Advances in Earthquake Geotechnics

Offshore Wind Farms have established themselves as a matured technology to decarbonize energy sources to achieve net-zero. Offshore wind farms are currently being constructed in many seismic-prone zones and the codes/best practice guidelines are not fully developed. The aim of the paper (which is based on the keynote lecture presented at the 7th International Conference on Recent Advances in Geotechnical Earthquake Engineering) is as follows: (a) discuss the potential seismic risks to the offshore wind farm system and its components such as turbines, cables, and offshore substations; (b) present the different analysis and design methods for offshore wind turbine foundations; (c) highlight the performance-based design considerations and how to assess risks. Future research needs are highlighted.

The Kutch Basin in Western India has experienced one of the deadliest earthquakes on 26 January 2001 with 20,000 fatalities and an economic loss of around 5 billion USD. Though the strong motion records of the Kutch region indicate the effects of basin on the ground motion, there are no detailed numerical studies concerning the basin effects. Therefore, a sincere attempt is made to study the basin effects in Kutch region. Initially, a three-dimensional linear seismic site response analysis is carried out for the simplified Kutch Basin of size (150 × 90 × 1.5) km subjected to ricker wave, using the spectral element code SPEED. The average shear wave velocity profile of the Kutch region reported in the literature is adopted in the study. It is found out from the 3D numerical analysis that the maximum amplification occurs at the corner of the basin where the interference of waves reflected from multiple edges happen. Then, a two-dimensional non-linear seismic site response analysis is carried out for the numerical Kutch Basin model using a finite difference programme FLAC. The 2D numerical Kutch model has the similar shape and depth to width ratio as the actual Kutch Basin and is subjected to the recorded earthquake motion at SIVLAKA station (SIV) in Kutch region. The amplification pattern obtained from the 2D numerical model compares well with that of field studies. Further, based on the developed 2D model, parametric studies are carried out and basin amplification factors are computed for various locations of the basin.

The number of artificial sources of ground-borne vibrations is escalating with an increase in the construction activities of rail and road traffic systems. These ground-borne vibrations may disturb the stability of nearby old structures. The geo-mechanical problems associated with ground-borne vibrations can be solved by analyzing the soil–structure interaction using various numerical methods. Theoretical evaluation and mitigation of ground-borne vibration in sub-structural systems has been explained in the present study with the help of practical application. First, the open and infill trench methods of vibration mitigation and their efficiency are discussed. Then the assessment of blast-induced vibrations soil parameters and the response of the railway track system under cyclic loading are briefly reviewed. An emphasis has been given on evaluating the effect of vibrations induced by construction activities such as tunneling, blasting, piling, on heritage structures in the nearby area. The impact of proposed tunneling operations on adjacent heritage structures has been assessed and explained through a case study.

Geotechnical, geological and geophysical investigations for seismic microzonation and site-specific earthquake hazard analysis adopted in Gujarat, western India, are explained. Geology of the area is studied to understand basic earthquake hazards. Seismicity and tectonics are studied up to 50 km distance in detail and 300 km distance in general. To know the nature of soil layers and drainage patterns, a geomorphological map is prepared by remote sensing and ground check. Depth and seismic shear velocity of near-surface soil/rock layers are estimated at 2 km grid by 30–90 m borehole drilling and geophysical methods like MSWA, analysis of seismograms, microearthquake recording and PS-logging. The 2D and 3D soil models are prepared. Deep layers and fault details are estimated by gravity, seismic reflection and magnetotelluric geophysical surveys. The Peak Spectral Acceleration (PSA) is estimated on the grid pattern with a spacing of 0.5 km. The methodology is divided into three parts: (i) Establishment of Engineering Bed Layer (EBL) (a layer above which the soil effect is to be estimated) from borehole data, SPT N-Values and seismic shear velocity of soil layers, (ii) Estimation of strong ground motion at EBL using strong motion simulation technique and (iii) Estimation of surface strong-motion parameters and soil amplification by passing the EBL ground motion through ground model prepared for each borehole. The 1D ground response analysis is conducted through SHAKE program and strong-motion parameters are estimated at surface. The PGA maps and spectral acceleration (Sa) maps for 0.1–1.25 s are prepared. Liquefaction potential is estimated.

