- Email: [email protected]
Monitoring the Quarry Pit Development
Available online at www.sciencedirect.com
ScienceDirect Procedia Chemistry 19 (2016) 721 – 728
5th International Conference on Recent Advances in Materials, Minerals and Environment (RAMM) & 2nd International Postgraduate Conference on Materials, Mineral and Polymer (MAMIP), 4-6 August 2015
Monitoring the Quarry Pit Development M.A.M. Fadela, H. Zabidia*, K.S. Ariffina a
School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, 14300, Nibong Tebal, Penang, Malaysia
Abstract Planning and maintaining intact pit slopes in a quarry/mine pit is an important aspect for a sustainable quarry development and operation. It is a main concern in ensuring the constructed pit slopes and benches are stable and safe during quarrying operation activities as well as after permanent closure of a quarry pit. An appraisal for slope instability and failures is conducted at the quarry pit of the Lafarge RawangSdn. Bhd. which is located in Rawang, Selangor. Determination of the failures level, integrity and governing geological parameters the contributed to the failures, stability and the hazards are done by field observation, geological mapping and geophysical surveys. The earth resistivity and seismic imaging surveys were conducted to image the subsurface condition such as extent and thickness of limestone bedrock and overburden, including suspicious features like the presence of underground seepage, sinkhole and soil/rock inferred strength indirectly. Occurrences of a few slope failures are considered minor, small scale and localized in nature. The rock slope failures mainly due to structural parameters of the in-situ rock which imposed a heavy jointing intersection and rock bedding plane that coincidently constructed parallel to pit slope face. © by by Elsevier B.V.B.V. This is an open access article under the CC BY-NC-ND license © 2016 2016The TheAuthors. Authors.Published Published Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia. Peer-review under responsibility of School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia Keywords:Slope failure; Resistivity survey; Seismic refraction survey
1. Introduction The open pit method is used to extract the rock’s material from the ground. Open pit method is the process of mining any near surface deposit by means of a surface pit excavated using one or more horizontal benches.1To
* Corresponding author. Tel.: +6-04-599-6124; fax: +6-04-5941011. E-mail address: [email protected]
1876-6196 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia doi:10.1016/j.proche.2016.03.076
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sustain quarry development and its operation, the important aspects need to focus are planning and maintaining intact pit slopes in a quarry/mine pit. The purpose of the benches is to control the depth of the blast holes, the slope of the pit walls, and the dangers of high wall faces.1It is a main concern in ensuring the constructed pit slopes and benches are stable and safe during quarrying operation activities as well as after permanent closure of a quarry pit. Slopemanagement rather than one ensuring slope stability is followed; there is expectation of manageable slope instability at acceptable levels of risk rather than a design that focuses on achieving stable pits wall (slope and benches). The slope design is brought into the quarry pit design through proper quarry pit planning, which is usually an iterative process between the slope designer and the quarry planner. The stability of the slope mainly influenced by rock mass, joints and fissures, and blasting vibration.2 Upon the slope failure, the landslide may occurs and become a threat to the workers and any machineries.3 Stabilizing techniques such as controlled blasting and slope support actually increase the operating cost with normally quarry operator try to minimize. Therefore, in achieving a practical pit slope design and construction that equate with operating constraints, requires interaction and compromise between the quarry planner, geologist, geotechnical engineer and other workers. The Lafarge Rawang’s quarry pit is located within the constraint N-S zone, which is enveloped by urbanization that confined the quarry expansion especially on the western site. At the eastern limits, the pit slope already reached concessionary border and buffer zone limit, where permanent slope was established (involved competent, hard rock slope). On the western site, the pit slope has been constructed involving a few design elements and materials. The slope mainly constructed on ground partly adjacent to the diverted Sg. Rawang by some natural limestone bedrock, overburden material from quarry pit (slime from tin tailing and top soil) and variety in sizes of the rock fragments. As observed, the height, inclination and bench sizes of the constructed are less consistent. The evaluation of the existing western slope and benches features in terms of their stability and their respective geotechnical conditions is done in order to find a solution for remedy and strengthening it. This especially focused ona 100m to 200m stretch section of constructed slope as shown in Fig. 1.
Fig. 1. Sections investigated across the benches (A-B) and along the slope (B-C) of the western slope site. Green dash lines indicated geophysical survey.
Fig. 2. Geology of the Rawang neighbourhood and the quarry.
