Study of Influence of Environmental Factors on Deep Shale Creep Properties

In order to study the influence of environmental factors on creep properties of deep shale, a series of creep tests of deep black shales are performed under different environmental conditions, including stress deviation, temperature, and chemical pH value. ,e influence of these conditions on creep properties of deep shale is studied. ,e results show that the creep and creep rate of shale will grow with the increase of temperature, stress difference, and acidity-alkalinity. We get nonlinear creep model of deep shale when pH � 4 and T � 30°C.,e critical stress difference of deep shale is no larger than 31.04MPa when the chemical pH value is 4 and the temperature is 30°C. By scanning shale after corrosion, we know that the effects of chemical pH value on the creep characteristics are mainly determined by the feldspar dissolution and corrosion caused by chemical action. Our work has important theoretical significance and practical value for evaluating rock engineering stability.


Introduction
Deformation and failure of the underground engineering in Shandong Tangkou Coal Mine showed that the engineering was mainly located in shale and mudstone area.e overburden of surrounding rocks is under the influence of gravity and coupled with the complex geological environment by the threat of high ground temperature, high pressure, and groundwater erosion, which produce rheological phenomena as time increases.If the rock is buried deeper, this condition will be more obvious [1][2][3][4][5].Shale is one of the primary surrounding rock materials of deep roadway, which exhibits strong rheological properties and significant deformation characteristics when it interacts with the underground complex environments, i.e., high temperature, high stress, and chemical corrosion [6][7][8], which badly threatens the stability of subterranean rock engineering and may even lead to serious accidents.
In recent years, more and more researchers have investigated creep characteristics of rock.Most of the research focuses on the creep properties of rock under condition of the single stress field [9][10][11].Zhang  implemented uniaxial and triaxial creep experiments on sand stone and found that, as temperature increases, the total creep deformation, initiative creep deformation, instantaneous thermal strain, and creep velocity increase.In terms of temperature field and stress field coupling [15,16], Wang et al. studied the effects of chemical corrosion on creep under uniaxial compressive.e results showed that the creep and creep strain rate increase with the degree of chemical corrosion.According to failure criteria of Mohr-Coulomb and Griffith, the primary factor of great deformation failure for a roadway is the main stress deviation [17,18].
e above studies of creep characteristics of rock are based on single stress eld or two elds, and most of them are developed for shale, greenschist, sandstone, etc. e creep behaviors of rock under the action of multiple elds are seldom studied.erefore, the creep characteristics of deep shale under the combined action of temperature, stress, and chemical corrosion are rarely reported.
ere is plenty of shale with clear bedding and high chemical activity in roadway of 1000 m deep in Tangkou Coal Mine.e creep characteristics of shale are inevitably a ected by the chemical action of the ground water, which are accompanied by the in uence of ground stress and temperature of the ambient environment.erefore, study of the in uence of stress di erence, temperature, and chemical corrosion on creep characteristics of deep shale can provide theoretical guidance and technical approach for the support design, disaster prediction, safety assessment, and long-term maintenance of deep mines under complex environment in this area.It can also be used as a reference for other underground tunnels, undersea tunnels, and deep roadways in other areas.
By simulating di erent conditions of stress deviation, temperature, and chemical corrosion, we carry out laboratory experiments of creep characteristics of deep shale and nd the creep behaviors under various environmental factors.

Shale Properties and Sample Preparation.
e shale samples used in this study were taken from the tunnel at a depth of 1000 m in the Shandong Tangkou Coal Mine.Samples are black shales with clear bedding and low hardness, which are integral and dense.Because some rock cores were disconnected for bumpy road from the Tangkou Coal Mine, all samples were processed into cylinders of 55 × 80 mm. Figure 1 shows all test samples.
To study the chemical corrosion e ects on the creep properties, each sample was immersed in the chemical solution of the same volume and the pH value of 4, 7, or 10 (Table 1), which was tightly sealed.After being soaked for 15 days, the sample reached natural saturation and was removed and sealed with plastic wrap.To protect the chemical eld of the shale from the impact of hydraulic oil, heat-resistant tape was twined, and then shrink lm was put by heat drying with a hair dryer in order to seal tightly.

