Rock Slope Stability Assessment of Limestone Hills in Northern Kinta Valley , Ipoh , Perak , Malaysia

Limestone, failure modes, rock slope stability assessment The uniqueness of karsttopography in Kinta Valley lies with the spectacular shape of the steep-sided limestone towers. However, theinstability ofthese hillslopes may affect the vulnerability of the surrounding area. Thus, this study was conducted with the objective to investigate the failure modes of 9 slopes in the vicinity ofnorthernKinta Valley, Ipoh, Perak. There were two types of failure modes identified in the study area,which are planar and wedge failures. Planar failures were identified on slope GL3of Gunung Lang and slope GR3of Gunung Rapat with the dip direction and dip angle of 2800/790 and 0040/ 640 respectively. Two wedge failures were identified on slope QXL1of Qing Xing Ling, Taman Saikat with dip direction and dip angle of 2520/820 and 3020/740 respectively. A wedge failure was identified on slope GL3 for Gunung Lang, slope GR1, slope GR3 for Gunung Rapat and slope QXL2 for Qing Xing Ling,Taman Saikat. The dip direction and dip angle for the respective wedges failure were 3450/650, 0360/ 490, 0060/ 640 and 0250/600. No failure was identified on slope GL1, and slope GL2,forGunung Lang and slope GR2, and slope GR4forGunungRapat.


INTRODUCTION
Karst topography in the Kinta Valley is characterized by the steep-sided limestone hills with many limestone morphological features such as caves and do lines.Kinta Valley was proposed to be developed as one of the national geoparks in Malaysia due to its beautiful landscape [1].
The literature study revealed that less research studies have been conducted on limestone hillslope stability assessment.The local researchers were focused on rock mass classification, landslides, rock fall and prediction of uniaxial compressive strength using ultrasonic velocities [2][3][4][5][6][7][8][9][10].Some researcher characterized the discontinuity surface roughness by establishing a polynomial relationship between JRC with peak friction angles schist and granite [11][12][13].A scientist, investigated the influenced of conditions of weathering to the geomechanical strength of Granites and Schists [14].
However, the geological structures such as jointing and fractures and daylighting of rock blocks that characterize the steep sided slopes may hasten the occurrence of geohazards such as rock slides and rock falls as stated [6].According to a research, geological hazards will affect the vulnerability of development in the encompassing areas [15].Generally, the main factor for rockfall events is due to structural failure [15].Chemical weathering from water dissolution and quarry activities were also classified as the factors causing rockfalls such as the incident at Gunung Tunggal [16].The impact of a rockfall can also affects its surrounding in which the air blast resulting from the fallen rock debris can be felt at a distance that is much further from the catastrophe area which could also affect nearby buildings [17,15].
The instability of limestone hills such as at Gunung Cheroh in the Kinta Valley was investigated by the Geological Survey of Malaysia [18].One of the incidents of rockfall tragedy that had occurred at Gunung Cheroh, Ipoh, Perak caused the death of 40 people in October 1973 [15].Rock falls from Gunung Pondok, Perak were also reported in a research paper [19].About these issues, this research was conducted to investigate the failure modes of 7 slopes of limestone hills in the Kinta Valley by using kinematic analysis as recommended by a researcher [20].The recorded rockfall events occurrences in the Kinta Valley and the consequences are shown in Table 1.

Geology of Study Areas
The study area is in the Kinta Valley, Perak as shown in Figure 1.The study areas cover the massive limestone bodies that are heavily jointed and fractured.The average size of limestone hills in the Kinta Valley are 1.08 km2 with the maximum elevation of up to 546 m based on the topographic map.The localized, highly weathered and poorly stratified schist was found at the bottom of a massive limestone body [18,21].This limestone bodies were named as Kinta Limestone Formation with the age of Silurian to Permian [2 2].The Kinta Valley is bordered by Trassic granite on both side.The limestone bodies were deposited on stable continental shelf in a marine environment [21].The calcareous sedimentation during Late Paleozoic was interrupted by argillaceous sedimentation from the deeper ocean [21].

METHODOLOGY
Discontinuity surveys were conducted using the scan line method as suggested by two researchers and ISRM where 10 discontinuity parameters were considered [23,24].The parameters are dip direction, dip of angle, discontinuity length (persistence), aperture, surface

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roughness, infilling, weathering, groundwater condition, number of joint set and block size.
The kinematic analysis was conducted by using Stereo32 software.The results were interpreted to identify the types of failure mode based on classification of failure modes [20].The peak friction angle for each slope was determined by using the polynomial equation developed by a researcher [25].Slopes of Gua Naga Mas; GNM1, GNM2, and GNM3 are composed of 4 to 5 major joint sets; slopes of Gua Kandu have 3 to 4 major joint sets; while the slope of Gua Tempurung, GT is composed of 4 major joint sets.The orientation of major joint sets and slope face for respective slopes are shown in Table 2.

