1. Introduction
Many coal mines and metal mines have reached a mining depth of more than one kilometer. Owing to the increase in mining depth, dangers to the safety of the mines significantly increases, even with minor negligence [^{1}]. Increased initial earth stress causes the surrounding rock to be in a state of rupture. The expansion of the surrounding rock rupture range is temporally dependent. This leads to a nonlinear continuous deformation and a difficulty in maintaining the stability of surrounding rock. For soft rock mass, the creep displacement is significant with a deformation ratio of tens of millimeters per day. If the supporting intensity is weak, the deformation ratio can reach tens of centimeters per day [^{2}]. The stress and displacement distribution of the rock surrounding the roadway rock influence the stability and safety of roadway support system. Therefore, it is of great significance to study the rheological properties of surrounding rock in deep roadway [^{3}]. Typical rheological failures of the roadway are shown in Fig. 1.
The axisymmetric deformation analysis of the rock around a circular tunnel is a classic problem. The ground responses of elasto-perfectly-plastic, elasto-brittle-plastic and strain-softening have been studied. Park and Kim [^{5}] derived analytical solutions of the stress and deformation of the surrounding rock using an elasto-brittle model based on different elastic deformation laws in the plastic zone. Sharan [^{6}-^{7}] derived the closed-form expressions of stress and deformation in the elasto-brittle-plastic Hoek-Brown rock mass for circular tunnels. Jiang et al. [^{8}] studied the stress and deformation solution of circular tunnels in the elasto-plastic-brittle rock mass on the account that the deep underground rock mass possesses a higher plastic deformation capacity. Stille et al. [^{9}] combined the Mohr-Coulomb (M-C) criterion with the elasto-brittle-plastic model, and considered the dilatancy characteristics of the surrounding rock after yielding. Brown et al. [^{10}] proposed the distribution law of stress and strain fields of surrounding rock, and considered the material characteristics of plastic softening and volume expansion.
Strictly speaking, the deformation and stress responses of the roadway obtained by elastic and elastoplastic mechanics were instantaneous in the past. In fact, the deformation and stress which were measured and maintained by the engineering support were caused by rheology. Therefore, solving the practical problems of rock mechanics cannot be separated from the analysis of rheology. In consideration of the elasto-viscoplastic constitutive theory, Perzyna proposed an elasto-viscoplastic model in 1966 [^{11}]. Subsequently, Perzyna’s general theory of elasto-viscoplasticity was applied to metal alloys by Lemaitre and Chaboche [^{12}], and was extended by Pellet et al. [^{13}] to the study of the time-dependent behavior of rocks. Boidy et al. [^{14}] and Bonini et al. [^{15}] combined numerical simulations with Lemaitre’s viscoplastic model. In turn, Pellet [^{16}] combined the Burger and the elastoplastic models, and calibrated the constitutive parameters using back-analyses of convergence data.
Barla et al. [^{17}], Debernardi et al. [^{18}], and D. Sterpi et al. [^{19}], have conducted corresponding studies of deep tunnels using numerical simulations at squeezing conditions. Roatesi [^{20}] obtained the numerical solution using the elasto-viscoplastic Cristescu constitutive law, which was used to perform an analysis of the evolution of the damage zones around a circular lined cavity in a viscoplastic material subjected to a nonisotropic far stress field. Many scholars have conducted corresponding research on the analytical solution of surrounding rock regarding rheology. Specifically, Sulem [^{21}], Ladanyi [^{22}], Gill [^{23}], and Wang [^{24}] et al., analyzed the stress and strain of the surrounding rock of the roadway based on the viscoelastic model, but neglected the plastic mechanical properties. This led to the overestimation of the support pressure in the support design.
In recent times, research related to the viscoplastic analysis of the surrounding rock was less extensive. Fritz [^{25}] used the viscoplastic constitutive relation of the surrounding rock in a manner similar to the form of the linear elastic Hooke's law, and the elastic-viscoplastic analysis was carried out by ensuring that the stress and strain satisfy the basic equations of the problem and the stress intensity condition. This study was based on the generalized Bingham model in accordance to the correspondence principle that has been extensively adopted in plastic and viscoelastic mechanics. Additionally, its three-dimensional elastic-viscoplastic constitutive equation is established by rigorous mechanical analyses, and the stress and deformation solutions of elastic-viscoplastic surrounding rock are obtained in circular roadways. Combined with engineering examples, the variation law of stress and displacement of the surrounding rock was analyzed at different times given its significance for roadway engineering design and construction.
