IGC. 50 th. 50 th INDIAN GEOTECHNICAL CONFERENCE LIQUEFACTION ANALYSIS OF ALLUVIAL SOIL DEPOSIT

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1 5 th 5 th INDIAN GEOTECHNICAL CONFERENCE 17 th 19 th DECEMBER 215, Pune, Maharashtra, India LIQUEFACTION ANALYSIS OF ALLUVIAL SOIL DEPOSIT D. Das 1, A. Ghosh 2 ABSTRACT Kolkata, one of the oldest metropolitan cities of India, which is situated on the east bank of river Hugli, lies on the boundary between zone III and IV, with an equivalent PGA range of g in accordance to the prevailing seismic zoning map of India (BIS, 22) prepared based on the peak ground acceleration (PGA) induced by the maximum considered earthquake (MCE). The most prominent tectonic feature in the Bengal Basin is the NE SW-trending Eocene Hinge Zone (EHZ), also known as the Calcutta Mymensing Hinge Zone, which is 25. km wide and extends to a depth of about 4.5 km below Kolkata. The city is mainly formed by alluvial deposit, which is erratic in nature and is situated on a sedimentary deposit that is above the crystalline basement. Large number of Standard Penetration Test (SPT) and laboratory test data of several boreholes (more than seventy borehole data) has been collected from several locations. A computational tool has been developed in MATLAB environment to analyze the liquefying depth of the soil profile based on simplified procedure. The tool converts the field SPT blow count to clean sand equivalent N1,6cs and it further analyzes the Cyclic Stress Ratio (CSR), Cyclic Resistance Ratio (CRR), Magnitude Scaling Factor (MSF) and factor of safety against liquefaction for each borehole based on the Peak Ground Acceleration (PGA) and Moment Magnitude (Mw) values for different locations. These results are further analyzed based on the susceptibility criteria. The analysis revealed that though the liquefying depth is approximately 13. m to 15. m, the susceptibility criteria reveals that the layer clayey silt/silty clay with decomposed wood which is roughly extending between 5. m to 12. m is highly susceptible to liquefaction as the natural moisture content to liquid limit (Wc/LL) is well above.85 in most of the locations and plasticity Index value also lies in between 12% to 18% in most cases which reveals that the soil is resistant but may liquefy and in some occasions it is less than 12% which reveals the soil is susceptible to liquefaction. Keywords: Liquefaction, SPT, Computational tool, Factor of Safety 1 Das_Debarshi, Post Graduate Student, Department of Civil Engineering, IIEST, Shibpur, India, debarshi94@gmail.com 2 Ghosh_Ambarish, Professor, Department of Civil Engineering, IIEST, Shibpur, India, ambarish@civil.iiests.ac.in

2 5 th 5 th INDIAN GEOTECHNICAL CONFERENCE 17 th 19 th DECEMBER 215, Pune, Maharashtra, India LIQUEFACTION ANALYSIS OF ALLUVIAL SOIL DEPOSIT *D. Das, Post Graduate Student, Department of Civil Engg., IIEST, Shibpur,debarshi94@gmail.com A. Ghosh, Professor, Department of Civil Engineering, IIEST, Shibpur, ambarish@civil.iiests.ac.in * Corresponding author ABSTRACT: Kolkata is mainly situated on newly laid alluvial soil deposit and lies at the boundary of seismic zone III and IV with major part of it falling in zone IV as per latest studies. The Peak Ground Acceleration of most of the regions is well above.16g, with some regions having as high as.23g. In this work, numbers of borehole and laboratory data were collected from various sites of Baguiati, Newtown, Rajarhat and adjoining areas to analyze the seismic liquefaction hazard of the region and also the highly susceptible layer(s) identified based on the stress based approach and susceptibility criteria. The change in CSR and CRR based on latest studies are also presented in this paper. INTRODUCTION Newly laid soil deposits that are being deposited by rivers, lakes and wind actions are loosely packed and their density is also less. Moreover, if the soil particles are not well graded then the void present in the soil is high. These kinds of soils have high susceptibility towards liquefaction since they tend to densify when an earthquake occurs. This phenomenon increases the pore water pressure and subsequently decreasing the strength of the soil. Cities situated on flood plain, especially on the banks of rivers or sea or any other newly laid sedimentary soil deposit may undergo major hazard due to earthquake. One of the major hazards for these cities due to earthquake is liquefaction, since the ground water table is near to the surface. With the aim of analyzing the liquefaction hazard due to earthquake for these types of soil deposits, liquefaction analysis of Kolkata soil was done by the authors and the results are shown here. Geology of Kolkata Kolkata is the third largest urban agglomeration in India. The city is located about 15 km north of the Bay of Bengal, right over the Ganges Delta and on the eastern bank of river Hooghly. Adi Ganga use to flow through the city near Kalighat area. The flat topography of Kolkata has an average elevation of 6.4 m above mean sea level, the highest location is about 9.5 m at Netaji Subhas Chandra Road (Clive Street), and the lowermost place is Mechhua at an elevation of 5.5 m. The population of Kolkata, which was 1.5 million in the year 191, had a phenomenal increase to 14 million as per the census report of 211, thus making it one of the most densely populated regions in the world. The city is situated on a sedimentary deposit with a thickness of the order of 7.5 km that is above the crystalline basement which can be further divided into quaternary sediments which is m, followed by 4,5 5,5 m of tertiary sediments, 5 7 m trap wash of cretaceous trap and 6 8 m permian-carboniferous Gondwana rocks. The city is highly developed, with many old buildings, bridges, subways, tall structures, huge shopping malls, and lifeline facilities that require earthquake disaster mitigation and safety regulations. Seismotectonic Details The prevailing seismic zoning map of India (BIS, 22) prepared based on the peak ground acceleration (PGA) induced by the maximum considered earthquake (MCE) further constrained by the geologic and seismotectonic considerations, thus scaling it down to the design basis earthquake

