Liquefaction Potential Evaluation Based on Site Classes – A Performance Based Approach

Lots of research work is being carried out in evaluating the liquefaction susceptibility. The main objectives of these studies are to identify the regions which are vulnerable to liquefaction. In the present study, an attempt has been made to predict the liquefaction susceptibility based on corrected SPT values required to prevent the liquefaction for given return periods. The evaluation of liquefaction susceptibility requires the calculation of two parameters, seismic loading and the soil resistance. In most of the studies, the seismic loading will be evaluated based on probabilistic methods and the evaluation of soil resistance will be done based on deterministic analysis. In the present study these parameters were evaluated based on the probabilistic methods. The contour curves showing the spatial variation of SPT values required to prevent the liquefaction for return periods of 475 and 2500 years are presented here. The liquefaction hazard curves, based on SPT values for some of the selected cities in the study area are also presented here

Nepal earthquake of April 25, 2015

A powerful earthquake of magnitude (M) 7.8 occurred on April 25, 2015 at the plate boundary between Indian plate and Eurasian plate. The epicenter of this earthquake is located approximately 80 km northwest of Kathmandu, Nepal. This epicenter location is near to the district Gorkha for which it is also called as Gorkha earthquake. The convergent movement of the Indian plate with the Eurasian plate resulted in the strain accumulation along major faults and hence produced many significant earthquakes along the boundary in past. This earthquake is as severe as its predecessor event, the 1934 Nepal-Bihar earthquake where the fatalities was 10,600 while 8000 and above during this event. We describe in detail the seismotectonic aspects of the 2015 Nepal earthquake (Mw 7.8) and the damage caused by it. We also provide a background on the seismicity of the Himalayan region

Site Response Study of Deep Soil Column in Lucknow, India

Earthquake activities in the many parts of world had shown the importance of local soil condition in the propagated wave motion. In this paper an attempt has been made to estimate site effects of deep soil column in Lucknow, Indo-Gangetic basin for scenario earthquakes at Himalayan plate boundary. Based on previous study, the synthetic ground motion has been generated using Stochastic Finite Fault model (FINSIM) for two scenario earthquakes at seismic gaps. One seismic gap called as western location/seismic gap is located about 307.88km from site and gives the peak ground acceleration of 0.11g at site. Another one is the Central seismic gap/location is located in central seismic gap at 229.77km from the site and gives the Peak Ground Acceleration (PGA) of 0.218g. The local soil layer details with standard penetration test N value have been collected for the main location in Lucknow from literates. The general soil found for this site is silty sand and silty clays having SPT N value up to above 100 up to a depth of 30m. The soil details extrapolated up to 100m by assuming linear variation from 30 m. The site response analysis has been carried out using equivalent linear and non linear approaches by employing SHAKE 2000 and Deep soil program. Input has been assigned at 30m 50, 75 and 100m to find effective depth of input motion. This study shows that the ground motions are being amplified within a depth of 50 to 80 m, but these results need to be further confirmed with large number of data. The input ground motions are amplified 1.06 to 2.5 times due to the soil condition

Correlation Between Low Strain Shear Modulus and Standard Penetration Test ‘N’ Values

In this study an attempt has been made to develop correlation between standard penetration test (SPT) N values and low strain shear modulus (Gmax). The field experiments of Multichannel Analysis of Surface Wave (MASW) are carried out at 38 locations close to boreholes having Standard Penetration Test N values and in-situ density. These experimental data were generated and used for seismic microzonation of Bangalore, India. In-situ densities of subsurface layers were obtained from undisturbed soil samples collected from the boreholes. Shear wave velocity (Vs) profile with depth were obtained for the same locations or close to the boreholes using MASW. The low strain shear modulus values have been calculated using measured Vs and soil density. About 215 pairs of SPT N and shear modulus values are used for regression analysis and correlation between them are developed. The differences between fitted regression relations using measured and corrected N values were analyzed and presented. More details of correlation between shear modulus versus measured and corrected SPT N values and comparisons are presented elsewhere

