A Boominathan

Pile foundations are frequently used in very loose and weak deposits, in particular soft marine clays deposits to support various industrial structures, power plants, petrochemical complexes, compressor stations and residential multi-storeyed buildings. Under these circumstances, piles are predominantly subjected to horizontal dynamic loads and the pile response to horizontal vibration is very critical due to its low stiffness. Though many analytical methods have been developed to estimate the horizontal vibration response, but they are not well validated with the experimental studies. This paper presents the results of horizontal vibration tests carried out on model aluminium single piles embedded in a simulated Elastic Half Space filled with clay. The influence of various soil and pile parameters such as pile length, modulus of clay, magnitude of dynamic load and frequency of excitation on the horizontal vibration response of single piles was examined. Measurement of various response quantities, such as the load transferred to the pile, pile head displacement and the strain variation along the pile length were done using a Data Acquisition System. It is found that the pile length, modulus of clay and dynamic load, significantly influences the natural frequency and peak amplitude of the soil-pile system. The maximum bending moment occurs at the fundamental frequency of the soil-pile system. The maximum bending moment of long piles is about 2 to 4 times higher than that of short piles and it increases drastically with the increase in the shear modulus of clay for both short and long piles. The active or effective pile length is found to be increasing under dynamic load and empirical equations are proposed to estimate the active pile length under dynamic loads. © 2007 - IOS Press and the authors. All rights reserved.

The aim of this study is to investigate the response of metro tunnel systems subjected to internal blast loading using Explicit Finite Element analysis. Usually, these metro systems consist of two tunnels running parallel to each other. The recent terror strikes exposed the vulnerability of tunnels to explosion. In this study, a typical metro system with two tunnels of 5m diameter running parallel to each other is considered. The tunnels are embedded in clayey stratum with a burial depth of 9m below the ground surface. The tunnel-soil interaction was also considered in this numerical model. The effect of parameters like explosive quantity and tunnel spacing on the response of tunnels was investigated. The explosion tunnel yields for the explosive quantity greater than 50 kg of TNT. The influence of blast wave on the adjacent tunnel is less for the spacing greater than 2.2 times the internal diameter of the tunnel.

Dynamic experiments in lateral mode were carried out on model aluminium single piles in a simulated elastic half-space filled with clay soil to determine dynamic constants of the soil-pile system and to study the bending behaviour of piles. Model piles with various lengths were subjected to steady-state harmonic vibrations with different magnitudes of force of 7-30 N applied over a wide range of frequencies from 2 Hz to 50 Hz. The load transferred to the pile, pile head displacement and strain gauge readings at different locations on the pile were measured. It is observed consistently that the magnitude of the applied force and the pile length significantly affect the natural frequency of the soil-pile system. It is found that rigid piles behave linearly even at the higher magnitudes of applied force, but that flexible piles behave non-linearly as the magnitude of the applied force increases, which leads to a substantial reduction of the lateral stiffness of the soil-pile system. Damping of the soil-pile system is found to increase with an increase in pile length and magnitude of the applied force, owing to the occurrence of radiation and hysteretic damping. Based on experiments carried out on model piles embedded in clay at low confining pressure, it is found that the maximum dynamic bending moment of long flexible piles is about four times higher than that of short rigid piles. The maximum bending moment under dynamic loads occurs at deeper depth than the corresponding depth for static loads, which indicates an increase of the active length of piles under dynamic load.

This paper presents the results of two-way cyclic lateral load tests carried out on model pile groups embedded in soft marine clay. The tests are conducted on 1 × 2, 2 × 2 and 3 × 3 pile groups having length to diameter ratio (L/D) of 15, 30 and 40 with the spacing to diameter ratio (S/D) of 3, 5, 7 and 9. The experimental results are presented in the form of load-deflection curves and bending moment profiles. Cyclic group efficiency, critical spacing, critical cyclic load level and cyclic p-multipliers are evaluated. It is found that the lateral capacity of the 3 × 3 group reduces by about 42% after 50 cycles of loading. The cyclic p-multipliers of 3 × 3 pile group are found to be 0.41, 0.25 and 0.29 for leading, intermediate and rear rows respectively. The test results are compared with the numerical analysis carried out by p-y method using GROUP program. The analysis carried out with experimentally evaluated p-multipliers predicts load-deflection and bending profiles of pile groups reasonably well, but underestimates the depth to maximum bending moment by about 15%. © 2010 Springer Science+Business Media B.V.

