Mechanical properties of Jintan mine rock salt under complex stress paths

International Journal of Rock Mechanics & Mining Sciences 56 (2012) 54–61

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Technical Note

Mechanical properties of Jintan mine rock salt under complex stress paths Yintong Guo n, Chunhe Yang, Haijun Mao State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan 430071, China

a r t i c l e i n f o Article history: Received 29 September 2011 Received in revised form 22 April 2012 Accepted 24 July 2012 Available online 22 August 2012

1. Introduction Rock salt has the properties of low-permeability and creep, which can be useful for natural gas and oil storage. It has become a favored medium for waste disposal and oil and gas storage [1]. In the past few years, underground storage of natural gas has begun to be constructed in the middle and lower reaches of Yangtze River, Jintan city of China. The mechanical properties of rock salt have been researched by many researchers. The effects of cyclic loading on compressive strength, elasticity and timedependency were studied by Kittitep [2]. It was reported that axial strain–time curve compiled from loci of the maximum load of each cycle showed a time-dependent behavior similar to that of creep tests under static loading. Rock salt dilatancy boundary investigation based on combined acoustic emission and triaxial compression tests was carried out by Alkan [3]. It was shown that the dilatancy boundary depended on both stress loading rate and pore pressure. High pore pressure can accelerate the dilatancy. In laboratory, the mechanical properties of bedded rock salt under uniaxial and triaxial compression tests were studied by Liang [4]. It was found that with increasing s3 , the anhydrite–halite composite lithology deformation showed strain hardening and strong trend towards ductile behaviors as the halite bands tended to dominate. Also Liang [5] studied the effect of strain rate on the mechanical properties of rock salt, and found that the strength of rock salt was only slightly affected by loading strain rate, which is different from other brittle rocks. The confining pressure s3 and axial stress effect on the time-dependent stress–strain behaviors of rock salt were analyzed by Yang [6]. It was suggested to model the creep strain from transient to steady-state and to fit well with the creep strain. Hunsche [7] reported a large number of uniaxial and triaxial test results and analyzed the confining pressure effects on the creep strain.

n

Corresponding author. Tel./fax: þ 86 27 87198721. E-mail address: [email protected] (Y. Guo).

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There are many investigations on mechanical properties of rock salt. However, most of the reports are creep mechanical properties and basic strength properties, but there are few studies on complex stress paths (loading–unloading conditions). With an actual salt storage cavern, the process of injection and withdrawal of natural gas can be divided into four stages: gas injection, high pressure constant, gas withdrawal, and low pressure constant. The kinds of stages should be given full consideration: market survey, season regulation, and long-life. During the whole process of gas storage, the surrounding formation has remained in cyclic loading–unloading conditions. The range of cavern pressure is from 5 MPa to 14 MPa in Jintan. Furthermore, one cycle occurs with compressed air energy storage plant in rock salt in one day. We conducted uniaxial compression, triaxial compression, unloading confining pressure and cyclic loading tests on rock salt in the laboratory. The results can be applied to engineering stability analysis of underground storage of gas and gas injection-withdrawal process.

2. Rock salt specimens and methodology 2.1. Specimens The rock salt specimens tested were obtained from a salt mine in Jintan city, Jiangsu Province of China. They were drilled from depths ranging between 900 and 1100 m. The specimens have very high purity (NaCl, Na2SO4, and CaSO4) with a slight amount (less than 10%) of insolubles (glauberite, argillaceous, anhydrite). To keep the natural mechanical properties, the specimens were prepared by artificial dressing, for which ISRM suggested methods were referred [4]. Three specimens (L/D ¼2.0) were prepared for uniaxial compression test, six specimens (L/D ¼2.0) for triaxial compression test, five specimens (L/D¼ 2.0) for unloading confining pressure test, and four specimens (L/D ¼2.0) for cyclic loading test. Generally, 3–5 specimens are suggested to

Y. Guo et al. / International Journal of Rock Mechanics & Mining Sciences 56 (2012) 54–61

2.2. Experimental methodology The main purpose of this experiment is to study the effect on mechanical properties of rock salt different stress paths. The initial stress state of cavern surrounding formation is under three-directional stress. During the cavern operation, the regularities of stress distribution and variations are complex. According to the actual process of cavern, we designed three experimental methods. They are as follows:

30 YD-2

25 Axial Stress (MPa)

be prepared for each test to guarantee reliability [4]. However, the successful ratio of rock salt core drilling was very low and difficult to dress, so we used one specimen in this study.