2. General Geology This monitoring activity that carried out to appraise the slope instability and failures at the quarry pit of the Lafarge RawangSdn. Bhd is done around the problematic area where it is situated at the western site slope. Fig. 2 shows the local geological map of the Rawang neighborhood. Geologically, the quarry is underlain by a thick
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sequence of Kenny Hill rock formation. The predominant type of rock is massive limestone with vast occurrence ofphyllite, slate, shale and sandstone.4 In the quarry area, limestone generally characterized by very massive bedrock and often formed in places either as thick layered or jointed bedrock. The rock generally hard, competent, white to greyish white with black (carbonaceous) patches/lenses, fine to medium grained varieties. Structurally, the limestone is interbedded with shale and generally striking in the N-S trending, coinciding with regional structure of Peninsular Malaysia. The limestone bedrock outcrops are moderate to heavy jointed in places. The rock mass often bisected at least by 3 prominent major joint sets, and 1-2 less prominent joint sets. The joint surface can be classified as tight and rough in natures with milky white, calcitic veinlets.Generally, the limestone bedrock is outcropping and striking in N-S direction (e.g. 340°/80° (steeper) or gentle (325°/23°). Some of the observed joints sets are 300°/90°, 300°/80°, 80°/58°, 60°/NN, 160°/22°, 340°/80°, 100°/NN, 60°/48°, 325°/22°. These joint set can be classified as J1: 140°/32°, J2: 330°/60° and J3: 20°/82° and mainly close spacing. Bedding and joints plays an important role in control the shape and the size of rock fragmentation. In very close spacing joint sets and exposed slope, will likely to produce small blocks of rock during blasting or local weathering. 2.1. Western Slope Features The 125m slope stretch pertaining to B1b slope and B1a bench were examined as shown in Fig. 3 (GPS coverage stretch).Variety of rock/slope materials were encountered representing by massive and highly bedded sections to highly blocky and jointed rock fragmented rock masses, filling mixture of sand, silt and rubble materials were seen characterized the western man-made slope features.
Fig. 3. Schematic cross-section diagram of western slope profile with bench slope facing 80NE.
At the northern 350°N part of the quarry are, overburden comprising medium to loose, excavated material made up of brownish to yellowish grey, silty, clayey sand component (overburden/transported materials predominated the top of B1b slope section. Presence of local normal faulting was also observed (B1b slope). Appendix A shows some of the photographs taken along the 125m B1b stretch. Most of the upper slopes were constructed by gravity filling/backfilling by means of haulage/waste materials and mainly comprising limestone rubbles (10-50cm), irregular in shape from the blasted quarry/rock face slope (QFS). Soil overburden as well as weathered rock (WRS) predominate the lower slope section towards north (B1b and B1a- haulage road section-QFS). Observation from top level bench (OBS) downward to the B1b and lower of the slope face (quarry/rock face-QFS), in average, the slopes show, the height (15m), inclination (75°-85°) and bench width are less uniform. 2.2. Failures Mode A few pit slope failure modes/causes were noticed, mainly minor, isolated and locally occurred along this stretch (B1b). The main controlled partially or wholly contributed to and subjected to the, highly jointed and close spacing joints sets (loose materials), intersection of joint set and bedding that resulted in wedge failure, poor top soil infilling
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(above rock cliff), bedding plane failure (planar failure), presence of underground water seepage buildup that reduce the soil cohesiveness (internal friction), elevated pore pressure in pit slope/toe, presence of local faulting, and probably presence of deep seated underground water drainage or sinkhole (karst geomorphology) and nearby stream (Sg. Rawang). Failures could be minimized by an integrated measures during slope-bench design, during construction and the slope maintenance especially on overburden slopes in a long run. Poor in slope planning and design are normally resulted of inappropriate selection, construction or effects of the slope face angles, slope height, catch bench, overall slope angle (top bench to lower bench), slope orientation (e.g. avoiding parallel to rock bedding plane), poor vegetation/landscape (erosion control), drainage system for reducing oversaturated ground and increase effective pore pressure, and movement of heavy vehicle. 3. Geophysical Survey Geophysical techniques are considered the best non-destructive and remote sensing methods to infer the image of subsurface geology and other related features. Geophysical methods are quick implemented, cheap and cover wide area5. It provides a large scale characterization of the physical properties under undisturbed condition.6Two geophysical imaging methods were used to map the subsurface condition of the effected slope, i.e. 2D seismic refraction and earth resistivity survey. These surveys are conducted along the top crest of the pit which is parallel and adjacent to Sg. Rawang (L1- A-B direction) and along the OBS1 (L2- A-B direction) bench, across the slope (A-B) as shown in Fig. 1 and Fig. 3. 3.1. Earth Resistivity Survey Resistivity survey was established along line L1 (level 30m top bench) and CL1 as shown in Fig. 3. The typical range for resistivity values for limestone are generally in the range of 500 to 10x107 ohm/m (Ωm-1). Resistivity values for clayey sand, silty sand and gravel is often between 30 and 250 Ωm-1 as shown in Fig. 4.
