Centrifuge testing of dual row retaining walls in dry sand: The influence of earthquake sequence and multiple flights

Multi-hazard threats from tsunami events preceded by large earthquakes have been put into sharp focus in recent times. Dual row retaining walls are soil-structure systems that can have large lateral capacity with a small horizontal extent, making them ideal for the next generation of coastal protection. However, the dynamics of their behaviour is a complex interaction problem. Centrifuge tests, with multiple earthquakes within a single flight as well as multiple flights with varied embedment ratio were conducted to elucidate the mechanics of these systems when founded in dry sand. The structural response shows permanent, plastic deformations during the early cycles dependent mainly on the PGA superposed with more elastic vibrations during prolonged shaking. The development of the soil stresses and stiffnesses mobilised is used to explain the overall system response. Finally, the recorded structural and soil behaviour during swing up and down are combined to show that the soil stress and strain state is effectively reset between flights. Overall, useful methods for judging the progressive response of a complex soil-structure system are presented which can help justify future comparisons between experimental datasets and understand the implications of practical dynamic design

Accuracy of distributed optical fiber temperature sensing for use in leak detection of subsea pipelines

Accurate and rapid detection of leaks is important for subsea oil pipelines to minimize environmental risks and operational/repair costs. Temperature-sensing optical fiber cables can provide economic, near real-time sensing of leaks in subsea oil pipeline networks. By employing optical time domain reflectometry and detecting the Brillouin scattered components from a laser source, the temperature gradients can be detected at any location along an optical fiber cable attached to the oil pipeline. The feasibility of such technology has been established in the literature along with a case study on a land-based pipeline. In this paper the accuracy of an optical fiber-based temperature sensing system is investigated. A mathematical model that simulates the process of temperature sensing is developed and the results are presented. An experimental investigation is carried out with two different laboratory setups to establish the spatial resolution and accuracy of the optical fiber cable detection system, and the experimental results are compared with predictions from the theoretical model. Based on these comparisons it has been established that the optical fiber cable detection system is capable of providing an accurate and rapid assessment of the location of a leak along a subsea pipeline. Furthermore, the sensing system can be used to give an indication of the scale of the oil leak using the temperature gradients detected by the system.The first author would like to acknowledge the support received under the UROP program from the Centre for Smart Infrastructure and Construction (CSIC) at the Department of Engineering, University of Cambridge.This is the accepted manuscript. The final version is available from ASCE at http://ascelibrary.org/doi/abs/10.1061/%28ASCE%29PS.1949-1204.0000189

Finite Element Analysis of Floatation of Rectangular Tunnels Following Earthquake Induced Liquefaction

Underground structures such as tunnels, pipelines, car parks etc. can suffer severe damage during strong earthquake events. As many of these structures are buoyant, soil liquefaction due to earthquake loading can result in their floatation. In this paper, the floatation of rectangular tunnels, normally constructed by the cut-and-cover method, is investigated using dynamic finite element analyses. Sinusoidal and more realistic earthquake input motions are considered. The acceleration response of the tunnel and the soil surface following soil liquefaction is investigated. The generation of excess pore pressures in the soil around the tunnel and the consequent floatation of the tunnel are observed for both types of input motions. It will be shown that the amount of tunnel uplift depends on the type of input motion with the sinusoidal motion leading to a significantly larger uplift compared with the more realistic Kobe motion. Further, the effect of soil permeability on the floatation of the rectangular tunnel is investigated. It will be shown that tunnels can suffer floatation in finer soils with low permeabilities, whilst coarser soils with high permeability can lead to tunnel settlements owing to the rapid re-consolidation of the liquefied soils. The average axial strains in the soil above the tunnel will be shown to decrease with decreasing permeability

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

An Experimental Investigation of the Liquefaction Resilience of Dual Row Retaining Walls Using Centrifuge Modeling

Dual row retaining walls can be used as embankment structures and coastal defense against wave or even tsunami loads. However, their resiliency to a preceding earthquake and liquefaction of the foundation soil is not well understood. In this paper, dynamic centrifuge modeling is used to investigate the seismic response of a dual row retaining system where the walls are founded in a loose, saturated, and level deposit susceptible to earthquake-induced liquefaction. A combination of instrumentation techniques, ranging from measurements of the wall bending and displacements to the pore pressures and horizontal earth pressures acting on the walls, are used to elucidate the mechanics of the soil-structure system. The experiments reveal that the cyclic loading of PGA 0.4 g does not induce a catastrophic collapse. An understanding of the liquefaction phenomena, including shear-induced dilation and coseismic migration of the pore fluid toward a region of relative suction between the walls, is required to interpret the observed resilience of the wall-soil system to liquefaction. A ratcheting mechanism for the wall deformations, consistent with the coseismic measurements of the soil and structural response, is proposed. For the geometries tested, the findings challenge conventional conservative design recommendations, such as the need for sheet-pile walls to extend beyond a liquefiable layer to prevent the liquefaction-induced collapse of dual row wall systems

On the dynamic response of flexible dual-row retaining walls in dry sand

Dual-row retaining walls can be utilised to form embankment structures for protection against tsunamis or as port structures. The dynamic response of these walls involves a complex interaction between the soil and structural elements which is not well understood. In this paper, for the first time a combination of centrifuge and finite-element modelling is used to better understand the mechanical response of the combined wall–soil system. The measured variation of the horizontal stresses leads to bending moment distributions featuring large, singly outward bending or double curvature with significant inward bending of the wall near the ground level. The numerical analyses are used to understand the stress state and highlight the dynamic variations of the vertical effective stress that drive this previously unexplained behaviour. The shear stresses that can develop at the wall–soil interface govern the mechanism by which the vertical effective stresses can vary. Further consideration of the dynamic soil stress state suggests a purer interpretation of the limiting loads in soil–structure interaction problems relative to the traditionally defined dynamic earth pressure coefficients. Combining the altered vertical soil stresses with earth pressure coefficients depending only on the soil friction angle adequately bounds the horizontal stresses that develop in the soil. </jats:p