Numerical Investigation of Blast-Induced Damage to the Rock Pillar Between a Tunnel and an Adjacent Water-Filled Karst Cave

Tunnel construction adjacent to a water-filled karst cave is very prone to sudden water and mud inrush and to other geological hazards. This paper presents a numerical study of the damage that the blasting of the tunnel face causes to the rock pillar between a tunnel and an adjacent water-filled karst cave. The problem is analysed using a plane-strain dynamic finite element model in LS-DYNA (version R11.0), in which the limestone is described by the RHT damage model, the explosive by the JWL equation of state and the detonation products, and the karst water by a multi-material ALE formulation with fluid–solid coupling; the static karst water pressure is applied before the detonation through an implicit–explicit stress initialisation. Thirty-six working conditions are analysed, covering explosive densities of 1300–1500 kg/m3, detonation velocities of 7250–7650 m/s, karst cave cross-sectional sizes of 3.5 m and 5.7 m, and cave water pressures of 1.0 MPa and 1.2 MPa. The findings indicate that the cavern water pressure governs whether the fissure zone becomes through-going, whereas the extent of the damage is controlled mainly by the explosive parameters. Across the 36 working conditions, the maximum length of the fissure zone varies from 206 cm to 295 cm. A quantitative comparison of the individual factors shows that the explosive density and the detonation velocity are the dominant controls: raising the density from 1300 kg/m3 to 1500 kg/m3 or the detonation velocity from 7250 m/s to 7650 m/s lengthens the fissure zone by 31–44 cm, i.e., by 13–19%. The influence of the karst cave size is secondary (enlarging the cave from 3.5 m to 5.7 m gives an increment of only 2–14 cm, i.e., 1–6%), and the influence of the cave water pressure over the engineering-relevant range of 1.0–1.2 MPa is of the same modest order (an increment of 5–12 cm, i.e., 1.9–4.8%). The role of the water pressure is therefore not to dominate the overall trend of the fissure zone but to control whether the fissure zone crosses the safety threshold: the same pressure increment pushes those combinations that already lie close to the relative-spacing threshold past it, raising the number of water-inrush conditions from 6 to 9 out of 18. Identifying the relative weight of the controlling factors, together with the threshold role played by the water pressure, provides the quantitative basis for blasting optimisation in tunnels adjacent to water-filled karst caves. For the conditions analysed, the density of the explosive in sections of high water pressure should be kept below 1400 kg/m3 and the detonation velocity below 7450 m/s.