A numerical investigation into effects of single and multiple frequency earthquake motions

During dynamic centrifuge modelling for earthquakes there is a decision to be made about the type of base input motion that should be imparted to the soil model. Motions can either be a tone burst of single frequency or a multi-frequency event simulating a real earthquake. In this paper a series of numerical analyses is reported which endeavours to capture the effects of loading rate on liquefiable soil. Non-linear dynamic finite element analysis in terms of effective stress was carried out using the FE code SWANDYNE. The numerical model was subjected to four types of input motion. Results are presented in terms of excess pore pressure ratios for different frequencies. It was seen that the interplay between frequency content of the seismic motion and generation of excess pore pressure could change the dynamic response of a system. It was concluded that simplicity of the input motion leads to a soil response that is less difficult to analyse.     

Applications of wavelet analysis to the investigation of the dynamic behaviour of geotechnical structures

Harmonic wavelet analysis is a tool that has been developed over the past 15 years for the analysis of non-stationary signals in the time–frequency domain. This paper discusses the use of this technique and its application to the problems of soil dynamics and earthquake engineering. Specific reference is made to the use of this technique in investigating the performance of liquefiable slopes in centrifuge model earthquakes and the investigation of earthquake accelerograms.

Harmonic wavelet analysis will be shown to be a versatile tool that can reveal information unavailable traditional time or frequency domain analysis.     

Mitigation of the seismic motion near the edge of cliff-type topographies

Concentration of damage of buildings near the edge of hard or dense cliff-type topographies has been observed during a number of recent earthquakes. These observations have been interpreted by numerical dynamic analyses that illustrate the amplification of accelerations near the edge of slopes. The paper studies the effect of mitigating these accelerations using anchors both experimentally and numerically. The main issue is that if the part of the slope in which topographic amplification occurs is connected to that in which the acceleration is less, then the accelerations have to become more uniform. The experimental study involves dynamic centrifuge tests at the Cambridge University facility both with and without anchors. The numerical procedure was verified by the seismic response of the centrifuge tests. It was then applied to the study of the effect of anchors of a typical field case, the Aegion slope, under two different input motions. In all cases anchors were found effective in mitigating the seismic motion near the edge.     

Seismic performance of buildings with structural and foundation rocking in centrifuge testing

Rocking motion, established in either the superstructure in the form of a 2‐point stepping mechanism (structural rocking) or resulting from rotational motion of the foundation on the soil (foundation rocking), is considered an effective, low‐cost base isolation technique. This paper unifies for the first time the 2 types of rocking motion under a common experimental campaign, so that on the one hand, structural rocking can be examined under the influence of soil and on the other, foundation rocking can be examined under the influence of a linear elastic superstructure. Two building models, designed to rock above or below their foundation level so that they can reproduce structural and foundation rocking respectively, were tested side by side in a centrifuge. The models were placed on a dry sandbed and subjected to a sequence of earthquake motions. The range of rocking amplitude that is required for base isolation was quantified. Overall, it is shown that the relative density of sand does not influence structural rocking, while for foundation rocking, the change from dense to loose sand can affect the time‐frequency response significantly and lead to a more predictable behaviour.     

Geographically distributed hybrid testing & collaboration between geotechnical centrifuge and structures laboratories

Distributed Hybrid Testing (DHT) is an experimental technique designed to capitalise on advances in modern networking infrastructure to overcome traditional laboratory capacity limitations. By coupling the heterogeneous test apparatus and computational resources of geographically distributed laboratories, DHT provides the means to take on complex, multi-disciplinary challenges with new forms of communication and collaboration. To introduce the opportunity and practicability afforded by DHT, here an exemplar multi-site test is addressed in which a dedicated fibre network and suite of custom software is used to connect the geotechnical centrifuge at the University of Cambridge with a variety of structural dynamics loading apparatus at the University of Oxford and the University of Bristol. While centrifuge time-scaling prevents real-time rates of loading in this test, such experiments may be used to gain valuable insights into physical phenomena, test procedure and accuracy. These and other related experiments have led to the development of the real-time DHT technique and the creation of a flexible framework that aims to facilitate future distributed tests within the UK and beyond. As a further example, a real-time DHT experiment between structural labs using this framework for testing across the Internet is also presented.     

Calibration of Strain Gauged Square Tunnels for Centrifuge Testing

A series of dynamic centrifuge tests were conducted on square aluminum model tunnels embedded in dry sand. The tests were carried out at the Schofield Centre of the Cambridge University Engineering Department, aiming to investigate the dynamic response of these types of structures. An extensive instrumentation scheme was employed to record the soil-tunnel system response, which comprised of miniature accelerometers, total earth pressures cells and position sensors. To record the lining forces, the model tunnels were strain gauged. The calibration of the strain gauges, the data from which was crucial to furthering our understanding on the seismic performance of box-type tunnels, was performed combining physical testing and numerical modelling. This technical note summarizes this calibration procedure, highlighting the importance of advanced numerical simulation in the calibration of complex construction models.     

Modelling Solitary Waves and Its Impact on Coastal Houses with SPH Method

The interaction between solid structures and free-surface flows is investigated in this study. A Smoothed Particle Hydrodynamics (SPH) model is used in the investigation and is verified against analytical solutions and experimental observations. The main aim is to examine the effectiveness of a tsunami-resistant house design by predicting the wave loads on it. To achieve this, the solitary wave generation and run-up are studied first. The solitary wave is generated by allowing a heavily weighted block to penetrate into a tank of water at one end, and the near-shore seabed is modelled by an inclined section with a constant slope. Then, the SPH model is applied to simulate the three-dimensional flows around different types of houses under the action of a solitary wave. It has been found that the tsunami-resistant house design reduces the impact force by a factor of three.     

Liquefaction remediation by vertical drains with varying penetration depths

Vertical drains have been used as remediation against earthquake-induced soil liquefaction for many years. These are seen to begin fluid dissipation from deeper deposits first. Drains are not necessarily installed to the full depth of the liquefiable layer. To determine the effect of this on the efficiency of drain systems, centrifuge test results are presented. It is seen that not installing all drains through the full liquefiable depth significantly retards their performance, due to the dominance of vertical dissipation. It will be shown that a standard design chart may over-predict an improvement in drain performance.     

An investigation into the seismic behaviour of dams using dynamic centrifuge modelling

Dynamic response of dams is significantly influenced by foundation stiffness and dam-foundation interaction. This in turn, significantly effects the generation of hydrodynamic pressures on upstream face of a concrete dam due to inertia of reservoir water. This paper aims at investigating the dynamic response of dams on soil foundation using dynamic centrifuge modelling technique. From a series of centrifuge tests performed on model dams with varying stiffness and foundation conditions, significant co-relation was observed between the dynamic response of dams and the hydrodynamic pressures developed on their upstream faces. The vertical bearing pressures exerted by the concrete dam during shaking were measured using miniature earth pressure cells. These reveal the dynamic changes of earth pressures and changes in rocking behaviour of the concrete dam as the earthquake loading progresses. Pore water pressures were measured below the dam and in the free-field below the reservoir. Analysis of this data provides insights into the cyclic shear stresses and strains generated below concrete dams during earthquakes. In addition, the sliding and rocking movement of the dam and its settlement into the soil below are discussed.