Fig. 4. Resistivity values for some common rocks, minerals, chemicals and metals.7 (Loke, 1999)
Based on the field observation and these resistivity values, the coverage area were anticipated to be overlained by thick sequence of loose to compact of poorly graded silty sand to poorly graded sand at the lower section and gravely sand near the top section which could be up to 20-25m thick. However, they are variable different in saturation (moisture and conductive mineral content). On the top bench, limestone bedrock could be encountered at the depth just 20m below surface (generally around 20m above msL) with different competency, freshness and saturation as portrays in Fig. 3, Fig. 5 (a), (b) and (c). Generally, fresh and intact limestone bedrock could be located
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at the greater depth. In general, the results from the resistivity survey show high saturated pocket of resistivity value at the transverse profile of the slope. These values generally represent loose/fracture bedrock with different level of saturation. This also might indicate presence of sinkhole features or weathered pocket in the limestone bedrock.
Fig. 5. (a) Inversed modelling for L1; (b) Inversed modelling for L2; (c) Inversed modelling for CL1.
3.2. 2-D Seismic Refraction Survey Three seismic refraction survey lines were established as shown in Fig. 1 and Fig. 3. Two profiles are surveyed along slope of L1 (level bench 30m), L2 (level bench 24m) in N-S direction. The third profile is laid CL1 across slope profile (80°E). General earth materials’ with their indicative primary velocity are shown in Table 1. The results of the 2D pseudo-section subsurface images of the seismic refraction survey along L1, L2 and CL1 respectively are shown in Fig. 6. Seismic refraction 2D subsurface model of L1, generally shows that upper section is underlain by soft to firm materials (overburden or top soil) up to 20m thick at one particular point, as shallow as 8m. This material could be made up mixture of loose to dry sandy to gravelly sand material as well as clay mixture locally. Competent limestone bedrock may be encountered at the depth below 8m below msL of the bench.
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M.A.M. Fadel et al. / Procedia Chemistry 19 (2016) 721 – 728 Table 1. General earth material with their indicative velocity values.8 (Reynolds, 1995) Description
Primary velocity, Vp (m/s)
Sand
200-1000
Clay
330-380
Soil
350-500
Limestone
1700-4200
Meanwhile, L2 which was conducted at lower bench level 24m msL indicated that hard, fresh limestone bedrock (Vp>2000m/s) located very near the surface, especially shallow towards the south section. Loose or soft materials which could comprising silty clayey sand and lost rock boulder (Vp= 330-600m/s) become apparent in thickness (backfilled overburden) toward north, and could be up to 12-15m thick. The overall thickness of the limestone bedrock seemed decreasing towards north. Cross section CL1 line down slope profile (A-B) generally with Vp = 300-1000m/s indicated this western slope is made up of slightly firm to very firm overburden (backfilled overburden) composed of silty sand or gravelly sand and loose boulders below surface. However, this seismic data should be used cautiously as estimation only to show general distribution pattern and must be confirmed with drilling or the past available historical data. The rigidity of the underlain rock mass is weathered limestone bedrock to fresh/component are generally represents by Vp higher than 2500m/s (Fig. 6> green and blue colour shade).
Fig. 6. (a), (b), (c) Seismic refraction models of L1, L2 and CL (downslope).
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4. Discussion Fig. 7 which representing various scenario and categories of slope characteristics and failure modes encountered in relation to the western site pit slopes. a.
b.
Fig. 7. (a) Transition one. Separating grey, highly fractured limestone (left-QFS) with highly weathered (WRS), ferrogeneous reddish brown limestone characterized with gullies and water rills (right) (Facing 220°). Located at B1b, QFS-WRS contact.
