Slope stability study for optimum design of an opencast project

Transcription

1 Journal of Scientific & Industrial Research Vol. 65, January 2006, pp Slope stability study for optiu design of an opencast project V K Singh* Slope Stability Departent, Central Mining Research Institute, Dhanbad Received 05 July 2005; revised 13 Septeber 2005; accepted 27 October 2005 The stability of an opencast ine slope, investigated by the liit equilibriu ethod, indicates that 148 deep open pit is stable with 60 overall slope angle. Geotechnical apping was undertaken to deterine the critical orientation of structural discontinuities. Geoechanical properties were deterined in the laboratory and subsequently odified to approxiate those of in situ rock ass. Sensitivity analysis was done to deterine the influence of the slope design paraeters on the safety factor. It was also used in deterining the ost suitable reedial easure for the critical slope. Slope onitoring did not reveal any oveent in and around the pit. Keywords: Rock excavation, Safety analysis, Slope stability, Surface ining Introduction The geotechnical study and slope design was conducted of an open pit copper ine (500 long, 200 wide) situated in Rajasthan, India. The ore body is 40 wide and 313 long. The ultiate pit depth is 148. The ore body is striking NNW-SSE with an average dip of 65 due west. It swelled centrally and pinching towards south. Mineralization is confined to aphibole felspathic quartzite that is exposed towards the hangingwall. Footwall is characterised by felspathic quartzite. A dyke dipping at 80 towards hangingwall is also present in the pit (Fig. 1). Since the ineralized body is sall and steeply dipping, profitability of the ine is largely dependent on the steepest possible final slope angle. Earlier, the open pit was designed with 45 overall slope angle. Therefore, the ine anageent sought the study for an optiu slope design of the open pit. The rock discontinuities were apped at the exposed benches of the pit as per ISRM 1. Geotechnical Field Investigations Geotechnical apping, using detail line apping ethod, was carried in and around the partially developed open pit. The apping locations, arked as A, B, C and D (Fig. 1), were selected at different working levels of the footwall and the hangingwall side of the ine to represent the coplete rock ass fabric of the ine. *Tel: E-ail: vks_slope@yahoo.co The statistical analysis of the orientation data was done (Fig. 2) with "SNAP" coputer prograe 2. The ean orientations of the joint sets are alost siilar at all the four locations. Therefore, the orientation data of the four locations were grouped to represent the discontinuity orientation of the whole pit. The contour plot of the grouped data is shown in Fig. 2. The statistical analysis of the discontinuity data indicated the presence of four steeply dipping sets and a single sub-horizontal joint set (Table 1). The joints are oderately to widely spaced (20-90 c, ISRM 1 ). The spacing of the bedding plane is greater than 3 and the rock ass ay be described as assively bedded. The wide spacing of the joints contributes to a rock quality designation (RQD) rating in excess of 80 per cent. Discontinuity sets J1 and J2 ay be classed as systeatic joint sets. J3 and J4 are sub-systeatic and J5 is non-systeatic joint set (ISRM 1 ). J1, J2 and J3 joint sets can be said to have high persistence, being traceable to distances of 20 in the direction of dip. Attepts were ade to easure the roughness precisely in the field. Although only a few discontinuities could be found suitable for roughness easureent due to inaccessibility of large exposed discontinuity surface. The dip direction and dip aount were obtained with the help of Clar copass by putting 5, 10, 20 and 40 c dia circular discs at various positions over a few discontinuity surfaces 3. The effective large-scale roughness angle, easured fro 40 c circular disc 4, in the direction of potential sliding is 3 for the joints of aphibole felspathic quartzite and felspathic quartzite both. The joints are

