Deformation Mechanism Of A Layered Creeping Slope

Paper 3B 10 — SINOROCK2004 Symposium Int. J. Rock Mech. Min. Sci. Vol. 41, No. 3, CD-ROM, © 2004 Elsevier Ltd.

THE DEFORMATION MECHANISM OF A LAYERED CREEPING COAL MINE SLOPE AND THE ASSOCIATED STABILITY ASSESSMENTS T.H. Yang¹, T. Xu1, R.Q. Rui 2, C.A. Tang¹ ¹) Center for Rock Instability and Seismicity Research, Northeastern University, Shenyang, 110004, P.R.China [email protected] 2 )School of Highway Engineering, Changsha University of Science & Technology, Changsha, 410076, P.R.China

Abstract: Fuxin Haizhou open pit coal mine slope , located in Fuxin city, P.R. China, is one of the most noted large-scale open pit coal mine slopes with problems related to the safety stability. Characterization of the structural geometry of the slope creep deformation and reconstruction of its development history are believed to be pivotal in understanding what has happened and what will happen to the slope. Based on lots of creep tests combined with thorough field investigation on weak layers, a creep model of weak layer was thus proposed and the creep deformation and failure mechanism of bedding creep slope was also discussed. According to time dependent long term strength, Fuxin Haizhou open-pit coal mine slope is exemplified to investigate dynamic stability of creep slope under different conditions and cases, and an effective assessment and prediction method for assessing and predicting the tertiary creep or instability of this kind slope was also proposed. The analytical results for predicting the creep slope displacement are well tallied with the field data. Keywords: Bedding creep slope, Creep test, Stability assessments, Creep limit strain , Slope failure, Tertiary creep

1. INTRODUCTION Slope failure is a common natural disaster in rocky regions caused by mining and the most dangerous type of sliding due to the quick movement of rock mass. For the stability analysis of high slopes, knowledge of the rheological mechanical properties of large-scale engineering rock mass is important. This issue is most essential if these slopes are deformed by deep creep with an imminent danger of a transition to rapid sliding. Obviously a better understanding of the nature of the slope is of critical importance in determining the engineering design and safety of large-scale rock mass structures. Although investigations have recently been carried out on some slopes (Lam & Fredlund, 1993, Hencher, Liao, et al., 1996; Stead & Eberhardt, 1997; and Deng, Zhu, et al., 2000), but the deformation and failure mechanisms of bedding creep slope is still not fully understood as of today. The Haizhou open-pit coal mine is located in the region of the western part of Fushun city, Liaoning province, P.R.China. The northeastern monoclinic bedding slope of the Haizhou open-pit coal mine is typically controlled

by weak layer. The slope began to creep in 1987. Mining activities in the open-pit stopped in 1993, and the slope steadily crept at the speed of 0.5-0.64 mm/d and slope deformation tended to accelerate. Some measures were taken to stabilize the slope since 1996 and the stability of the slope was slightly improved and strengthened. In the present study, the creep deformation characteristics of the bedding creep slope at Haizhou open-pit coal mine was investigated to assess the slope stability, address both the consequence of slope failure and the hazard or probability of failure and better understand the creep failure mechanism of slope.

2. SITE INVESTIGATIONS 2.1 Geology in the area The northern slope in the Haizhou open pit coal mine consists of gently inclined bedding strata with a dip of 18°~20°in the direction SE. The slope is cut off by fault FE1 and can be sub-divided into three geological rock mass groups, as shown in Fig. 1: (1) the footwall of the fault is in laminative

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Paper 3B 10 — SINOROCK2004 Symposium Int. J. Rock Mech. Min. Sci. Vol. 41, No. 3, CD-ROM, © 2004 Elsevier Ltd.

structure, consisting of sandy shale, sandstone and carbonaceous shale; (2) the hanging wall of the fault is in bedding cataclastic structure, consisting of coal, coarse sandstone and gravel; (3) the weak intercalations of carbonaceous shale and siltstone. There are mainly four weak layers in the footwall of the fault, i.e., no.7, 8, 9 and the upper no.9. The upper no.9 layer is the weakest in the physicomechanical properties among these weak layers.

