Subgrading method for surrounding rock of soft rock tunnel

The invention discloses a subgrade grading method for surrounding rock of a soft rock tunnel. The subgrade grading method comprises the following steps: grading index determination and grade division; calculating a digital characteristic value of each grading index; employing MATLAB to generate a corresponding cloud model graph, and reading the test value certainty degree; obtaining four test parameter values of sublevel sections needing to be divided through an indoor test; determining the weight of each grading index; and calculating a comprehensive certainty degree and judging a grade. According to the method, four parameters of a non-passing rock sample are obtained through an indoor test, soft rock tunnel surrounding rock subgrade classification is carried out in combination with a targeted classification method, IV-grade and V-grade surrounding rock is divided into five subgrades, namely IV-a, IV-b, IV-c, V-a and V-b, corresponding supporting measures are formulated according to different subgrades, mor