55

YD-1

20 YD-3 15 10 5 0 1

0

4

2 3 Axial Strain (%)

5

Fig. 1. Stress–strain curve of rock salt under uniaxial compression.

2.2.1. Plan 1 Uniaxial and triaxial compression tests Uniaxial and triaxial compression tests were conducted to obtain basic parameters, which provided mechanical parameters for designing unloading s3 and cyclic loading tests are followed for the test procedure. ISRM suggested methods.

2.2.3. Plan 3 Cyclic loading fatigue test Using sinusoidal load for cyclic loading the specimens are loaded cyclically at 1 Hz. In cyclic loading test, the maximum stress level (the ratio of maximum stress to static uniaxial strength) is varied from 0.90 to 0.75 and the minimum stress level (the ratio of minimum stress to static uniaxial strength) is varied from 0.45 to 0.375. In order to distinguish the hysteretic circle of cyclic loading test, we define a hysteretic circle to be recorded when the axial strain cumulative reaches 0.01 mm.

3. Analysis of experimental results 3.1. Deformation properties of rock salt 3.1.1. Uniaxial and triaxial deformation properties Three uniaxial compression tests and five triaxial compression tests were performed by MTS-815 mechanical testing machine. The results are shown in Table 1. Fig. 1 shows the stress–strain curve of rock salt under uniaxial compression. The strength of Table 1 The test results of uniaxial and triaxial compression. Specimen Length (mm)

Diameter (mm)

Peak stress (MPa)

Elastic modulus (GPa)

Poisson’s ratio

Confining conditions

YD-1 YD-2 YD-3 C1 C2 C3 C4 C5

49.94 49.68 49.82 50.08 50.34 50.66 50.14 50.28

24.99 25.38 24.42 39.12 48.59 67.25 78.99 87.44

4.57 3.06 3.01 6.78 9.12 10.86 10.24 12.09

0.32 0.28 0.31 0.27 0.19 0.23 0.35 0.12

Uniaxial Uniaxial Uniaxial 2.5 MPa 5 MPa 10 MPa 15 MPa 20 MPa

99.48 101.24 100.72 99.70 100.32 99.80 101.10 100.62

20MPa

15MPa Deviatoric Stress (MPa)

2.2.2. Plan 2 Pre-peak unloading confining pressure tests Divided into three stages: (1) The hydrostatic pressure (s1 ¼ s2 ¼ s3 ) is exerted gradually with the stress rate of 0.05 MPa/s on the specimens, setting s3 of 2.5, 5, 10, 15, 20 MPa; (2) Keeping s3 constant, increases principal main stress s1 is gradually increased with the stress rate of 0.05 MPa/s, until the stress level on any point reaches to a value between uniaxial and triaxial compressive strengths; (3) Keeping principal main stress s1 constant, s3 is gradually decreased with the stress rate of 0.05 MPa/s, until the specimen is damaged [3].

80 20MPa

15MPa

60

10MPa

10MPa 5MPa 40

5MPa 2.5MPa

2.5MPa 20

0 -6

-4

-2

0 Strain (%)

2

4

6

Fig. 2. Stress–strain curve of rock salt under triaxial compression.

rock salt range is from 24.42 MPa to 25.38 MPa, and the average value is 24.93 MPa. The UCS of pure salt is usually reported to be in the range of 15  32 MPa by Hansen [8]. As shown in Fig. 1, the peak-strain to failure is more than 2%, and it has a stage of plastic deformation, which is different from other hard rocks. The elastic modulus range is from 3.01 GPa to 4.57 GPa; the Poisson’s ratio range is from 0.28 to 0.32. Fig. 2 shows the stress–strain curve of rock salt under triaxial compression. It can be shown that the peak strength increases from 39.12 MPa to 87.44 MPa, with increasing s3 from 2.5 MPa to 20 MPa. the peak strength has an obvious increase with s3 . That is because the frictional strength component increases with normal stress in all elastic rock or rocks, which display Mohr–Coulomb type strength [4]. Under the condition of s3 ¼2.5 MPa, the stress– strain curve has a peak strength turning point; other stress–strain curves demonstrate the properties of hardness and exhibit a visco-plastic deformation.