Figure 1. Cross-Section of Slope E24 in Haizhou Open-pit Coal Mine. Legends: 1-Monitoring point, 2-Pore water pressure hole and pressure head, 3-Borehole, 4-The upper no.9 weak layer, 5-Mining area, 6-Surface fissures,7Reconnaissance adit, 8-Horizontal drainage hole, 9Fault FE1 , 10-Quaternary alluvium, 11-Phreatic water, 12-Bedrock fissured aquifers, 13- Bedrock confined fissured aquifers, 14- Bedrock fissured aquifers in fault FE1 , 15-Confined aquifers in fault FE1 , 16Pressure water head before drainage, 17-Pressure water head after adit driving, 18-Evaluated water table after drainage.

2.2 Geohydrology There are mainly three types of aquifers in this mining area. (1) Quaternary alluvium fissured aquifer The aquifer, consisting of coarse sandstone and gravel, has a depth of 2-8m and is supplied with phreatic water and rainfalls. (2) Jurassic bedrock fissured aquifer This aquifer is a Jurassic bedrock fissured aquifer, and the weak layers in the aquifer can more or less act as aquicludes. Especially, the upper no.9 weak layer is mainly composed of gouges and act as aquifuge in the slope. The bedrock aquifer in the fault FE1 can be subdivided into three aquifers: II-1 aquifer located above the upper no.9 weak layer in the footwall of fault FE1 has lower pressure due to its good drainage to the slope surface. II-2 aquifer located below the upper

no. 9 weak layer in the footwall of fault FE1 in the state of confined pressure due to its weak drainage to slope as well as rich water supply from the alluvium. The monitoring results show that the II2 aquifer has a higher pressure head when the adit E24 cut through the upper no.9 layer. II-3 aquifer located in the hanging wall of fault FE1 has a lower pressure due to its good drainage along the slope surface. (3) FE1 cataclastic gouge fissured aquifer The dip and the fall height of normal fault FE1 are about 70°, 50m, respectively. The width of the cataclastic gouge in normal fault FE1 is 5~10m, which makes the aquifers almost impermeable; meanwhile, the aquifers can also obtain water supply from II-2 aquifer. Therefore, the aquifers in the fault FE1 have a higher water pressure. VWP-52611900 water piezometers were employed to monitor the water pressure in the borehole except for the no.2 collapsed borehole. The obtained monitoring results of water pressure in the boreholes were shown in Table 1. Table 1 Monitoring results of ground water pressure in the borehole of slope No. Level,m Depth,m Aquifers Water pressure, MPa Pressure head, m

1 +189 34.4 II-2

3 +149 29.4 II-1

4 +97 23.0 III

5 +82 27.5 III

6 +44 32.0 II-1

7 +115 29.4 II-2

0.00

0.04

0.24

0.26

0.16

0.36

0

123

98

80

28

121

The water pressure at borehole 7 markedly decreased by 5 meters and slightly rose several days later when reconnaissance adit E24 cut through the no.9 upper layer. The main possible reason for this is that the influence field of drainage adit on the decrease of pressure is limited. Some horizontal drainage holes at the interval of 50 meters at the∇-25 level were set up to ensure the pressure water head of II-2 aquifer fall close to the no.9 upper layer. The distribution of the water pressure in the slope and the obtained results were also shown in Fig. 1.

3. DEFORMATION CHARACTERISTICS Since on March, 1987, the remarkable creep deformation of the upper and footwall of the fault FE1 along the fault plane took place among the

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Paper 3B 10 — SINOROCK2004 Symposium Int. J. Rock Mech. Min. Sci. Vol. 41, No. 3, CD-ROM, © 2004 Elsevier Ltd.

mining zone E22.5~E25. The hanging wall of the fault FE1 moved upward along the fault plane and the footwall moved downward. Correspondingly, tensile fissures coalesced and formed in the northern ground surface, endangering the safety of the six railways at the 38~ 85 and 86~ 102 levels.

the pushing forces of the footwall of fault induced by the mining activities in the bolted rock mass after 1987. The displacement rates of the monitoring holes are almost equal except for the monitoring hole 07, which shows that the deformation of the slope in this area is in the steady-state or secondary creep state. In addition, the monitoring results about the ground surface displacements show that the tension and uplift deformation of the hanging wall of the fault FE1 took place. The average displacement rates of monitoring holes are presented in Table 2. It can also be seen from the monitoring device underground that a shear-slip plane forms along the upper no.9 weak layer, which is well tallied with the calculated and analyzed results from the data obtained.

Figure 2. Ground

surface and underground displacement curves of slope E24.