Fig. 7. (b) Massive, grey, interbedded and jointed, limestone outcrop slope. Dip/dip direction: 45-50°/300°. QFS.
Most of the slope failures are minimum and restricted to specific benches located especially in the lower bench section with high slope angle, i.e. bench B1b and B2a levels (Fig. 3) – i.e. along the flag pole marker sections. So far, the failures do not create an alarming situation that threating or interruption to the quarry operation, causalities and downtime. Relatively, slopes at the lower section (rock slope/quarry face-QFS) possessed higher slope angle. This is generally greater than 45° (steep slopes). These failures are often in small scales, localized and in various mode and size. Within this 150m stretch along B1 (B1a and B1b) bench levels (haulage road) at least 5 minor slope failure size at the beginning, it may trigger and culminating bigger size and out of controlled situation in the future should remediation effort to correct this problem is not taken. Along B1a and B1b slope stretches, a few slope failure conditions and modes were encountered either involved the slopes partially or wholly constructed involved natural rock mass, partly rock and soil (overburden/backfilled), and natural soil slope. Root causes such pit slope failure in many sections (B1a and B1b) are governed mainly by nature of the rock mass, overburden nature (soil), backfilled materials and characteristic of the effected sections. These including lithology types, weathering grade/mineral alteration, slope angle, structural features (close spaced of intersected joint sets, bedding orientation), hydrogeological conditions (seepage/seeping), presence of excessive water/moisture due to prolong raining. Slightly to water saturated zone (in electrical resistivity survey) may represent underground water seepage and sinkhole features, which consider weak zones. Groundwater flow can have a detrimental effect on pit slope stability. Fluid pressure acting within discontinuities (joint/crack/bedding plane) and pore surface in the rock mass/filling materials reduce the effective stress, with a consequent reduction in effective shear strength. 5. Conclusion Slopes within western slope can be divided into 3 types, i.e. type A: quarry rock face slope (QFS), weathered rock slope (WRS) and type C: overburden (OBS). The slope failures along the B1b and immediate B2b slope (type A-QFS), and other slope types (OBS-type A and type B- WRS) along the various levels of benches within western slope site which comprising backfilling soil/materials and especially quarry face rock slopes (QFS) in the lower bench sections B1b-B2a) are due to various and complex geological, geotechnical and lastly hydrogeological parameters. Overall slope design and construction needed are variable, where the criteria adopted in determination
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of slope angles, bench high, bench width are acceptable. However, in failing to address a few complex issues such as bedding plane orientation and joint/bedding and closed joints spaced and intersection has resulted in many small scale slope failures and localized in occurrences. Geophysical imaging indicated underground seepage may cause some instability to the rock slope at the lower section B1b and B2a which has reduced the effective shear strength of the rock. Presence of heavy jointed in bedded limestone and close spaced have induced stability of the slope, thus culminated failure in future. Presence of poor, less cohesive soil liquefaction effect at some slope on the effected slopes have further culminating the problem. However does not affect the overall pit slope stability of the western slope. Such failures needed to be rectified, where advanced geotechnical information on rock/material are needed to redesign and rectify this slope back to it better and safe condition. Reinforcement measures, where complex and simple engineering solution could not addressed, protection like wire mesh fitting, anchoring and grouting is advisable to stabilize the slope. References 1. Howard L. Hartman & Jan M. Mutmansky. Introductory Mining Engineering. 2nd ed. New Jersey: John Wiley and Sons, Inc; 2012. 2. Qiao L, Li Y. Engineering geological model of high-steep slope damage in open pit mines. J UnivSciTechnol Beijing 2004;26 (5):461–4 3. Matsushi Y, Hattanji T, Matsukura Y. Mechanisms of shallow landslides on soil-mantled hillslopes with permeable and impermeable bedrocks in the Boso Peninsula, Japan. Geomorphology 2006;76 (10):92–108 4. S. C. Hutchinson & D.N.K. Tan. Geology of Peninsular Malaysia; 2009. 5. C. E. Liu. Soil Exploration in Soils and Foundations. 7th ed. Jurong: Pearson. 6. A. S. Godio. Geophysical characterization of a rockslide in an alpine region, Engineering Geology 2008. p. 273-286. 7. M.H. Loke. Electrical imaging surveys for environmental and engineering studies. 1999. p. 4. 8. J. Reynolds. An introduction to applied and environmental geophysics. John Wileys& Sons Ltd; 1995.
