机械化作业条件下考虑覆跨比修正的大断面隧道稳定性评价方法

Stability Evaluation Method of Large Section Tunnel Considering Modification of Cover-Span Ratio under Mechanized Operation Condition

进行双线高铁隧道全断面或大断面机械化开挖时对围岩稳定性评价的准确性要求较高,而现有行业规范对隧道稳定性评价时未考虑埋深和跨度的影响,因而需要进行考虑覆跨比修正的大断面隧道稳定性评价方法研究。首先,以黄(冈)黄(梅)高速铁路刘元隧道为背景,以隧道开挖后拱顶正上方某点围岩的第三主应力偏转为判据,确定大断面隧道的临界埋深;然后,采用强度折减法,数值求解不同围岩级别和覆跨比(埋深与跨度比)下的隧道安全系数,应用数理统计方法发现不同围岩级别下隧道安全系数与覆跨比及与围岩基本质量指标(BQ)存在的关系;最后,考虑覆跨比、地下水、围岩初始应力及结构产状因素的影响,得到修正的BQ,借此对埋深100 m以下的机械化作业大断面隧道稳定性进行评价,并将评价方法应用于刘元隧道和重庆市城市快速干道慈母山2号隧道工程进行验证。结果表明:不同围岩级别下隧道安全系数与覆跨比满足严格的幂函数关系,而与BQ存在一定的线性关系;将修正后的BQ作为评价隧道稳定性的定量指标,更符合现场实际情况。

When the full section or large section mechanized excavation of the double-track high-speed railway tunnel is carried out,the accuracy requirements for the stability evaluation of surrounding rock are high.However,the existing industry standards do not consider the influence of buried depth and span when evaluating the tunnel stability.Therefore,it is necessary to study the stability evaluation method of large section tunnels considering the modification of cover-span ratio.First of all,taking Liuyuan Tunnel of Huanggang-Huangmei high-speed railway as the background,the third principal stress deflection of surrounding rock at a point directly above the vault after tunnel excavation was used as the criterion to determine the critical buried depth of large section tunnel.Then,the strength reduction method was used to numerically solve the safety factor of tunnel under different surrounding rock levels and cover-span ratios(buried depth and span ratio).The relationships between the safety factors of tunnels and cover-span ratios and the basic quality(BQ)index of surrounding rock under different surrounding rock levels were found by using mathematical statistics method.Finally,considering the influence of cover-span ratio,groundwater,initial stress of surrounding rock and structural occurrence factors,the modified BQ was obtained to evaluate the stability of mechanized large section tunnel with a buried depth of less than 100 m.The evaluation method was applied to the projects of Liuyuan Tunnel and Cimushan No.2 Tunnel of the Urban Expressway in Chongqing for verification.The results show that under different surrounding rock levels,the tunnel safety factors meet the strict power function relationships with the cover-span ratios,and have certain linear relationships with BQ.The modified BQ is used as a quantitative index to evaluate the tunnel stability,which is more in line with the actual situation of the site.