3.1.2. Deformation properties of unloading s3 During the process of excavation, the formation would experience unloading–loading proceeding; due to the variation of cavern pressure, the mechanical properties could be changed; and under the condition of loading and unloading, the behaviors of rock salt are essentially different [9]. Five unloading s3 tests were performed by MTS-815 mechanical testing machine. The initial s3 values are 2.5, 5, 10, 15, 20 MPa. Fig. 3 shows the stress–strain curve of rock salt under unloading s3 . It can be found that the values of slopes of axial and radial strain curves were small, and axial and radial strain increased rapidly during unloading s3 . An obvious radial expansion under is

Y. Guo et al. / International Journal of Rock Mechanics & Mining Sciences 56 (2012) 54–61

40

40

30

30

Deviatoric Stress (MPa)

Deviatoric Stress (MPa)

56

20

10

20

10

0 -1

-2

-3

0 0

1

2

-3

-2

-1

Strain (%)

0 Strain (%)

60

Deviatoric Stress (MPa)

Deviatoric Stress (MPa)

30 20

40 30 20

10

10

0 -1

3

50

40

-2

2

60

50

-3

1

0 1 Strain (%)

2

3

4

-4

-3

-2

0 -1 0 Strain (%)

1

2

3

60

Deviatoric Stress (MPa)

50 40 30 20 10 0 -4

-2

0

2

4

6

Strain (%) Fig. 3. Stress–strain curves of rock salt under unloading s3 . (a). s3 ¼2.5 MPa, (b). s3 ¼5.0 MPa, (c). s3 ¼10.0 MPa and (d). s3¼ 15.0 MPa.

shown the condition of maximum principal stress keeping s1 constant. The specimen shows plastic deformation and is different from brittle failure of hard rock under unloading s3 . Fig. 4 shows the s3 vs. strain curve under unloading s3 . The results are shown in Table 2. At the terminal of unloading s3 , the axial strains of majority are more than the radial strain. In the initial stage of unloading, the strain value has a linear relationship with s3 . Axial and radial strains increase sharply with decreasing s3 . It shows unrecoverable plastic deformation.

3.1.3. Deformation properties of cyclic loading During the past few years, considerable efforts have been made to study the rock response under cyclic loading. It was reported that the fatigue properties of rock material were dependent on the maximum stress, amplitude, loading waveform and frequency, etc. [10–13]. Cyclic loading often causes rock to fail at

a lower level than its determined compressive strength. Cyclic loading tests can be divided into three sections: the first stage, static loading, is the axial stress, which increases from zero to the top limit stress of cyclic loading; the second stage is cycle fatigue; and the third stage is fatigue failure. The tests were conducted with an axial displacement controlling loading system. The static strength of rock is crucial for cyclic loading tests. In order to provide reference and comparison for cyclic loading, we obtained the average uniaxial compressive strength of rock salt 24.98 MPa. In uniaxial cyclic loading test, the maximum stress level (the ratio of maximum stress to static strength) is varied from 0.90 to 0.75 and the minimum stress level is varied from 0.45 to 0.375 (the ratio of minimum stress to static strength). The specimens show fatigue damage when the upper and lower limit stress is the same as those of the other brittle rock. The hysteretic circle is too closed to distinguish. In order to

3

6

2.5

5

2

4

σ3 (MPa)

σ3 (MPa)

Y. Guo et al. / International Journal of Rock Mechanics & Mining Sciences 56 (2012) 54–61

1.5 1

3 2

0.5

1

0 -0.6

-0.4

-0.2

57

0 0 0.2 Strain (%)

