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Top1. Introduction
Local site condition plays a major role in the earthquake motion for structural design. A study by Seed and Idriss (1982) showed that the peak ground acceleration (PGA) recorded on the rock is comparable to soil; however, the difference is more pronounced at lower acceleration levels (lower than 0.1g). The shape of the response spectrum is very dependent upon local site condition. In their study, Seed et al. (1976) discovered that sites with weak, flexible soil yield higher spectral acceleration at periods larger than 0.4s, compared to stiff soils. In general, flexible soil sites tend to amplify the long-period motions while stiffer soils will amplify the shorter-period motions. The effect of flexible soil is evident during the Mexican earthquake in 1985. Mexico City, located 400km from the epicenter, is heavily damaged by the earthquake compared to locations that are much closer. The field report by EEFIT (Booth et al., 1986) concluded that the motion amplified by the local site condition is very large, even though the motion attenuated by the distance is considered to be harmless. The amplification is found to be 10 times of rock site, at the period of about 2s. The effect is due to the 40m soft superficial clay underlaying the city and damages medium rise structures.
Kuala Lumpur, Malaysia is a city that was considered to be safe from earthquake threat due to its far location from nearby faults. However, the increasing tremors felt due to far-field earthquakes increase awareness of engineers on the importance of earthquake loads to the design of structures (Shoushtari et al., 2016). Previous study shows that the magnitude of peak ground acceleration (PGA) from the far-field earthquake is relatively low at approximately 0.02g (Nabilah and Balendra, 2012). However, local soil properties could amplify the motion further, possibly damaging structures resting on it. Balendra and Li (2008) conducted seismic hazard assessment on three sites in Singapore, namely Marine Parade, Katong Park and Katong sites, all with clay layers. The soil amplification factors vary from 10 to 12, which resulted in the spectral acceleration of 0.06 to 0.1g. The fundamental period of the soils varies from 1 to 2s, which will affect primarily the high-rise buildings resting on it. However, Megawati and Pan (2009) reported an amplification of 4.8 for flexible soil in the southern part of Singapore at similar periods. Marto et al. (2011) developed 4 synthetic time-histories to evaluate the effects of soil amplification to the ground motion in Putrajaya and Kuala Lumpur, Malaysia. From their study, the average amplification factor in Putrajaya is 1.94 for ground type D and 2.17 for ground type E. Several weak soils are identified in Bandar Puteri Puchong, Mutiara Damansara and Bandar Petaling Jaya in Kuala Lumpur, based on a study done by Husen et al. (2008). Majid et al. (2007) proposed design response spectra for Northern Peninsular Malaysia (300 to 500km from Kuala Lumpur) based on Uniform Building Code for various soil classifications. 50 soil investigation reports consisting of 193 borehole data are collected and the soil amplifications are evaluated using Nonlinear Earthquake Site Response Analysis software (NERA, 2001).
Pitilakis et al. (2012) in their research highlighted the possibility that the soil factor for flexible soil (ground types C, D and E) could be higher than the value proposed in Eurocode 8. Hence, a more thorough study is recommended to address this issue. For regions that are highly affected by large far-field earthquakes (example Kuala Lumpur, Singapore and Hong Kong), other country’s seismic design methodology cannot be adopted blindly (Su et al., 2015). A study by Lam et al. (2014) for flexible soil in Singapore found that the soil amplification could be up to 4 times the bedrock motion at the high period range. This is significantly higher than the value given in the code.