Assessment of Small Strain Dynamic Soil Properties of Railway site Agartala, India by Bender Element Tests Rajat Debnath NIT Agartala: National Institute of Technology Agartala Rajib Saha ( [email protected] ) NIT Agartala Sumanta Haldar IIT Bhubaneswar: Indian Institute of Technology Bhubaneswar Research Article Keywords: Shear modulus, Shear wave velocity, Bender element test, Indo Bangla Railway Project, Agartala, Peat soil. Posted Date: April 27th, 2021 DOI: https://doi.org/10.21203/rs.3.rs-292690/v1 License: This work is licensed under a Creative Commons Attribution 4.0 International License. Read Full License Assessment of small strain dynamic soil properties of railway site Agartala, India by Bender Element tests Rajat Debnath Ph.D. Scholar, Civil Engineering Department, National Institute of Technology Agartala Jirania-799046, Tripura (w), India. Email: [email protected] Rajib Saha Associate Professor, Civil Engineering Department, National Institute of Technology Agartala Jirania-799046, Tripura (w), India. Email: [email protected] Sumanta Haldar Associate Professor of Civil Engineering School of Infrastructure Indian Institute of Technology Bhubaneswar (IIT Bhubaneswar) Bhubaneswar – 751013, Odisha, India. Email: [email protected] Submitted for possible publication in Natural Hazards Address for correspondence: Rajib Saha Associate Professor, Civil Engineering Department, National Institute of Technology Agartala Jirania-799046, Tripura (w), India, Mobile No. 09436125024 Email: [email protected] Abstract Small strain dynamic properties of soil is the primary input parameter in seismic ground response analysis studies. This study examines the small strain shear modulus (Gs or Gmax) and damping ratio (ξ) using bender element tests on subsoil samples along with evaluation of liquefaction potential (Sandy soil) collected from a railway construction site at Agartala, India. The objective of this study is to develop a database and empirical relationship on dynamic properties of soil Agartala which may help to carry out site specific seismic hazard studies of Agartala city. In present study, both undisturbed and disturbed soil samples were collected from twelve boreholes near the study area which is mainly of soft marshy ground with localized deposits of peat layer. Similar type of soil is often found in various parts of Agartala basin and other regions of India. Experimental results indicate that Vs, Gmax and ξ vary within a wide range depending upon the type of soil and a closed form empirical equation are proposed to calculate Gmax for different types of soil which would be applicable for evaluating dynamic properties for similar nature soil available across various sites of India. Keywords: Shear modulus, Shear wave velocity, Bender element test, Indo Bangla Railway Project, Agartala, Peat soil. Introduction Determination of small strain dynamic parameters of soil such as Vs, Gmax, and plays a vital role in carrying out several earthquake geotechnical engineering applications, such as, ground response analysis, soil-structure interaction analysis, liquefaction potential evaluation and earthquake resistant design of different foundations as well as geotechnical structures. Bender Element (BE) test is found to be well recognized to obtain accurate values of Vs, Gmax and from element testing (Lings et al. 2001, Lee et al. 2006, Jung et al. 2007, Kumar et al. 2010, Jaya et al. 2012, Gu et.al. 2013, Gu et.al. 2015, Cai et al. 2015, Ogino et al. 2015). Vs measurement by BE test was originally introduced by Shirley and Hampton (1978) and further utilized widely by many researchers. BE testing is widely accepted due to its various advantages, such as, (i) low cost experiment and easy to perform in the laboratory, (ii) strains produced in the piezo-electric transducer are in the range of 10-6 which is well within the elastic limit of all varieties of soil and hence this test is non-destructive, (iii) methods adopted for data interpretation are simpler, and (iv) BE can be incorporated with other apparatus such as oedometer, and triaxial cell (Dobry et al. 1987, Fam et al. 1995, Brignoli et al. 1996, Brocanelli et.al. 1998, Valle et al. 2012). Several researchers highlighted on field based geophysical experiments which may be applied to obtain Vs profile of the ground (Zhang et al. 2004, Xu et al. 2006, Sil et.al. 2013). However, determination of Vs from field experiment is an indirect method based on several assumptions. Several correlations based on field Vs measurement and standard penetration test (SPT) also presented in many literatures using which an estimation Vs can also be determined, (Jafari et.al. 1997, Chien et.al. 2000, Kiku et.al. 2001, Hasancebi et.al. 2007, Dikmen et al. 2009, Anbazhagan et al. 2012, Sil et al. 2013). Hence, It clearly indicates that BE results are more reliable since it is a direct assessment technique and several dynamic parameters, such as Vs, Gmax, and can be calculated from a single experiment. In addition, advanced tests such as, resonant