0.4

-3

0.8

0.6

-2

-1

0 Strain (%)

1

2

3

16

12 10

14 σ3 (MPa)

σ3 (MPa)

8 6 4

12

10 2 8

0 -3

-2

-1

0 1 Strain (%)

2

3

4

-6

-4

-2

0 Strain (%)

2

4

6

25

σ3 (MPa)

20 15 10 5

-4

-2

0

0

2

4

6

Strain (%) Fig. 4. s3 -strain curves under unloading s3 . (a) s3 ¼2.5 MPa, (b) s3 ¼5.0 MPa, (c) s3 ¼10.0 MPa, (d) s3 ¼15.0 MPa and (e) s3 ¼ 20.0 MPa.

Table 2 Test results of rock salt under unloading s3 . Specimen Begin point of unloading s3 (MPa)

End point of unloading s3 (MPa)

Begin of axial strain (%)

Terminal of axial strain (%)

Begin of radial strain (%)

Terminal of radial strain (%)

U1 U2 U3 U4 U5

0.15 0.70 3.40 10.70 7.26

0.31 0.49 0.99 3.71 1.30

0.67 1.96 2.90 5.40 4.03

 0.13  0.22  0.48  1.87  0.49

 0.50  2.32  2.33  2.90  3.03

2.5 5 10 15 20

analyze the evolution laws of hysteretic circle under cyclic loading, we choose one curve of specimen YDX-04 for an example. Fig. 5 shows the stress–strain curve of specimen YDX-04 under

uniaxial cyclic loading. Fig. 6 shows the the magnified stress– strain curve of specimen YDX-04 under uniaxial cyclic loading, each stage with ten hysteretic circles. Figs. 5 and 6, and Table 2, show that the whole cyclic loading process can be divided into three stages: initial deformation, constant-velocity deformation, and accelerated deformation. In the first stage, axial strain increases greatly and there is has a large accumulation of strain. In the second stage, axial strain increases stably and strain accumulating rate is slow, therefore, it has occupied an absolute stage of fatigue life. In the third stage, axial strain increases rapidly and the specimen reaches fatigue damage. The accumulated deformation of the three stages leads to fatigue damage. The original damage and the damage originated under the first static loading are neglected. At the initial stage of cyclic loading, the hysteretic circle is sparse, and then it becomes more and more close, finally, the hysteretic circle changes to sparse. It presents a stage of sparseness–closeness–sparsness. It can be found that

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Y. Guo et al. / International Journal of Rock Mechanics & Mining Sciences 56 (2012) 54–61

Axial Stress (MPa)

25 20 15 10 5 0

0

1

2 3 Axial Strain (%)

4

5

Fig. 5. Stress–strain curve of specimen YDX-04 under uniaxial cyclic loading.

Axial Stress (MPa)

25 20 15 10 5 0 1.2

1.3

1.4 Axial Strain (%)

1.5

1.6

Axial Stress (MPa)

25 20 15 10 5 0 2.5

2.55

2.6

2.65

Axial Strain (%)

Axial Stress (MPa)

