Rajib Sarkar

Seismic Behavior of Soil-Pile-Structure Interaction in Liquefiable Soils: Parametric Study

Nonlinearity of the soil medium plays a very important role on the seismic behavior of soil-pile-structure interaction. The problem of soil-pile-structure interaction is further complicated when the piles are embedded in liquefiable soil medium. A finite-element code was developed in MATLAB to model three-dimensional soil-pile-structure systems. Frequency dependent Kelvin elements (spring and dashpots) were used to model the radiation boundary conditions. A work-hardening plastic cap model was used for constitutive modeling of the soil medium. The pore pressure generation for liquefaction was incorporated by a two-parameter volume change model reported in the literature. In this paper, a 2×2 pile group in liquefiable soil is considered and a parametric study is conducted to investigate its seismic behavior. The effects of loading intensity and stiffness of the soil on the seismic behaviour of the soil-pile system are investigated, considering nonlinearity and liquefaction of the soil medium for a wide ran...

Effects of Separation on the Behavior of Soil-Pile Interaction in Liquefiable Soils

Separation and sliding between soil and pile have a significant effect on the complex behavior of pile groups under dynamic condition. In addition to separation between soil and pile, the behavior of soil medium surrounding the piles is nonlinear during strong excitations. Therefore, soil is modeled using a nonlinear work-hardening plastic cap model. Because of shaking, there is an increase in pore water pressure. To simulate it, the pore-pressure generation capability is added in modeling the behavior of the soil medium. A full three-dimensional finite-element soil-pile model has been developed with approximate radiation condition at infinity simulated by frequency dependent Kelvin elements. The model and the process of computation have been verified with the established literature. The effects of separation on the dynamic stiffness and seismic response of single pile and pile groups have been examined considering work-hardening soil-plasticity without and with pore-pressure generation capability for a wide range of frequencies of excitation. Real-time earthquake motions are also applied and the effects of separation on the responses of pile groups are investigated. Significant effects of separation of the soil medium on the response of the soil-pile system have been observed and approximately quantified.

A Review of Seismic Damage of Mountain Tunnels and Probable Failure Mechanisms

Severe cases of damages of mountain tunnels following 1995 Hyogoken-Nanbu (Japan), 1999 Chi-Chi (Taiwan), 2004 Mid-Niigata Prefecture (Japan) and 2008 Wenchuan (China) earthquakes have challenged the traditional belief of tunnel structures being seldom damaged in seismic events. These experiences are a reminder that seismic behaviour of mountain tunnels must be further studied in detail. Such investigations assume greater significance as more number of tunnels are being planned to be constructed to meet the infrastructural needs of mountainous regions all around the world. In this paper, seismic damages of mountain tunnels have been reviewed. Prominent failure patterns have been identified based on the case histories of damages. Damages in the form of cracking of tunnel lining, portal cracking, landslide induced failures, uplift of bottom pavement, failures of sidewalls, shearing failure of tunnel liner and spalling of concrete have been majorly observed. Based on the damage patterns and earthquake data, main factors leading to instabilities have been discussed. Probable failure mechanisms of mountain tunnels under seismic loading conditions have been explained. Seismic analyses of a circular lined tunnel in blocky rock mass have been carried out through discrete element based approach. The significant role of different seismic parameters like frequency, peak ground acceleration has been identified. Moreover, effect of tunnel depth on the seismic response of tunnels has been investigated. It is believed that the present study will help in advancing the present state of understanding with regard to the behavior of tunnels under seismic conditions.

Effect of soil nonlinearity and liquefaction on dynamic stiffness of pile groups

Abstract Determination of the behavior of pile groups under dynamic condition is a complex problem. The complexity becomes more intricate when the soil medium surrounding the piles behaves nonlinearly due to higher cyclic loading under dynamic condition. Further if the soil is saturated then the behavior of pile groups is also affected due to liquefaction. In this paper, nonlinear dynamic analyses of pile groups have been performed with and without generation of pore water pressure.A three-dimensional finite element soil-pile model has been developed with proper radiation boundary conditions. Frequency dependent Kelvin elements have been used for simulating radiation conditions at infinity. The numerical model and the process of computation have been verified with the established literature. For soil nonlinearity, work-hardening plastic cap model has been considered for further investigation. The pore pressure generation for liquefaction has been incorporated by two-parameter volume change model reported in literature.The effects of nonlinearity and liquefaction of soil medium on dynamic stiffness of single pile and pile groups have been investigated. Effects of frequency of excitation have been examined. It has been observed that the work-hardening soil plasticity and generation of pore water pressure has a significant influence on dynamic behavior of pile groups.

