Effect of Axial In-Situ Stress in Deep Tunnel Analysis Considering Strain Softening and Dilatancy

Abstract

In many previous tunnel analyses, the axial in-situ stress was ignored. In this work, its effect on the deformation and failure of the surrounding rock of a deep tunnel was revealed, considering the objective strain softening and dilatancy behavior of the surrounding rock. Analysis based on the incremental plastic flow theory was conducted, and C++ was used to write a constitutive model for numerical simulation to verify and further analyze this effect. Then, the results were validated by the field monitoring data of a coal mine gateway. Results show that the effect of the axial in-situ stress σa0 is more significant when strain softening is considered, compared with the results of a perfectly elastoplastic model. When the axial stress σa is σ1 or σ3 at the initial yield, an increase or decrease in σa0 intensifies the deformation and failure of the surrounding rock. When σa is σ2 at the initial yield, 3D plastic flow partly controlled by σa may occur, and an increase in σa0 intensifies the deformation and failure of the surrounding rock. The effect of σa0 will be amplified by considering dilatancy. Considering both strain softening and dilatancy, when σa0 is close to the tangential in-situ stress σt0 or significantly greater than σt0 (1.5 times), σa will be σ2 or σ1 at the initial yield, and then 3D plastic flow will occur. In the deformation prediction and support design of a deep tunnel, σa0 should not be ignored, and the strain softening and dilatancy behavior of the surrounding rock should be accurately considered.

Evolution Mechanism and Control of Floor Heave in the Deep Roadway with Retained Bottom Coal

“…The increase in the elastic modulus of the coal body with prestressed and non-pre-stressed supports is determined according to equation ( 9) and reference [36]. The foundation coefficients of the two sides of the coal body before and after the bottom corner support is installed are calculated according to equation ( 7) and equation (8).…”

“…The first factor, in situ stress, is the local stress resulting from roadway excavation. Radial unloading and shear loading are the main causes of deformation and failure in the surrounding rock [8][9][10]. For the floor, an increase in horizontal in situ stress-as the initial value of tangential stress-aggravates the deformation and failure of the rock mass [11,12].…”

Transfer and dissipation of strain energy in surrounding rock of deep roadway considering strain softening and dilatancy

“…For a rectangular roadway, the deformation and plastic zone depth are larger at the middle (width direction) of the roof; due to symmetry, the radial stress σ r , tangential stress σ t , and axial stress σ a at these positions are always the principal stresses. Therefore, based on the stress adjustment process given by Yi et al, 30 a microunit at the middle of the roadway roof is considered for the analysis.…”

“…As the maximum principal stress σ 1 ( σ t ) increases and the minimum principal stress σ 3 ( σ r ) decreases, the microunit yields and enters the plastic stage. Only the shear failure is discussed herein because the failure mode of the surrounding rock of a deep roadway is mainly shear failure, and tensile failure only occurs near the excavation boundaries 30,31 . The M‐C yield criterion f , which is widely adopted in engineering, 36,37 and the nonassociated plastic flow rule, which well describes the plastic behavior of rock masses well and is widely used in roadway deformation calculations, 38,39 are used for the analysis and given as follows:where is the plastic principal strain increment, c is the cohesion, φ is the friction angle, ψ is the dilation angle, N α = (1 + sin( α ))/(1 − sin( α )) is a transformation function, g is a plastic potential function, and λ is a constant that can be determined based on the consistency condition.…”

“…As a result, a large amount of initial strain energy is converted into plastic strain energy and dissipated during the surrounding rock deformation process. Additionally, the consideration of strain softening and dilatancy causes a significant increase in the plastic zone and strain in the deep roadway surrounding rock, 30,31 inevitably affecting the transfer and dissipation of strain energy.…”