25 20 15 10

during cyclic loading, the damage evolves from zero to unit. Therefore, this method is effective and useful only in terms of describing the relative evolution process. Changing the upper limit and mean stress would impair fatigue proceeding significantly. Initial axial strain (the axial strain before cyclic loading) and the ratio of cyclic axial strain (the average strain for each cycling) would be increased with the increase upper limit and mean stress. The necessity of cyclic accumulated deformation for fatigue damage would be reduced, which impairs fatigue proceeding significantly and cuts down the cyclic life, especially for the total cyclic numbers. We should avoid the project proceeding under upper limit stress condition. The whole fatigue process and fatigue life decrease with the increase of the maximum stress level increasing. When the applied maximum stress keeps a lower level, the second segment becomes flatter; the specimen has a longer fatigue life. Furthermore, when the ratio of upper limit stress is 0.75, the specimen has no obvious surface damage after 14789 cycles; it shows few effects on mechanical behaviors. When the applied stress level in cyclic loading does not cause rock failure during a very large number of cycles, the specimen becomes ‘strain hardened’, in turn increasing the uniaxial compressive strength [10]. It can be suggested that the ‘threshold value’ of fatigue fell between 75 and 80% of uniaxial strength. The axial strain reduces and part of that is recovered, which can be considered as elastic deformation, others recovered can be considered as irreversibility deformation. It contains plastic deformation and damage irreversibility deformation during unloading stage. The accumulation of irreversibility deformation is along with the cyclic loading until the specimen damage. The accumulation of axial strain can reach 2–3%. However, it is about 0.5% for brittle and hard rock. 3.2. Contrast of the failure characteristics of uniaxial compression to cyclic loading There is a distinction between fatigue damage and static uniaxial compression deformation. However, there is some relationship between them [14,15]. Fig. 7 shows that the fatigue failure curves of rock salt are controlled by complete stress–strain curves. It is shown that there are few differences of terminal axial strain between cyclic loading and uniaxial compression. The controlled axial strain error of rock salt can reach up to 10%. However, the strain error can be controlled no more than 1% for sandstone, granite and marble [16]. Creep deformation would be accumulated under long time uniaxial compression. Fig. 8 shows the specimens before and after uniaxial compression test. Figs. 1 and 8 show that as the damage strain reaches 2%, the specimen has more than one fracture plane under uniaxial compression. However, the fracture is along axis direction to be distinguished clearly. It would still have load capacities. Fig. 9 shows the specimens before and after cyclic loading test. It can be shown that the specimen has an instantaneous collapse under cyclic loading. The main part of the specimen falls as a powder on the test platform. Only a small part of residue can be collected. The internal fracture is in full development and causes instantaneous energy release, leading to falling around.

5

3.3. Contrast of unloading s3 to triaxial compression test 0 3.8

3.9

4 Axial Strain (%)

4.1

4.2

Fig. 6. Magnified stress–strain curves of specimen YDX-04 under uniaxial cyclic loading. (a) Initial cyclic loading stage. (b) Constant-velocity cyclic loading stage. (c) The accelerated cyclic loading stage.

The results of unloading s3 and triaxial compression are in Tables 3 and 4. Fig. 10 shows unloading s3 and triaxial compression–strain curve. Under the condition of the same maximum principal stress s1 , when unloading s3 is the same as triaxial compression, the axial and radial strain variations are

Y. Guo et al. / International Journal of Rock Mechanics & Mining Sciences 56 (2012) 54–61

Table 3 Test results of rock salt under uniaxial cyclic loading.

30 25 Axial Stress (MPa)

59

20 15 10 5

Specimen Upper limit stress (MPa)

Lower limit stress (MPa)

Initial axial strain (%)

Numbers Failure Axial of cycling conditions strain of failure (%)

YDX-02 YDX-03 YDX-04 YDX-05

11.25 10.63 10.00 9.38

1.77 1.56 1.29 0.91

3.97 3.99 4.09 3.89

22.50 21.25 20.00 18.75

1041 2167 6449 14789

Failure Failure Failure Unfailure

0

Axial Stress (MPa)

0

1

2 3 Axial Strain (%)

4

5

Table 4 Triaxial unloading and compression test results.

30

Specimen Correspondence pressure

25 U2 U3 U4 U5 U5 C1 C2 C3 C3 C4

20 15 10 5 0 0

1

2 3 Axial Strain (%)

4

5

Correspondence axial strain

Correspondence radial strain

(MPa)

Maximum principal stress s1 (MPa)

(%)

(%)

2.5 5 10 10 15 2.5 5 10 10 15

38.35 57.29 62.51 59.04 59.04 38.35 57.29 62.51 59.04 59.04

0.94 2.26 5.40 2.84 1.84 0.71 1.93 2.20 2.48 1.17

 0.70  1.64  2.90  1.75  0.90  0.42  1.20  1.46  1.65  0.64

Note: Correspondence pressure is triaxial unloading from a higher to a lower confining pressure same as the compression confining pressure strain.

Fig. 7. Fatigue failure curves of rock salt controlled by complete stress–strain curves. (a) YDX-2, (b) YDX-4.

