NATURE OF PROJECT: ROCK FALL HAZARD
CLIENT: PERAK HANJOONG SIMEN SDN BHD
LOCATION: PADANG RENGAS, PERAK
INTRODUCTION
Gunung Pondok consists of limestone that could give the risk to the people who live close to the hill. There are several villages nearby the east hill of Gunung Pondok namely Kampung Sungai Ati, Kampung Pondok, Kampung Pulau, and Kampung Padang Rengas. Light Detection and Ranging (LiDAR) and geological field survey were conducted to assess the physical characteristics of the Gunung Pondok (east hill). The LiDAR field data acquisitions were conducted using Terrestrial Laser Scanning (TLS) and Unmanned Aerial Vehicle (UAV) method. The geological discontinuities mapping, and identification of rock material were conducted using the scanline method and Point Load Index Tests at three outcrops, namely GP1, GP2, and GP3. The surveys were performed from 9th to 22nd September 2016.
Figure 1: Gunung Pondok Hazard Hill
Figure 2: Site condition of Gunung Pondok for TLS data capturing
RESULTS AND DISCUSSION
i) LiDAR Survey
LiDAR survey that was conducted provides the Digital Elevation Model (DEM) of the hazard hill. Several factor maps were generated such as hillshade map, elevation map, contour map, slope gradient map, slope aspect map as well as aerial photograph. These maps were used in rockfall trajectory analysis to visualize the running distance of the rock. Based on the LiDAR survey, a natural hazard hill estimated dimension of the total unstable rock mass being 100m long, 110m wide and 80m height with estimated weight of 2.38 million ton of limestone rock.
ii) Discontinuity Mapping and Kinematic Analysis
Discontinuity mapping and kinematic analysis conducted to determine the behavior of rock discontinuities (bedding, joint, fault) that effects the stability of the hill. The potential failures at the site were wedge failure, planar failure and toppling failure. Two (2) cavities were also identified near the base of hazard hill. The results of the analysis utilized in rock mass classifications based on Slope Mass Rating (SMR) method.
Figure 3: Location of outcrop GP1, GP2 and GP3, Gunung Pondok, Padang Rengas Perak
iii) Slope Mass Rating (SMR)
Slope Mass Rating (SMR) assessment was conducted utilizing the results from kinematic analysis, discontinuity mapping and point load test. Based on the analysis, the probability of failure of the slope ranged from 0.2 (stable) to 0.9 (completely unstable).
Figure 4: Slope stability distribution at eastern hill of Gunung Pondok, Padang Rengas, Perak
iv) Rock Fall Trajectory and Hazard Zoning
The 875m length slope was divided into 43 sections (25m differences) which are CH0 to CH875 to be simulated. The rockfall trajectory analysis conducted based on Colorado Rockfall Simulation Program (CRSP) and Lump Mass Method. The CRSP and Lump Mass Method were developed based on equation of motion to estimate energy, velocity, and height of bouncing of rock. The analysis shows that the maximum rolling distance for slope ranged from 56m to 440m from slope toe. The rock blocks from slope CH188, CH213, CH475, CH500, CH513, CH525, CH538, CH550, CH563, CH575, CH588, CH688, CH700, CH713, CH725, CH738, CH763, CH775, CH788, CH800, CH813, CH825 and CH838 were simulated to fall toward Kampung Sungai Ati.
Figure 5: Example of rockfall run out analysis conducted by using Rocfall software
The hazard zones were determined based on the results of rock fall analysis and Guideline by JMG (2013). The hazard zones divided into three (3) zones which are high hazard zone, intermediate hazard zone and low hazard zone. Distance of high hazard zone, intermediate hazard zone and low hazard zone in entire study areas were ranging from 56 m to 440 m, 12 m to 473 m and 381 m to 566 m. The distance of high hazard zone is the maximum rolling distance. The distance of intermediate hazard zone was calculated by minus the 1.5 times of height of slope (1.5H) with maximum rolling distance of rock fall. The distance of low hazard zone is equal to the 1.5 times of height of slope, H.
Figure 6: Hazard zones and slope stability distribution for Gunung Pondok, Padang Rengas, Perak
SUGGESTION AND RECOMMENDATION
The results of the analysis have been presented to the State’s Exco in 2016. The state together with the Perak Hanjong management are on negotiation to choose the best method to avoid harm to the people. As for GMT, based on the results and analysis, the long-term best mitigation measure to reduce the risk from the hazard hill is to establish and demarcate a buffer zone around the hazard hill.
CONCLUSION
The hazard assessments of 49 slopes (875 m) using Slope Mass Rating (SMR) rock mass classification suggested that the probability of failure of slopes at study area ranging from 0.2 to 0.9. The total width of slope with probability of failure of 0.2, 0.4, 0.6 and 0.9 are 200.25 m, 337.25 m, 281.25 m and 56.25 m respectively. The stability of slopes varies from Completely Unstable to Stable. The rock fall analysis recommended that the maximum rolling distances for slope ranging from 56 m to 440 m from slope toe. The rock fall simulation results suggested that the distance of high hazard zone, intermediate hazard zone and low hazard zone in this study area were ranging from 56 m to 440 m, 12 m to 473 m and 381 m to 566 m.