2 48 J SCI IND RES VOL 65 JANUARY 2006 Fig. 1 Geological ap of the copper open pit Table 1 Mean orientation of joint seats (719 observations) Joint set Mean orientation of joint seats Dip direction Dip aount, degree J1 N011±07 82±3 J2 N338±07 82±5 J3 N100±10 81±5 J4 N219±18 24±8 J5 N273±08 81±6 sooth planar. The surface atching ethod is also being to estiate shear strength 5. The ajority of joints (>90%) are closed. A few open fractures observed in the field appear to be the result of blasting operation. A sall nuber of joint aperture are filled with siliceous aterial which is derived by the weathering of the parent rock. The lack of hydraulic channels within the rock ass, the presence of properly aintained surface drains and an annual rainfall of only 500 ean that the slopes of the open pit can be regarded as effectively drained. Rock type Table 2 Realistic value of friction angles Aphibole Felespathic Quartizite Laboratory deterined friction angle (φ r ) of saw cut saple ean Large scale ean field roughness angle (i) Realistic value of friction angle (φ r +i) ean Felspathic Quartizite Geotechnical Laboratory Investigations Scale effect that is associated with laboratory testing of shear strength of discontinuities ay be overcoe through easureent of the residual angle of friction in the laboratory Realistic estiate of the shear strength of discontinuities ay be ade by adding the residual friction angle, deterined in the laboratory, to the large scale roughness angle, which is easured in the field 14,15. Residual friction angles were deterined fro saw cut saples in a shear box apparatus and the values obtained were added to large-scale roughness angles (Table 2).

3 SINGH: SLOPE STABILITY STUDY FOR OPTIMUM DESIGN OF AN OPENCAST PROJECT 49 Table 3 Back analysis for friction angle of failed hangingwall slope Slope height = 20.0 Slope angle = 75 o Dip of failure plane = 55 o Rock density = 2.68 ton/ 3 Water density = 1.0 ton/ 3 ZW = 3.5 Tension crack depth = 7 HW = 10 F = 1 Friction angle for drained slope without tension crack = 55.0 o Friction angle for drained slope with tension crack = 55.0 o Friction angle for undrained slope with tension crack = 64.6 o Friction angle for undrained slope without tension crack = 61.6 o Back analysis for friction angle of failed footwall slope Slope height = 20.0 Slope angle = 75 o Dip of failure plane = 50 o Rock density = 2.62 ton/ 3 Water density = 1.0 ton/ 3 ZW = 4.0 Tension crack depth = 8 HW = 10 F = 1 Fig. 2 Density plot of grouped orientation data of the discontinuities by SNAP An assessent of the angles of friction so derived was verified by back analysis of two bench scale slope failures. It is necessary to estiate the effective cohesion of joint surfaces that had been obilized at the tie of failure to arrive at the angles of friction. After having coplete configuration of the failed slopes, firstly it was decided to back calculate the effective friction angle obilized during the failure (Table 3). This back analysis was done with the assuption that effective cohesion along the failure plane would have been zero. The values of friction angles (Table 3) are very high to that of the laboratory tested values (Table 2). So, it was considered that there would have been soe effective cohesion along the failure plane. Accordingly, the back analyses were done with possible cobination of friction angles and corresponding cohesion (Table 4); wide variation was observed in the effective cohesion values for a given value of friction angle for four different geoining conditions. This variation is ainly due to different groundwater condition. So, to get the best estiate of the effective cohesion and friction angle, it is essential to know the ost likely groundwater condition during the slope failure. Friction angle for drained slope without tension crack = 50.0 o Friction angle for drained slope with tension crack = 50.0 o Friction angle for undrained slope with tension crack = 58.7 o Friction angle for undrained slope without tension crack = 55.1 o ZW is depth of water is tension crack HW is depth of water in slope without tension crack F is factor of safety 1 t/ 3 = 9.81kN/ 3 The rocks of the copper open pit are well jointed with oderate spacing. The ine is located in sei arid region with average annual precipitation at about 50 c and is generally torrential resulting into a runoff fro the surface in excess of percolation. The topography of the area is hilly which enables quick run-off to the rainwater. The pit reains dry, except in the rainy season. Suitable surface drains are provided to divert the rainwater away fro the pit. These surface drains are properly aintained, especially in rainy season, to keep the effective. The presence of tension crack cannot be ruled out due to the presence of well-developed nearly vertical joint set J1. So, it was concluded that the applicable geoining condition for the copper open pit slope ight be regarded as "drained slope with tension crack".