ScienceDirect Procedia Chemistry 19 (2016) 721 – 728
5th International Conference on Recent Advances in Materials, Minerals and Environment (RAMM) & 2nd International Postgraduate Conference on Materials, Mineral and Polymer (MAMIP), 4-6 August 2015
Monitoring the Quarry Pit Development M.A.M. Fadela, H. Zabidia*, K.S. Ariffina a
School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, 14300, Nibong Tebal, Penang, Malaysia
Abstract Planning and maintaining intact pit slopes in a quarry/mine pit is an important aspect for a sustainable quarry development and operation. It is a main concern in ensuring the constructed pit slopes and benches are stable and safe during quarrying operation activities as well as after permanent closure of a quarry pit. An appraisal for slope instability and failures is conducted at the quarry pit of the Lafarge RawangSdn. Bhd. which is located in Rawang, Selangor. Determination of the failures level, integrity and governing geological parameters the contributed to the failures, stability and the hazards are done by field observation, geological mapping and geophysical surveys. The earth resistivity and seismic imaging surveys were conducted to image the subsurface condition such as extent and thickness of limestone bedrock and overburden, including suspicious features like the presence of underground seepage, sinkhole and soil/rock inferred strength indirectly. Occurrences of a few slope failures are considered minor, small scale and localized in nature. The rock slope failures mainly due to structural parameters of the in-situ rock which imposed a heavy jointing intersection and rock bedding plane that coincidently constructed parallel to pit slope face. © by by Elsevier B.V.B.V. This is an open access article under the CC BY-NC-ND license © 2016 2016The TheAuthors. Authors.Published Published Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia. Peer-review under responsibility of School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia Keywords:Slope failure; Resistivity survey; Seismic refraction survey
1. Introduction The open pit method is used to extract the rock’s material from the ground. Open pit method is the process of mining any near surface deposit by means of a surface pit excavated using one or more horizontal benches.1To
* Corresponding author. Tel.: +6-04-599-6124; fax: +6-04-5941011. E-mail address: [email protected]
1876-6196 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia doi:10.1016/j.proche.2016.03.076
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M.A.M. Fadel et al. / Procedia Chemistry 19 (2016) 721 – 728
sustain quarry development and its operation, the important aspects need to focus are planning and maintaining intact pit slopes in a quarry/mine pit. The purpose of the benches is to control the depth of the blast holes, the slope of the pit walls, and the dangers of high wall faces.1It is a main concern in ensuring the constructed pit slopes and benches are stable and safe during quarrying operation activities as well as after permanent closure of a quarry pit. Slopemanagement rather than one ensuring slope stability is followed; there is expectation of manageable slope instability at acceptable levels of risk rather than a design that focuses on achieving stable pits wall (slope and benches). The slope design is brought into the quarry pit design through proper quarry pit planning, which is usually an iterative process between the slope designer and the quarry planner. The stability of the slope mainly influenced by rock mass, joints and fissures, and blasting vibration.2 Upon the slope failure, the landslide may occurs and become a threat to the workers and any machineries.3 Stabilizing techniques such as controlled blasting and slope support actually increase the operating cost with normally quarry operator try to minimize. Therefore, in achieving a practical pit slope design and construction that equate with operating constraints, requires interaction and compromise between the quarry planner, geologist, geotechnical engineer and other workers. The Lafarge Rawang’s quarry pit is located within the constraint N-S zone, which is enveloped by urbanization that confined the quarry expansion especially on the western site. At the eastern limits, the pit slope already reached concessionary border and buffer zone limit, where permanent slope was established (involved competent, hard rock slope). On the western site, the pit slope has been constructed involving a few design elements and materials. The slope mainly constructed on ground partly adjacent to the diverted Sg. Rawang by some natural limestone bedrock, overburden material from quarry pit (slime from tin tailing and top soil) and variety in sizes of the rock fragments. As observed, the height, inclination and bench sizes of the constructed are less consistent. The evaluation of the existing western slope and benches features in terms of their stability and their respective geotechnical conditions is done in order to find a solution for remedy and strengthening it. This especially focused ona 100m to 200m stretch section of constructed slope as shown in Fig. 1.
Fig. 1. Sections investigated across the benches (A-B) and along the slope (B-C) of the western slope site. Green dash lines indicated geophysical survey.