Legends: 1, Horizontal displacement from 1989 to 1990; 2, Horizontal displacement from 1991 to 1993; 3, Vertical displacement from 1989 to 1990; 4, Vertical displacement from 1991 to 1993; 5, Displacement rate from 1989 to 1990; 2, Displacement rate from 1991 to 1993.

Table 2 Monitoring results of ground surface displacement of slope E24 No.

Date

uh

u& h

mm

mm/d

Date

uh

u& h

mm

mm/d

03

89.4-

231

0.48

91.8-

154.6

0.46

04

90.8

201

0.42

93.1

159.4

0.48

05

194

0.40

170.0

0.51

06

153

0.32

175.4

0.53

07

329

0.68

337.9

1.01

08

171

0.35

197.0

0.59

Notes: u h stands for horizontal displacement and u& h for horizontal displacement rate. As can be seen from the monitoring line E24 in the Fig. 2, the horizontal displacements tended to the mining gob. The “Five-Seven” mining zone, which lies near the hanging wall of the fault, is 57 meters from the fault along the cross-section E24. The mining coal seam is located in the bolted rock mass at the southern of the fault FE1 and the roof of coal seam is only 15~25 meters from the ground surface. The strata movement was activated under

4. SLOPE FAILURE MECHANISM 4.1 Mechanical sub-division of slope The slope can be subdivided into three mechanical basic elements (as shown in Fig. 3). (1)Bedding creep elements along the upper no. 9 weak layer in the footwall of the fault FE1 . Its deformation and failure is mainly governed by the physical and mechanical properties of the upper no. 9 weak layer. Since the upper no. 9 weak layer is rich in water content and in plastic and rheological state, the footwall of the fault FE1 creeps along the upper no. 9 weak layer. (2) Crush and uplift elements in the crushed and cataclastic zone. The dip of the fault FE1 is about 60~70°. The cataclastic gouge in the fault plane is rich in water and is characterized by the plastic flow, thus the uplift of slope will occur when the yield strength of the gouge is reached under the loading of the creep elements. (3) Bolted elements in the slope. The hanging wall of fault has a role in supporting and anchoring the slope in this area. The bolted rock mass will fail and landslide of the slope will take place once the pushing force of creep elements in the footwall of fault is bigger than the resistance force of the bolted rock mass.

4.2 Analysis of progressive failure mechanism A higher confined water pressure in II-2 aquifer acting on the footwall of the fault and the floor of the upper no. 9 weak layer deteriorates the uplift of the hanging wall of the fault and makes the hanging wall and the lower bolted slate rock mass

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Paper 3B 10 — SINOROCK2004 Symposium Int. J. Rock Mech. Min. Sci. Vol. 41, No. 3, CD-ROM, © 2004 Elsevier Ltd.

in the slope creep, and leads to the progressive failure of the whole slope in the end. The strength of weak layer decreases with the creep deformation time and the residual pushing force acting on bolted rock mass increases with the creep time. As a result, the development of shear failure of the bolted rock mass will progressively continue with time. The angle between shear faulting plane of bolted rock mass and the vertical direction under the pushing force of the creeping rock mass is (45°- /2). is the internal friction angle of the rock mass. The formation of the failure plane firstly begins at the intersecting point between the fault and the upper no.9 weak layer and gradually propagates toward the bolted rock mass. The propagation of failure plane arrests when the residual pushing force is less than the shear strength of the bolted rock mass. Otherwise, the propagation of failure plane will continuously develop and lead to the progressive failure of bolted rock mass.

Figure 3. Schematic for slice stability analysis of slope E24. Lengends: 1, anchorage part; 2, crush and uplift zone; 3, creeping part; 4, shear deformation; 5,-uplift deformation; 6, horst deformation; 7, slip deformation; 8, slice index; 9, drainage adit; 10water head before drainage; 11-water head after adit driving; 12- Evaluated water table after drainage.