2. Problem definition
2.1 Mechanical model
The purpose of this paper is to obtain the distributions of stress and displacement of surrounding rock around a circular tunnel in elastic-viscoplastic rock mass. The characteristics of brittle rock mass are that brittle failure occurs and steps into the post-failure region once the stresses exceed the elastic limit.
When the tunnel is located in brittle rock mass, the deformation can be divided into two parts after stress redistribution, namely, the elastic region and the fractured region as shown in Fig. 2. The emerging conditions of the two parts can be described in accordance to two conditions: a) when the stress of the surrounding rock does not reach the plastic condition, the deformation is considered to be in the elastic state, and b) when the stress reaches and exceed the plastic yield condition (that is, when no deformation occurs and when the stress decreases at the same time), the deformation is considered to be in the fractured region. For convenience of analysis, the stress-strain curve of the actual rock is replaced by the fold line in Fig. 2.
The plane strain problem requires the deformation of long roadway to be completed instantaneously. In fact, the deformation of the surrounding rock is gradually carried out. In this paper, the surrounding rock is assumed in an elastic state at the beginning. Stress concentration of surrounding rock leads to plastic yielding and fracture of rock and causes viscoplastic deformation with the excavation. Finally, when the stress distribution of the surrounding rock reaches equilibrium again, the viscoplastic deformation no longer changes with time.
Fig.3 shows a circular tunnel with radius a in a homogeneous infinite isotropic rock mass subjected to a hydrostatic stress p _{0} in the cross section of the tunnel. A support pressure p _{ i } is applied on the inner surface. The surrounding rock consists of three parts: the initial plastic region, the viscoplastic region and the elastic region. The initial plastic zone is generated due to excavation of rock mass, and it is transiently formed. The viscoplastic zone continues to develop with time on the basis of the above. There is no stress disturbance beyond the viscoplastic zone, it is in an elastic region.
Assuming the rock mass is uniform, isotropic and the tunnel is a problem of plane strain, the equations are expressed in polar coordinates, the radial stress, radial strain, tangential stress and tangential strain are expressed as
2.2 Governing equations
The differential equation of equilibrium for axisymmetric problems in each region can be expressed as (not considering the body force of rock mass)
The geometric equation of this axisymmetric problem can be denoted as
In this study, the M-C yield criterion was considered when the plastic yield and rupture of the surrounding rock occurred. The strength theory is suitable for homogeneous isotropic rock, and the calculated results were also generally consistent with the actual situation. The M-C criterion cannot only describe the failure characteristics of brittle materials, but also describes the damage characteristics of the friction materials. It can be uniformly represented as:
where N and S are material parameters. For the intact and failure rock mass, N and S can be expressed by the cohesion strength and the friction angle as follows:
where c is the cohesive strength and φ is the frictional angle. The subscript “c” and “p” denote the fractured region and the plastic region, respectively.
The complexity of the elasto-plastic problem lies in the nonlinear relationship between the stress and the strain. According to the theory of plasticity, plastic strain depends on the plastic potential, corresponding M-C criteria, the plastic potential [^{27}, ^{28}] here is:
where
The plastic strain can be obtained from the potential theory, as shown in the following equation:
On the basis of the plastic potential (4), combined the above results with the elastic strain, the total strain of rock mass induced by excavation can be deduced as
Where
It is assumed that after the stress of the surrounding rock exceeds the yield strength
One dimensional constitutive equation of generalized Bingham model is succinct, as shown in the following equation:
where dot “ ^{∙ “} denotes the derivative of time,
By the correspondence principle, the three-dimensional constitutive equation of the generalized Bingham model can be denoted as:
where
It is generally considered that the volume deformation of the rock mass is elastic. Here, in order to describe the general situation, it is considered that the volume deformation of the surrounding rock and the deviatoric strain tensor both present the elastic-viscoplasticity.