Test Methods.
Test equipment is the triaxial creep test machine, which was jointly developed by Qingdao University of Science and Technology and Lear Ltd. of Yantai in Shandong (Figure 2).e axial loading system is composed of a common tension and compression test machine used in the course of mechanics of materials, which is the axial force source equipment of the test machine.
e triaxial environmental chamber is a core unit of the triaxial creep test machine.Oil in the test chamber provides temperature and con ning pressure by the server control system for the sample.
We set the test temperature as three levels: 30, 50, and 70 °C.Each temperature was rated to a group, and each group had three di erent pH value samples.Previous experiments have concluded that the average uniaxial compressive strength of shale is respectively 63.69, 38.80, or 47.30 MPa when the chemical pH value is 4, 7, or 10. is experiment adopted the classi cation loading mode, and the total load series was 5.And the con ning pressure was 3 MPa.e nal-level stress deviation was 38.8 MPa.erefore, the rstlevel stress deviation was 7.76 MPa.In addition, stress deviation was controlled at 7.76, 15.52, 23.28, 31.04, and 38.8 MPa (Table 2).A heating rate of 2 °C/min was applied.To ensure uniform heating of the sample, we heated the oil to a predetermined temperature and kept the constant temperature for 3 h.e test used the displacement control mode, which simultaneously applied con ning pressure and axial pressure to a predetermined value at a loading rate of 0.005 mm/s, which remained constant for 24 h.Con ning pressure remained 3 MPa during the test process, and axial pressure used the classi cation loading mode.We used a 5 mm displacement sensor to measure the axial displacement of the sample.When the stress deviation is 7.76 MPa, the creep strain of shale is small, and the creep rate is slow and soon stable.e experimental data fit well with Burgers model, and the creep constitutive equation is as follows:

Effect Factors on Creep Properties of Shale
where ε is the total strain of Burgers; Δσ is stress deviation; E 1 and E 2 are the elastic modulus; and η 1 and η 2 are viscosity coefficients.e correlation coefficient of fitting is 0.98, and the model parameters are shown in Table 3.
When the stress deviation is 23.28 MPa, creep curve shows no sudden destruction trend, and the experimental data still fit well with Burgers model.However, the creep strain rates are larger, and the time that the shale takes to enter the stable creep stage is relatively longer.
When the stress deviation is 38.80 MPa, the creep characteristics are completely different from that of the lowstress deviation.Creep strains develop rapidly with time, and creep rates are very large.e strain-time curve becomes more and more concave to the strain axis, which shows creep with obvious nonlinear characteristics.Under this condition, the experimental data cannot fit well with Burgers model.
erefore the creep equation of Burgers model cannot fully satisfy the creep characteristics of shale.
Replacing nonlinear element with the conventional linear element in order to describe the nonlinear rheological properties, the creep equation is as follows: where σ s is the yield strength of the rock and η 3 is the viscosity coefficient of the nonlinear viscoplastic model.By using nonlinear least square method of MATLAB and programming, the correlation coefficient is 0.96, and the parameters of the model are shown in Table 3.
Isochronous stress-strain curves of creep test are shown in Figure 3(c).An inflection point occurs when the stress difference is greater than a threshold value, which indicates that the creep deformation of shale has a steady development trend when the stress difference is smaller than this threshold value, and creep strain increases sharply when the stress difference is greater than this threshold value.We define the D-value of stress as the critical stress deviation value of creep. is value can be identified from Figure 3(c).For underground roadway design, the critical stress deviation value of creep is no greater than 31.04MPa under ambient condition of pH � 4 and T � 30 °C.

Effects of Temperature.
e strain-time curves are shown in Figures 4(a We can more clearly see the effects of temperatures on the creep of shale by drawing the stress deviation-creep strain curves under different temperatures (Figures 4(c) and  4(d)).
e ambient temperature affects the critical stress deviation value.e critical stress difference for temperature lower than 50 °C is no greater than 31.04MPa, and it is reduced to 23.28 MPa when temperature is 70 °C.
Table 4 gives creep strains of shale after the loads stabilize for 1 h and 10 h under different temperatures.As seen from the above charts and table, the rising temperature only causes an increase of the creep deformation under low-stress deviation, but there is not much effect on the creep duration curves.erefore, the creep equation is still simulated by the Burgers model.However, when the stress deviation increases to 31.04 MPa, the ambient temperature not only leads to a sharp increase of creep, but also significantly changes the other creep characteristics (Figure 4(b)).Creep curves at 70 °C are significantly nonlinear curves.When the stress deviation continues to increase to 38.80 MPa, the effects of temperature on creep characteristics are more remarkable, and the creep and creep rate of shale are very great, and shale quickly reaches a state of destruction.