5. CREEP TEST AND MODEL OF WEAK LAYER Weak intercalated layers in the slope are characterized by controlling the deformation and failure of slope. A series of creep tests on the carbonaceous siltstone have been conducted. On the basis of fatigue theory (Fan, 1993) and the creep testing results, the creep equation of the weak layers is established as follows (Rui, 1998) :

τ=

A0 σ ⋅ γ m (1 + m ) = c + σ m ⋅ tgΦ α (1 + δ ⋅ t ) H (1)

where τ is shear stress acting on the weak layer, MPa; A0 is transient shear modulus, also the intercept at the τ coordinate in the σ - τ coordinates, MPa; t-shear duration, d; γ is shear strain, %; σm is normal stress, MPa; H is the tensile strength, also the intercept at the σ coordinate in the σ - τ coordinates, MPa; m is the parameter of stain hardening; δ , α are both testing constant, c is the cohesion, MPa; Φ is the internal friction angle. Based on the creep testing results, the parameters of equation (1) can respectively be obtained: A0=9.66MPa, H=25kPa, m=0.385, δ =0.417, α =0.184. Testing results show that rock and soil only experience the initial creep and attenuated creep when the condition τ0 < τ < τ∞ is met and initial creep, steady-state creep, tertiary creep and failure when the condition τ > τ∞ is met. τ0 is transient shear strength and τ∞ is the long-term shear strength. With the increase of shear stress τ , steady-state creep and tertiary creep of rock and soil will tend to easily occur. In the present creep tests, the strain transiting from steady-state creep to tertiary creep is defined as creep limit strain. The creep tests on the upper no.9 weak layer show that the creep limit strains under different normal stress levels are approximately equal. Testing results of creep limit strains of the upper no.9 weak layer are shown in Table 3. Table.3 Results of creep limit strain of the upper no.9 weak layer

σm kPa 50 100 150 200 Average

Creep limit displacement

Substitution of equation (2)

10-2mm 136.1 160.3 179.0 162.6 159.5

Creep limit strain

γF

0.02126 0.0250 0.02797 0.0250 0.02481

into equation (1) leads to

A0 γ Fm α 1+δ ⋅t A0 tgφ = γ Fm α H (1 + δ ⋅ t ) c=

(2)

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Paper 3B 10 — SINOROCK2004 Symposium Int. J. Rock Mech. Min. Sci. Vol. 41, No. 3, CD-ROM, © 2004 Elsevier Ltd.

This equation can be called as relation equation of shear strength with respect to time.

some measures based on the dynamic stability mechanism of creep slope.

6. STABILITY ASSESSMENTS OF CREEP SLOPE

7. PREDICTION OF TERTIARY CREEP The strain rate transiting from secondary creep to tertiary creep of slope is named as creep limit strain, denoted asγ& . It is a benchmark for judging the transition from secondary or steady-state creep to tertiary or accelerated creep and is also a criterion for assessing the stability of slope in the long term. The creep tests for the upper no. 9 weak layer show that the creep limit strain γ& is linear to

The relation equation (2) of shear strength with respect to time shows that the direct shear strength of the upper no. 9 weak layer is time-dependent. Meanwhile, the stability of slope was also analyzed using Sarma limit equilibrium methods (Yang & Rui, 2001) and the obtained results are shown in Table 4. The final slope was formed in 1991. The shear strength of the upper no.9 weak layer and the coefficient of stability both gradually decrease with time. Water pressure becomes the dominant controlling factor influencing the stress state in the slope under these conditions. Table 4 presents the analyzed results of slope stability. The coefficient of the slope stability in 1993 and 1997 are 1.171 and 1.000, respectively. The slope in 1997 is in the state of limit equilibrium. The testing tunnels excavated in 1993 had drained the local underground water in the slope and the coefficient of the slope stability reached to 1.05, but the designing standards on the slope stability was still not meet. Further drainage and de-pressure measures were taken to insure the level of water pressure below the upper no.9 weak layer (see Fig. 1) and the stability of slope was remarkably improved after 1997. Theoretical analysis and testing results show that the deformation and failure of creep slope can be controlled by taking

the value of

τ − τ∞ τ 0 − τ∞

at the different of shear

stress (equation 3), as shown in Table 5.

γ& = 144

τ − τ∞ × 10 − 6 τ 0 − τ∞

(3)

Thus, the limit displacement velocity of the bedding strata can be obtained from the length of the sliding part of the slope:

V = γ& × L

(4)

Based on the coefficient of slope stability: F 0 = τ 0 / τ; F∞ = τ∞ / τ , the following equation can be got:

γ& = 144

1 − F∞ × 10 −6 F 0 − F∞

(5)

where, F 0 is the coefficient of stability from

Table 4 Results of creeping shear tests in the upper weak layer of No.9 Time t(yr) 0.006 0.1 0.4 1 c (kPa) 16.42 4.84 3.37 2.69 Shear index 16.0 12.69 12.42 12.25 φ(°) Water pressure in 1.171 1.060 1.028 1.019 1993 Coefficient of Water pressure stability 1.312 1.120 1.086 1.070 after tunneling Further drainage 1.366 1.172 1.137 1.122

