J. SE Asian Appl. Geol., Sep–Dec 2010, Vol. 2(3), pp. 202–207

EVALUATION OF ROCKFALL OCCURRENCE INPADANG–BUKITTINGGI ROAD,WESTSUMATRA, INDONESIA

Wahyu Wilopo∗, Dwikorita Karnawati, Salahuddin Hussein, and Ignatius Sudarno

Department of Geological Engineering, Gadjah Mada University, Yogyakarta, Indonesia

Abstract

A big earthquake occurred on September 20, 2009 inPadang, West Sumatra, Indonesia. This earthquakecaused damage and loss of both property and lives.The earthquake also triggered the emergence of rockfalls in several areas in Padang, especially in the roadof Padang to Bukitinggi. After the earthquake inSeptember, several big rockfalls occurred more thanthree times that cost lives. Based on field investi-gations, the incidence of rock falls was caused byfield conditions which was prone to rockfall, such assteep slope and exposure of rocks to intensive struc-tures that then triggered by earthquake and rainfall.Therefore, fast action is needed to be taken in orderto minimize the impact of the rockfall disaster.Keywords: Rockfall, slope, rock structure, earth-quake, rainfall

1 Introduction

Rockfalls are major hazards in rock cuts forhighways and railways in mountainous ter-rains such as in Indonesia. While rockfalls donot pose the same level of economic risk aslarge scale failures which can close major trans-portation routes for days at a time, the num-ber of people killed by rockfalls tends to be ofthe same order as people killed by all otherforms of rock slope instability. The hazard ofrockfalls is higher in areas with intense seis-

∗Corresponding author: W. WILOPO, Department ofGeological Engineering, Faculty of Engineering, GadjahMada University, Jl. Grafika 2 Yogyakarta, 55281, Indone-sia. E-mail: wwilopo@gadjahmada.edu

mic activity where earthquakes are the princi-pal triggering factor (Marinos and Tsiambaos,2002). On September 20, 2009, a big earth-quake occurred in Padang, West Sumatera, In-donesia. This earthquake caused some buildingdamages and also triggered the landslide androckfall in many places. A big rockfalls haveoccurred in the road from Padang to Bukit-tinggi especially around Anai Waterfall, situ-ated at the foot of a 280 m high andesite rockcliff (Figure 1). This road is the main route toBukittinggi from Padang. Three people alreadydied because of this disaster after the Septem-ber 20, 2009 earthquake. Therefore, actions areurgently required in order to minimize the ef-fect of rockfall disaster.

2 Engineering geological conditions

The geological formations encountered in thearea consist of pumice tuff (Katowo, et al.,1996). The rock slope overhanging the roadfrom Padang to Bukitinggi consists of andesite.A section of this road cuts the great Sumateranfault, therefore this area is very complex in termof geological structures. Two major fault zoneswith NE-SW and NW-SE strike intersect the for-mation, forming the valley as shown in Figure1. The pumice tuff and andesite rock mass isstrongly fractured and weathered, intersectedby numerous major vertical and horizontal frac-tures which have a parallel strike to the faultstructures that forms the rock face (Figures 2aand 2b). The spacing of the discontinuities isrelatively small (less than 1 m); however, there

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are also big boulders in the top of slope. Inplaces, mainly due to stress relief, the rockmassis very blocky and the size of the rock blocksis smaller especially on the upper part and thecrest of the slope where the wall of road is situ-ated.

3 Rockfall analysis

The stability conditions of the rock slope aremainly controlled by the following factors: a)due to spacing, large blocks are prone to fall, b)lack of persistency of the discontinuity planes,which results in instabilities only in specificparts of the slope, c) lack of weak zones, whichwould result in large shear failures, d) the rockface is in a state of relaxation due to the highinclination of the slope (Marinose, et al, 2009).The rock slope stability analyses were based onthe prevailing mode of instability of each po-tential rock failure. The principal failure type isrockfall due to a sort of toppling and bouncing(Figure 3). The rock blocks were delineated andtheir geometry and mass was determined. Dueto the inaccessible nature of the slope, the as-sessment of the above characteristics was basedon the topographic map of the rock cliff. Thesecharacteristics were grouped and specific sec-tions (A to G) were formed for separate anal-ysis (Figure 4). The surveying and mapping ofthe high rock cliff were based on a new geodeticmethodology and using the topographical mapfrom secondary data. The height of the sourceof fallen-potential blocks is at the minimum of70 meters above the road, while in some placesit reaches 280 meters. The size of the unsta-ble materials which are more probable to fall isonly between 0.5 m3 to about 5 m3 (Fig. 4a), ex-cept in the area between Sections C - D (Figure4a ) where the size can go up to 20 m3. In thearea shown in Figure 4, the potentially unstableblocks have very large dimensions.

