Karst is an unfavorable geology that is very undesirable in tunnel construction. It is extremely risky due to the complexity and fuzziness of shield excavation in karst strata. To reduce the construction risk of shield tunneling in karst strata, a risk assessment method based on cloud model and fuzzy analytic hierarchy process (FAHP) is proposed. A risk assessment index system is established, including four first‐level indices of karst geological conditions, hydrogeological conditions, tunnel design, and shield construction, as well as seventeen second‐level indices. Fuzzy analytic hierarchy process is used to determine the evaluation indices weights. The numerical characteristics of each index and the risk cloud diagram are obtained via the backward cloud generator and forward cloud generator, respectively. The certainty degrees of the assessment indices corresponding to each risk level are calculated, and the final risk level is determined based on the comparison of the risk cloud diagram and the principle of maximum certainty. The proposed model is applied to the risk analysis and calculation of shield tunnel engineering, and the risk assessment results indicate that the comprehensive risk level of shield tunnel construction in karst strata is level II, which is consistent with the actual project. It demonstrates that the proposed method can provide a reference for the risk assessment of shield tunnels construction in karst strata.

Rock mass blasting is a complex process that involves the coupling of both discontinuous and continuous media. This paper aims to reveal the dynamic failure process between adjacent boreholes under an elliptical bipolar linear charge structure using the SPH‐FEM (smooth particle hydrodynamics and finite‐element method) coupling algorithm numerical simulation method. The numerical simulation results are compared with the existing experimental results, which proves the rationality of the algorithm. According to the numerical simulation results, the shaped jet will first shock the hole wall and form a stress concentration zone that will guide the formation of cracks during the stress wave propagation process. In the case of double‐hole blast loading, there is a tendency for cracks coalescence to develop between adjacent boreholes due to the superposition of stresses between the double holes and the increase in damage and plastic strain. The best blasting results will be achieved with this structure when the distance between adjacent holes is 110 cm. Finally, the superiority of elliptical bipolar linear blasting in engineering blasting was verified through field experiments. The results of this study provide a reference for subsequent applications of elliptical bipolar structures in the field of rock blasting.

Based on LS‐DYNA numerical simulation analysis and comparison with laboratory tests, the blasting crack development dynamic evolution mechanism of elliptical bipolar linear shaped charge is analyzed. The development law of rock crack and optimal radial decoupling coefficient under different blast hole diameters were studied. The results revealed that the blasting with elliptical bipolar linear shaped charge had a remarkable effect on the directional crack formation, and the maximum effective stress of rock close to the position of shaped charge in the direction of concentrating energy is about 2.3 times of that in the direction of nonconcentrated energy. Moreover, the directional crack could be formed by blasting with elliptical bipolar linear shaped charge with different hole diameters, whilst the length of the main crack was related to the radial decoupling coefficient. Particularly, the main crack reached the longest when the radial decoupling coefficient was 3.36.