In special geology conditions such as silt-soil, foundation pits are prone to instability and severe deformation. In this paper, a composite soil nailing structure was studied and its effect on a silt-soil symmetrical foundation pit investigated. The factors affecting the stability of the pit as well as its deformation characteristics were also explored. The results show that excavation depth of the foundation pit has a significant impact on its stability. The soil outside the foundation pit is in the form of a parabola, and the uplift of the soil mainly occurs at the bottom. The horizontal displacement of soil on the side wall of the foundation pit presents a “bulk belly” form. In addition, the axial force of soil nails is larger in the middle part, and smaller at both ends in the shape of a spindle. Moreover, the horizontal displacement is positively correlated with the inclination and spacing of the soil nails, but negatively correlated with the diameter and depth of the mixing pile inlay. Furthermore, the inclination and spacing of the soil nails, the diameter, and embedded depth of the mixing pile have their own critical values for stability of the foundation pit. Specifically, in this paper, with respect to soil nails, inclination should be below 30° and prestress value should not exceed 20 kN. With respect to the mixing pile, the diameter should be less than 1.5 m; when the embedded depth of the mixing pile exceeds the critical depth, the limiting effect of the mixing pile on horizontal displacement is not significant. This research provides important takeaways for the design of a composite soil nailing structure for symmetrical foundation pits.

Under the effect of initial stress and excavation disturbance, there exists interaction between rock mass and rockbolt in deeply buried tunnels. In order to fully explore the mechanism of rock mass supported with rockbolts, this article studied the time-dependent behavior of the rock mass supported with discretely mechanically or frictionally coupled (DMFC) rockbolts. The interaction model elastic solutions under distributed force model were analyzed, then the viscoelastic analytical solutions were conducted to describe the rheological properties of the coupling model, and the solutions were acquired by setting the constitutive models of the rockbolt and rock mass in terms of a one-dimensional Kelvin model and a three-dimensional Burgers model based on material properties and dimension. Several examples were performed and the influence of initial stress σ₀, the viscosity parameters η₁ and η₂ of the three-dimensional Burgers model as well as the pre-tension T₀ on reinforcement effect were analyzed. According to the proposed model, the smaller η₂ is or the larger the pre-tension T₀ is, the more effective the support effect. However, when the pre-tension is too large, the support effect is no longer significantly enhanced. In addition, the early reinforcement effect is controlled by the first creep stage in the Burgers model while the ultimate support effect is mainly influenced by the viscosity coefficient of the second creep stage in the Burgers model. This research can provide an important theoretical reference to guide the parameter design of rockbolt reinforcement engineering in a circular symmetrical tunnel.

The shear behavior of rock discontinuities controls the stability of rock masses to a great extent. In this paper, laboratory shear tests were performed on rock-like materials with different cracks to study the effect of nonpersistent joints on the shear behavior of rock masses. The results show that the variation trends of the shear stress-displacement curves of specimens with different cracks are generally similar and have the same stage characteristics. When the crack length is relatively short, the elastic stage is prolonged, the peak shear strength decreases, and the shear displacement corresponding to the peak shear strength and the residual shear strength increases with the increase of the crack length. When the crack length is relatively long, the elastic stage is shortened, the peak shear strength decreases, and the shear displacement corresponding to the peak shear strength increases with the increase of the crack length. The peak shear stress gradually decreases with the increase of the crack length. The shear strength of the specimens with unilateral cracks is much higher than that of the specimens with bilateral cracks. The shear strength of the specimens is affected not only by the crack length but also by the crack distribution. The acoustic emission (AE) count peak occurs when the shear stress drops sharply and has an inverse “S”-type variation trend with the increase of the crack length. The inclination angle of the fracture decreases, the roughness of the fracture surface decreases, and the proportion of the wear area on the fracture surface increases gradually with the increase of the crack length. The AE source decreases with the increase of the crack length, and their locations are obviously asymmetric. This work can greatly contribute to the insight into the shear failure mechanism of rock discontinuities with nonpersistent joints.