SLOPE STABILITY ANALYSIS OF QUARRY FACE AT KARANG SAMBUNG DISTRICT, CENTRAL JAVA, INDONESIA

Size: px

Start display at page:

Download "SLOPE STABILITY ANALYSIS OF QUARRY FACE AT KARANG SAMBUNG DISTRICT, CENTRAL JAVA, INDONESIA"

Basil Reed
5 years ago
Views:

1 International Journal of Civil Engineering and Technology (IJCIET) Volume 9, Issue 1, January 2018, pp , Article ID: IJCIET_09_01_083 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed SLOPE STABILITY ANALYSIS OF QUARRY FACE AT KARANG SAMBUNG DISTRICT, CENTRAL JAVA, INDONESIA Rini Asnida Abdullah*, Dedy Yusufianshah, Mohd Asmawisham Alel, Siti Nurafida Jusoh, Mohd Azril Hezmi, Nor Zurairahetty Mohd Yunus and Ahmad Nazri Ali Faculty of Civil Engineering, Universiti Teknologi Malaysia, Johor Bahru, Malaysia Masagus Ahmad Azizi Mining Engineering Department, Trisakti University, Jakarta Barat 11440, Indonesia *Corresponding Author: ABSTRACT The district of Karang Sambung lies on interbedded calcareous sandstone and siltstone with an acceptable topography, leading to locals creating a small quarry. The excavation has been done traditionally at the quarry, which creating steep slopes on the quarry face that endanger workers. Therefore, stability analysis is necessary to ensure the safety of the quarry face. The quarry face was divided into two regions, and geological mapping and scanline mapping was carried out. The slopes were then analysed by empirical, kinematic, limit equilibrium, and finite element methods. Based on analyses, the slope has a high probability of wedge failure. Key words: slope stability, rock mass classification, kinematic analysis, limit equilibrium method, finite element method. Cite this Article: Rini Asnida Abdullah1, Dedy Yusufianshah, Masagus Ahmad Azizi, Mohd Asmawisham Alel, Siti Nurafida Jusoh, Mohd Azril Hezmi, Nor Zurairahetty Mohd Yunus and Ahmad Nazri Ali, Slope Stability Analysis of Quarry Face at Karang Sambung District, Central Java, Indonesia. International Journal of Civil Engineering and Technology, 9(1), 2018, pp INTRODUCTION The district of Karang Sambung, located in Central Java, Indonesia, lies above various valuable rock resources. One of its potential ores is the sandstone, which lies beneath a hilly area. Due to excavation by shovels, hoes and pans, the quarry face is at risk of failure. The quarry is still an active quarry, making it critical to analyse slope stability to ensure the safety editor@iaeme.com

Rini Asnida Abdullah1, Dedy Yusufianshah, Masagus Ahmad Azizi, Mohd Asmawisham Alel, Siti Nurafida Jusoh, Mohd Azril Hezmi, Nor Zurairahetty Mohd Yunus and Ahmad Nazri Ali of workers and the

STUDY SITE The site is located in the District of Karangsambung, Kebumen Regency, Central Java, Indonesia, with coordinates of 49S 353170 N and 9159789E.

Mining activity has created a slope with a height of 6 to 7 meters and a width of up to 75 meters. The slope has a steep angle with an inclination of 80ᵒ to 90ᵒ.

2 Rini Asnida Abdullah1, Dedy Yusufianshah, Masagus Ahmad Azizi, Mohd Asmawisham Alel, Siti Nurafida Jusoh, Mohd Azril Hezmi, Nor Zurairahetty Mohd Yunus and Ahmad Nazri Ali of workers and the surrounding area. The aim of this study is to assess the stability of the quarry face. RocScience software was utilised for modelling, as explained below. 2. STUDY SITE The site is located in the District of Karangsambung, Kebumen Regency, Central Java, Indonesia, with coordinates of 49S N and E. The slope was created by local mining activities. The area consists of calcareous sandstone that is interbedded with thin layers of siltstone. A geological map is shown in Figure 1. Mining activity has created a slope with a height of 6 to 7 meters and a width of up to 75 meters. The slope has a steep angle with an inclination of 80ᵒ to 90ᵒ. The top of the rock mass is moderately to highly weathered, while the bottom of the slope is fresh to slightly weathered. This condition could lead to failure, due to slope instability from improper mining methods. Figure 1 Geological map in study site 3. ROCK MASS CLASSIFICATION Scanline mapping was performed in two regions, Region 1 and Region 2 (Figure 2). Region 1 is located at the northern part of the slope with UTM coordinates 49S N and E, with a strike and dip orientation of N 220ᵒ E / 85ᵒ. The slope height is 7 m and the width is 10 m. Three major discontinuities were measured, rock bedding (J1) with an average orientation of N 115ᵒ E / 10ᵒ, the Joint set (J2) with an average orientation of N 265ᵒ E / 75ᵒ, and the Joint set (J3) with average orientation of N 165ᵒ E / 80ᵒ. Figure 2 Rock mass in Region-1 (Left) and Region-2 (Right) editor@iaeme.com