Fig. 2. Geology of the Rawang neighbourhood and the quarry.
2. General Geology This monitoring activity that carried out to appraise the slope instability and failures at the quarry pit of the Lafarge RawangSdn. Bhd is done around the problematic area where it is situated at the western site slope. Fig. 2 shows the local geological map of the Rawang neighborhood. Geologically, the quarry is underlain by a thick
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sequence of Kenny Hill rock formation. The predominant type of rock is massive limestone with vast occurrence ofphyllite, slate, shale and sandstone.4 In the quarry area, limestone generally characterized by very massive bedrock and often formed in places either as thick layered or jointed bedrock. The rock generally hard, competent, white to greyish white with black (carbonaceous) patches/lenses, fine to medium grained varieties. Structurally, the limestone is interbedded with shale and generally striking in the N-S trending, coinciding with regional structure of Peninsular Malaysia. The limestone bedrock outcrops are moderate to heavy jointed in places. The rock mass often bisected at least by 3 prominent major joint sets, and 1-2 less prominent joint sets. The joint surface can be classified as tight and rough in natures with milky white, calcitic veinlets.Generally, the limestone bedrock is outcropping and striking in N-S direction (e.g. 340°/80° (steeper) or gentle (325°/23°). Some of the observed joints sets are 300°/90°, 300°/80°, 80°/58°, 60°/NN, 160°/22°, 340°/80°, 100°/NN, 60°/48°, 325°/22°. These joint set can be classified as J1: 140°/32°, J2: 330°/60° and J3: 20°/82° and mainly close spacing. Bedding and joints plays an important role in control the shape and the size of rock fragmentation. In very close spacing joint sets and exposed slope, will likely to produce small blocks of rock during blasting or local weathering. 2.1. Western Slope Features The 125m slope stretch pertaining to B1b slope and B1a bench were examined as shown in Fig. 3 (GPS coverage stretch).Variety of rock/slope materials were encountered representing by massive and highly bedded sections to highly blocky and jointed rock fragmented rock masses, filling mixture of sand, silt and rubble materials were seen characterized the western man-made slope features.
Fig. 3. Schematic cross-section diagram of western slope profile with bench slope facing 80NE.
At the northern 350°N part of the quarry are, overburden comprising medium to loose, excavated material made up of brownish to yellowish grey, silty, clayey sand component (overburden/transported materials predominated the top of B1b slope section. Presence of local normal faulting was also observed (B1b slope). Appendix A shows some of the photographs taken along the 125m B1b stretch. Most of the upper slopes were constructed by gravity filling/backfilling by means of haulage/waste materials and mainly comprising limestone rubbles (10-50cm), irregular in shape from the blasted quarry/rock face slope (QFS). Soil overburden as well as weathered rock (WRS) predominate the lower slope section towards north (B1b and B1a- haulage road section-QFS). Observation from top level bench (OBS) downward to the B1b and lower of the slope face (quarry/rock face-QFS), in average, the slopes show, the height (15m), inclination (75°-85°) and bench width are less uniform. 2.2. Failures Mode A few pit slope failure modes/causes were noticed, mainly minor, isolated and locally occurred along this stretch (B1b). The main controlled partially or wholly contributed to and subjected to the, highly jointed and close spacing joints sets (loose materials), intersection of joint set and bedding that resulted in wedge failure, poor top soil infilling
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(above rock cliff), bedding plane failure (planar failure), presence of underground water seepage buildup that reduce the soil cohesiveness (internal friction), elevated pore pressure in pit slope/toe, presence of local faulting, and probably presence of deep seated underground water drainage or sinkhole (karst geomorphology) and nearby stream (Sg. Rawang). Failures could be minimized by an integrated measures during slope-bench design, during construction and the slope maintenance especially on overburden slopes in a long run. Poor in slope planning and design are normally resulted of inappropriate selection, construction or effects of the slope face angles, slope height, catch bench, overall slope angle (top bench to lower bench), slope orientation (e.g. avoiding parallel to rock bedding plane), poor vegetation/landscape (erosion control), drainage system for reducing oversaturated ground and increase effective pore pressure, and movement of heavy vehicle. 3. Geophysical Survey Geophysical techniques are considered the best non-destructive and remote sensing methods to infer the image of subsurface geology and other related features. Geophysical methods are quick implemented, cheap and cover wide area5. It provides a large scale characterization of the physical properties under undisturbed condition.6Two geophysical imaging methods were used to map the subsurface condition of the effected slope, i.e. 2D seismic refraction and earth resistivity survey. These surveys are conducted along the top crest of the pit which is parallel and adjacent to Sg. Rawang (L1- A-B direction) and along the OBS1 (L2- A-B direction) bench, across the slope (A-B) as shown in Fig. 1 and Fig. 3. 3.1. Earth Resistivity Survey Resistivity survey was established along line L1 (level 30m top bench) and CL1 as shown in Fig. 3. The typical range for resistivity values for limestone are generally in the range of 500 to 10x107 ohm/m (Ωm-1). Resistivity values for clayey sand, silty sand and gravel is often between 30 and 250 Ωm-1 as shown in Fig. 4.
