Rock slopes in most mining areas are prone to instability, because of the rock mass conditions are strongly effected by both internal rock properties and external conditions. Therefore, instability of rock slope attracts a high level importance for researchers, especially in the conventional mining practice area. The research area is located in Padalarang area, West Bandung, Indonesia, geographically at 6˚50’15.6” S – 107˚25’31.1” E, which is an area of andesitic rock mining. Identification of rock failure type and rock mass classification were carried out to determine the potential of rock failure, recommended excavation direction and methods. Primary data was collected using a scanline method to determine the discontinuity characteristics of three adjacent slopes namely SP-1, SP-2, and SP-3. A quantitative assessment was undertaken in the form of Rock Mass Rating (RMR), kinematic analysis, and Slope Mass Rating (SMR) from 442 discontinuities. RMR shows that all three slopes are classified as good rock with value 71, 65, and 65 respectively. Kinematic analysis shows that SP-1 has potential of wedge sliding, with an intersection percentage of 53.7%, SP-2 and SP-3 has planar no limit sliding with percentage 47.85% and 58.86%. SMR shows that SP-1, SP-2, and SP-3 are classified respectively as IIA stable, IIB partially stable, and VA unstable. The evaluated safe slope angel for excavation is achieved when the slope cut at no more than 65˚. The optimum recommended direction and angle of slope excavation are N160˚E/65˚ at SP-1, N250˚E/65˚ at SP-2 and SP-3, with a recommended excavation method is blast to loosen.
Rock slopes in most mining areas are prone to instability, because of the rock mass conditions are strongly effected by both internal rock properties and external conditions. Therefore, instability of rock slope attracts high level of importance for researchers, especially in the conventional mining practice area. This research was carried out in the andesite mining area at Padalarang, West Bandung District, West Java Province, Indonesia and geographically at 6˚50’15.6” S – 107˚25’31.1” E. This area is located about 31 km, west of Bandung. The topography elevation ranging from 600 meters to 800 meters above sea level and slope steepness ranging from 0% to 15% based on classification of van Zuidam (1985). This paper briefly describes the several techniques and methods used to evaluate the rock slope stability, including kinematic analysis, rock mass rating analysis, slope mass rating analysis, and consideration of the road adjacent to the mining area to determine excavation direction.
Detailed geological mapping and observation of the fracture attributes such as orientation, length, termination, aperture width, nature of filling, surface roughness, surface shape, and spacing were carried out. Lithological samples were collected for analysis of mechanical properties of the rock. Mechanical properties were analysed by point load test to measure the rock’s strength. A detailed petrography analysis of the study area was carried out using polarizing microscopes.
Kinematic analysis was performed using polar projection of fracture orientation compared to the direction of the slope on the stereograph to predict type of slope failure such as wedge failure, planar, or toppling (Price and Cosgrove, 1990). Futhermore, the results of kinematic analysis were then used to design the optimum excavation direction. Rock Mass Rating (RMR) is a commonly used method proposed by Bieniawski (1989). This method is used to predict the stability value of the slope which is technically used to determine the type of slope reinforcement. Slope Mass Rating (SMR) proposed by Romana (1985) was undertaken by adding a correction factor to adjust the slope orientation and plane of weakness.
Katili (1975) suggests that the subduction activity of the Australian Plate under the Eurasian Plate during the Eocene has resulted in a pattern of spreading of Tertiary volcanic rocks in Java with a west-east (WE) trend orientation. There are three dominant structural patterns found on the island of Java according to Pulunggono and Martodjojo (1994), namely Meratus Pattern with northeast-southwest direction (NE-SW), Sunda pattern with north-south direction (NS), and Java Pattern which has west-east direction (WE).
Sapiie et.al (2010) shows that all of the lithology units in the Padalarang area have been folded by the ENE-WSW thrust fault system in the south. The existence of thrust fault system makes several lithological units show duplication or repetition. The thrust fault is cut off by a NW-SE dextral strike-slip fault.
Based on the characteristics of Tertiary sediment distribution, Martodjojo (1984) divides the stratigraphy of West Java into three sedimentary mandalas, namely the Continuous Exposure Mandala, Bogor Basin Mandala, Banten Sedimentation Mandala, and Southern Mountain Mandala. The area of research in general is included into Bogor Basin Mandala, which has the characteristics of deposition mecanism is gravitational flow with the majority fragments are limestone. Bogor Basin Mandala consist of Bayah Formation, Batuasih Formation, Rajamandala Formation, Citarum Formation, Saguling Formation, Tuff Unit, and Alluvium Unit. The regional geology of the research area of Padalarang area is illustrated in Figure 1.