4 50 J SCI IND RES VOL 65 JANUARY 2006 Table 4 Back analysis for cohesion of hangingwall slope Slope height = 20.0 Slope angle = 75 degree Dip of failure plane = 55 degree Rock density = 2.68 ton/ 3 Water density = 1.0 ton/ 3 ZW = 3.5 F = 1 Tension crack depth = 7 HW = 10 I C CC CCC CCCC φ Back analysis for cohesion of footwall slope Slope height = 20.0 Slope angle = 75 degree Dip of failure plane = 50 degree Rock density = 2.62 ton/ 3 Water density = 1.0 ton/ 3 ZW = 4.0 F = 1 Tension crack depth = 8 HW = 10 I C CC CCC CCCC φ ZW, HW and F as entioned is Table 6 C is cohesion for drained slope without tension crack (ton/ 2 ) CC is cohesion for drained slope with tension crack (ton/ 2 ) CCC is cohesion for undrained slope with tension crack (ton/ 2 ) CCCC is cohesion for drained slope without tension crack (ton/ 2 ) φ is friction angle. 1 ton/ 2 = 9.81 kpa After getting the best estiate of likely ground water and geoining condition, now it is possible to estiate the effective shear strength for the failed slopes. For 35 friction angle (Table 4), cohesive strength for the joint surface of aphibole felspathic quartzite is 4.88 ton/ 2 for drained slope with tension crack condition. For 33 friction angle, the cohesion coes to be 5.09 ton/ 2 for the joint surface of felspathic quartzite. These values for joint surfaces of both the rock types were considered to be the realistic and the sae were used for the slope design purpose. Residual friction angles (32 and 30 ) were adopted, as final friction angles during stability analysis, for the J4 joint set and bedding planes in the aphibole felspathic quartzite and felspathic quartzite rock ass due to the rearkable planarity of both types of structure. The considerable persistence of the bedding plane and J4 joint set in the dip direction led to the assignent of a zero value of cohesion for use in the stability analysis. The hoogeneity/heterogeneity of the rock ass, at different locations and also at different depths in the open pit, was deterined with two-way analysis of variance technique 16. The fractal ethod can also be used to easure the statistical hoogeneity 17. Extensive diaetrical point load testing with liited uniaxial copressive strength test were done at different depths on NX size borehole saples of both the rock types drilled at nine different locations of the open pit (Table 5). It was used to develop a relationship (Table 6) between pint load index (Is) and uniaxial copressive strength (σ c ). This relationship was used to deterine the copressive strength of the rock types at different depths of the boreholes to be used for two-way analysis of variance technique (Table 7). By coparing the estiated variance ratio for respective degree of freedo at 5% level of significance (Table 8), it was observed that the strength is not significantly varying at different location and depth of the pit. Rock type Aphibole feldspathic quartzite Table 5 Coparison of uniaxial copressive and point load test results Point load index Uniaxial copressive strength ton/ 2 ton/ 2 No. of saple Mean Standard No. of saples Mean Standard tested deviation tested deviation Feldspathic quartzite

5 SINGH: SLOPE STABILITY STUDY FOR OPTIMUM DESIGN OF AN OPENCAST PROJECT 51 Slope Design by Liit Equilibriu Method Liit equilibriu ethod is the ost widely accepted and coonly perfored design tool in slope engineering. Slope Design of Hangingwall Slope Kineatic test of hangingwall slope (Fig. 3) is to know the types of failure possible. The critical discontinuity ust lie within 20 of the slope face for plane failure to occur. Only the J1 joint set strikes approx parallel to the hangingwall slope face. The ean dip direction of J1 joint set is N011, which is Rock type Depth of saple (MRL) Table 6 Correlation between σ c and Is Aphibole feldspathic quartzite Rock type Ratio between σ c and Is oblique to the ean dip direction of the hangingwall slope face of N030 (Fig. 3). Structures with dip directions between N010 to N050 (i.e. within 20 of the slope face) were therefore analyzed with the use of the SNAP progra to deterine the nuber of joints with dips greater than 35, the condition for sliding to occur (Table 9). The ajority of joints dip between 45 and 90, but less than 8.7% of the 172 Table 7 Uniaxial copressive strength at different depth Mean uniaxial copressive strength, kg/c 2 Bore hole nuber CS-1 CS-5 CS-6 CS-7 CS-10 CS-11 CS-13 CS AFQ AFQ AFQ AFQ AFQ AFQ FQ AFQ FQ FQ AFQ ` 389 FQ AFQ FQ FQ FQ FQ FQ kg/c 2 = 98.1 kpa AFQ stands for aphibole fespathic quartizite. FQ stands for felspathic quartizite Correlation coefficient Feldspathic quartzite Aphibole feldspathic quartzite and feldspathic quartzite together Table 8 Suary of coputations for analysis of variance Source of variation Variation of bore holes Variation of depth Degree of freedo Su of squares Mean su of squares Estiated variance ratio F Tabulated variance ratio Interaction Residual Total 319

