向变形规律曲线影响较小;采用平面应变模型并结合本文提出的经验公式可对城门形隧洞上下台阶开挖下的纵向变形规律曲线进行预测。工程实例验证表明,采用经验公式计算所得曲线与监测数据比较吻合。利用本文经验公式能简便、较准确地预测围岩变形规律,结合监测数据进而可对隧洞总变形进行初步预测。

The longitudinal deformation profile （LDP） is important in determining the deformation magnitude of tunnel wall as a function of distance to tunnel face. But the widely used empirical equations were derived from numerical analysis of full-face excavation of a tunnel with circular cross-section. It is not clear if the equations can still be used for city-gate shaped tunnel under top heading and bench excavation. Therefore, the LDP for top heading and bench excavation is analyzed by FLAC3D. The difference of LDP between full face and top heading and bench excavation is investigated by FLAC3D under different geo-mechanical properties of surrounding rock and insitu stress conditions. Finally, the method on determining the LDP and its empirical equation are suggested for large tunnel using top heading and bench excavation and the equation is verified against measured data of the upper side walls at a diversion tunnel of Zipingpu project. The results show that there is similar shape for LDP curves between full face and top heading and bench excavation, but the displacement ratio at face is obvious different between them; the LDP for side wall does not vary obviously with the varied horizontal stress when the ratio of horizontal to vertical initial stress is lager than 1.0; the LDP for city-gate shaped tunnel under top heading and bench excavation can be formulated in virtue of the proposed empirical equation and numerical analysis with plain strain model, and it is verified against measured data well. The empirical equation can be easily and reliably applied to deformation analysis, and the total deformation magnitude for large tunnel can be predicted well with the monitored data.