This study deals with the determination of the engineering properties of sand–rubber tire shred mixtures. The rubber tire shreds of fine size are mixed with river sand in various controlled proportions and tested under dense, fully saturated conditions. The monotonic direct shear tests, one-dimensional compression, and the permeability tests are conducted on these mixtures to determine their shear behaviour, compressibility, and drainage characteristics. The increase in rubber content results in the reduction of shear strength, constrained modulus and the increase of compressibility of the mixtures. The brittleness index which is a measure of ductility and the energy absorption capacity is also discussed. Intriguingly, the permeability of the mixtures reduces with the increase in rubber content. The contact angles of sand-water and the rubber–water interfaces determined using Goniometer explains this behaviour.

Lifeline structures which are employed worldwide for the conveyance of water, sewage, oil, natural gas, and other material, gets uplifted during an earthquake due to the liquefaction of the soil in which they are buried. From the damages observed from the past earthquakes, it can be seen that many of these lifeline structures lie in the region with high liquefaction potential and large ground displacement. Therefore, there exists a need to understand the uplift mechanism of an underground structure during an earthquake. In this study, the uplift mechanism of a 5 m diameter underground structure buried at a depth of 1.1 times the diameter of the structure from the surface in a 16 m deep potentially liquefiable soil as reported in Chian et al. (2014) is analyzed using a finite difference program FLAC3D. The soil model with the dynamic boundaries is subjected to sinusoidal motion applied at the base of the model. The cyclic behavior of the soil represented by Finn-model involves the dynamic pore pressure generation from the volumetric strain induced by the seismic excitation. The results obtained emphasizing the pore pressure response at different level of the structure was compared with the centrifuge model and 2D numerical analysis results published by Chian et al. (2014). It was found that the results obtained from the three dimensional analysis using Finn-model gives slightly higher results as obtained from the two dimensional analysis using the Wang model.

Spatial variation of ground motion has a significant impact on the response of structures that extend over long distances parallel to the ground. Large, relatively rigid foundations are known to average spatially varying ground motion, within its envelope. This paper discusses the response of a typical containment structure (CS) supported by a 40m diameter circular raft in a deep soil site in the Indo Gangetic Plain, subjected to spatially varying ground motion. A three dimensional model of the structure is developed by the Sub Structuring method using the SASSI2010 program. Both foundation and near-field soil are modelled using brick elements, while the soil layers are modelled as horizontal viscoelastic layers. The methodology is evaluated using the analytical transfer functions developed by Luco and Wong (1986), and the comparison is found to be good. The coherency function proposed by Abrahamson (2006), for the deep soil site is then used in the analysis, for the deep soil site. It is found that the response of the structures when subjected to spatial variation in ground motion decreases with increase in embedment depth as well as size of the foundation. A 45degree angle of incidence of the plane wave is found to result in highest response in the structure.

The Foundation Input Motion (FIM) arising out of kinematic soil-pile interaction forms an integral part of the seismic analysis of pile-supported structures. Several studies in the recent past have recognized the importance of the filtering effect produced by pile foundations. This paper presents a comprehensive investigation of the influence of raft embedment on the Foundation Input Motion from piled rafts. A hybrid numerical methodology based on the finite element-boundary element method is employed to rigorously model rectangular piled rafts in varying soil conditions. Analysis of transfer functions in the frequency domain suggests that the translational response is suppressed by increasing embedment of the raft. An increase in rocking response is also synchronous with this decrease in translational motion. A transient response analysis carried out using eight different earthquake time histories showed that the spectral acceleration curve is altered by the embedment effect. A set of simplified expressions are proposed to incorporate the embedment effects in the spectral acceleration ratio curve of a pile group. These proposed expressions have the potential to enable a quick estimation of the effect of embedment in the seismic design of pile-supported structures.

The effects of pile spacing, number of piles, and configuration on displacement and bending response of pile groups in clay under dynamic lateral loading were investigated. The displacement response of pile group in clay is strongly nonlinear. Pile-soil-pile interaction is predominant for the groups with closer spacing and with greater number of piles. Group interaction causes reduction in the group stiffness and increase in damping of the pile group. Strong group interaction leads to significant differences in bending profiles of different row piles of the groups. Dynamic lateral loading increases the maximum bending moment and active pile length. Copyright © A. S. Elnashai and N. N. Ambraseys.