| | Serial number | Assessment indices | Risk level | | I | II | III | IV |
| | U11 | Karst distribution characteristics | Very homogeneous | Homogeneous | Inhomogeneous | Very inhomogeneous | | U12 | Degree of karst development | Undeveloped | Slightly developed | Moderate developed | Strongly developed | | U13 | Scale of cave development (m) | [0, 3) | [3, 5) | [5, 10) | [10, +∞) | | U14 | Stability of the cave roof | Very stable | Stable | Unstable | Very unstable | | U15 | Cave filling situation | Completely filled | Partially filled | Empty | Filled by karst water | | U21 | Groundwater level (m) | [0, 10) | [10, 20) | [20, 40) | [40, +∞) | | U22 | Underground water richness | Poor | Slight | Moderate | Rich | | U23 | Seasonal distribution of rainfall | Very homogeneous | Homogeneous | Concentrated | Very concentrated | | U31 | Shield tunnel length (m) | (0, 500] | (500, 1000] | (1000, 3000] | (3000, +∞) | | U32 | Tunnel buried depth (m) | (+∞, 20] | (20, 10] | (10, 5] | (5, 0] | | U33 | Ratio of tunnel burial depth to shield diameter | (+∞, 3] | (3, 2] | (2, 1] | (1, 0) | | U34 | Maximum slope of tunnel (‰) | (0, 25] | (25, 30] | (30, 35] | (35, 40] | | U41 | Tunnel support | Very reasonable | Reasonable | Unreasonable | Very unreasonable | | U42 | Distance of shield tunneling test (m) | [200, 300] | [100, 200) | [50, 100) | [0, 50) | | U43 | Shield tunneling speed (Loops/day) | (0, 5] | (5, 10] | (10, 15] | (15, 20] | | U44 | Grouting pressure (MPa) | [0.15, 0.25) | [0.25, 0.634) | [0.634, 1) | [1, +∞) | | U45 | Distance of shield deviation from design axis (mm) | [0, 10) | [10, 30) | [30, 50) | [50, +∞) |
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