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During tunnel construction, the excavation face frequently encounters heterogenous weathered composite strata that considerably reduce tunnel face stability. To assess the stability of the tunnel face under such conditions, a 3D analysis model is established using the limit analysis approach. The model comprehensively assesses the failure of the entire tunnel face, incorporating the Hoek-Brown criterion to account for the nonlinear failure characteristics of rock. In addition, improvements are made to the shape of the sliding surface in the cover layer to consider the influence of the arching effect. The proposed model is validated by comparisons with existing theoretical methods and numerical simulation results. Further parameter analysis is conducted to investigate the influence of stratum parameters, stratum thickness and tunnel depth. The results indicate that tunnel face instability is more likely to occur when the rock mass quality of the higher layer is greater than that of the bottom layer. The rock mass in the bottom layer has the most remarkable impact on the tunnel face stability compared with the cover layer and higher layer. The stability of the tunnel face initially decreases and then increases with the tunnel depth due to the rock-arching effect. These research results can provide useful references for tunnel face support design in composite strata. [ABSTRACT FROM AUTHOR]

To study the force and deformation characteristics of subsea mudmat-pile hybrid foundations under different combined loads, a project at a water depth of 200 m in the South China Sea was studied. A numerical model of a subsea mudmatpile hybrid foundation is developed using the numerical simulation software FLAC3D. The settlement of the seabed soil, the bending moments of the mudmat, and the displacements and bending moments along the pile shaft under different load combinations, including vertical load and horizontal load, vertical load and bending moment, and horizontal load and bending moment load, are analyzed. The results indicate that settlement of the seabed soil is reduced by the presence of piles. The settlement of the mudmat is reduced by the presence of piles. Different degrees of inclination occur along the pile shaft. The angle of inclination of pile No. 1 is greater than that of pile No. 2. The dip directions of piles No. 1 and No. 2 are identical under the vertical load and bending moment and are opposite to those under the other combined loads. The piles that are located at the junctions between the mudmat and the tops of the piles are easily destroyed.