6 52 J SCI IND RES VOL 65 JANUARY 2006 Table 10 Factor of safety for hangingwall slope Overall slope height=148 Cohesion = 4.8 ton/ 2 Friction angle =35 0 Unit weight = 2.68 ton/ 2 Failure plane inclination Tension crack depth, F1 F2 F3 F4 60 o overall slope o overall slope Fig. 3 Kineatic analysis to know the types of failure Table 9 Dip ring analysis (172 observations) for hangingwall slope (18 ring of 5.0 degree each) Sr No Range, Nuber % joints in the range of dip directions, considered occur in the dip rings fro 40 to 60. However, the proportion of joints that daylight increases sharply in the dip range (Table 9). Furtherore, these joints have approx the sae strike and dip direction as the hangingwall slope face. F1 is factor of safety for slope with water up to half of the depth of the tension crack, F2 factor of safety for drained slope with tension crack, F3 factor of safety for drained slope without tension crack, F4 is factor of safety for slope with water up to half of the depth of slope, but without tension crack. If slope angle (> 60 ) is specified, the nuber of critical joints that could daylight in the slope face would be significant. The ain objective in the overall slope design is to iniize the risk of failure. Hence, it is iportant to iniise the nuber of potential failure planes that could daylight in a given slope. Plane failure analysis for overall slope was done to calculate the factor of safety for four different conditions (Table 10). The F2 condition (that of a drained slope with tension crack) was adopted as representative of the situation at the ine site (Table 10) because of the ine's location in a seiarid region and the potential for developent of tension cracks along the steeply dipping J1 joint set. A depth of 20 was assued for tension cracks in accordance with the observed persistence of the J1 set. A preliinary stability analysis indicated that joints with dips of less than 56 would be stable should they daylight in a hangingwall slope of 60 dip. So, detailed stability analysis with joint dip less than 56 was considered to be unnecessary. Siilarly it was judged that analyzing the stability of joints with dips greater than would be inappropriate as these joints would not daylight in a slope. So, it was decided to conduct stability analysis with 58 joint dip

7 SINGH: SLOPE STABILITY STUDY FOR OPTIMUM DESIGN OF AN OPENCAST PROJECT 53 Table 11 Dip ring analysis (75 observations) for footwall slope (18 rings of 5.0 degree each) Sr No Range, Nuber % Table 12 Factor of safety for footwall slope Overall slope height=77 Cohesion = 5.1t/ 2 Friction angle =33 0 Unit weight = 2.62 t/ 3 Failure plane inclination, Tension crack depth, 60 overall slope F1 F2 F3 F overall slope (the ost critical joint orientation) for an overall slope angle (60-65 ). The 60 hangingwall slope (drained slope with tension crack, F2 of Table 10) is stable with failure plane dipping at 58 with the available joint shear strength. The sae 60 slope becoes unstable with 55 joint dip (Table 10). Only 10 joints (5.8%) are having dip less than 58, which ay for critical plane failure geoetry (Table 10). As such, these few observations are not representative of the overall structure and also do not coe fro the sae area. At the sae tie, 65 slope becoes unstable with 55 and 58 joint inclinations both and critically stable with 63 joint dip with critical tension crack depth (Table 10). Hence, ore critical joints are exposed in 65 slope face. Therefore, 60 overall slope angle is a logical choice. Slope Design of Footwall Slope The kineatic analysis of footwall slope indicated that J5 is the only critical joint set for plane failure which has a dip direction of N270 (Fig. 3). A dip ring analysis revealed that only one joint is found to be dipping at less than 55 in the relevant range of dip directions of N250 -N290 (Table 11). However, about 9 percent joints are having dip between 60 and 65. Hence, it was decided once ore to carry out stability analysis with 58 joint dip, the ost critical orientation for a footwall slope angle in the range The single daylighting joint (Table 12) is stable in 60 footwall slope, because it is dipping at an angle (20-25 ) less than friction angle (33 ). However, the 65 slope is critically stable with few joints having inclination between 60 and 65. Hence, 60 pit slope angle is safe fro plane failure condition. Wedge Failure Analysis The kineatic analysis of the structural data indicated that the footwall and hangingwall slopes would be free of critical wedge geoetry (Fig. 3). Moreover, the etabasic dyke will not cause unstable condition because of its location in the iddle part of the pit, which will be reoved in the course of ining. At depth, the steeply dipping dyke is favourable oriented within the hangingwall slope and so would have no undesirable effect on slope stability. Sensitivity Analysis of Slopes The ain ai of sensitivity analysis was to deterine the influence of different paraeters on the safety factor. The decisive influence of water was observed in tension crack, in coparison to other paraeters, on safety factor (Fig. 4). The slopes are stable without water in tension crack with any cobination of cohesion and friction angle. Hence, the slope is stable in the ost likely geoining condition (drained slope with tension crack). For half