This article presents an integrated approach for seismic analysis of pile foundations and demonstrates the applicability through an example. The approach involves various steps starting from identifying the seismic sources, analyzing local soil conditions, ground response analysis, and dynamic analysis of pile foundations. The complete nonlinearity of the soil (in case of liquefied soils) is incorporated in the analysis through nonlinear effective stress-based ground response and liquefaction analysis. A single pile and a 2 × 2 pile group are considered and dynamic analysis with varying earthquake intensities of ground motions have been applied to analyze the effect of intensity on pile response. The obtained results are presented in terms of pile displacements and bending moments. A state-of-the-art strain-hardening model has been utilized to simulate the response of pile in liquefied stratum. The proposed approach can be adopted for the design of pile-supported structures in seismically active regions as well as requalification studies of existing pile-supported structures.

The response of a saturated sand deposit to seismic motion is a very significant and challenging problem of soil dynamics, and a completely satisfactory generalized solution does not yet exist. A qualitative and quantitative prediction of the phenomena leading to permanent deformation or unacceptably high buildup of pore pressure is therefore essential to guarantee the safe behavior of engineering structures under transient consolidation and dynamic conditions. The significant approaches to model the behavior of a two-phase porous medium are usually categorized as uncoupled and coupled approaches. In the uncoupled analysis, the response of saturated soil is modeled without incorporating the interaction between soil and fluid, and then the pore pressure is accounted separately through a pore pressure generation model. In the coupled analysis, a mathematical framework is developed for computation of displacements and pore pressures at each time step. A comparison has been performed considering both approaches based on the consideration of soil non-linearity. The coupled analysis resembles closely with the liquefaction phenomena as compared to uncoupled approach. Hence, the usual decoupled and factor of safety approach may not be considered as most appropriate in the analysis of such dynamic behavior.

Ground motion prediction equation (GMPE) is a major contributing component in establishing seismic hazard values, design coefficient, and risk level for disaster management in the region. Many seismic hazard analyses at a regional level are carried out using GMPEs developed elsewhere. In this study, realistic GMPE function form, suitable GMPEs with ranks and weights for different distance segments and the best design spectrum shape for the active region of the Himalayas is presented by analysis of regional recorded earthquake data. About 241 earthquake data recorded in rock sites in the Himalayan region has been collected and used here. The functional form of GMPE for the Himalayan region is selected by considering the mixed-effect analysis of the residual of the recorded and simulated ground motion. About 43 GMPEs applicable to the Himalayan region are used in the study, of which 12 were developed for the region. We have carried out a systematic analysis by Log-likelihood (LLH) method to test the applicability of the GMPEs for the Himalayan region using recorded earthquake data. Ranks and weights of applicable GMPEs are estimated for the segmented distance of <100 km, 100–300 km, and more than 300 km. These are helpful in seismic hazard analyses of different cities in North India. Further, we arrived at the design spectrum shape using bedrock recorded data through the Tripartite plot of acceleration, velocity, and displacement-control period and factors. It is noticed that the design spectrum shape arrived is different from the currently used design spectrum shape in the Indian seismic code. Based on regional data and study; there may be a need to reach a design spectrum shape for different soil classifications.

During an earthquake event, structures on soft and loose soil pose more complicated seismic behaviour compared to similar structures on the rock or stiff soil. This is due to various reasons including ground motion amplification in soft soil, kinematic interaction, large deformations at foundation level influencing the eigenvalue and damping properties of the structure, material and geometric damping of the soil and so on. Several past studies have indicated that the deformations at the foundation base, particularly the rocking of shallow foundation and subsequent energy dissipation may pose a beneficial effect on the structure through reducing floor acceleration, column moment, and ductility demands of the structural members. However, adverse consequences, such as excessive permanent and transient settlement and tilting of the foundations are also associated with rocking shallow foundations. In this background, the present study aims to investigate the utility of geosynthetics as a potential improvement of the subsoil to reduce the earthquake-induced settlement of low-rise steel-moment-resisting frame (SMRF) structures.

Two-dimensional finite element analysis was performed to investigate the effect of earthquake force on a concrete gravity dam resting on the rock foundation system with shear zone. The compressive stress in the dam structure, the shear force distribution, bending moment distribution and settlement of raft and pile, and axial force within pile were studied using pseudo-static method and time-history dynamic analysis. From the results, it is observed that the maximum compressive stress under earthquake loading is more than the permissible limit within the dam body. While compressive stress is within the permissible limit in the other locations of the ground and dam. The maximum shear force and bending moment within the raft is observed to be at the nearest pile-head to the dam toe. Similarly, the shear force, bending moment, and axial force are maximum within the pile nearest to the dam heel. The results obtained from pseudo-static analysis and dynamic analysis were also compared and it is observed that both the analyses predicted results in a comparable range, except for the pile load. The dynamic analysis predicted a much higher value of pile load in comparison to the pseudo-static analysis.