Dem Modelling of Granular Materials During Cyclic Loading

This paper summarises the potential of the Discrete Element Method (DEM) to simulate the behaviour of granular materials under cyclic loading conditions. DEM simulations were carried out on two different assemblies of granular particles (e.g. sand and ballast). It is shown that DEM is capable of simulating the cyclic behavior of granular materials (e.g. liquefaction, post liquefaction and cyclic densification of ballast) similar to the laboratory experiments. Moreover, an instrumented track at Bulli, NSW, Australia, was modeled using DEM to examine the lateral movement of granular materials, including the particle breakage during cyclic loading. The results of these simulations captured the lateral response of ballast in accordance with the field observations and the evolution of micro-mechanical parameters such as a distribution of the contact force and bond force developed during cyclic loading is presented to explain the mechanism of particle breakage

The function of basal geogrids in minimizing rutting of geocell reinforced subgrades

Rutting is a common phenomenon encountered in flexible pavements supported by weak subgrades. Reinforcing the weak subgrades is one of the promising alternatives to alleviate the pavement surface rutting. This paper presents the results of laboratory model tests on a circular plate supported by geocell reinforced sand subgrades. A series of tests were carried out by varying the height of the geocell mattress with an additional layer of basal geogrid placed underneath the geocell mattress. The surface settlements (rutting) were measured through displacement gauges. Strain gauges were placed along the width of the basal geogrid to verify their performance as a base layer. A substantial reduction in surface rutting is observed in the case of geocell reinforced beds with basal geogrids. A seven fold improvement in bearing capacity was obtained with the provision of an additional geogrid layer over unreinforced subgrades. Overall, a basal geogrid layer provides higher structural support mobilized through membrane effect to the geocell reinforced pavement layer

Principles and Practices of Seismic Microzonation: Case Studies in India

This paper presents an overview of the principles and practices of seismic microzonation with some case studies in India. India has experienced major damages and loss of life due to earthquakes. Macrozonation map in Indian seismic code IS-1893 is frequently revised soon after a major earthquake in the country. New revision which was published in 2002 after Bhuj earthquake in 2001 contains four macro zones. These zones are based on geology and limited seismology input without considering geotechnical aspects such as site effects and liquefaction. In order to understand the earthquake vulnerability of major urban centers and prepare new zonation map, the Govt. of India has initiated microzonation of 63 cities in India after 2001 earthquake. Many microzonation studies are under progress and few of them have been completed. This paper presents an overview of these studies. Seismic microzonation of Jabalpur urban area is the first work in India towards seismic microzonation of Indian cities. Jabalpur study has provided many learning lessons to other studies. Preliminary microzonation of Delhi has been completed and detailed one is under progress. Seismic Hazard and Microzonation Atlas of the Sikkim Himalaya has been published with geological and seismological background. Microzonation of Guwahati was done based on geology, geomorphology, seismotectonics, soil characteristics, pre-dominant frequencies, peak ground acceleration, seismic hazard and demography. Seismic Microzonation of Dehradun has been prepared based on shear wave velocity with site response. First order Microzonation of Haldia has been developed based on peak ground acceleration, predominant frequency and elevation map. Different maps and results were presented for Gujarat microzonation based on noise survey and after shock data. None of these studies included the geotechnical aspects. The geotechnical aspects were fully incorporated in the recently completed Microzonation work of Bangalore and the ongoing study of Chennai microzonation. An overview of seismic microzonation studies in India is presented in this paper

A Note on the Effect of Non-Plastic Fines on the Liquefaction and Reconsolidation Volumetric Strain Behaviour of Sands

This paper discusses the influence of non- plastic fines on the liquefaction and reconsolidation volumetric strain behaviour of sand based on the strain controlled cyclic triaxial experimental results. The investigations were carried out on sand samples collected from earthquake-affected area of Ahmedabad city of Gujarat state in India. Laboratory investigations were conducted on clean sand (prepared from the natural sand) samples with varying amount of non-plastic fines. Experimental results highlight a considerable influence of non-plastic fines on the resistance to liquefaction and reconsolidation volumetric strains behaviour during cyclic loading