8 54 J SCI IND RES VOL 65 JANUARY 2006 Fig. 4 Sensitivity analysis for hangingwall slope Fig. 5 Sensitivity analysis for footwall slope water filled tension crack condition, the slope is stable only with cohesion greater than 5 ton/ 2. For fully suberged tension crack condition, the slope is only stable with cohesion greater than 6.5 ton/ 2. The available cohesive strength along joints ay decrease with tie due to weathering and the slope ay becoe unstable. Therefore, it is necessary to avoid any entry of water in tension crack and it's further extension to greater depth. Any visible tension crack should be filled with pereable aterial. Any ipereable aterial covers the top of the crack. The pereable aterial within the crack will allow ground water flow across the tension crack. The ipereable aterial at the top of tension crack will not allow surface water to enter within the crack. The suitably filled-up tension crack will change the condition fro "undrained slope with tension crack" to "undrained slope without tension crack". Half suberged slope without tension crack (Fig. 4) is stable even with the lowest values of cohesion and friction angle. For constant aterial properties of the sliding surface, an increase of the cohesion by 2.0 ton/ 2 causes an increase of safety factor by 0.39 (Fig. 4). The influence of the friction angle by contrast is saller. In this way, this sensitivity analysis is very useful for selecting a ore justified and suitable reedial easure. Proper drainage will influence the stability ost, and artificial reinforceent will also be beneficial to retain the axiu available shear strength of joints. The sensitivity analyses of footwall slope have also revealed the sae results (Fig. 5). Controlled Blasting A presplit blasting prograe was adopted in order to iprove the stability of the ultiate pit walls. Perfection of the presplitting at this site was achieved by conducting any experiental blasts in the ine (Table 13). The 150 dia vertical holes are drilled to the depth of Decoupled explosive charge (1 kg) was attached to the cordex at a spacing of 1 extending throughout the hole. Slurry explosive (8 kg, cartridge dia 50 ) is charged in each hole. To avoid an under fragented toe, an additional cartridge of 6.25 kg (125 ) is also placed at the botto of each hole (a total 14 kg explosive in each hole). Steing, kept to 1.8, is less than the noral. The presplit holes are drilled at 1.5 spacing. The next row of holes is drilled en echelon at a distance of 2.5 (burden), which is average back- Spacing Table 13 Paraeters for drilling Burden Depth of holes Height of bench Dia of hole Rearks O. B. drilling Ore drilling Pre-split drilling

