Understanding the Mining Induced Stresses by Actual

National Seminar on Innovative Practices in Rock Mechanics, Bengaluru, 2014

Understanding the Mining Induced Stresses by Actual Measurement and Numerical Modelling in a Deep Lead Zinc Mine R.K. Sinha*, D.S. Subrahmanyam, G. Shyam and S. Sengupta National Institute of Rock Mechanics, Bangalore unit, Karnataka, *[email protected]

Abstract A deep underground Lead-Zinc mine in India with a strike length of 1700 m has been developed up to a depth of 600 m to extract lead and zinc. The management of the mine decided to design and develop stopes below the mined out area for increased productivity to sustain the business. For the design of the stopes a detailed stress measurement programme was carried out using hydraulic fracturing method at different depths from the developed excavations available near the proposed stope. The result indicated that the mining induced stress had a magnitude higher than the pre-mining stress in conjunction with the rotation of the direction of the maximum compression (maximum principal horizontal stress) by to with respect to the pre-mining stress tensor. The post-mining stress was oriented almost transverse to the ore body as against sub parallel to the orebody during pre-mining stage. A 3-D numerical modelling of the mine using pre-mining stress tensor as input parameter substantiated the field result for the postmining stage. The generation of post-mining stress helped in understanding the impact of mining on the stress distribution which was used for design and sequencing of the stoping operation for the safe and optimal extraction of the ore. Keywords: Hydraulic fracturing, stope, pre-mining stress, mining induced stress

mRL, 500 mRL being the Surface mRL) is complete and presently developments are underway below 0 mRL up to a depth of -119 mRL to extract ore below 0 mRL. A detailed stress measurement programme was undertaken to determine the nature of induced stress redistributed at the future stoping area due to adjoining mining activities. Two levels were selected at -55 mRL and -87 mRL below 0 mRL and stress measurements were carried out by hydraulic fracturing method inside boreholes (Fig. 2).

1. Introduction Dariba lead-zinc mine, an important captive underground mine of Hindustan Zinc Limited is situated in village Dariba, District Rajsamand, Rajasthan (Fig. 1).

1.

2. Geology of the Study Area The mine area is constituted mainly by a sequence of meta-sedimentary rocks consisting of quartz mica schist, calcareous biotite schist and graphite mica schist (from footwall to hanging wall). Calc-silicate bearing dolomite occurs within the graphite mica schist horizon towards its contact with the calcareous biotite schist. The formations, in general, strike N-S with moderate to steep easterly dips. Cross-beds and laminations are observed in mineralised schist and Dolomite bands. The area between the South and North Lodes is traversed by 2-10 m wide meta-basic dykes. Nature of mineralisation is synsedimentary, later remobilised and recrystallised during subsequent polyphase deformation and

Fig. 1. Location of Rajpura Dariba Mine

The predominant host rocks of Dariba mine are dolomite and graphite mica schist. The Main lode extends over a strike length of 1700 m and is separated by a barren stretch of 300 m into two ore bodies namely South and North Lode. The formations, in general, strike N-S with moderate to steep easterly dips [2]. Mining up to depth of 500 m (500 mRL to 0

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metamorphism. Ore forming minerals are mainly sphalerite, galena, chalcopyrite and fahlore (tetrahedrite-tennantite). Preferred concentration of silver has taken place in fahlore and chalcopyrite. Pyrite and Pyrrhotite are ubiquitous.

Mining up to 8 mRL is complete and presently it is active at 0 mRL and -100 mRL (Fig. 2).

4. In-situ Investigations The in-situ stress measurement was carried out by using HTPF method (Hydraulic Tests on Preexisting Fracture) as introduced by Cornet [5]. The straddle packer assembly (hydraulic fracturing assembly, Fig. 3) was used for fracture

LONGITUDINAL VERTICAL SECTION (R.D.MINE) MAIN SHAFT

NORTH FAN

AUX SHAFT

OUTH LODE

NORTH LODE O.W.

10000cum/min CAP. 4000cum/min 40 0 0 0gal o l n

Fill B.H.

11 0 00gall o n

20 00 00ga lo l n

FILL B.H.

initiation/opening and further extension.

L -1+L- 2 + L- 3 375mRL

N 301

S-305

N 303

N 302

N 304

P 4

S P S P 3 2 1 1

S P S P 2 3 4 5

P

P

TP T

3

31 5 32

S-1 P-1 S- 2 P -2

S 7

S 6

S 5 P 6

S- 3

P S 8 8

P 7

SUMP3 (870m )

P- 3

P3 P10

2

SPMP 4

NP 2 SS 8

SS 6

SS 9

SS 7

S14

S8

P1

S6

P4

P2

S13

S11

P5

P3

S9

S7

P6

P4

S15 S13

P P

180 mRL

D.R.

