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PROBABILISTIC SEISMIC HAZARD ASSESSMENT AND DYNAMIC BEHAVIOR OF PORT STRUCTURESAuthor: Swathi Priyadarsini 201512661 Date: 2021-12-27 Report no: IIIT/TH/2021/130 Advisor:K S Rajan,Neelima Satyam AbstractRisk due to induced ground deformations has been observed from past seismic events. From the geographical statistics, it is evident that 60% of the Indian land mass is vulnerable to earthquakes of MSK intensity of VII or higher. Ground motion intensity due to an earthquake changes as it disseminates through the soil media from bedrock to the surface. The ground response because of an earthquake mainly depends on the magnitude of the event, geological details of the location, surface topography, fault mechanism, propagation path length between source and the site, dynamic properties of the soil medium (Abrahamson and Shedlock, 1997) and other local site conditions. Ground characteristics may result in amplification (causing resonance) or ground motion attenuation. Frequency of the ground motion is controlled by amplification mechanism. Source, path and site characterization were considered as the deciding factors of seismic ground motion intensity. Local site conditions are also considered as triggering factors of liquefaction. Therefore, estimation of local site effects can be substantial in seismic hazard assessment of an area. Probabilistic seismic hazard assessment, dynamic site characterization and ground response analysis provide an indispensable base for quantifying the seismic hazard associated with an area. Though design engineers are provided with macro level solutions for geotechnical and geological problems such as hazard maps, earthquake catalogs etc., from the past literature as well as practical experiences it was evident that they are not reliable in predicting the exact strong ground motion. In addition to this rapid infrastructure development with poor construction practices in terms of quality, mushrooming of structures and increased population growth rate also makes the country very vulnerable for earthquake damage. Hence it is very important to carry out the dynamic site characterization and site specific hazard assessment considering geotechnical, geological, geophysical and seismotectonic features. In this thesis, an attempt has been made to carry out seismic hazard assessment studies in the study area i.e., Vishakhapatnam. Also as an application of the results from hazard assessment studies, earthquake resistant design of pile supported wharf has been carried out considering the local site conditions in the study area. Vishakhapatnam lies in east coast region of southern India, falls under seismic zone II (IS: 1893, 2016) according to seismic zonation map of India. It has two operational ports making it the financial capital of the state and prominent in terms of trade and economy. Addressing the seismicity in the study area, seismic activity in the Eastern Ghats mobile belt region of southern part of India and Bay of Bengal has increased due to subduction of Burma tectonic plate towards the Bay of Bengal, which resulted in reactivation of older inactive faults and also new faults development (Reddy and Chandrakala, 2004). Therefore, increasing seismic risk and importance of the study area has motivated the researchers to conduct detailed ground response analysis and estimation of local site effects. Probabilistic seismic hazard assessment (PSHA) has been carried out to quantify the seismicity associated with the study area. Extensive geotechnical data has been collected from various private and public organizations. Using the available geotechnical data 1D and 2D soil profiles have been generated. Rock outcrops and landfilled sites have been identified from the soil profiles. Equivalent linear ground response analysis has been carried out to estimate the peak ground acceleration at various locations in the study area using DEEPSOIL (Youssef, 2009). The surface PGA values obtained have been used in liquefaction hazard assessment using stress based method (SBM) and energy based method (EBM). Hazard maps were generated from the estimated values of PGA and factor of safety against liquefaction (FL) from both the approaches. The city has been characterized into different zones of varying hazard with respect to PGA and FL. Soft soils may amplify or de-amplify the ground motion resulting in increased or decreased ground shaking intensity and remains as one of the deciding factor in seismic hazard intensity (Neelima and Towhata, 2016). Hence to estimate the effect of local site conditions, microtremor testing has been carried out at 75 locations and predominant frequency map has been developed for the study area using H/V spectral ratio method (Nakamura, 1989). The dynamic parameters are further used as input in seismic design and analysis of pile supported wharf structure in the study area. As mentioned earlier, the study area is having two operational ports contributing to the economy of the state and the country. Therefore designing the port structures considering the seismicity and local conditions will be of great importance. As in India, port structures are not designed considering the dynamic forces leading to failure of many structures during an earthquake. Liquefaction is the major failure mode observed in port structures since past. Therefore in the present research, liquefaction failure in pile supported wharf structures is considered for detailed investigation. A pile supported wharf structure in Vishakhapatnam Port Trust (VPT) has been considered for analysis. Initially Liquefaction hazard assessment has been carried out to assess the potential hazard associated with the soils beneath the structure. From the liquefaction assessment, it is observed that the soil beneath the wharf is prone to liquefaction in both free field and existing load conditions. Therefore the pile support system has been redesigned against liquefaction failure. Further pushover analysis has been carried out for the designed foundation system to obtain fragility curves along with site specific response spectra. The seismic hazard parameters, uniform hazard response spectrum, hazards curves from PSHA are further useful in construction of power plants and other prominent structures in the study area. The peak ground acceleration at surface and bedrock, H/V frequency and H/V amplitude from the study indicate the variation in local site conditions and will be very useful in the analysis and design of structures considering earthquake forces. Hazard maps generated from the current study will be a great tool in engineering practice. Liquefiable sites can be identified and mitigation measures can be adopted such as soil modification, stone columns etc., against liquefaction. Looking through the literature available, it is seen that many countries have included performance based seismic design of port structures in their standards, few countries have even started using it. There is a great need to develop guidelines for seismic design of port structures in India, which works on the concept of performance rather than force based, to minimize losses and hazards due to earthquake in the near future. Therefore the dynamic analysis of pile supported wharf and development of seismic fragility curves helps in improving the performance of a structure by estimating its repair cost and operational status. The repair cost is the cost of retrofitting involved in bringing back the capacity of a structure to its original condition. As the container wharves are accountable for loading and unloading of cargos, it is vital to understand the performance of such structures during earthquakes. This study provides new perceptions into the seismic performance of pile supported wharves by means of 3D modelling and advanced seismic analyses of a pile supported wharf along with soil structure interaction. Full thesis: pdf Centre for Spatial Informatics |
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