Seismicity in slope stability practice in Malaysia.

Ting, W.H. ; OOI, T.A. ; Ahmad, Aminuddin 等

Recent earthquake experiences have revealed that Malaysia is vulnerable to the earthquake originating from active plate boundaries in terms of tremors. Bridges, high-rise buildings and dams are greatly exposed to these seismic activities during their lifetime. This paper presents the seismic design requirements in terms of different g values used in various dam slope stability designs in Malaysia. The design g value applied in each dam site is dependent on the information, advice and guidance given by the Malaysian Meteorological Service (MMS) as the national information centre for seismology.

INTRODUCTION

Malaysia, in particular Peninsular Malaysia, has been considered as a non-seismic risk country until the tsunami incidence on December 26, 2004. In 1982, the Public Works Department, Malaysia, cautioned in the Keynote address to the Conference on Tall Buildings in Kuala Lumpur that Malaysia is actually located in the 'ring of fire' that marks the areas affected or likely to be affected by earthquakes and predicted that Malaysia will experience in the future severe earthquakes arising from the Sumatra and Andaman fault that runs the length of Sumatra and the Andaman Sea to the north of the Straits of Malacca (Yunus, 1982). The tsunami that occurred on 26 December 2004 can be said to be the prediction that came true. The interest in earthquake design amongst the Malaysian engineers in fact already started then. The Komtar Towers in Penang and the Penang Bridge both built in the 1980s in fact were designed for seismic forces.

TECTONIC SETTING OF MALAYSIA

Malaysia is surrounded by the two most seismically active plate boundaries namely, the plate boundary between the India Plate (part of the huge Indian-Australian Plate that underlies the Indian Ocean and the Bay of Bengal) and the Burma Plate (part of the Eurasian Plate) on the west at the Sunda (Java) Trench off Sumatra, and (b) the plate boundary between Eurasian and Philippine Plates on the east. Large earthquakes originating from in and around these boundaries resulted in tremors being felt in Malaysia (Tibballs, 2005). The maximum observed intensities of these tremors on the Modified Mercalli (MM) scale are VI for Peninsular Malaysia and VII for East Malaysia (Mohd Rosaidi bin Che Abas, 2001). Figure 1 shows the major tectonic plates and plate boundaries around Malaysia.

[FIGURE 1 OMITTED]

Two recent major earthquake events occurred in Sumatra on the 4 June 2000 and 26 December 2004 with measured magnitudes of 7.9 and 9.3 respectively on the Richter scale. These events had resulted in people having to evacuate from their homes and office buildings in panic and most significantly the loss of life during the tsunami that struck Kuala Kedah and Penang Island on 26 December 2004. These events clearly indicate that although Peninsular Malaysia is classified as a seismically stable area, we are still vulnerable to the effect of earthquakes that originated from the Sumatran active fault. Large earthquakes from this fault have created considerable ground motions over the western part of Peninsular Malaysia. The locations of epicentres of earthquakes near Peninsular Malaysia and in Sabah and Sarawak are shown in Figures 2 to 4. Earthquakes in plate boundary areas usually cluster in place and time. At a particular earthquake source location, clusters may recur after a long period of time perhaps thousands or hundreds of thousands of years and large amount of energy will have to be released (Seismology Research Centre Victoria Australia, 2000).

[FIGURES 2-4 OMITTED]

THEORY AND PRINCIPLE OF SEISMIC FORCES ON DAM SLOPES

In most earthquake hazard studies, the main concern is the response of the structure to earthquake vibrations. There are ground motion hazards, surface rupture, landslip, liquefaction or tsunami. East Malaysia is affected by low magnitude earthquakes that measured below 6.0 on the Richter scale, which is a measure of the earthquake magnitude (M) that includes the logarithm of the amplitude of the motion recorded on seismograph. The earthquake is assumed to produce ground acceleration intensity below 0.15g recorded by the accelero-graph. These intensity measurements are essential in determining the risk to structural damage (Azlan et al., 2001). The seismic effects are then used in the design of bridges, high-rise buildings and dams in Malaysia since these structures could be exposed to important seismic effect during their service lifetime. It is determining the risk to structural damage (Azlan et al., 2001). The seismic effects are then used in the design of bridges, high-rise buildings and dams in Malaysia since these structures could be exposed to important seismic effect during their service lifetime. It is also noted that the liquefaction of poor soil deposits is negligible since in Malaysia we do not have high waste dump and thick natural soft clay deposit as in Sweden and other parts of the world. However, the high-rise condominiums located in the alluvium over the cavernous Limestone formation in Kuala Lumpur have experienced the effect of earth tremors. During earthquake, landslides in dam slopes and slumping of dam embankment may occur and some failures may be of major consequences. These failures are attributed to increase in shear stresses caused by seismic loading. Decrease or loss of strength during cyclic loading due to ground motion may have caused major and catastrophic failures. Figure 5 shows the seismic force acting on a potential sliding mass. The seismic force is equal to the weight of the potential sliding mass times a seismic coefficient in terms of g (ground acceleration intensity). Safe slope gradients for dam slopes are determined using slope stability analysis program using the limit equilibrium method to check for slip failure conditions for the undrained, drained and rapid draw-down conditions. The input for g values is normally shown as earthquake loading coefficients.