Effects of soil nonlinearity and liquefaction on seismic response of pile groups

Abstract The seismic behavior of pile groups is very complicated if piles are passing through liquefiable soil. In this paper, the effects of soil nonlinearity and liquefaction on seismic response of pile groups are investigated. A three-dimensional finite element soil-pile model was developed with Kelvin elements for simulating radiation conditions at infinity. The effect of soil nonlinearity was investigated considering work-hardening plastic cap model for the soil. The liquefaction of the soil medium was simulated using two parameter volume change model.Seismic responses of a single pile and pile groups were examined considering soil nonlinearity and liquefaction. The study was performed for range of frequency of excitation. Finally analysis is carried out for real time earthquake motion. A significant effect of soil nonlinearity and liquefaction of the soil medium on the seismic response of pile groups was observed and it was also noticed that this effect is very much dependent on peak ground acceleration and frequency characteristics of the input motion.

Assessment of Vulnerability of Rock Slope Considering Material and Seismic Variability

Stability analysis of rock slopes has always been a critical and challenging task for the geotechnical engineering professionals. The complexities associated with the stability analysis arise due to the heterogeneous, anisotropic and variable nature of the rock mass. Assessment of slope stability becomes further challenging under earthquake motions which are random in nature. Thus, uncertainties in both material and loading parameters are required to be considered for a robust assessment of the vulnerability of slopes in geologically complex and seismically active regions.In the present study, the influence of variability in geological properties on the slope stability has been considered within the framework of First Order Reliability Method (FORM). Reliability analysis has been performed for a typical slope profile using the Response Surface Method (RSM) and FORM leading to the identification of critical design parameters along with the quantification of the system performance in terms of reliability index. Subsequently, dynamic time history analyses have been performed for generating the seismic fragility curves of the rock slope as a function of increasing earthquake intensity. Thus, the study attempts to present a methodology for assessing the vulnerability of a rock slope with due consideration of the variation in both the material properties and seismic loading.

Bearing capacity of circular footing supported on coir fiber-reinforced soil

Abstract A series of triaxial compression tests on sandy soils reinforced with varying percentages of coir fibers have been numerically simulated to observe its effect on the elastic properties and friction angle of the sand. Test results indicated that with increase in fiber content, the elastic properties and friction angle of soil improved significantly. Three dimensional numerical investigation has also been carried out to investigate the load settlement characteristics of a circular footing resting on fiber-reinforced sand using the properties obtained from the triaxial simulations. This study provides an insight into the effectiveness of addition of coir fibers in the sand of different strengths on the bearing capacity of circular footing.

Seismic Analysis of a 275 m Tall RCC Multi-flue Chimney: A Comparison of IS Code Provisions and Numerical Approaches

Tall chimneys play pivotal role in proper functioning of many industrial facilities like thermal, nuclear power plants, petrochemical and steel industry. Because of the non-uniformity of the diameter along the height of the chimney and also the variation of dead weights at different levels, seismic analyses of chimney have become critical and the design values need to be recommended with caution. In this paper, a RCC multi-flue chimney of 275 m height has been considered for study. Design forces in the chimney were obtained in accordance with the stipulated recommendations of IS 1893 (Part 4) 2005 for based on equivalent static lateral force method. Then dynamic response spectrum modal analyses were carried out for calculating the seismic forces developed in the chimney. The results from the response spectrum method were compared with the values obtained from the codal procedures for different base conditions viz. fixed base, circular raft resting on soil and annular raft resting on piles. Next, spectrum compatible time history for the particular project site has been considered and detailed dynamic analyses using time history were carried out for the chimney structure. The variations in the design force values as compared to the simplified methods are discussed and the importance of the proper dynamic analyses in designing of multi-flue RCC chimney is highlighted.