Geology of Research Area
The stratigraphy of the research area consists of claystone units which are synchronized with the Marl Member of Rajamandala Formation and limestone units that are synchronized with the Limestone Member of Rajamandala Formation. This formation was deposited along the shelf edge in the southern margin of West Java from Late Oligocene to Early Miocene. Then, this formation was intruded by an andesite unit (Figure 2).
Claystone units are characterized by grey appearance and react with acids. Limestone units generally are grain supported, poorly sorted, fragment (35%) consists of larger foraminifera (12%) such as heterostegina and nummulites, solitary coral (10%), detrital (8%), planktonic foraminifera (3%), and bentonic foraminifera (2%), 1-2.5 mm, matrix lime mud (40%), sparry calcite cement (25%). Whereas the andesite intrusion is classified as pyroxene andesite, holocrystalline, porphyritic, phenocryst (20%) consists of plagioclase (12%) and pyroxene (8%), 1-2 mm, in a groundmass (80%) consisting of plagioclase microlite (75%) and opaque minerals (5%), texture consists of cummulocryst, glomerocryst, zoning, mineral inclusion, and opaque rim.
The Figure 1: Geological map of Padalarang area (Sapiie et.al, 2010).
The geological structures developed in this area are shear fracture, joint, and fault. In the southern region, the thrust fault shows an E-W orientation. The thrust fault is cut by a NW-SE sinistral strike-slip fault. The andesite intrusion is cut by NW-SE minor normal fault.
Figure 2: Geological map of research area.
RESULT AND DISCUSSION
Rock Mass Rating (RMR)
Discontinuity data on each slope were calculated based on several parameters of Bieniawski’s Rock Mass Rating (1989) as shown in Table 1. Uniaxial Compressive Strength (UCS) is obtained based on Point Load Index (PLI) of 3.4MPa which has an equivalent UCS of 76.8MPa, which is classified as strong rock with equalization rating of 7. The value of Rock Quality Designation (RQD) on SP-1, SP-2, and SP-3 slopes respectively are 90%, 89%, and 85%, so the equalization rating of the RQD is 17. The average spacing of discontinuities on the entire slope is 20-60cm, so spacing equalization rating is 10. The condition of discontinuity was obtained from five sub-parameters; length of discontinuity (persistence), opening (aperture), roughness, infillings, and state of weathering. Each slope shows various value that have equalization rating for SP-1, SP-2, and SP-3 respectively are 26, 20, and 20. The last parameter is the condition of groundwater that was observed in both the wet and dry seasons. There was no water observed in the dry season, while a flow of water was observed during the wet season, so it has equalization rating of 11. From the summary of equalization rating of those parameters, we found out that SP-1, SP-2, and SP-3 slopes have RMR value respectively 71, 65, and 65 and classified as good rock.
|Spacing||20 – 60cm||10||20 – 60cm||10||20 – 60cm||10|
|Persistence||<1 – 20m||6||<1 – 20m||3||<1 – 20m||3|
||4||0.1 – 1mm||4||0.1 – 1mm||4|
|Weathering||Slightly weathered||5||Slightly weathered||5||Slightly weathered||5|
|Groundwater Condition||Discontinuity dry||11||Discontinuity dry||11||Discontinuity dry||11|
|RMR||Good Rock||71||Good Rock||65||Good Rock||65|
Table 1: Summary of the RMR value each slope.
The correlation between discontinuity direction and slope direction determines the type of rock sliding that is likely to occur. Analysis of the type and percentage of the potential rock sliding on each slope was performed using Dips 6 software. On the SP-1 slope, the discontinuities show two dominant sets with N10˚E – N60˚E and N40˚W – N50˚E which intersect on the slope with the orientation of N193˚E/65˚. Based on that correlation, the largest rock sliding potential is wedge sliding (53.7%). On the SP-2 slope, the discontinuities show two dominant sets with N70˚W – N90˚E and N40˚W – N60˚W which intersect on the slope with the orientation of N245˚E/77˚. Although the two sets intersect each other, the direction of discontinuities is suitable with the slope orientation, so based on this correlation, the largest sliding potential that can be formed is planar no limit sliding (47.85%). On the SP-3 slope, the discontinuities also show two dominant sets with N0˚E-N20˚E and N300˚E-N310˚E which intersected on the slope with the orientation of N263˚E/82˚. Although the two sets intersect each other, the direction of discontinuities is suitable with the slope orientation, so based on this correlation, the largest sliding potential that can be formed is planar no limit sliding (58.86%) (Figure 3).
