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Back-analysis can be regarded as the reverse process of forwarding analysis. The forward analysis in geotechnical engineering is to utilize the geometric dimensions, constitutive relations, material parameters, and boundary load conditions determined by the rock mass medium to solve the physical quantity information (strain, stress, Displacement) process. In this paper, a novel back analysis program based on BP neural network is proposed, which can realize automatic correction and adjustment of parameters and adapt to most tunnel projects. Subsequently, the analysis program is applied to the front and back joint modeling of the Nagasaki tunnel project. The mechanical parameters and initial stresses of the back analysis are utilized to conduct finite element calculations, and the displacements of all monitoring points of the multi-point displacement measurement are calculated. The results are in good agreement with the measured values. The proposed back-analysis method in this paper can also be applied to the determination of other tunnel rock mass parameters.

The mechanical characteristics and stress state caused by water jet loading are influenced by many factors that have restricted studies on the jet mechanism. Using the basic theory of elasticity and fluid dynamics, the mechanical effects of water jet loading are analyzed. Based on the simplified jet distribution pressure, the elastic analytical solution of the stress distribution in a semi-infinite plane under the jet distribution pressure is derived, and the stress distribution characteristics are discussed. In addition, the Mohr–Coulomb yield criterion served as a fracture criterion, enabling establishment of the relationship between jet pressure and rock strength. The results indicate that the compressive stress is symmetrically distributed around the jet axis with the maximum at the center. Conversely, the shear stress is Anti-symmetrically distributed around the jet axis with the maximum at the point (0.5r, ± 0.65r). The initial fracture of rock under the impact of water jet is mainly shear compression failure. The Mohr–Coulomb yield criterion is useful for estimating the relationships between the rock failure zone diameter with the water jet impact pressure.

The subject of this investigation is the stress evolution characteristics of rock impacted by transverse plain water jet (PWJ), including the application of stress evolution in the interpretation of the laws of breaking rocks (including artificial rock). Based on the semi-infinite plane theory, an analytical solution of the stress evolution of rock subjected to transverse jet loading is obtained, and the evolution of the stress state variable F of the rock microelement based on the Mohr–Coulomb criterion is also obtained. Then, elastic brittle failure and elastic-brittle damage of rock microelements are analyzed. Besides, an expression of the damage variable D based on elastic-brittle failure assumption is proposed. The analysis shows that the peak values of the F evolution curve determines the failure of the rock microelement based on the elastic-brittle failure hypothesis. The area SF surrounded by the evolution curve F and the line (F = F0) is positively correlated with the damage variable D, which can be used to qualitatively explain the time dependence of breaking rock using a transverse PWJ. In addition, the method based on damage variable D presented in this paper shows great potential in quantitative analysis of time dependence of rock failure.