Figure 3

Histograms of the eight parameters considered to predict the quality of tunnel surrounding rock. The numbers on the X-axis represent the characteristic of different parameters. (1) The numbers 1, 2, 3, 4, and 5 for rock hardness represent hard, slightly hard, slightly soft, soft, and extremely soft, respectively. (2) 1, 2, 3, 4, and 5 for rock mass integrity represent complete, slightly complete, slightly broken, broken, and extremely broken, respectively. (3) 1, 2, 3, 4, and 5 for rock mass structure represent integral/extremely-thick layered, block/thick layered, fractured block/medium-thick layered/interlayer/thin layered/mosaic, fragmented, and granular, respectively. (4) 1, 2, 3, 4, and 5 for weathering degree represent fresh, slight, medium, severe, and extreme, respectively. (5) 1, 2, 3, and 4 for structural plane integrity represent good, ordinary, bad, and very bad, respectively. (6) 1, 2, 3, and 4 for groundwater represent dry/wet, moist/dripping, rain-like dripping/linear water, and tubular water/gushing water, respectively. (7) 1, 2, 3, and 4 for in-situ stress represent low, medium, high, and extremely high, respectively.