16 14 12 σ3 (MPa)

10 8 Unloading Unloading Traixial Traixial

6 4 2

Before test

After test

0 -2

-4

0

2

4

6

Strain (%) Fig. 8. The specimens before and after uniaxial compression test. Fig. 10. Unloading s3 and triaxial compression–strain curve.

proceeding. Under the condition of unloading s3 , with s3 decreasing gradually, cohesion decreases and friction angle plays a leading role in the damage proceeding [5]. 3.4. Fatigue accumulative damage law

Before test

After test

Fig. 9. Specimens before and after cyclic loading test.

larger than those of the loading conditions. Under the same initial stress condition, the changed magnitude of stress required for rock failure is comparably less than that of the triaxial compression. The damage is because of increasing axial stress in the state of triaxial compression until it is up to loading capacity. On the macroscopic view, cohesion plays a leading role in the damage

It has influence on the strength of rock materials under loading conditions. Considering damage mechanics, the concept of continuum damage mechanics is proposed by Lemaitre [17]. Based on fundamental principle of hypothesis strain equivalence, The onedimensional damage problem is studied, and the damage constitutive equation is

s ¼ Eð1DÞe

ð1Þ

where s is the actual stress of undamaged material, e is the actual strain of undamaged material, E is the actual modulus of undamaged material, and D is the damage variable, such that when D ¼ 0 the material is undamaged, and when D ¼ 1 the material is completely deteriorated. The damage constitutive equation of

60

Y. Guo et al. / International Journal of Rock Mechanics & Mining Sciences 56 (2012) 54–61

uniaxial compression can be written as ð2Þ

Without consideration of a single cyclic damage, assuming that s is a constant, calculating the derivative of e, it can be written as

s dD ¼ 2 de Ee

ð3Þ

Assuming e0 is the initial axial strain before cyclic loading, so D ¼ 0. ed is the end axial strain after cyclic loading, so D ¼ 1. The solutions are obtained by the integral to formula (3) Z Z D s e1 dD ¼ de ð4Þ E e0 e2 0 D¼



ð6Þ

when D ¼ 1, and e ¼ ed in view of Eq. (5) it follows that

s

1 ¼ 1 1 E e0  ed

ð7Þ

so

ee0 ed ed e0 e

ð8Þ

According to the defined damage variation, we get the variation between damage variation and cyclic numbers. Fig. 11 shows the curves of damage variation–cyclic numbers. It can be seen that damage can be divided into three stages. In the Initial cyclic stage, damage variation increases obviously with the increasing of

26 24 22 0

300

0

2000

600 Cyclic Numbers

900

1200

29

26

23

20 4000

6000

8000

Cyclic Numbers Fig. 12. The curves of deformation modulus–cyclic numbers. (a) YDX-02 and (b) YDX-04.

cyclic numbers, and it accounts for 50% of the total damage variation; in the second stage, damage variation increases slowly with the increasing of cyclic numbers and it accounts for most of the total cyclic numbers. However, the damage variation only accounts for 30% of total damage variation; and in the third stage, damage variation has an obvious increase until fatigue damage.

2.5 2 Axial Strain (%)

28

32

ð5Þ

C2 ¼ 0

1.5

3.5. Relationship between deformation modulus and cyclic numbers 1 0.5 0 0

200

400

600 800 Cyclic Numbers

1000

1200

3 2.5 Axial Strain (%)

30

20



s 1 1  þ C2 E e0 e

when D ¼ 0, and e ¼ e0 in view of Eq. (4) it follows that

D¼

Deformation Modulus (GPa)

32

s Ee

Deformation Modulus (GPa)

1D ¼

2 1.5 1

Fig. 12 shows that the curves of deformation modulus–cyclic numbers for YDX-02 and YDX-04. The results of deformation modulus and cyclic numbers are in Table 5. The deformation modulus is the instantaneous modulus instead of the static elastic modulus during the cyclic loading. As illustrated in Fig. 12 and Table 5, it can be seen that deformation modulus also can be divided into three stages. In the First stage, deformation modulus increases with the increasing of cyclic numbers, because of the closed pore and crackunder loading condition; in the second stage, deformation modulus decreases within small amplitude and it accounts for most of the total cyclic life; and in the third stage, only for a few cyclic numbers, it is sharply reduced.