Gua Naga Mas
Three slopes were investigated at Gua Naga Mas which were labelled as GNM1, GNM2 and GNM3.The outcrops of the slopes are shown in Figure 3.The stereographs of respective slopes are shown in Figure 4.The peak friction angle for GNM1 was 55⁰.The peak friction angle for both GNM2 and GNM3 was 49⁰. Figure 5 shows the results of kinematic analysis for respective slopes.Based on the kinematic analysis, four wedge failures and a planar failure were identified at slope GNM1 and the dip directions/dip angles for respective wedge and planar failures were 005⁰/54⁰, 354⁰/59⁰, 124⁰/52⁰, 360⁰/50⁰ and 063⁰/70⁰, respectively.The mode of failure identified at slope GNM2 was wedge failure with the dip direction/dip angle of 021⁰/64⁰ whereas two wedge failures and a planar failure were identified at slope GNM3.The dip directions/dip angles for respective wedge and planar failures at slope GNM3 were 336⁰/49⁰, 301⁰/68⁰ and 270⁰/71⁰, respectively.Table 3 shows the summary of rock slope assessment at Gua Naga Mas according to failure mode, failure direction, peak friction angle and joint roughness coefficient.

RESULTS AND DISCUSSION
A total of three research stations were located at Gua Naga Mas, Gua Kandu, and Gua Tempurung.The slopes at Gua Naga Mas were labelled as GNM1, GNM2 and GNM3.The slopes at Gua Kandu and Gua Tempurung were labelled as GK1, GK2, GK3 and GT respectively.Figure 2   44⁰.No mode of failure was identified at slope GK3.Table 4 shows the summary of rock slope stability assessment at Gua Kandu according to failure mode, failure direction, peak friction angle and joint roughness coefficient.

Gua Kandu
Three slopes were investigated at Gua Kandu and were labelled as GK1, GK2 and GK3.The locations of the respective slopes are shown in Figure 6.
The stereographs of the respective slopes where are shown in Figure 7.
The peak friction angle for slope GK1, GK2 and GK3 were 49˚, 36˚ and 55˚ respectively.Figure 8 shows the results of kinematic analysis for the respective slopes.The mode of failure for slope GK1 were wedge and planar failures with the dip directions/dip angles for wedge and planar failures as 231⁰/49⁰ and 217⁰/49⁰, respectively.The mode of failure at slope GNM2 was wedge failure with the dip direction/dip angle of 154⁰/

Gua Tempurung
The slope investigated at Gua Tempurung was labelled as GT.The location of the slope is shown in Figure 9.The stereograph of the slope is shown in Figure 10.The peak friction angle for GT was 49⁰. Figure 11 shows the results of kinematic analysis for slope GT.Two wedge failures were identified on slope GT with dip directions/dip angles of 011⁰/49⁰ and 321⁰/48⁰.Table 5 shows the summary of rock assessment at Gua Tempurung according to failure mode, failure direction, peak friction angle and joint roughness coefficient.

CONCLUSION
The slope stability assessment using kinematic analysis revealed that there were wedge failures and planar failures at both Gua Naga Mas and Gua Kandu.Only wedge failure was identified at Gua Tempurung.Slope GNM1 consist of four wedge failures and planar failure with failure directions of 005⁰/54⁰, 354⁰/59⁰, 124⁰/52⁰, 360⁰/50⁰ and 063⁰/70⁰ respectively.Slope GNM2 consist of a wedge failure with the dip direction/dip angle of 021⁰/64⁰.Two wedge failures and a planar failure were identified on slope GNM3.The dip directions/dip angles for respective wedge and planar failures on slope GNM3 were 336⁰/49⁰, 301⁰/68⁰ and 270⁰/71⁰ respectively.Slope GK1 consist of wedge and planar failure with dip direction/dip angle of 231⁰/49⁰ and 217⁰/49⁰ respectively.The mode of failure on slope GNM2 was wedge failure with the dip direction/dip angle of 154⁰/44⁰.No mode of failure was identified on slope GK3.Slope GT was identified to have two wedge failures with dip direction/dip angle of 011⁰/49⁰ and 321⁰/48⁰ respectively.

Figure 11 :Table 5 :
Figure 11: Kinematic analysis of slope GT for Gua Tempurung, Gopeng, Perak Table 5: Summary of rock slope assessment at Gua Tempurung according to failure mode, failure direction, peak friction angle and joint roughness coefficient.

Table 3 :
Summary of rock slope stability assessment at Gua Naga Mas according to failure mode, failure direction, peak friction angle and joint roughness coefficient.

Table 4 :
Summary of rock slope assessment at Gua Kandu according to failure mode, failure direction, peak friction angle and joint roughness coefficient.