[FIGURE 5 OMITTED]

DESIGN OF SLOPE STABILITY FOR DAMS

It is common practice in Malaysia to meet seismic design requirements by using different g values for dam slope stability design. Table 1 shows the different g values used. The design g value applied in each dam site is dependent on the information, advice and guidance given by the Malaysian Meteorological Service (MMS) as the national information centre for seismology. The iso-seismal maps from the Malaysian Meteorological Service (MMS) are used as an indicator for zones that received different levels of recorded intensity on the Modified Mercalli (MM) scale. The maximum observed intensity was VI for Peninsular Malaysia and VII for East Malaysia on MM scale (Mohd Rosaidi bin Che Abas, 2001). Figure 6 shows the iso-seismal map for Peninsular Malaysia.

[FIGURE 6 OMITTED]

CONCLUSIONS

Seismic factors have been considered in the design of dam slope stability in Malaysia. The design requirements in terms of different g values (ground acceleration intensity) used in slope stability analyses for dams are dependent on the information, advice and guidance given by the Malaysian Meteorological Service (MMS) as the national information centre for seismology. Based on distinct differences in seismicity, higher design g values are used foe East Malaysia when compared to Peninsular Malaysia. As for the Peninsular Malaysia, the west coast has a higher seismic intensity in MM scale when compared to that of the east coast, it is therefore expected to experience greater effect of the tremors. In view of recent earthquake activities that have affected Malaysia, it is hoped that Malaysia will have its own seismic hazard assessment map for various parts of Malaysia on structures such as bridges, high-rise buildings, dams etc. Only then will it be able to apply its own seismic regulations on all structures in order to safeguard the general public in terms of loss of lives and properties, mitigation measures in disaster prevention and environmental protection.

REFERENCES

Azlan Adnan et al. (2001). "Seismic Hazard Assessment of Malaysia", Seismic Risk Seminar 2001, Malaysia.

Mohd Rosaidi bin Che Abas. (2001). "Earthquake Monitoring in Malaysia", Seismic Risk Seminar 2001, Malaysia.

Seismology Research Centre Victoria Australia (2000). "Review of Seismicity, Kelalong Dam for GHD ", June 2000. Technical Study Report, 25.

Tibballs, G. (2005). "Tsunami: The World's Most Terrifying Natural Disaster", Carlton Books Ltd., London.

Tungah Surat. (2001). "Case History of Earth Tremors in Malaysia", Seismic Risk Seminar 2001, Malaysia.

Yunus, M. M. Y. (1982). "Keynote Address- Building High", Proceedings Asian Regional Conf. on Tall Buildings and Urban Habitat, Kuala Lumpur, Institution of Engineers, Malaysia, K-1 to K-8

W.H. TING

Zaidun Leeng Sdn Bhd, Tingkat 5, Jalan Bukit Nanas, 50250 Kuala Lumpur, Malaysia.

T.A. OOI

TAO Consult Sdn Bhd, 17A, Jln Awan Hijau, Taman Overseas Union Batu5, Off Jln Klang Lama, 58200 Kuala Lumpur, Malaysia

AMINUDDIN AHMAD

Zaidun Leeng Sdn Bhd, Tingkat 5, Jalan Bukit Nanas 50250 Kuala Lumpur, Malaysia

B.K. TAN

Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor Darul Ehsan, Malaysia

Table 1 Different g values used in dam slope stability design
in Malaysia

 Dam Locations Design g values

1. Gemencheh, Negeri Sembilan 0.10g
2. Sungai Selangor, Selangor 0.10g
3. Sg. Kinta, Perak 0.15g
4 Bakun, Sarawak 0.20g
5. Kelakong, Sarawak 0.30g