3 Slope Stability Analysis of Quarry Face at Karang Sambung District, Central Java, Indonesia Region 2 is located to the south of Region1 with UTM coordinates 49S N and E, with a 6 m height and facing N 260 E. The lower part of the slope has a dip of 70, while the upper part has a dip of 90. Scanline mapping was done in front of the slope with a 7 m horizontal measurement. The slope on this section consist of three major discontinuities which have similar orientations as in Region 1. Sedimentary rock bedding (J1) had an average strike and dip of N 120 E / 10, joint set (J2) had an orientation of N 265 E / 70, and another joint set (J3) had an orientation of N 150 E / 85. In general, there are 3 major discontinuity sets in the area. Some discontinuities have an open aperture without infilling, while some are filled with weak material which might increase the probability of failure. From the scanline survey, the quality of the slopes was categorised based on the RMR (Beinewiski, 1989). Table 2 summarises the RMR rating for both regions. Based on the RMR rating, Region 1 has an RMR value of 52 while Region 2 has an RMR value of 47, meaning that both regions are classified as having fair rock mass. The RMR rating is only capable of classifying the rock mass condition, without giving any indication of slope stability. Nonetheless, fair rock mass is an indication that the rock mass is not in good condition and weathered, potentially contributing to instability. Table 1 Rock Mass Rating using horizontal scanline Horizontal Scanline Vertical Scanline Parameters Region-1 Region-2 Region-1 Region-2 Strength of Rock (MPa) Rating RQD (%) Rating Average Spacing (m) Rating Groundwater Condition Damp Damp Damp Damp Rating Joint Condition Avg. Persistence (m) Rating Avg. Aperture (mm) Rating Roughness Rough Rough Rough Rough Rating Infilling material Soft filling < 5 mm Soft filling < 5 mm Soft filling < 5 mm Soft filling < 5 mm Rating Weathering Moderately weathered Moderately weathered Moderately weathered Moderately weathered Rating Strike-dip orientation depending on slope Strike perpendicular to slope axis, drive with dip < 20ᵒ (Favourable) Strike parallel to slope axis with dip < 20ᵒ (Fair) Strike perpendicular to slope axis, drive with dip < 20 (Favourable) Strike parallel to slope axis with dip < 20 (Fair) Rating RMR Rock Classification Fair Rock Fair Rock Fair Rock Poor Rock An alternative measurement was then conducted by changing the direction of scanline from horizontal to vertical. Due to difficulties in onsite vertical scanline measurement, a virtual measurement was performed. Smaller spacing obtained from the vertical scanline in editor@iaeme.com

