Post-liquefaction reconsolidation of sand.

Loosely packed sand that is saturated with water can liquefy during an earthquake, potentially causing significant damage. Once the shaking is over, the excess pore water pressures that developed during the earthquake gradually dissipate, while the surface of the soil settles, in a process called post-liquefaction reconsolidation. When examining reconsolidation, the soil is typically divided in liquefied and solidified parts, which are modelled separately. The aim of this paper is to show that this fragmentation is not necessary. By assuming that the hydraulic conductivity and the one-dimensional stiffness of liquefied sand have real, positive values, the equation of consolidation can be numerically solved throughout a reconsolidating layer. Predictions made in this manner show good agreement with geotechnical centrifuge experiments. It is shown that the variation of one-dimensional stiffness with effective stress and void ratio is the most crucial parameter in accurately capturing reconsolidation.This is the author accepted manuscript. The final version is available from Royal Society Publishing via https://doi.org10.1098/rspa.2015.074

Influence of phase difference between kinematic and inertial loads on seismic behaviour of pile foundations in layered soils

A series of dynamic centrifuge experiments was conducted on model pile foundations embedded in a two-layered soil profile consisting of soft-clay layer underlain by dense sand. These experiments were specifically designed to investigate the individual effect of kinematic and inertial loads on a single pile and a 3 × 1 row pile group during model earthquakes. It was observed that the ratio of free-field soil natural frequency to the natural frequency of structure might not govern the phase relationship between the kinematic and inertial loads for pile foundations as reported in some previous research. The phase relationship obtained in this study agrees well with the conventional phase variation between the force and displacement of a viscously damped simple oscillator subjected to a harmonic force. Further, as expected, the pile accelerations and bending moments can be smaller when the kinematic and inertial loads act against each other compared to the case when they act together on the pile foundations. This study also revealed that the peak kinematic pile bending moment will be at the interface of soil layers for both single pile and pile group. However, in the presence of both kinematic and inertial loads, the peak pile bending moment can occur either at the shallower depths or at the interface of soil layers depending on the pile cap rotational constraint. First author funded by Commonwealth Scholarshi

Tuned Mass Damper Positioning Effects on the Seismic Response of a Soil-MDOF-Structure System

Tuned mass dampers (TMDs) are effective structural vibration control devices. However, very little research is available on the experimental investigation of TMDs and their performance in systems undergoing dynamic soil-structure interaction (SSI). Geotechnical centrifuge tests are conducted to investigate storey positioning effects of single and multiple TMDs in a soil-MDOF-structure system. The criteria for optimal storey positioning will be established and it is shown that storey positioning influences TMD performance more than the number of TMDs used. Non-optimal storey positioning was found to have the potential of reducing damping efficiency, amplifying peak structural response and inducing lengthier high-intensity motion.Engineering and Physical Sciences Research Council (Grant ID: EP/K503009/1

Use of viscous pore fluids in dynamic centrifuge modelling

The scaling laws that arise from dynamic centrifuge modelling contain an inconsistency regarding the scaling of time between dynamic and diffusion events. This problem can be resolved by reducing the permeability of the soil, with the help of high-viscosity pore fluids. Hydroxypropyl methylcellulose is a water-soluble cellulose ether that is widely used to create such fluids. In this paper, the effects that concentration, temperature, ageing and shearing rate have on the viscosity of hydroxypropyl methylcellulose solutions are examined and equations that quantify them are presented. This information is meant to act as a guideline for the preparation of high-viscosity pore fluids for dynamic centrifuge tests. This is the accepted manuscript currently embargoed pending publication. Permission is granted by ICE Publishing to print one copy for personal use. Any other use of these PDF files is subject to reprint fees

Comparison of the dynamic responses of monopiles and gravity base foundations for offshore wind turbines in sand using centrifuge modelling

Monopiles and gravity base foundations (GBF) are two of the most commonly used offshore foundations for wind turbines. As resonance can cause damage and even failure of wind turbines, understanding the difference between the dynamic responses of monopiles and GBFs under free vibration is important, however there is little experimental data regarding their natural frequency, especially for model tests at correct stress levels. This paper presents the results of novel monopile and GBF tests using a centrifuge to directly determine the natural frequency (fn) of the foundation-soil system. The natural frequencies of wind turbine monopiles and GBFs in centrifuge models were measured during harmonic loading by a piezo-actuator, with the results confirming that soil-structure interaction must be considered to obtain the system natural frequency as the frequency reduces substantially from fixed-base values. These results will contribute to preventing resonance damage in designs for wind-turbine foundations