For circular tunnel, the Eq. (8) is unfolded in a cylindrical coordinate system:
where
By substituting plane strain condition (
Under the condition of plane strain, axial stress is zero, therefore, the relationship between the viscosity coefficient
By substituting Eq. (11) into Eq. (9), adopting the form of Eq. (6), it is considered that the elastic-viscoplastic deformation is produced after the elastic deformation occurs,
where
2.3 Boundary conditions and initial conditions
The stress boundary condition, initial condition and contact conditions can be expressed as follows:
where
3 Deformation analysis of broken rock in the final state
In the final state of the surrounding rock, the fractured region and the elastic region are formed, and the stress components and the displacement can be expressed as follows [^{8}, ^{26}].
3.1 Cracked zone (a≤r≤R _{ c } )
Eqs. (1), (3), (3b) and (13) will be used to get the stress of the cracked zone:
where
By using the geometric Eq. (2), the constitutive Eq. (6) and the deformation coordination equation, the corresponding expressions of strain and displacement can be obtained:
where
3.2 Elastic zone (Rc ≤ r <∞)
The stress, strain and displacement of the elastic zone are easily obtained:
where
Using the condition of continuous displacement at the junction of the cracked zone and the elastic zone, i.e. the second equation of Eq. (13), we can get the solution for A:
By substituting Eq. (20) into Eq. (17), the expression of the strain and displacement in the cracked zone can be obtained. If the rock has not been broken, the above results can be returned to the famous Kastner solution, therefore, the aforementioned analysis is a supplement of Kastner solution.
4. Elastic-viscoplastic solution
Elastic deformation instantaneously occurs at the moment of tunnel excavation, and the surrounding rock near the boundary of the roadway may generate a viscoplastic area with radius
4.1 Transient elastic stress and deformation of surrounding rock
When the surrounding rock is in the elastic state, its stress component and displacement can be expressed as:
where variable superscript “e” represents elastic state, the subscript “0” represents the initial moment (t=0).
Through analyzing rheological test data [^{30}], elastoplastic deformation occurs, but the magnitude of the elastic deformation is far greater than the plastic deformation, the plastic deformation is ignored here, only elastic deformation exists. Hence, at the moment of
4.2 Initial plastic zone stress and relative displacement of surrounding rock
Due to the excavation of rock mass, the stress of the surrounding rock is redistributed, we assume that an initial plastic zone produces at the moment of excavation (i.e.
When the stress of the surrounding rock reaches the plastic yield condition, it enters the critical plastic state. By substituting the expression of elastic stress Eq. (21) into yield condition Eq. (3) and Eq. (3b), initial critical plastic zone radius is:
Combined with the basic equations (1), (2) and (11a), we can get an equation:
where
Eq. (23) is partial differential equation, its solution can be expressed as:
where,
By using the first and fourth equation of the boundary and initial conditions Eq. (13), and the equilibrium Eq. (1), the radial stress and the circumferential stress can be expressed as:
By substituting Eq. (25) into the second equation of Eq. (12a), and integrate Eq. (12a) on t, the displacement u can be obtained:
The displacement contact condition is replaced by Eq. (26), that is, the fourth equation of Eq. (13):
Combined Eq. (27) with Eq. (26), the displacement of the initial plastic zone is shown as:
The above equation,
4.3 Stress and relative displacement of viscoplastic zone of surrounding rock
Suppose total stress exceeds yield strength, outside the scope of the initial plastic zone (i.e.
where,
In order to separate the stress, strain, and displacement of the cracked zone from the corresponding variables of the viscoplastic zone, these variables are labeled with the superscript “vp”. In the expression of
In essence, the deformation of the surrounding rock refers to the process of constant stress adjustment. During the process of surrounding rock deformation, the stress in the viscoplastic zone changes with time. In addition, when time
The functions
With the continuous development of time, the viscosity of the surrounding rock gradually disappeared, finally, the viscoplastic zone no longer changes, the boundary between the viscoplastic zone and the elastic zone becomes the junction of the cracked zone and the elastic zone, the Bingham elasto-viscoplastic model degenerates into an elastoplastic model.