4. Discussion

0.5 0 0

2000

4000 Cyclic Numbers

6000

8000

Fig. 11. The curves of damage variation–cyclic numbers. (a) YDX-02, (b) YDX-04.

Underground storage of natural gas has been carried out for only a few years in China. Rock salt is a particular soft rock type with the characteristics of low permeability and strong rheology. The rock salt evaporates saline lake sediments of Tertiary age. The mechanical properties are different from those of the salt domes of American and Canada. In the past few years, most researchers

Y. Guo et al. / International Journal of Rock Mechanics & Mining Sciences 56 (2012) 54–61

Table 5 The test results of deformation modulus and cyclic numbers. Specimen

Deformation modulus (GPa) and cyclic numbers

YDX-02

1 26.68 500 26.18 1 25.81 800 31.04 1 24.67 2500 27.39 1 21.76 1000 27.63

YDX-03

YDX-04

YDX-05

3 27.44 700 25.55 3 26.76 1600 29.27 3 27.12 4500 27.78 2 21.42 3000 26.76

5 27.59 900 25.13 5 27.17 2000 29.37 5 27.75 6200 25.98 3 21.73 5000 28

10 28.33 1000 24.46 10 28.64 2080 29.31 15 26.35 6400 23.27 5 21.75 9000 27.01

50 29.59 1035 23.05 50 29.58 2165 28.37 50 25.01 6445 23.41 10 23.09 11 000 27.87

100 29.42 1039 22.81 200 30.62 2166 21.45 100 25.99 6448 22.42 50 25.01 13 000 26.54

300 27.37 1041 21.45 400 30.59 2167 20.46 800 26.16 6449 21.48 100 25.83 14 789 26.43

focused on creep and damage properties under different stress and temperatures, for the study on nuclear waste disposal and gas storage. Depending on the types of storage products, the injection-withdrawal durations range is from daily to season, and the minimum and maximum cavern pressures can be as low as 40% and as high as 90% of the in situ stress. The cavern pressure is cyclic for daily with compressed air energy storage plant in rock salt. Due to the complexity of cavern stress states, we considered and designed this experiment. From our research results, it is confirmed that the mechanical properties of rock salt under triaxial compression loading is different from those under unloading s3 . In the process of dissolving the mining cavern, we should be attaching importance to the properties. The strength of the surrounding formation for gas storage will change during injection and withdrawal periods. We suggest that the minimum gas pressure is not too low and the maximum gas pressure is not too high, in order to maintain the cavern stability. The cavern should try to be in less cycling.

5. Conclusions The results of this experimental study on complex stress paths of rock salt are presented. The main conclusions of the studies are as follows: 1) Rock salt has good deformation properties under triaxial compression condition; it has no obvious destruction surface under high s3 , no longer shearing damage. 2) Under the condition of cyclic loading, axial strain can be divided into three stages: initial deformation, constant-velocity deformation and accelerate deformation. The accumulation strain of three stages leads to specimen damage. 3) Under the conditions of the same maximum principal stress, when unloading s3 is the same as triaxial compression pressure, the axial and radial strain variations are larger than those of loading conditions; the changing magnitude of stress required for rock failure is comparably less than that of triaxial compression. 4) Changing the upper limit and mean stress would impair fatigue proceedings significantly. Initial axial strain and the ratio of cyclic axial strain would be increased with the upper limit and

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increasing mean stress. The whole fatigue proceeding and fatigue life decreased with the maximum stress level decreasing. 5) The failure modes of uniaxial compression and cyclic loading are different. Under uniaxial compression, the specimen has more than one fracture plane, the fracture along axis direction many be distinguished clearly and has loading capacity. Under cyclic loading, the specimen has an instantaneous collapse. The main part of the specimens falls as powder on test the platform. The internal fractures of specimens are full development and there is instantaneous energy release, leading to falling around. Within the range of our experimental results, the strength states around the caverns during gas withdrawal-injection can be shown. All of these conclusions are of great importance for gas storage.

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