为解决20~70 m超大跨洞室围岩分级难题，提出在初步分级阶段采用面积加权平均法确定围岩基本质量指标、在详细分级阶段增加跨度影响修正系数的超大跨扁平洞室围岩分级方法，跨度影响修正系数值根据洞室稳定系数与围岩基本质量指标和洞室跨度之间的关系来确定。采用三维数值模拟和强度折减法，通过模拟洞室的开挖全过程，计算Ⅰ—Ⅳ级围岩中20~70 m跨度洞室的稳定系数，分析得出洞室稳定系数与围岩基本质量指标和跨度之间的经验公式。结果表明： 1）当跨度由20 m增大至70 m后，洞室稳定系数减小了46.54%~50.67%； 2）当洞室跨度相同时，围岩基本质量指标越小，洞室稳定性越差。定义跨度影响修正系数为洞室稳定系数偏离基准跨度（20 m）洞室稳定系数的相对偏离度与基准跨度洞室稳定系数级差的比值。跨度影响修正系数取值范围为0~1.0，跨度增大对围岩稳定性的最不利影响相当于围岩级别降低1级。
To classify the surrounding rocks of superlargespan caverns of 20~70 m, a novel method is proposed. This method
uses the area weighted average to determine the basic quality indicators of the
surrounding rock during the preliminary classification stage and increases the
span influence correction coefficient during the detailed classification stage.
The value of the span influence correction coefficient is determined based on
the relationship between the stability coefficient of the tunnel and the basic
quality index of the surrounding rock, along with the tunnel span.
Additionally, a threedimensional numerical simulation
of the entire excavation process and strength reduction method are used to
calculate the stability coefficient of caverns with spans of 20~70 m in grade Ⅰ~Ⅳ surrounding rocks. Through analysis, an
empirical formula relating the stability coefficient of the cavern to the basic
quality index and span of the surrounding rock is obtained. The results reveal
the following insights: (1) As the span increases from 20 to 70 m, the
stability coefficient of the tunnel decreases by 46.54% to 50.67%. (2) When the
tunnel span remains constant, the smaller the basic quality index values of the
surrounding rock correspond to poorer tunnel stability. The span influence
correction coefficient is defined as the ratio of the relative deviation of the
tunnel stability coefficient from the reference span (20 m) to the grade
difference of the reference span tunnel stability coefficient. This correction
coefficient for span influence is 0~1.0, where the most adverse effect of
increasing the span on the stability of the surrounding rock is equivalent to
decreasing one grade in the grade of the surrounding rock.
