K. S. Vipin

A performance-based framework for assessing liquefaction potential based on CPT data

This article describes a new performance-based approach for evaluating the return period of seismic soil liquefaction based on standard penetration test (SPT) and cone penetration test (CPT) data. The conventional liquefaction evaluation methods consider a single acceleration level and magnitude and these approaches fail to take into account the uncertainty in earthquake loading. The seismic hazard analysis based on the probabilistic method clearly shows that a particular acceleration value is being contributed by different magnitudes with varying probability. In the new method presented in this article, the entire range of ground shaking and the entire range of earthquake magnitude are considered and the liquefaction return period is evaluated based on the SPT and CPT data. This article explains the performance-based methodology for the liquefaction analysis – starting from probabilistic seismic hazard analysis (PSHA) for the evaluation of seismic hazard and the performance-based method to evaluate the liquefaction return period. A case study has been done for Bangalore, India, based on SPT data and converted CPT values. The comparison of results obtained from both the methods have been presented. In an area of 220 km2 in Bangalore city, the site class was assessed based on large number of borehole data and 58 Multi-channel analysis of surface wave survey. Using the site class and peak acceleration at rock depth from PSHA, the peak ground acceleration at the ground surface was estimated using probabilistic approach. The liquefaction analysis was done based on 450 borehole data obtained in the study area. The results of CPT match well with the results obtained from similar analysis with SPT data.

Multiple source and attenuation relationships for evaluation of deterministic seismic hazard: logic tree approach considering local site effects

The most important seismic hazard parameters required to demarcate seismic zones are the peak horizontal acceleration (PHA) and spectral acceleration (SA). The two approaches for evaluation of seismic hazard are the probabilistic seismic hazard analysis and the deterministic seismic hazard analysis (DSHA). The present study evaluates the seismic hazard of the South Indian Peninsular region based on the DSHA methodology. In order to consider the epistemic uncertainties in a better manner, a logic tree approach was adopted in the evaluation of seismic hazard. Two types of seismic sources and three different attenuation relations were used in the analysis. The spatial variation of PHA (mean and 84th percentile values) and SA values for 1 Hz and 10 Hz at bedrock level (84th percentile values) for the entire study area were evaluated and the results are presented here. The surface level peak ground acceleration (PGA) values will be different from that of the bedrock level values due to the local site conditions. The PGA values at ground surface level were evaluated for four different National Earthquake Hazard Reduction Program site classes by considering the non-linear site response of different soil types. The response spectra for important cities in South India were also prepared using the deterministic approach and the results are presented in this paper.

Seismic Response of Twin Tunnels in Weathered Rocks

With rapid urbanization and wide use of underground space, the behaviour of underground structures under various conditions has gained significant attention. Twin tunnels are two closely constructed tunnels. Underground structures are generally supported by the surrounding medium like soil or rock. Due to this additional constraint from the ground, the underground structures are not as vulnerable as compared to the surface structures during an earthquake event. But recent earthquakes have shown that these structures are also prone to damages. Unlike surface structures, the soil-structure interaction is very important in the seismic analysis of underground structures. Moreover, in the case of structures like tunnels, rather than inertial forces the deformation behaviour is controlling the seismic behaviour. The various waves generated during an earthquake event generate additional forces and moments, which may jeopardize the stability of tunnels/underground structures. Among the different types of ground instability associated with seismic loading, the upward and downward movement of the ground due to seismic waves is studied here. In this study, a two dimensional numerical simulation using finite element method is done to understand the response of twin tunnels in weathered type of rock. The dynamic load is applied in the form of earthquake loading. The effect of relative spacing between tunnels is studied when they are subjected to earthquake loading. The results show that due to the dynamic loading the effects on ground settlement is significant especially when the relative spacing between the tunnels is less.