Centrifuge modelling of flexible retaining walls subjected to dynamic loading

This paper outlines the results of an experimental program carried out on centrifuge models of cantilevered and propped retaining walls embedded in saturated sand. The main aim of the paper is to investigate the dynamic response of these structures when the foundation soil is saturated by measuring the accelerations and pore pressures in the soil, displacements and bending moment of the walls. A comparison among tests with different geometrical configurations and relative density of the soil is presented. The centrifuge models were subjected to dynamic loading in the form of sinusoidal accelerations applied at the base of the models. This paper also presents data from pressure sensors used to measure total earth pressure on the walls. Furthermore, these results are compared with previous dynamic centrifuge tests on flexible retaining walls in dry sand.Consorzio interuniversitario (ReLUIS project), European Union (SERIES project)This is the author accepted manuscript. The final version is available from Elsevier via http://dx.doi.org/10.1016/j.soildyn.2016.06.01

LEAP-GWU-2015: Centrifuge and numerical modelling of slope liquefaction at the University of Cambridge

As part of the LEAP-GWU-2015 exercise, a dynamic centrifuge test was conducted at the University of Cambridge on a 5° slope of medium dense Ottawa F-65 sand. The model preparation and saturation details are presented in this paper. This paper presents the experimental data recorded during small and large magnitude sinusoidal ground motions. After the experiment, numerical simulations of the experiment were performed using the finite element code Swandyne. The results from these numerical analyses are compared with the centrifuge test data and the deformations observed during the post-test investigations. The numerical analyses replicated many of the salient features of the test, such as the overall generation of excess pore pressures and attenuation of accelerations in the liquefying ground. More subtle results, such as the de-liquefaction shocks and the asymmetric response due to differences in upslope and downslope accelerations were less well captured in terms of the expected spikes in the dynamic excess pore pressures and accelerations. Overall, the combination of centrifuge testing and numerical analysis were found to complement each other well

Durability of partial saturation to counteract liquefaction

Recently, artificially introducing gas/air into liquefiable soils has been presented as a method for reducing the risks from liquefaction. Although this method offers a simple and cheap solution, its use in practical applications is still very limited. This might be primarily ascribed to the concerns of practising engineers about the durability of gas/air bubbles in soils over time. This paper discusses the durability of entrapped air bubbles under various simulated field conditions that may potentially cause the dissolution, diffusion, compression and escape of air bubbles. Multiple series of 1g vertical sand column and high-g centrifuge tests were undertaken to provide insights into the problem. Air-induced partially saturated soils were prepared using an air-injection technique. The test results showed that the majority of entrapped air bubbles in soils can persist under several simulated field conditions for a sufficient period of time, indicating the long-term reliability of the mitigation accomplished.Ministry of Education, Turke

Estimation of the coefficient of lateral stress used in the calculation of loads on buried structures

© ASCE. The average vertical pressure on a buried structure can be calculated using the silo theory, which assumes the translation of a vertical prism of soil above the structure that is resisted by friction on the sides of the prism. One of the key assumptions made is the value of the coefficient of lateral stress, K. In this study, an assumption regarding the rotation of principal stresses in the yielding soil has been used to calculate the average coefficient of lateral stress acting at the side of a prism of yielding soil above the horizontal buried structure. The calculated value using the proposed method agrees well with experimental observation made in literature for the value of K, and is suggested for use in the estimation of loads on buried structures, where it is expected that the structure will yield relative to a stiff body of soil

Physical modelling of air injection to remediate liquefaction

Seismic liquefaction of loosely packed, saturated soils poses a significant threat to the built environment. Recently, air injection into liquefiable soil deposits has been introduced as an innovative and cost-effective liquefaction mitigation technique. However, few effective guidelines are available to the engineers for its application and performance. The way that air should be injected appropriately, most particularly, in the presence of structures, is not clearly defined. The distribution of retained air bubbles within the saturated soil medium and its effect on the seismic response also need further investigation. In an effort to offer insights into this problem, an experimental programme consisting of a series of centrifuge and 1g shaking table tests was undertaken. The results have shown that the use of higher air injection pressure provides a much wider and a more uniform air-entrapped zone, but increases the risk of soil deformations developed under the foundations. The distance from the air injector and preferential flow pathways influence the distribution of the retained air bubbles and seismic response of the soil models. Moreover, it was shown in a novel way that the air injection technique is not very effective at low confining stresses to reduce liquefaction-induced deformations beneath shallow foundations. The first author extends his thanks to the Ministry of National Education (MEB) of Turkey for their financial support