4.4 Calculation procedure
By inputting mechanical parameters of surrounding rock,
5. Example analysis
In order to illustrate the correctness of the derived equation, an engineering example is discussed. Tang Kou coal mine was excavated in crosscut, which is located in Jining, Shandong, China. After the construction of chambers and tunnels, large deformation and failure occurred. The stress measurement of the original rock was carried out at the depth of 1028 m in tunnel, the vertical and horizontal stress components of the 2# measuring point is 23.77 and 23.04 MPa, respectively. The stress of the original rock is basically isobaric in two directions, herein, the original crustal stress is taken as 23.4 MPa. The physical and mechanical indexes of rock mass as shown in Table 1.
Rock mass parameters | Value |
---|---|
Radius, a (m) | 1 |
Initial ground stress p _{0} (MPa) | 23.4 |
Modulus of elasticity E (MPa) | 5700 |
Viscosity coefficient η ( Days·MPa ) | 6000 |
Poisson ratio μ (-) | 0.3 |
Internal friction angle φ _{ p } (Deg.) | 35.82 |
Residual internal friction angle φ _{ c } (Deg.) | 22.99 |
Cohesive force c _{ p } (MPa) | 2.51 |
Residual cohesive force c _{ c } (MPa) | 1.43 |
Dilatancy angle ψ _{ c } (deg.) | 13 |
5.1 The solution of function
Eq. (33) is a transcendental equation of the function
By substituting the rock mass parameters into the Eq. (33), analytical solutions of
It is obvious that
5.2 Distributions of stresses
When the support force is 0 MPa, the radius of the initial plastic zone is 1.2197 m and the final plastic zone is 2.5236 m. There is a viscoplastic zone between the above two zones. The calculation time is taken as 0, 10, 50 and 500 days, respectively, the corresponding stress states are States I, II, III and IV. The distributions of radial and tangential stresses of the above four states are shown in Fig. 8. The state I is fully elastic, plastic deformation doesn’t occur. The radius of the state II is 1.7381 m, the radius of the state III is 2.3303 m, and the radius of the state IV is 2.5236 m. Thus it can be seen, the development of the viscoplastic zone is a process of increasing radius. When the radius no longer changes, it means that the viscoplastic zone no longer develops, and the surrounding rock reaches a state of balance. The difference of the radial stress and circumferential stress at the radius of the initial plastic zone decreases with time.
5.3 Comparison of elasto-viscoplasticity and elastic-brittle displacement solution
This paper mainly studies the variation of stress and displacement of surrounding rock with time. Here the cases of states II, III and IV are discussed especially. Fig. 9 shows the distributions of viscoplastic displacement corresponding to States II, III and IV, respectively. Obviously, the displacement of above four states has similar distribution forms. With the increase in radius, the displacement gradually decreases. The displacement is continuous at the interface of the initial plastic zone and the viscoplastic zone. The maximum displacement occurs on the free face. And the displacements of the three states are 0.019 m, 0.04 m and 0.049 m separately in the free face of the tunnel. The three points (i.e. A, B and C, shown in Fig. 9) represent the
The three-dimensional elasto-viscoplastic model established in this study is suitable for homogeneous isotropic rock. The results based on the elasto-viscoplastic model are in agreement with the static analytical solution of the elasto-brittle state. The proposed analytical solutions consider the progressive evolution process in plastic region, which is an improvement for the static analytical solution.
6. Conclusions
In this study, the elasto-viscoplastic analytical solutions of stress and deformation around a circular tunnel were derived according to the generalized Bingham model using the Mohr-Coulomb criterion and the non-associated flow rule.
The rock is assumed to be homogeneous and isotropic. When excavating tunnel, rock mass behaves elasticity. Then, when t=0^{+}, the radius of the initial plastic zone reaches
The emergence of viscoplastic zone causes redistribution of stress in the surrounding rock, that is, the peak stress is transferred to the deep surrounding rock, and rheological analyses can thus better describe the changing process of developed stress and strain in the surrounding rock of deep tunnel. This method can effectively predict the failure process of the roadway and provides a reference for engineering support.