4 Rini Asnida Abdullah1, Dedy Yusufianshah, Masagus Ahmad Azizi, Mohd Asmawisham Alel, Siti Nurafida Jusoh, Mohd Azril Hezmi, Nor Zurairahetty Mohd Yunus and Ahmad Nazri Ali both regions affected the RQD value. Therefore, RQD in Region 1 was reduced from 95% to 76%, while in Region 2 it was reduced from 97% to 66%. The changes in RQD affected the RMR classification. RMR calculations were recalculated based on the new RQD values in Table 1. The RMR value in Region 1 was reduced to 49, while in Region 2 it dropped to 40. Based on the rock classification, Region 1 is still classified as fair rock. However, the rock mass in Region 2 is classified as poor rock. The result for Region 2 explains the reason for slope failure observed in Region 2. In addition, different scanline directions might affect the RQD value, which then affects the RMR rating. Accuracy of scanline mapping is based on the placement and direction of the scanline itself and is a subjective observation. 4. KINEMATIC ANALYSIS Kinematic analyses were performed in DIPS software by RocScience (DIPS, 2017). The kinematic analysis of Region 1 shows 29% probability of wedge failure and 20% possibility of rock fall (Figure 3a and 3b). The failure was controlled by the two intersecting joint sets J2 and J3, which produced an intersection line parallel to the face slope. When the dip angle of the intersection line is smaller than the dip angle of the slope, wedge failure may occur (Wyllie and Mah, 2004). Rock fall is possible due to the steepness of the slope. Two joint sets (J2 and J3) intersect each other while the base plane is controlled by rock bedding, allowing rock blocks to fail. Smaller spacing of the discontinuities caused rock to be broken into smaller pieces, leading to rock fall. Region 2 shows the same type of failure but at a higher probability, with 49% probability of wedge failure and 23% probability of rock fall (Figure 2c and 2d). Wedge failure was caused by intersecting joints J2 and J3. Again, the slope in Region 2 has a dip of almost 90 and the occurrence of three joint sets breaks the rock mass into smaller pieces, increasing the probability of rock fall. Figure 3 Kinematic analysis on Region-1(a,b) and Region-2 (c,d) editor@iaeme.com

5 Slope Stability Analysis of Quarry Face at Karang Sambung District, Central Java, Indonesia 5. NUMERICAL MODELLING 5.1. Limit Equilibrium Method (LEM) Slope stability analysis requires the properties of the intact rock to be known. Therefore, rock samples were taken from the top, middle, and bottom of the slope. Secondary data of engineering properties of sandstone are summarized in Table 3. Density (Kg/m3) Table 2 Intact rock properties and input parameter for SWEDGE UCS (MPa) Young s Modulus (MPa) Poisson s Ratio Peak Residual τ (kpa) ø c (kpa) τ (kpa) ø c (kpa) Sandstone (U) Sandstone (M) Sandstone (L) Parameter Region-1 Region-2 Slope (Strike/dip) N 220 E / 85 N 260 E / 90 Slope height (m) 7 6 Joint set J2 (Strike/dip) N 275 E / 75 N 270 E / 75 Joint set J3 (Stike/dip) N 145 E / 80 N 150 E / 80 Unit Weight (Kg/m 3 ) 19.5 Cohession (Kpa) Friction Angle FOS Rocscience has specific software for each analysis based on the type of failure that can be done either by visual assessment or, as in this study, based on previous kinematic analysis. Slopes are further analysed in LEM using SWEDGE software (SWEDGE, 2017). Analysis on wedge failure was based on the Mohr-Coloumb failure criterion. Input data for SWEDGE and the Factors of Safety (FOS) of slopes are summarised in Table 2. Properties of sandstone (U) were chosen for this model. The lowest value of rock properties was chosen to simulate the worst condition. Two critical joint sets (J2 and J3) were analysed. As result, slope in Region 1 gave a Factor of Safety (FOS) of 0.79, while slope in Region 2 gave a FOS of Table 3 Summary of slope stability analyses Region Type of Analyses RMR Kinematic analysis LEM FEM Rock Fall Analysis Vertical scanline Horizontal scanline Region Wedge failure (29%) FOS : 0.79 FOS :1.78 End point location : (Fair Rock) (Fair Rock) Rock Fall (20%) 3.1 m Region Wedge Failure (49%) FOS : 0.58 FOS :1.31 End point location : (Poor rock) (Fair Rock) Rock Fall (23%) 3.6 m Description Used at the preliminary stage of the slope stability analysis to Used to define type of failure based on the orientation of the FEM is used to obtain SRF which can be compared Used to simulate the fall path. Important to identify the acknowledge the quality discontinuities. It of the rock mass. Easy to requires with result from LEM. The maximum end point location and impact apply on site. However, measurement of advantage of FEM energy. This can be this analysis is subjective which may give different result. discontinuities orientation. The result is then used to decide which software used for LEM. Unlike soil, stability in rock mass is controlled by discontinuities. Therefore, software which is used for LEM are specified for each type of failure. However, discontinuities cannot be modelled using this type of analysis. is that discontinuity parameter can be assigned in the model. used to determine the buffer zone and capacity of the remedial measures editor@iaeme.com