While sandy silty soils, especially in the river delta area are subjected to liquefaction due to high-intensity earthquakes, the geotechnical profession has gone far and wide in the determination of liquefiable/non-liquefiable zone based on the semi-empirical methodology since the 1970s. Moreover, cyclic triaxial, as well as simple shear tests, on soils have been exploited sequentially to recreate pre- and post-liquefaction situations. There are few laboratory studies to capture the liquefaction effect in soils when subjected to dynamic excitations. In this paper, the effect of manual tapping on the small–medium–large containers filled with soil–water, both visualization of the liquefaction phenomena and measurement of excess PWP has been explained. Simulation of underground utilities and overground buildings in a liquefiable soil has been done with rubber balls affixed with a needle. Capturing the upward movement of the rubber balls and downward movement of the needle fixed rubber balls vis-à-vis instant jumping of water level in the open standpipe piezometer, are some of the unique experimental findings. The results show that out of nine types of soils specimen, the Yamuna River sand in Delhi and the Khowai River sand in Agartala, India, are highly liquefiable, and equivalent PWP variation during dynamic excitation has been measured and simulated during the experiment.

The scarcity of conventional backfill materials has necessitated exploring marginal soils for use by stabilization. Efforts of researchers have indicated significant improvement in engineering properties of fine sand stabilized with the addition of synthetic fibres. Industrial establishments in coastal areas at sites comprised of fine sand or filled up ground are preferring the use of fibre-reinforced fine sand cushions to support the foundations due to good frictional characteristics and better drainage. As the design of vibrating bases requires the soil spring stiffness, it is essential to study the coefficient of elastic uniform compression (Cu) of fibre-reinforced fine sand for supporting vibrating bases. Hence, in the present study, the effect of polypropylene fibre addition in fine sand is studied through small-scale cyclic load tests. The effect of the addition of polypropylene fibres of 6 mm and 12 mm length at optimum fibre content (1% based on improved shear strength parameters) on Cu is evaluated to quantify the reduction in amplitude of vibration for damping ratios of 0–0.15. At optimum fibre content, the values of Cu of 6 mm and 12 mm fibre-reinforced fine sand decreased by about 40% and 58%, respectively. The study indicated a reduction in amplitude of vibration of about 25% for square and circular vibrating bases in fine sand with the addition of 1% of randomly distributed polypropylene fibre of 12 mm length due to improved elastic response and increased frequency ratio (r) from 1.5 in unreinforced to 2 in reinforced case.

Resilience against earthquakes has become important in the current scenario, when numerous events of seismic shaking keep on occurring, especially in regions of high population. For that, reliable seismic site characterization is extremely important. The seismic surface wave methods are the commonly adopted methods for seismic site characterization and subsequent applications. After many evolvements, the multichannel analysis of surface waves (MASW) has grown to be the widely chosen surface wave method across the globe. However, the method has numerous peculiar specifications regarding its procedures of data acquisition, analysis, and interpretations. Each step in the MASW can induce notable uncertainties in the results. Even a single mistake in any of the procedures can lead to erroneous results, which are difficult to be identified even by any third party. Many such cases have been reported earlier by researchers. Such practices may foster substantial losses because ultimately the MASW results are useful for the generation of the design response spectra. Due to all these reasons, there is an alarming need for the dissemination of knowledge about the guidelines for the reliable practice of MASW testing. This paper provides a comprehensive account of the fundamentals of the MASW method, the ways to implement its three steps of data acquisition, processing, and inversion, and the ways to improve confidence in the results. It presents a detailed and comprehensive literature review on the topic, including the historical developments and theoretical basis, subsequent evolvements, current practices, and recommendations for reliable testing. The amount of references has been kept very high to facilitate a thorough understanding of the topic and draw sufficient inferences. An elaborate description of the uncertainties in the test and how to deal with them has been presented. The parameters to be set while carrying out those three steps have been discussed, and suggestions are presented. The suggestions are based on the field experiences and literature review of the research works by many prominent researchers.