SP 5 SS 11

L- 2 + L- 3

L-1C

L-1D SP 1

SS 10

150 mRL NP 6

NP 1

NP 8

NP 9

NP 4

L- 2 + L- 4

OP

SP SP 22 23

P1 P3 P2

NP NP 5 10

NP 3

NP 7

50 mRL OP SP 1

SP 3

SP SP 2 4

NS 19

NS 14

NS 18

NS 12

NS 17

NS 11

NS 16

NS 13

NS 15

NB NB NB NB NB NB NB 6 9 7 10 8 11 12

INDEX Ore Body

-55 mRL -87 mRL

-100 mRL

Mined Out

-119 mRL

Fig. 2. Longitudinal vertical section showing locations of the hydraulic fracturing stress measurement sites

3. Mining Status In the scheme of mining with respect to Rajpura Dariba Lead-Zinc mine the following methods have been adopted: i) Vertical Retreat Mining (VRM) method. ii) Blast Hole Stoping (BHS) method. The VRM method is being adopted in the ore blocks where the ore body width is more than 10 m. VRM stopes are mined using downward drilled large diameter blastholes (165 mm) drilled by DTH drills from upper levels. These holes are charged and blasted using spherical charge technology. A slice of 2.5-3.0 m is blasted at a time, thus retreating in upward direction. Mucking is done by electric LHD at sill level. The empty stopes are filled by classified mill tailings mixed with cement. Stopes are being mined in a sequence in a Primary-Secondary fashion.

Fig. 3 Hydraulic fracturing equipment used at site

The straddle packer assembly consisted of a test interval of length 200 mm and two 250 mm steel reinforced packers. Diameter of the packer was 42 mm having a burst pressure of 70 MPa. The packers units were attached at two ends of the test interval. In the case of hydraulic fracturing experiments inside the 48 mm diameter boreholes at the present project, the straddle packer unit was operated by using 1500 mm long and 32 mm diameter dual line quick connecting tubes. The dual lines, one for packer inflation and the other for injection, were concealed in a single rod. The maximum injection rate of the electric pump was

BHS method is being adopted in the mining blocks where the ore body width is narrow and varying from 8-15 m. The entire strike length is filled with panels having lengths 20 m each, designated as primary and secondary stopes. The stopes are mined using DTH blastholes (115 mm diameter) drilled down from the upper drill level. Blasting is done against a slot raise. The stopes are backfilled with cement fill after removal of ore. Stopes are being mined in a sequence in a Primary-Secondary fashion [4]. The mine extends from 500 mRL to 100 mRL with the surface mRL of 500 m. 118


10 l/min using water for pressurisation. All the events of injection were recorded in continuous real time digital mode.

comparison of pre and post mining stress gradient. Table-3. Comparison between pre- and post-mining stress gradient

After all the hydraulic fracturing tests were conducted in all the boreholes, an impression packer tool with a soft rubber skin together with a magnetic single shot orientation device was run into the holes to obtain information on the orientation of the induced or opened fracture traces on the borehole wall. The stress tensors as revealed by hydraulic fracturing measurement during pre-mining and post-mining stages are given in Tables-1 [1] and 2 [2].

Orientation

Table-1. Pre-mining stress tensor as revealed by hydraulic fracturing stress measurement

Stress gradient of

Principal Stresses (MPa) 11.09 13.83

(MPa ) 18.28 22.76

Orientation

(MPa)

Rock Cover (m)

N 100 N 300

9.14 11.38

404 504

Stresses

Rock Type (MPa) (MPa) Orientation Rock Cover (MPa) K

1035 N -55 mRL Siliceous Dolomite 33.495 22.33

1150 N - 55 mRL Siliceous Dolomite 17.74 11.83

N 1200

N 1200

N 1100

589 m 15.58 2.36

557 m 14.74 2.27

557 m 14.74 1.20

N 100 to N 30 0

Post-mining Stage (400 to 0 mRL) N 1100 to N 1200

Stress gradient of

Remarks

Rotation of horizontal stress orientation due to stoping Change in stress gradient due to mining Change in stress gradient due to mining

A numerical modelling programme was undertaken to understand the role of mining on stresses and to compare the numerical modelling results with the actual measured result.