The application of FEM can overcome limitations in LEM, because instead of just the safety factor, the maximum shear strain, total displacement, and yield elements of the slope can be monitored

6 Rini Asnida Abdullah1, Dedy Yusufianshah, Masagus Ahmad Azizi, Mohd Asmawisham Alel, Siti Nurafida Jusoh, Mohd Azril Hezmi, Nor Zurairahetty Mohd Yunus and Ahmad Nazri Ali 5.2. Finite Element Method (FEM) Finite element analysis was performed using RS 2 software (RS 2, 2017). The application of FEM can overcome limitations in LEM, because instead of just the safety factor, the maximum shear strain, total displacement, and yield elements of the slope can be monitored (Mohamad et. at., 2015). In FEM, models were generated in two stages, an initial and an excavation stage. In the initial phase, slope was modelled as a solid mass, so that the initial stress can be fully generated, representing the hill before excavation. Then, the rock mass was excavated to the current condition of the slopes. Rock mass properties were assigned based three on regions as in Table 2(upper, middle and bottom intact rock properties). The model was simplified by reducing the number of joints to allow efficient computation. The model for Region 1 was generated with a 75 slope and three joint sets. From the analysis, the strength reduction factor (SRF) obtained was 1.78 (Figure 4), meaning the slope is stable. However, the analysis also shows that, some movement occurred in the middle of slope with a maximum of 200 mm displacement. Figure 4 FEM analysis for Region-1 Region 2 was modelled with a 70 slope at the bottom and an 80 angle at the middle. It consists of similar joint sets as Region 1, but was assigned a lower rock mass condition based on the RMR rating. The SRF obtained by the model for Region 2 was 1.31 (Figure 5). The slope in Region 2 gave a lower factor of safety due to the steeper slope angle and the concave slope shape. The model showed movement of rock blocks on the surface of the slope with 40 mm displacement at the middle of the slope. This agreed with site observations, where block movement has been seen, despite the factor of safety being greater than one. Figure 5 FEM analysis for Region editor@iaeme.com

Slope Stability Analysis of Quarry Face at Karang Sambung District, Central Java, Indonesia In both analyses, slope failure was controlled by discontinuities in the rock mass, supporting kinematic

Although finite element can provide information such as stress distribution, shear strain, and displacement, it also has a few limitations.

7 Slope Stability Analysis of Quarry Face at Karang Sambung District, Central Java, Indonesia In both analyses, slope failure was controlled by discontinuities in the rock mass, supporting kinematic analysis that showed the failure type was rock fall or wedge failure. Although finite element can provide information such as stress distribution, shear strain, and displacement, it also has a few limitations. A complicated model such as tight joint sets could not be run in this model. Therefore, the joint sets in the model were reduced. In addition, since the model is 2D, it was difficult to model joint sets which intersect each other. 6. ROCK FALL ANALYSIS Analysis of rock fall was carried out using RocFall software (RocFall, 2017) by constructing the slope face and assigning seeds. Seed, or sources of rock fall were located at different heights and locations, based on visual observation. Face roughness, dimension, and shape of the seed will affect the fall path. During model generation, the slope face roughness has to be modelled in more detail compared to the FEM. The dimension of the seed used were 30 cm x 30 cm x 20 cm, which is a rectangular block due to the lamination and jointing that created the rock block. The objective of this simulation is to analyse the impact energy and end point of the rock fall. This is useful for determination of the safe buffer zone. Figure 6 Rock fall analysis for (a) Region-1 and (b) Region-2 For Region 1, six different fall paths with various distance and impact energy were predicted. The maximum distance that the seed reached was 3.1 m from the foot of the slope (Figure 6a), however the kinetic energy produced at this maximum distance was only 0.4 kj. The highest kinetic energy 0.5 m from the bottom of the slope with 2.5 kj. This energy was produced by the seed which fell from the top of the slope. This result satisfies Newtons s second law where, as height increase, energy increases as well. A similar fall path was observed for Region 2. The maximum distance that the rock reached was 3.6 m (Figure 6b) and the highest kinetic energy was 0.5 m from the bottom of the slope with 1.6 kj energy. This was produced by a seed which was dropped from 5.5 m. 7. SUMMARY OF STABILITY ANALYSES Stability analyses on the quarry face were performed based. Table 3 summarises the results for each method, which suggest that the rock mass in Region 2 has lower quality than in Region 1, based on the RMR rating. Further analyses were performed, confirming that low rock mass contributed to the higher probability of failure in kinematic analysis and also to the lower safety factor in LEM and FEM analyses editor@iaeme.com

