Deformation control criterion of shield tunnel under lateral relaxation of soft soil

Metro shield tunnels under the lateral relaxation of soil(LRS)are susceptible to significant lateral deformations,which jeopardizes the structural safety and waterproofing.However,deformation control standards for such situations have not been clearly defined.Therefore,based on a specific case,a model test is conducted to realize the LRS of a shield tunnel in a sandy stratum to reveal its effect on segment liners.Subsequently,a deformation control criterion is established.The LRS is simulated by linearly reducing the loads applied to the lateral sides of the segment structure.During lateral unloading,the lateral earth pressure coefficient on the segment decreases almost exponentially,and the structural deformation is characterized by horizontal expansion at the arch haunches and vertical shrinkage at the arch vault and arch bottom.Based on the mechanical pattern of the segment structure and the acoustic emission,the deformation response of a segment can be classified into three stages:elastic and quasi-elastic,damage,and rapid deformation development.For a shield tunnel with a diameter of approximately 6 m and under the lateral relaxation of sandy soil,when the ellipticity of the segment is less than 2.71%,reinforcement measures are not required.However,the segment deformation must be controlled when the ellipticity is 2.71%to 3.12%;in this regard,an ellipticity of 3%can be used as a benchmark in similar engineering projects.

Anisotropy of strength and deformability of fractured rocks

Anisotropy of the strength and deformation behaviors of fractured rock masses is a crucial issue for design and stability assessments of rock engineering structures, due mainly to the non-uniform and non- regular geometries of the fracture systems. However, no adequate efforts have been made to study this issue due to the current practical impossibility of laboratory tests with samples of large volumes con- taining many fractures, and the difficulty for controlling reliable initial and boundary conditions for large-scale in situ tests. Therefore, a reliable numerical predicting approach for evaluating anisotropy of fractured rock masses is needed. The objective of this study is to systematically investigate anisotropy of strength and deformability of fractured rocks, which has not been conducted in the past, using a nu- merical modeling method. A series of realistic two-dimensional （2D） discrete fracture network （DFN） models were established based on site investigation data, which were then loaded in different directions, using the code UDEC of discrete element method （DEM）, with changing confining pressures. Numerical results show that strength envelopes and elastic deformability parameters of tested numerical models are significantly anisotropic, and vary with changing axial loading and confining pressures. The results indicate that for design and safety assessments of rock engineering projects, the directional variations of strength and deformability of the fractured rock mass concerned must be treated properly with respect to the directions of in situ stresses. Traditional practice for simply positioning axial orientation of tunnels in association with principal stress directions only may not be adequate for safety requirements. Outstanding issues of the present study and su^zestions for future study are also oresented.