The necessary support measures can be di-vided into two categories: a) those which offerprotection once the rockfall will occur, mainlyrock fall barriers and, b). those which apply anexternal force on the rock face e.g. tensionedrock anchors or patterned rock bolts. Othersupport, such as grouting of rock joints with

associated drainage, construction of buttressesin overhanging areas and removal of unstableboulders are necessary, however can be inap-plicable or very difficult to construct in highrock cliffs. The installation of a steel protec-tion net or sprayed concrete is not acceptabledue to the wide area. As discussed earlier, thescale of some potential failures is such that nostabilization measures can minimize or with-draw the risk of a potential rockfall after theirapplication. Even high-energy rock fall barri-ers would prove insufficient in the case thatthe rock blocks shown in Figure 3 are detachedfrom the cliff. A possible support solution, inthis case, would be to install tensioned wire-rope cables around the rock boulders to resisttheir movement and to grout any open discon-tinuities with appropriate drainage holes.

4 Rockfall risk assessment

In order to assess rockfall risk, a number of rat-ing systems have been developed. Pritchardet al. (2005) developed a rating methodology,which is applied to predict rockfall risk alongrailways. A similar system is the Rockfall Haz-ard Rating System (RHRS) (Pierson et al., 1990),which is most widely accepted. These sys-tems give a reasonable assessment of the rela-tive hazards due to rockfalls from cut slopes ad-jacent to highways and railways. In the presentstudy, a rockfall risk rating system for naturalrock slopes is proposed according to Marinoset al. (2009). It defines 19 rating parameters,grouped in 3 major categories on hazard andconsequences, which have a different weightfactor in the assessment of the total risk. Theparameters are presented in Table 1.

The rating system was applied at selected lo-cations along the rock cliff (Figure 4) since theparameter rating differs for each slope segment.The parameters that vary from one location toanother were: a) the size and number of rockblocks, b) the spacing and persistence of discon-tinuities, c) the seeder height, d) the width ofthe available catchments zone and e) the exis-tence of structures or human activity at the un-derlying area. The slope height and angle of theroad slopes vary significantly. The result of the

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Figure 1: Geological map of the research area (Katowo, et al., 1996)

Figure 2: (a) Slope of weathered pumice tuff; (b) Fractured andesite in Anai Waterfall

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EVALUATION OF ROCKFALL OCCURRENCE IN PADANG–BUKITTINGGI ROAD, WEST SUMATRA

Figure 3: (a) Andesitic materials that are still hanging in the top of slope; (b). A big andesite boulderfrom the rockfall

Figure 4: Risk zonation of rock slope based on rockfall rating system

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Table 1: Parameters of rockfall rating system for natural rock slopes to define risk (Marinos et al,2009)

application is a risk zonation of the cliff againstrockfall occurrence, presented on the risk mapshown in Fig.4. The map depicts the areas ofhaving a high and very high risk due to eitherincreased number of existing unstable bouldersor restricted area for their catchments or com-bination of both and the high of cliff. The riskcategories are presented in Table 2. The slopefoot area between sections C to D presents veryhigh risk due to the numerous unstable boul-ders on the high cliff with the size more than 5m3, therefore the installation of barriers is notpossible. The area between sections D to E hashigh risk, due to lower cliff than C- D, which of-fers possible conditions for installation of barri-ers. The area between sections B to C and E-Fdoes not have many boulders and stills manyvegetation with medium slope and low cliff.

5 Conclusions

The rock slope stability of the high cliff over-hanging the road from Padang to Bukittinggiwas studied, based on field investigation of un-stable boulders and prediction of their rockfalltrajectories. The main triggering factor of rock-fall in this road is rainfall, therefore it is neces-sary to close the road when rainfall occurs tominimize the negative impact. The applicationof active and passive support is very difficultto apply in the field due to high slope and thesize of boulder. It is evident that in the case ofboulders having weights higher than 10 tonnes,even the installation of high capacity rockfall

barriers cannot remove the hazard due to im-pact of falling rocks on structures, either be-cause the impact energy is extremely high or thecatchments zone is not sufficient for optimizedprotection.

References

Katowo, Gerhard W. Leo, S. Gafoer and T.C. Amin(1996) Geological Map of The Padang Quadran-gle, Sumatera,Geological Research and Develop-ment Centre, Republic of Indonesia.

Marinos P, Tsiambaos G (2002) Earthquake trigger-ing rockfalls affecting historic monuments and atraditional settlement in Skyros Island, Greece.Proc. of the International Symposium: Landsliderisk mitigation and protection of cultural and nat-ural heritage, Kyoto, Japan, pp. 343-346.

Marinos P., Tsiambaos, G., Saroglou, H., Marinos V.(2009 Rockfall hazard and risk for a high promon-tory: Monemvasia historical site, Greece, Land-slide Disaster Risk Reduction, Ed. K. Sassa et al.,Springer, 2009, pp. 206 – 209.

Pierson L A, Davis S A, and Van Vickle R (1990)Rockfall Hazard Rating System ImplementationManual. Federal Highway Administration Re-port FHWA-OR—EG-90-01. FHWA, U.S. Depart-ment of Transportation.

Pritchard M, Porter M, Savigny W, Bruce I, OboniF, Keegan T, Abbott B (2005) Rockfall hazard riskmanagement system: Experience, enhancements,and future direction. Landslide Risk Manage-ment: Proc. of the Int. Conference on LandslideRisk Management, Vancouver. Balkema.

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EVALUATION OF ROCKFALL OCCURRENCE IN PADANG–BUKITTINGGI ROAD, WEST SUMATRA

Table 2: Rockfall risk of natural rock sloped based on the Marinos, et al. (2009)

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