8 Rini Asnida Abdullah1, Dedy Yusufianshah, Masagus Ahmad Azizi, Mohd Asmawisham Alel, Siti Nurafida Jusoh, Mohd Azril Hezmi, Nor Zurairahetty Mohd Yunus and Ahmad Nazri Ali ACKNOWLEDGEMENT The authors would like to acknowledge the financial support under Geran Universiti Penyelidikan, Universiti Teknologi Malaysia (No: Q.J ) and Trisakti University, Indonesia for the research collaboration. REFERENCES [1] Bieniawski, Z. T. (1989). Engineering rock mass classifications: a complete manual for engineers and geologists in mining, civil, and petroleum engineering. John Wiley & Sons. [2] Cheng, Y.M. and Lau, C.K., 2014, Slope Stability Analysis and Stabilization: New Methods and Insight (2 nd ed.), CRC Press, pp [3] Das, B.M., 2005, Fundamental of Geotechnical Engineering (2 nd ed.), Thomson, Toronto, pp [4] Giani, G.P., 1992, Rock Slope Stability analysis, Balkema, Rotterdam, pp [5] Das, B.M., 2010, Principles of Geotechnical Engineering (6 th ed.), Thomson, California, pp [6] Hoek E., 2006, Practical Rock Engineering, Unpublished [7] Hoek, E. and Bray, J.W., 1981, Rock Slope Engineering (3 rd ed.), IMM, London, pp [8] Wyllie, D.C., and Mah, C.W., 2004, Rock Slope Engineering: Civil and Mining (4 th ed.), Spon Press, New York, pp [9] Brady, B.H.G. and Brown, E.T., 2006, Rock Mechanics for Underground Mining (3 rd ed.), Springer, Netherland, pp [10] Gupta, V., Bhasin, R.K., Kaynia, A.M., Kumar, V., Saini., Tandon, R.S., and Pabst, T., 2016, Finite Element Analysis of Failed Slope by Shear Strength Reduction Technique: a Case Study for Surabhi Resort Landslide, Mussoorie Township, Garwal Himalaya, Geomatics, Natural Hazzards and Risk 7:5, pp [11] Mithresh, K.P., Amalesh, J., Krishna, A.M., Dey, A., and Sreedeep, S., 2017, Stability Assesment of a Rock Slope Using Finite Element Modelling, International Conference on Geotechniques for Infrastructure Projets, Thiruvananthapuram [12] Mohammed Ali Mohammed Al-Baredm, Rini Asnida Abdullah, Nor Zurairahetty Mohd Yunus, Mohd For Mohd Amin and Haryati Awang, Rock Slope Assessment Using Kinematic and Numerical Analyses. Jurnal Teknologi. (E-ISSN: ) [13] Rao, K.S. and Singh, T, 2017, Two-dimensional Finite Element Based Parametric Analysis of South Portal Slope, Rohtang Tunnel, India, Procedia Engineering v.173, pp [14] RocScience,(2017) DIPS. Rocscience Inc. Toronto, Canada [15] RocScience,(2017) RocFall. Rocscience Inc. Toronto, Canada [16] RocScience,(2017) RS 2. Rocscience Inc. Toronto, Canada [17] RocScience,(2017) SWedge. Rocscience Inc. Toronto, Canada editor@iaeme.com

Stability Assessment of a Heavily Jointed Rock Slope using Limit Equilibrium and Finite Element Methods

Indian Geotechnical Conference 2017 GeoNEst 14-16 December 2017, IIT Guwahati, India Stability Assessment of a Heavily Jointed Rock Slope using Limit Equilibrium and Finite Element Methods Aswathi CK Amalesh