Yaoting Xiao

Soil-rock composite strata are frequently encountered during shield tunneling for underground projects such as metros, railways, and other underground building facilities. The strong heterogeneity and abrupt variations in soil and rock mechanical properties often induce excavation instability, thereby adversely affecting construction safety and efficiency. Based on an urban shield tunneling project, the interface effects on cutting force and the failure characteristics of soil–rock composites under shield cutter penetration are investigated in this study using the finite–discrete element method (FDEM). Firstly, the FDEM input parameters are calibrated, after which the cutting force, crack number, acoustic emission (AE), breaking process, displacement, and stress fields with different interface inclinations (a) during penetration are analyzed. The influence of the interface strength reduction factor (λ) on the cutting force and failure mode is then discussed. The results indicate that the cutting force–penetration curves can be classified into two phases: in stage I, the cutting force grows smoothly, and some cracks are initiated in the sandstone layer with a few AE events; in stage II, the cutting force drops with violent fluctuation, and both tensile and shear cracks develop rapidly in the sandstone layer, accompanied by many AE events. Generally, cracks mainly propagate in the sandstone layer, and less shear failure occurs in the silty clay layer. As a increases, the peak force of cutter A increases, while that of cutter B decreases. Additionally, tensile cracks between the cutters are connected when a is 30° and 45°, resulting in rock bridge breakage. However, a larger λ leads to a greater peak cutting force. The total crack length tends to decrease with increasing α and λ with an a of 30°, indicating that larger interface inclinations and higher interface strengths are associated with less damage to the composite specimens. These findings provide new insights into the fragmentation mechanisms of soft–hard composite ground and can contribute to the efficient construction of underground building and transportation infrastructure.

The mechanical properties of soil in soft soil area are poor, and the settlement of the underlying layer in the composite foundation accounts for a large proportion of the total settlement. At present, most of the research focuses on the settlement of the reinforced area, and the research on the settlement of the underlying layer is of great significance for the settlement of soft soil composite foundation. The differences in load transfer modes of soil and pile are analyzed, and based on the Boussinesq solution and Mindlin solution, a calculation method for the stress and settlement of the underlying layer in flexible and rigid pile composite foundation is proposed. The relative displacement of soil and pile in flexible pile composite foundation is small, and the negative friction can be ignored, but the influence of effective pile length should be considered. The relative displacement of soil and pile in rigid pile composite foundation is large, so the negative friction should be considered. Part of soil top stress is transmitted to the pile via negative friction, and then the pile axial force is transmitted back to the soil via positive friction. In addition to effective pile length, the change of stress transfer path caused by negative friction should also be considered in settlement calculation.

The classical Richards Equation of water infiltration in unsaturated soil is a partial differential equation, which is nonlinear and difficult to obtain analytical solution. Taking time as the least action variable, the infiltration path as a functional of time is established based on Richards Equation. The vertical infiltration problem of unsaturated soil under gravity is transformed into the functional extremum problem, and solved by Euler-Lagrange Equation. The results show that the diffusion coefficient D(θ) has a function relationship with the depth of wetting front. When the type of diffusion coefficient is known, the moisture content of wetting front at specified depth, the moisture content of the farthest wetting front and the maximum entropy distribution of soil moisture content can be obtained at the optimal path. Taking the optimal path as the standard, the accuracy of solving Richards Equation by Boltzmann transformation and linear transformation is tested.

In the traditional research of slope stability, it is difficult to continuously obtain the moisture content of soil in long time sequences. In combination with the precipitation, temperature, and vegetation cover data in the study area, the variable infiltration capacity hydrological model is used to estimate the daily variation of soil moisture content, which is used as a calculation condition for the analysis of slope stability. The results show that, from 1970 to 2010, the spatial‐temporal distribution of soil moisture content in the Weihe River Basin showed an increasing trend. Moreover, the shear strength of soil decreased, but the range was stable at approximately 2%. The strength reduction method based on the M‐C inscribed circle criterion (DP3) shows that the slope stability factor k had an increasing trend for over 40 y. The portion of the period with a k value higher than 1.2 is more than 85% of the entire period, that with a k value between 1 and 1.2 is approximately 9%, and that with a k value less than 1 is approximately 5.5%. The R/S analysis results show that the Hurst coefficients of the τ‐t curve and the k‐t curve are 0.5568 and 0.5888, respectively, and that the slope is in a state of no variation. Based on these factors, the present and future of the slope is in a stable state. This scheme is a method of studying slope stability based on hydrology, and it provides a modern alternative for soil shear strength calculation and geological hazard assessment.