column (RC) and torsional shear (TS) test were also performed by many researchers to validate the results of BE test (Kumar et al. 2010, Gu et al. 2013, Yang et.al. 2013, Cai et al. 2015, Dammala et al. 2019). It was reported that BE results were found to be in good agreement with RC and TS tests. On the other hand, a number of studies also highlighted the importance of confining pressure (CP) and range of input frequency which influences Vs measurement during BE test (Jung et al. 2007, Karl et al. 2008, Kumar et al. 2010, Jaya et.al. 2012, Gu et al. 2013, Cai et al. 2015, Prasanth et al. 2018, Dammala et al. 2019). However, a considerable research on dynamic characterization of various types of soil based on BE experiment was reported (Brignoli et al. 1996, Brocanelli et al. 1998, Lings et al. 2001, Reddy et al. 2010, Murillo et al. 2011, Valle et al. 2012, Li et al. 2012, Payan et al. 2016). Several studies presented empirical correlations for different types of soil to calculate Vs, Gmax, and, based on BE test results (Andrus et al. 2000, Arroyo et al. 2003, Kumar et al. 2010, Bai et al. 2011). Few studies highlighted dynamic characterization of peat clay using BE test (Kramer et al. 2000, Kishida et al. 2006, Karl et.al. 2008, Rahman et.al. 2015). Further, several works on BE testing were also performed in India on mainly alluvium sandy soil (Jaya et al. 2008, Kumar et al. 2010a, 2010b, Jaya et al. 2012, Dammala et al. 2017a,b, 2019, Prasanth et al. 2018). In view of the above context, it is evident that site specific assessment of dynamic characteristics is important and rational for performing seismic design of various civil infrastructures. Present study is an attempt in this direction to perform dynamic characterization of soil collected from proposed India Bangladesh railway project site at Agartala, the capital of NE state Tripura by performing BE test and subsequent analysis on liquefaction potential of that zone. The prime focus of present study is to obtain the dynamic characteristics of subsoil of study area upto maximum 20-25 m depth below ground level since the study area lies within Indo-Bangla potential seismic zone (Tripura fold zone) and close to Chittagong fault line. Further, as per Bureau of Indian Standards (BIS) IS: 1893 (Part 1)-2002 the study area was recognized as severe earthquake prone zone of the country. Figure 1 presents the site location map along with GPS coordinates from where both disturbed (DS) and undisturbed (UDS) soil samples were collected for testing. On the other hand, the seismic hazard of Agartala is growing up tremendously as it is the second highest populated city with population density of 10,119 persons per square kilometer after Guwahati (capital of Assam) at NE India. The growth is increased manifold times and in faster rate since infrastructural development is underway after recognition of smart city. In addition, Agartala was selected as business corridor of South-East Asian countries through Bangladesh which accordingly enhances the global importance in the upcoming days. Further to be noted that in the recent past a moderate earthquake of Mw of 5.7 occurred on 3rd January 2017 at Tripura which had resulted significant liquefaction in north district of Tripura (Das et al. 2018, Anbazhagan et al. 2019, Saha et al. 2020). Hence, evaluation of liquefaction potential of the present site with similar soil properties, which is located at an approximate distance of 80 km from the reported liquefaction region is also considered in the scope of present study. Probabilistic approach based on Vs proposed by Cetin et al. (2004) is adopted in present study to evaluate liquefaction susceptibility. Finally, present experimental study will help to generate database as well closed form empirical expressions on small strain dynamic properties, such as, Vs, Gmax, along with liquefaction potential of typical Agartala soil. The outcome of this study will help to perform seismic site-specific hazard studies on Agartala and other regions with similar soil properties in a reasonably accurate manner and vulnerability studies of different types of structures located in those regions. Seismicity of the study area and site characterization of target site As per seismological zonation, the whole North-East (NE) India including Agartala, the capital of Tripura (23.750 – 23.900N and 91.250 – 91.350E) categorized under highest seismic zone V. The seismo-tectonics of NE area is complicated because of interaction between the active north-south convergence along the Himalaya, the east-west convergence and folding within the Indo-Burma ranges which attribute a deformation known as subduction (Kayal 2008). Two third boundary of Tripura including Agartala and some parts of Assam, Mizoram are sharing international border with Bangladesh. In past, several moderate (Mw 5 to 7) to few mega range (Mw 7) earthquakes had occurred in this region mainly due to the presence of numerous active faults and this region was recognized as Indo-Bangla potential seismic zone.