Slope Mass Rating (SMR)
Slope Mass Rating (SMR) system is basically the summary of RMR value and other parameters; F1, F2, F3, and F4. Tabel 2 contains the orientation of discontinuity including strike or trend and dip or plunge. It also contains the strike and dip of the slope. These data will be calculated to determine the SMR value of each slope with the Equation 1:
SMR = RMR + (F1xF2xF3) + F4 (1)
Where, F1 is the relationship between the trend/strike of joint (discontinuity) and strike of slope. In SP-1 as it has wedge sliding type, the F1 is the angle between trend of joint and strike of slope and it has equalization value
Figure 3: Potential of block failure in each slope.
of 0.15. In SP-2 and SP-3 as they have planar no limit sliding type, the F1 is the angle between strike of joint and strike of slope and have equalization value respectively of 0.4 and 0.85. F2 is the dip or plunge of joint. Each of the slopes has the same F2 value of 1. F3 is the relationship between the plunge/dip of joint and dip of slope. In SP-1 as it has wedge sliding type, the F3 is the number of the plunge of the joint minus the dip of slope and it has equalization value of -50. In SP-2 and SP-3 as they have planar no limit sliding type, the F3 is the number of dip of joint minus dip of slope and they have equalization value respectively of -50 and -60. F4 is the excavation method used in the slope. All of the slopes are mechanically excavated with the equalization value of 0. By using the formula above, the SMR value of SP-1, SP-2, and SP-3 respectively are 63.5, 45, and 14 which were classified by Romana, 1985 in Singh and Goel, 2011 as stable good rock class IIB, partially stable normal rock class IIIB, and completely unstable very bad rock class VA (Table 3).
Table 2: Slope orientation and discontinuity set of each slope.
|Slope||Sliding type||Strike slope (as)||Dip slope (bs)||Strike/trend joint (aj/i)||Dip/plunge joint (bj/i)|
|SP2||Planar no limit||245˚||77˚||275˚||72˚|
|SP3||Planar no limit||263˚||82˚||270˚||70˚|
Table 3: SMR value set of each slope.
|Case of Slope Failure||Value||Case of Slope Failure||Value||Excavation Method||Value|
|SP1||71||85˚||0,15||1||-6˚||-50||Mechanical Excavation||0||63.5||Good (IIB)||Stable|
|SP2||65||30˚||0,4||1||-5˚||-50||Mechanical Excavation||0||45||Normal (IIIB)||Partially Stable|
|SP3||65||7˚||0,85||1||-12˚||-60||Mechanical Excavation||0||14||Very Bad (VA)||Unstable|
Method of Excavation
Based on the value of the RMR seen in Figure 4, a safe slope angle for excavation is performed when the slope is cut at no more than 65˚, according to Bieniawski (1993). Based on the value of the mechanical properties of the point load index and the spacing between fractures on each slope, the most optimum method of excavation to use in the research area is blast to loosen (Franklin et al, 1971) (Figure 5). Determining the optimum direction of excavation is done by considering the lowest total probability of failure and the feasibility of the road position. In Figure 6, showing the probability of failure analysis of the 10˚ excavation interval, it was found that each slope has a N160˚E direction on the SP-1, N250˚E on the SP-2 and SP-3, with a total probability of failure of each respectively 103.2%, 59.36% and 73.59%.
Figure 4: RMR and SMR value of each slope.
Figure 5: Assessment of rock masses with reference to excavatability classification system by Franklin et al. (1971), showing blast to loosen in all slope.
Overall field observations, laboratory analysis, data processing, rock failure kinematics, and analysis of all rock slopes data in the Padalarang andesite mining area, West Java, Indonesia can be summarized as follows. The classification of rock mass is very important to determine the stability of the slope including the potential for rock sliding that is likely to occur. The slope stability is strongly influenced by the orientation of the fractures with respect to the direction and angle of the slope. Therefore, it is very important to carefully determined the preferred direction and method of excavation to minimise the risk of rock cut slope instability, so it will not endanger the mining area and the road above it.
Figure 6: The optimum direction of excavation is based on the kinematic assessment of the percentage of poles that can induce failure for a given slope direction.
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