3 D.Das, A.Ghosh (DBE) for urban codal provisions, places the entire city of Kolkata at the boundary between zone III and IV, with an equivalent PGA range of g. Calcutta Mymensing Hinge Zone, more prominently known as Eocene Hinge Zone (EHZ) trending in the NE-SW is the most prominent tectonic feature in the Bengal Basin. It is 25. km wide and extends to a depth of about 4.5 km below Kolkata. This zone triggered an earthquake of magnitude M w 6.2 in 1935, with the epicentre at lat N, long E, that mostly affected Bangladesh, but a prolonged tremor was felt at Kolkata. Study Area Kolkata, the capital of West Bengal is a fast growing and expanding metropolis. Lot of construction works is undergoing in various sectors such as infrastructure, realty, hospitals, schools etc. Since the city is expanding a major portion of construction is being carried out in reclaimed land. So, if proper mitigation measures are not taken during design and construction the vulnerability of the structures from an earthquake hazard is very high. To study the vulnerability of the soil strata towards liquefaction more than seventy borehole data were collected from different site locations of Newtown, Rajarhat, Baguihati, and some parts of Saltlake, Keshtopur as well and their adjoining areas with a minimum borehole depth of 2. m to study the soil stratification and seismic behaviour of the soil. The data were collected from these locations keeping in mind that the major volume of construction work is being carried out in these locations. Moreover most of the areas in these locations are reclaimed land. Figure 1 shows the location of all the boreholes taken for the study. Fig. 1 Satellite view of the borehole location. FRAMEWORK OF THE COMPUTATIONAL TOOL FOR LIQUEFACTION ANALYSIS The stress-based approach for evaluating the potential for liquefaction triggering, initiated by Seed and Idriss (1967), compares the earthquakeinduced cyclic stress ratios (CSR) with the cyclic resistance ratios (CRR) of the soil. The soil's CRR is correlated to the in-situ parameter