Fig. 4. Resistivity values for some common rocks, minerals, chemicals and metals.7 (Loke, 1999)
Based on the field observation and these resistivity values, the coverage area were anticipated to be overlained by thick sequence of loose to compact of poorly graded silty sand to poorly graded sand at the lower section and gravely sand near the top section which could be up to 20-25m thick. However, they are variable different in saturation (moisture and conductive mineral content). On the top bench, limestone bedrock could be encountered at the depth just 20m below surface (generally around 20m above msL) with different competency, freshness and saturation as portrays in Fig. 3, Fig. 5 (a), (b) and (c). Generally, fresh and intact limestone bedrock could be located
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at the greater depth. In general, the results from the resistivity survey show high saturated pocket of resistivity value at the transverse profile of the slope. These values generally represent loose/fracture bedrock with different level of saturation. This also might indicate presence of sinkhole features or weathered pocket in the limestone bedrock.
Fig. 5. (a) Inversed modelling for L1; (b) Inversed modelling for L2; (c) Inversed modelling for CL1.
3.2. 2-D Seismic Refraction Survey Three seismic refraction survey lines were established as shown in Fig. 1 and Fig. 3. Two profiles are surveyed along slope of L1 (level bench 30m), L2 (level bench 24m) in N-S direction. The third profile is laid CL1 across slope profile (80°E). General earth materials’ with their indicative primary velocity are shown in Table 1. The results of the 2D pseudo-section subsurface images of the seismic refraction survey along L1, L2 and CL1 respectively are shown in Fig. 6. Seismic refraction 2D subsurface model of L1, generally shows that upper section is underlain by soft to firm materials (overburden or top soil) up to 20m thick at one particular point, as shallow as 8m. This material could be made up mixture of loose to dry sandy to gravelly sand material as well as clay mixture locally. Competent limestone bedrock may be encountered at the depth below 8m below msL of the bench.
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Primary velocity, Vp (m/s)
Sand
200-1000
Clay
330-380
Soil
350-500
Limestone
1700-4200
Meanwhile, L2 which was conducted at lower bench level 24m msL indicated that hard, fresh limestone bedrock (Vp>2000m/s) located very near the surface, especially shallow towards the south section. Loose or soft materials which could comprising silty clayey sand and lost rock boulder (Vp= 330-600m/s) become apparent in thickness (backfilled overburden) toward north, and could be up to 12-15m thick. The overall thickness of the limestone bedrock seemed decreasing towards north. Cross section CL1 line down slope profile (A-B) generally with Vp = 300-1000m/s indicated this western slope is made up of slightly firm to very firm overburden (backfilled overburden) composed of silty sand or gravelly sand and loose boulders below surface. However, this seismic data should be used cautiously as estimation only to show general distribution pattern and must be confirmed with drilling or the past available historical data. The rigidity of the underlain rock mass is weathered limestone bedrock to fresh/component are generally represents by Vp higher than 2500m/s (Fig. 6> green and blue colour shade).
Fig. 6. (a), (b), (c) Seismic refraction models of L1, L2 and CL (downslope).
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4. Discussion Fig. 7 which representing various scenario and categories of slope characteristics and failure modes encountered in relation to the western site pit slopes. a.
b.
Fig. 7. (a) Transition one. Separating grey, highly fractured limestone (left-QFS) with highly weathered (WRS), ferrogeneous reddish brown limestone characterized with gullies and water rills (right) (Facing 220°). Located at B1b, QFS-WRS contact.