Effects of Chemical pH Value.
e strain-time curves are given at T � 30 °C under different pH values and stress differences in Figures 5(a We can identify the characteristicsc from the isochronous stress-strain curves (Figures 5(d) and 5(e)).We can see that the curves of pH � 7 are approximately a family of lines, and curves of pH � 10 and the higher stress deviation are approximately a family of nonlinear curves, which show the creep of shale has nonlinearity at the viscoplastic stage.As is apparent from a comparison with the previous correlation curves of pH � 4, the creep and creep rate of shale will grow with the increase of stress difference or acidity-alkalinity.

Creep Mechanism of Shale under
Temperature and Chemical Corrosion

Creep Mechanism of Shale under Chemical Corrosion.
To observe the damage of chemical solutions to rock, we soaked samples in the chemical solution of pH 4 and 7 before creep test.In this experiment, equipment of scanning electron microscopy of JSM-6700F was adopted with a multiple of 10000 times (Figure 6). Figure 7 shows the identi cation results for the microstructure sheets of the shale under hydrochemical environment of pH 4 and 7 for 15 days.As seen from Figure 7(a), shale particles are dense and evenly distributed, and there are no obvious hole under pH 7. e apparent erosion phenomenon occurs in the hydrochemical environment of pH 4. e shale from Shandong Tangkou Coal Mine is mainly composed of quartz and feldspar debris.e shale is eroded and produces voids due to the hydrochemical solution corrosive e ects.e higher acidity-alkalinity, the greater the instantaneous elastic strains at each level of the load.Hydrochemical corrosion causes di erent initial damage to shale, and it has an aftere ect on the creep characteristics of shale.With the

Conclusions
From performing systematic indoor creep tests of deep shale under di erent temperatures, stress deviations, and chemical pH values, we get the following conclusions: (     Shock and Vibration and creep rate of shale will grow with the increase of temperature, stress difference, and acidityalkalinity. (2) e effects of temperatures and stress differences on shale creep are linked, i.e., the higher the temperature, the lower the critical stress deviation.e critical deviation stress values of T � 20 and 50 °C are not larger than 31.08MPa, but when the temperature is raised to 70 °C, the critical stress difference value is larger than 23.28 MPa.
(3) e effects of chemical acidity-alkalinity and stress differences on the creep characteristics of shale are interrelated.e critical stress deviation will reduce with the increase of acidity-alkalinity.e critical difference of stress of pH � 4 is no larger than 31.08MPa, but when the pH value is 7, the isochronous stress-strain curves are approximately linear.However, when the pH value is 10 and the stress deviation value is higher, it presents a nonlinear trend, though it is not obvious.erefore, in practical engineering applications, it should be noted that the allowable design strength is reasonably determined by the creep critical stress deviation based on local actual underground environment.(4) By using scanning electron microscopy structure, the effects of chemical pH on the creep characteristics in this paper are mainly determined by feldspar dissolution and corrosion caused by chemical action.

Shock and Vibration
et al. performed triaxial creep tests of mudstone and indicated triaxial creep of weak rock is nonlinear: the creep deformation is three times more than the transient deformation.Xu et al. studied lateral strain and axial strain variation with time under different confining pressures and the rupture mechanism of rheological properties of rock.ey established a new sevencomponent nonlinear viscoelastoplastic rheological model of rock.Li et al. performed triaxial creep study of shale and concluded that creep produces two phases: steady creep and transition creep and also obtained creep variables related to time at the steady-state creep stage.By studying two fields, such as the temperature field and stress field [12-14], Li et al.

Figure 1 :
Figure 1: All shale samples used for creep test.
) and 4(b) at pH value of 4 under different temperatures and stress deviations.
) and 5(b).Strain-time curves of different pH values at a stress deviation of 38.80 MPa (Figure 5(c)) are plotted.

Table 2 :
Schemes of creep test.
increase of temperature.e increase of temperature causes instantaneous elastic strain.e compressibility of shale is larger under constant stress, it takes a longer time to enter stable creep stage, and creep after 24 h is greater.

Table 3 :
Creep parameters of deep shale at pH 4 and T 30 °C.
1) Creep characteristics of shale closely associate with environmental factors, such as temperature, chemical corrosion, and stress di erence.e creep

Table 4 :
Creep strains of shale under di erent stress levels and temperatures (unit: %).