2 2.28 11.99

7 1.67 11.92

10 1.53 11.82

20 1.27 11.68

1.006

0.992

0.988

0.983

1.060

1.044

1.040

1.035

1.112

1.095

1.092

1.087

94

110

Table 5 Creeping limit strain rate of the upper weak layer of No.9 Limit strain rate, γ& (×10-6 d-1)

τ − τ∞ τ 0 − τ∞

15

16

31

32

63

78

0.31

0.07

0.29

0.38

0.29

0.78 0.80

126

141

188

0.68 0.66 0.99

1.03

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transient shear strength; F∞ is the coefficient of stability from long-term shear strength. According to the data of testing tunnel in 1993, the average coefficients of stability F 0 and F∞ at E24 zone are 1.171 and 0.972, respectively. Substitution of F 0 and F∞ into equation (5) leads to γ& =144×0.14×10-6/d. The theoretical average controlling length at E24 zone is 30.2m and the average displacement rate is 0.609mm/d. Compared with the obtained field data, i.e., average displacement rate 0.513mm/d, we can find that the theoretical result agrees well with the obtained field data. It is further shown that the tertiary creep of the slope would have occurred after four years’ steady-state creep (1989-1993) if no effective measures were taken for the slope. For other similar conditions or cases, such as further drainage and de-pressure, based on the theoretically obtained creep limit displacement rate of the slope and the field measured data, we can assess and predict the tertiary creep of the slope in the long term.

8. CONCLUDING REMARKS In this paper, Fuxin haizhou open-pit coal mine slope is exemplified to investigate the deformation and failure mechanism of creep slope and associated stability assessments. On the basis of the theoretical analysis of slope deformation and creep tests of the weak layer, assessment method of slope stability and prediction of the tertiary creep were proposed. Moreover, the theoretical analysis is well tallied with the field test findings. Some preliminary conclusions were drawn as follows: 1 The slope has experienced creeping and sliding along the upper no. 9 weak layer and caused tensile deformation of the ground surface. Bedding rock mass slipped and crushed the fault, which led to the uplift of the bolted rock mass in the hanging wall of the fault FE1 . Meanwhile, the higher confined water pressure in II-2 aquifer acting on the floor of the upper no.9 weak layer deteriorated the progressive failure of the bolted rock mass in the hanging wall of the fault FE1 . 2 Based on the creep model of weak layer, creep limit strain at the point of tertiary creep was determined and the time-dependent long-term shear strength equation was established to assess the

dynamic stability of slope under different conditions. 3 Creep limit strain rate equation at tertiary creep of slope with various stability states was established and an effective assessment and prediction method for the occurrence of tertiary creep of slope was also proposed based on the field measured data about displacement rates of slope.

9. ACKNOWLEDGEMENTS The study presented in this paper is jointly supported by the National Natural Science Foundation of China (No. 50204003, 50134040 and 50174013).

10. REFERENCES Deng Q.L., Zhu Z.Y., Cui Z.Q., et al. 2000. Mass rock creep and landsliding on the Huangtupo slope in the reservoir area of the Three Gorges Project, Yangtze River, China. Engineering Geology 58: pp.67–83. Fan, G.Q. 1993. Rheologic Mechanical of Rock and Soil (In Chinese). Beijing: China Coal Industry Press. Hencher, S.R., Liao, Q.H. & Monaghan, B.G. 1996. Modeling of slope behavior for open-pits. Trans. Instit. Min. Metall.-Sect. A, 105: pp.A37-A47. Lam, L. & Fredlund, D.G. 1993. A general limit equilibrium model of three-dimensional slope stability analysis. Can. Geotech. J. 30(6): pp.905-919. Rui yongqin. 1998. Stability of Creeping Slope and its Instability Prediction on Deformation Progress (In Chinese). PhD thesis. Xuzhou: China University of Mining and Technology press. Stead, D. & Eberhardt, E. 1997. Developments in the analysis of footwall slopes in surface coal mining. Engineering Geology 46 (1): pp.41-61. Yang Tianhong, Rui Yongqin , Tang Chunan. 2001. Study of Deformation Mechanism on Fuxin Haizhou Open-pit Bedding Stratum Slope (In Chinese). hydrology and engineering geology 28(4) : pp.36-39.

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