9 SINGH: SLOPE STABILITY STUDY FOR OPTIMUM DESIGN OF AN OPENCAST PROJECT 55 Fig. 6 Working plan showing location of onitoring stations break of blast. A 4 spacing is kept between the holes. Another row of holes is drilled at distance of 3.5 (burden) with 4 spacing, which is the usual burden, and spacing of holes. The 150 dia vertical holes of the ain blast are charged at the rate of 0.75 kg/ 3. The total volue of rock break in a single blast is used to be about One long delay detonator (LDD) of 500.s. (p + sec) was connected between the presplit holes and the first hole of the production blast. The row of presplit holes is fired before the ain charge. The other holes of the production blast were subsequently fired with short delay detonator (SDD) of 25.s. The blasting results are excellent at the ine by keeping the above paraeters. After blasting, a sooth bench face is fored. Slope Monitoring A slope-onitoring prograe has subsequently been undertaken. The observation stations are located on the crest of the open pit and on in-pit benches. Monitoring stations have also been installed on 384 and 404 bers (Fig. 6). An electronic distance eter (Wild DI4L) and a precise level (Wild NA2) have been used to deterine displaceents of the rock ass. Till date, no oveent has been observed. Conclusions A new ethod was introduced to deterine the optiu discontinuity orientation and can also be used for optiu slope design. A correlation has been developed between point load index and uniaxial copressive strength. It is very useful during physical

10 56 J SCI IND RES VOL 65 JANUARY 2006 characterization of the slope aterial of any open pit. The two-way analysis of variance technique was successfully utilized for the first tie in the process of open pit slope design to deterine the heterogeneity/ hoogeneity of the slope aterial at different depths and at different locations of an open pit. The whole open pit was designed with the axiu 60 overall slope angle; before this geotechnical study, the open pit was designed at 45 overall slope angle. The height of the individual benches was 10 to 12. It is the steepest slope angle peritted by Directorate General of Mines Safety (DGMS) for 148 deep open pit ine (20 bench height) in India. Initially, 10 high benches are developed but at the pit liits two benches are joined together to ake ultiate bench height of 20. The open cast ining has been successfully copleted up to 148 depth. Acknowledgeents Author is grateful to Director, CMRI, Dhanbad, for giving perission to publish the paper. Author is also thankful to the ine anageent for providing all necessary inforation and facility during field study. References 1 ISRM, Suggested ethods for the quantitative description of discontinuities in rock asses, Coission on the Standardization of Laboratory and Field Tests in Rock Mechanics, Jeran P W & Mashey J R, A coputer progra for the stereographic analysis of coal fractures and cleats, United States Departent of the Interior, Bureau of ines, IC 8454, Hoek E & Bray J W, Rock Slope Engineering, 3rd edn (Inst Min Met, London) Fecker E & Rengers N, Measureent of large-scale roughness of rock planes by eans of profilograph and geological copass, Proc Int Sy Rock Fracture, Nancy, 1971, Zhao J, Joint surface atching and shear strength, J Rock Mech Min Sci, 34 (1997) Coulson J H, The effects of surface roughness on the shear strength of joints in rocks, Technical report MRD 270, Missouri river division, Corps of engineers, Oaha, Nebraska, Barton N, Review of a new shear strength criteria for rock joints, Eng Geo, 7 (1973) Hoek E & Londe P, The design of rock slopes and foundations, General report 3rd Cong ISRM, Denver, Barton N & Choubey V, The shear strength of rock joints in theory and practice, Rock Mech, 10 (1977) McMahon B K, Soe practical considerations of the estiation of shear strength of joints and other discontinuities, Proc Int Syp Fundaentals of Rock Joints, Lulea, 1985, Bandis S C, Mechanical properties of rock joints, Proc Int Syp Rock Joints, Norway, 1990, Yu X & Vayssade B, Joint profiles and their roughness paraeters, Int J Rock Mech Min Sci & Geoech Abstr, 28 (1991) Huang S L, Oelgke S M & Speck RC, Applicability of fractal characterization and odelling to rock joint profiles, Int J Rock Mec Min Sci & Geoech Abstr, 29 (1992) Mohaad N, Reddish D J & Stace L R, The relation between in-situ and laboratory rock properties used in nuerical odelling, J Rock Mech Min Sci, 34 (1997) Hoek E & Brown E T, Practical estiates of rock ass strength, Int J Rock Mech Min Sci, 34 (998) Croxton F E, Cowden DJ & Klein S, Appllied General Statistics, 3rd edn (Prentice-Hall, New Delhi) 1982, Kulatilake P H S W, Fiedler R & Panda B B, Box fractal diension as a easure of statistical hoogeneity of jointed rock asses, Eng Geol, 48 (1997)