Table-2. Post-mining stress tensor as revealed by hydraulic fracturing stress measurements 1035 N -87 mRL Siliceous Dolomite 36.88 24.59

Premining Stage

5. Numerical Modelling Numerical model was developed using Examine3D, a three-dimensional stress analysis and data visualization program for underground excavation in rock [8]. Stope excavations were made by use of polylines and node lines. Between the two node lines the skin of the excavation was extruded. The skin of excavation was discretised using the default discretisation scheme of the program with a mesh density factor of unity. Later on the meshes were subdivided using the default scheme twice. Examine3D has an option of using the gravity as well as the in situ stress data. The in-situ stress values can be fed in the form of principal stresses with their respective dip and dip directions. The measured value of the regional stress was utilised with an option of incorporating the stress gradient. The view of major stress contours on a plane slicing the crown/sill pillar is shown in Fig. 4. The results of the stress output as revealed by the numerical model are given in Table-4.

The observations and conclusions thereof are: i. The K values showing an increasing trend at 1135 N with increasing rock cover. With rock cover of 557 m, K = 2.27, and with a rock cover of 589 m, K = 2.36. ii. The K value at 1150 N with a rock cover of 557 m is only 1.20. This implies that the mining induced stress did not influence the site as much as at 1135 N with the same rock cover. This may be due to less (?) stoping activities above the measurement site and its proximity to the unmined barren patch above 180 mRL. iii. As the stoping progressed there is a relatively higher concentration of horizontal stresses at the lower levels compared to the vertical stress. The K value is on the higher side which is due to mine related induced high horizontal stresses at 55 mRL and -84 mRL. Table-3 shows the

Fig. 4 Plot of major principal stress on a section along the crown pillar

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Table-4. Stress output as revealed by Examine3D

Major principal stress Intermediate principal stress Minor principal stress

ML -87 (-87 mRL) Magni- Dip Strike tude (MPa) 15.21 17 289

ML-55 Magnitude (MPa) 14.89

13.41

68

71

9.06

12

165

Dip

Strike

14

267

13.19

48

14

9.90

37

166

authorities and staff of Hindustan Zinc Limited are also thankfully acknowledged.

8. References 1. Sengupta S, Chavan AS, Joseph D and Raju NM, 2001. Final report on in-situ stress measurement by and hydraulic fracturing method at South and North Lodes of Rajpura Dariba Mine, Hindustan Zinc Limited, Rajasthan, NIRM Report No. GS 9402. 2. Sengupta S, Subrahmanyam DS, Sinha RK and Shyam G, 2011. Determination of in-situ stress for the design of stopes at Rajpura Dariba Mine, Hindustan Zinc Limited, Rajasthan, NIRM Report No. GE-2011.

The numerical modelling studies reveal that the measured value of the stresses do not agree reasonably with the computed values except may be the orientation of maximum horizontal principal stress (Table-5).

3. Whyatt JK, Williams TJ, and Blake W, 1995. InSitu Stress in Lucky Friday mines. US Department of the Interior, Bureau of Mines, Report of Investigations 9582. NTIS stock number PB96131685, 1995 1-33

Table 5 Stress magnitude and orientation as revealed by numerical model Stresses

Orientation of maximum horizontal principal stress ( )

Field measurements post-mining stage (184 mRL to 0 mRL) N 1100 to N 1200

4. Hindustan Zinc Limited Internal Notes.

Numerical modelling

5. Cornet FH, 1986. Stress determination from Hydraulic Tests on Pre-existing Fractures the HTPF Method. Proc. Intl Symp, Rock Stress and Rock Stress Measurements, CENTEK Publ., Lulea, pp. 301-311 6. Mesys Gensim and Plane software manual (1992)

N 870 to N 1090

7. Mindata Australia, operating and analysis manual 8. Users Guide, Examine3D: A three-dimensional stress analysis and data visualization program for underground excavation in rock. 1999. Toronto, Ontario, Canada: Rocscience Inc.

The non-convergence of the numerical modelling result with that of actual measurements is difficult to explain. One of the reasons may be full mining geometry during actual measurement could not be replicated in the modelling, due to the limitation of incorporating actual material discontinuity and inhomogeneity in the modelling.

6. Discussion and Conclusions The availability of stress results during premining stage and subsequent measurement of stresses at the post-mining stage has refined our understanding of the in-situ stress vis-à-vis mining. The change in the orientation of the major compression from a favourable N 10-30 0 (Strike of ore body N-S and crown pillar oriented parallel to ore body) during pre- mining stage to unfavourable N 110-120 0 at the post mining stage has prompted to properly sequencing of the stoping operation along the strike and to upgrade the support system at the footwall of the orebody where the full haulage system is based.

7. Acknowledgements We are thankful to the Director, National Institute of Rock Mechanics, for the permission to publish the work. The help and cooperation of the 120