基于分段单孔反演的隧址区多断层地应力分布特征

Research on In-situ Stress Distribution Characteristics of Multiple Faults in a Tunnel Site Using Segmented Single Borehole Inversion Method

目前,针对具有多个原位地应力测试钻孔的隧址区初始地应力场反演,往往一次性将全部实测值用以计算值的回归反分析,没有考虑到隧道纵向长度远超其他方向且钻孔分布较远的特点,单个钻孔表征的临近区域地应力分布特征或反演精度被其余钻孔实测值“拉低”,导致整体反演结果容易陷入局部优解。依托拉林铁路桑珠岭隧道、叙毕铁路斑竹林隧道、沿江高速火山隧道、木寨岭公路隧道等工程,采用多元线性回归法结合最小二乘法寻优准则,分析了隧址区仅有单个钻孔反演精度较高的原因,提出了对具有多个钻孔的隧址区分段并分别进行单钻孔回归反演的新方法。在此基础上,重点讨论了木寨岭公路隧道断层破碎带区域地应力分布特征。结果表明:分段单孔反演较多孔反演进一步提高了反演精度;地应力的分布在断层前后呈现出先急剧增大后急剧减小的特征;除自重应力与埋深存在一定关联外,两项水平主应力随断层间距或厚度的增大突变性加剧;断层之间间距、断层倾角、断层厚度均是影响地应力方向偏转大小的重要因素,断层之间间距或断层倾角越小、断层越厚,地应力方向偏转越显著。

Currently, for the back analysis of the in-situ stress field around a tunnel site with multiple in-situ stress-test boreholes, all measured values are often used at once, for the regression of the calculated value. This analysis does not take into account factors such as the longitudinal length of the tunnel being much longer than that of other directions and the boreholes being more widely distributed. Additionally, the characteristics of in-situ stress distribution or back analysis accuracy in the area adjacent to a single borehole are “pulled down” by the measured values of other boreholes, thereby causing the overall back analysis result to fall into a local optimal solution. Taking the Sangzhuling Tunnel of the Lalin Railway, the Banzhulin Tunnel of the Xubi Railway, the Huoshan Tunnel of the Yanjiang Highway, and the Muzhailing Highway Tunnel as examples, the multiple linear regression method combined with the least square method was used to analyze the cause of the high precision of single borehole inversion in a tunnel site. A new method for segmenting the tunnel site with multiple boreholes and inverting the single borehole was proposed. The analysis was focused on the in-situ stress distribution characteristics of fault fracture zones around the Muzhailing Highway Tunnel. The results showed that the segmented single borehole inversion further improves the inversion accuracy compared with the multiple boreholes inversion. Furthermore, the distribution of in-situ stress before and after the fault shows a sharp increase followed by a sharp decrease. Besides the correlation between the gravity stress and the buried depth, the abrupt change of the two horizontal principal stresses increases with the increase in the distance or thickness of fault. Moreover, the distance between faults, the dip angle of the fault, and the thickness of the fault are all important factors affecting the deflection of ground stress direction;the smaller the distance between faults, the lower the fault dip;the thicker the fault, the more significant the deflection of the ground stress direction.