4 5 th 5 th INDIAN GEOTECHNICAL CONFERENCE 17 th 19 th DECEMBER 215, Pune, Maharashtra, India Standard Penetration Test s blow count (N) with required modifications. The coding is developed in MATLAB environment in two distinct parts. The first part converts the field SPT value (N) to N 1,6cs N 1,6cs = N 1,6 + ΔN 1,6 (1.1) N 1,6 = N C N C E C B C R C S (1.2) and, ΔN 1,6 = exp( (1.3) FC = Fines Content of the soil In this work, the field SPT (N) values are modified based on the modification factors given by Skempton (1986), Youd et al. (21), Schmertmann and Palacios (1979), Idriss and Boulanger (24 and 28). The equivalent clean sand adjustments, ΔN 1,6, are thus empirically derived from the liquefaction case history data, and accounts for the effects that fines content have on both the CRR and SPT penetration resistances. The adjustment for the SPT-based correlation is based on Idriss and Boulanger (28). The computation of N 1,6cs is an iterative process and this N 1,6cs is further used to compute the factor of Safety. The next part of the coding calculates the Factor of Safety of the soil against liquefaction. The main components of this part are Cyclic Stress Ratio, Cyclic Resistance Ratio, Stress Reduction Factor and Factor of Safety. The cyclic stress ratio, CSR, as proposed by Seed and Idriss (1971), is defined as the average cyclic shear stress,, developed on the horizontal surface of soil layers due to vertically propagating shear waves normalized by the initial vertical effective stress,, to incorporate the increase in shear strength due to increase in effective stress. It is represented as CSR = =.65 ) (2.1) Here, a max = Peak Ground Acceleration in terms of g. r d = Stress Reduction Factor, as per Idriss (1999). r d = exp[α(z) + β(z) M] (2.1.1) α(z) = sin( +. (2.1.1a) β(z)= sin M = Moment Magnitude (2.1.1b) The Cyclic Resistance Ratio (CRR) of soil is usually correlated to modified SPT blow count. The soil's CRR is also affected by the duration of shaking and is correlated with Magnitude Scaling Factor (MSF) and effective overburden stress (which is expressed through a Kσ factor).the correlation for CRR, given by Idriss & Boulanger (28) was developed for a reference M = 7.5 and σ v ' = 1 atm, and then adjusted to other values of M and σ v ' using the following expression: CRR M,σv = CRR M=7.5,σv =1 MSF K σ (2.2) CRR M=7.5,σv =1 = exp((,6 +.,6,6. +,6..8 (2.2.1) Effect due to duration of an earthquake for triggering of liquefaction is incorporated by Magnitude Scaling Factor (MSF). It calculates the number and relative amplitude of loading cycles. It is given by MSF=1+(MSF max 1) (8.64 exp. (2.2.2) MSF max = (, (2.2.2a) The overburden stress factor (K σ ) developed by Boulanger (23) is given by K σ = C σ ln 1.1 (2.2.3) C σ =..,6.3 (2.2.3a) The above equations are shown in two flowcharts. These flow charts give a clear idea on the steps of the coding.

5 D.Das, A.Ghosh Fig2.1: Flowchart for calculation of N 1,6cs Nath et al. (214) gave a new perspective of multi-criteria holistic seismic hazard assessment for Kolkata based on an enriched homogeneous earthquake catalog. The PGA of different areas in the present work is incorporated from their work. The analysis of soil strata for seismic liquefaction vulnerability is followed by liquefaction susceptibility criteria of the soil strata to identify the most susceptible layer(s) vulnerable to seismic liquefaction. In the present study, liquefaction susceptibility criterion, proposed by Bray and Sancio (26) is followed as this criterion is developed because Chinese criteria are not reliable for determining the liquefaction susceptibility of fine -grained soils. The criteria states that: Loose soils with PI 12 and w c /LL.85 were susceptible to liquefaction. Loose soils with 12 PI 18 and w c /LL.8 were systematically more resistant to liquefaction. Soils with PI 18 tested at low effective confining stresses were not susceptible to liquefaction. Additionally, the results of the cyclic testing program provide insights regarding the effects of confining pressure, initial static shear stress, and stress-path on the liquefaction of fine-grained soils. Additionally, there may be cases where: Sensitive soils with PI 18 undergo severe strength loss as a result of earthquake induced straining. Fig2.2: Flowchart for calculation of Factor of Safety So the proposed criteria should be applied with engineering judgment. SOIL STRATIFICATION OF THE STUDY AREA The general soil stratification, as revealed from the soil exploration data is as follows: A fill layer is observed at the top with varying thickness with depth upto 2.5 m. Though, mostly it is upto 1.5 m depth. It is generally composed of loose soil. A thin layer of greyish clayey silt is observed in most of the locations with varying thickness. Greyish clayey silt/silty clay with decomposed wood layer exist roughly between 5. m to 12. m depth in almost all observations except in Dumdum area. The layer varies widely upto 16. m depth. The above observations were also cited by Dastidar and Ghosh(1967) and Das and Chattopadhyay(29). OBSERVATION FROM LIQUEFACTION ANALYSIS The SPT results of the boreholes were analyzed by the computational tool developed. The results obtained from the analysis are mainly divided in three locations namely Rajarhat, Newtown and Baguihati. The graphs given below show the change in CSR value as per the latest study and the CSR value change.