Fig. 7. (b) Massive, grey, interbedded and jointed, limestone outcrop slope. Dip/dip direction: 45-50°/300°. QFS.
Most of the slope failures are minimum and restricted to specific benches located especially in the lower bench section with high slope angle, i.e. bench B1b and B2a levels (Fig. 3) – i.e. along the flag pole marker sections. So far, the failures do not create an alarming situation that threating or interruption to the quarry operation, causalities and downtime. Relatively, slopes at the lower section (rock slope/quarry face-QFS) possessed higher slope angle. This is generally greater than 45° (steep slopes). These failures are often in small scales, localized and in various mode and size. Within this 150m stretch along B1 (B1a and B1b) bench levels (haulage road) at least 5 minor slope failure size at the beginning, it may trigger and culminating bigger size and out of controlled situation in the future should remediation effort to correct this problem is not taken. Along B1a and B1b slope stretches, a few slope failure conditions and modes were encountered either involved the slopes partially or wholly constructed involved natural rock mass, partly rock and soil (overburden/backfilled), and natural soil slope. Root causes such pit slope failure in many sections (B1a and B1b) are governed mainly by nature of the rock mass, overburden nature (soil), backfilled materials and characteristic of the effected sections. These including lithology types, weathering grade/mineral alteration, slope angle, structural features (close spaced of intersected joint sets, bedding orientation), hydrogeological conditions (seepage/seeping), presence of excessive water/moisture due to prolong raining. Slightly to water saturated zone (in electrical resistivity survey) may represent underground water seepage and sinkhole features, which consider weak zones. Groundwater flow can have a detrimental effect on pit slope stability. Fluid pressure acting within discontinuities (joint/crack/bedding plane) and pore surface in the rock mass/filling materials reduce the effective stress, with a consequent reduction in effective shear strength. 5. Conclusion Slopes within western slope can be divided into 3 types, i.e. type A: quarry rock face slope (QFS), weathered rock slope (WRS) and type C: overburden (OBS). The slope failures along the B1b and immediate B2b slope (type A-QFS), and other slope types (OBS-type A and type B- WRS) along the various levels of benches within western slope site which comprising backfilling soil/materials and especially quarry face rock slopes (QFS) in the lower bench sections B1b-B2a) are due to various and complex geological, geotechnical and lastly hydrogeological parameters. Overall slope design and construction needed are variable, where the criteria adopted in determination
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of slope angles, bench high, bench width are acceptable. However, in failing to address a few complex issues such as bedding plane orientation and joint/bedding and closed joints spaced and intersection has resulted in many small scale slope failures and localized in occurrences. Geophysical imaging indicated underground seepage may cause some instability to the rock slope at the lower section B1b and B2a which has reduced the effective shear strength of the rock. Presence of heavy jointed in bedded limestone and close spaced have induced stability of the slope, thus culminated failure in future. Presence of poor, less cohesive soil liquefaction effect at some slope on the effected slopes have further culminating the problem. However does not affect the overall pit slope stability of the western slope. Such failures needed to be rectified, where advanced geotechnical information on rock/material are needed to redesign and rectify this slope back to it better and safe condition. Reinforcement measures, where complex and simple engineering solution could not addressed, protection like wire mesh fitting, anchoring and grouting is advisable to stabilize the slope. References 1. Howard L. Hartman & Jan M. Mutmansky. Introductory Mining Engineering. 2nd ed. New Jersey: John Wiley and Sons, Inc; 2012. 2. Qiao L, Li Y. Engineering geological model of high-steep slope damage in open pit mines. J UnivSciTechnol Beijing 2004;26 (5):461–4 3. Matsushi Y, Hattanji T, Matsukura Y. Mechanisms of shallow landslides on soil-mantled hillslopes with permeable and impermeable bedrocks in the Boso Peninsula, Japan. Geomorphology 2006;76 (10):92–108 4. S. C. Hutchinson & D.N.K. Tan. Geology of Peninsular Malaysia; 2009. 5. C. E. Liu. Soil Exploration in Soils and Foundations. 7th ed. Jurong: Pearson. 6. A. S. Godio. Geophysical characterization of a rockslide in an alpine region, Engineering Geology 2008. p. 273-286. 7. M.H. Loke. Electrical imaging surveys for environmental and engineering studies. 1999. p. 4. 8. J. Reynolds. An introduction to applied and environmental geophysics. John Wileys& Sons Ltd; 1995.