6 5 th 5 th INDIAN GEOTECHNICAL CONFERENCE 17 th 19 th DECEMBER 215, Pune, Maharashtra, India N1,6cs PGA =.23 PGA = CSR (a) PGA =.23 N1,6cs CRR (b) Mw = 6.5 Mw= 7. 4 PGA =.16 4 N1,6cs 3 2 N1,6cs 3 2 Mw = Mw = CSR (c) PGA =.23 PGA = CRR (d) N1,6cs CSR (e) (f) Fig3: (a),(c) & (e) represents the variations between CSR vs N 1,6cs graphs for Baguiati, Newtown and Rajarhat respectively. (b), (d) & (f) represents the variations between CRR vs N 1,6cs graphs for Baguiati, Newtown and Rajarhat respectively. N1,6cs CRR Mw = 7. Mw = 6.5

7 5 th 5 th INDIAN GEOTECHNICAL CONFERENCE 17 th 19 th DECEMBER 215, Pune, Maharashtra, India OBSERVATION FROM SUSCEPTIBILITY CRITERIA The layer that is highly susceptible to liquefaction in Rajarhat area by this procedure is: Clayey silt with decomposed wood since the Plasticity Index is greater than 18 or in between 12 and 18 in many observations, the w c /LL well above 1. which is more than the limiting criteria.85. The layer that is highly susceptible to liquefaction in Newtown area by this procedure is: Clayey silt with decomposed wood the Plasticity Index is greater than 18 or in between 12 and 18 in many observations, the w c /LL well above 1. and some ranging as high as 1.7 which is much more than the limiting criteria.85. The layer that is highly susceptible to liquefaction in Baguihati area by this procedure is: Clayey silt with decomposed wood since Plasticity Index is greater than 18 in almost all occasion and the w c /LL varies from.5 to.99. CONCLUSION From the liquefaction analysis based on the state of the art procedure and most recent findings and from Susceptibility Criteria, the following conclusions are drawn. The change in Cyclic Stress Ratio is quiet significant in comparison of Cyclic Resistance Ratio. So it can be concluded that the degree of liquefaction vulnerability is quiet high as it was previously thought. The old PGA of Kolkata was.16 as compared to the newly found PGA which is as high as.23 in some areas and more than.16 in all areas. So the vulnerability of old structures is high for an earthquake. Clayey silt with decomposed wood layer which is roughly extending between 5. m to 15. m is highly susceptible to liquefaction. So, to mitigate the liquefaction hazard of the region the densification of this layer is essential. For this purpose proper ground improvement techniques should be taken. REFERENCES 1. Bray, J. D. and Sancio, R. B. (26) Assessment of the liquefaction susceptibility of fine-grained soils, Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol. 132, No. 9, pp Chakraborty, P., Pandey, A. D., Mukherjee, S. and Bhargava, A. (24) Liquefaction Assessment For Microzonation Of Kolkata City 13th WCEE Vancouver, B.C., Canada 3. Das, D. and Chattopadhyay, B. C. (29) Characterization of Soil over Kolkata Municipal Area., Guntur. 4. Idriss, I. M. and Boulanger, R. W. (24) Evaluating the potential for liquefaction or cyclic failure of silts and clays. Centre for Geotechnical Modelling, UCB, Report No. UCD/CGM-4/1 5. Idriss, I. M. and Boulanger, R. W. (21) SPT- Based Liquefaction Trigerring Procedure. Centre for Geotechnical Modelling, UCB, Report No.UCD/CGM Idriss, I. M. and Boulanger, R. W. (214) CPT And SPT Based Liquefaction Triggering Procedure. Centre for Geotechnical Modelling, UCB, Report No UCD/CGM-14/1 7. Nath, S. K., Adhikary, M. D., Maiti, S. K., Devaraj, N., Srivastava, N. and Mohapatra, L. D. (214) Earthquake scenario in West Bengal with emphasis on seismic hazard microzonation of the city of Kolkata, India. Nat. Hazards Earth Syst. Sci., 14, , Seed, H. B. (1976) Evaluation of Soil Liquefaction Effects on Level Ground During Earthquakes, Liquefaction Problems in Geotechnical Engineering, ASCE Annual Convention and Exposition, Philadelphia, PA, October, pp Seed, H. B. and Idriss, I. M. (1971) Simplified Procedure for Evaluating Soil Liquefaction Potential. J. Geotechnical Engg. Div., ASCE, 97(9),