372 research outputs found
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Liquefaction induced displacement and rotation of structures with wide basements
Earthquake induced liquefaction can cause structures with shallow foundations to experience large settlement and rotation, and can cause subsurface structures to uplift. The performance of structures with basements, which intuitively combine these two problems, is not understood. In this paper, data from three dynamic centrifuge tests on structures with wide basements are examined. The ratio of upward to downward vertical forces was varied, and a symmetric and asymmetric superstructure was tested. Digital image correlation was used to capture the soil displacements, providing novel insight into the co-seismic soil-structure interaction. The inclusion of wide basements was shown to reduce the overall settlement of structures by providing an increased uplift force during the liquefi ed period. For symmetric structures, symmetric soil displacements occurred around the basement during consecutive half-cycles of sinusoidal shaking, resulting in negligible accumulation of rotation. In contrast, significant rotation was accumulated for an asymmetric structure as a result of the P -delta effect due to the eccentric mass.The first author would like to thank the Engineering and Physical Science Research Council (EPSRC), United Kingdom, for their financial support through a Doctoral Training Account (DTA) studentship
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Centrifuge simulation of heave behaviour of deep basement slabs in over-consolidated clay
High demand on land in major cities is driving construction of basement structures to create additional space. Long-term heave of base slabs is a pertinent problem in deep basement construction in over-consolidated clay strata, such as the London clay. Sub-structures must be designed to withstand soil pressures and displacements that evolve gradually for many years after construction is complete. This paper discusses an
ongoing research project using centrifuge modelling to quantify the development of long-term heave by shortening the time-scale through dimensional similarity. The excavation process is simulated by draining of a heavy fluid (sodium polytungstate) and a model basement structure is instrumented to record the evolution of heave movements with time. This paper presents the preliminary results of a centrifuge test, which captured the magnitude of short-term differential and total heave deformation, the changes in support loads in horizontal props, and the evolution of pore pressures around the basement structure. Challenges encountered in this experimental technique and plans for further experimental work are discussed.This research project is supported by the EPSRC Centre for Doctoral Training in Future Infrastructure and Built Environment in the University of Cambridge
A new macro-element model encapsulating the dynamic moment–rotation behaviour of raft foundations
The interaction of shallow foundations with the underlying soil during dynamic loading can have both positive and negative effects on the behaviour of the superstructure. Although the negative impacts are generally considered within design codes, seldom is design performed in such a way as to maximise the potential beneficial characteristics. This is, in part, due to the complexity of modelling the soil–structure interaction. Using the data from dynamic centrifuge testing of raft foundations on dry sand, a simple moment–rotation macro-element model has been developed, which has been calibrated and validated against the experimental data. For the prototype tested, the model is capable of accurately predicting the underlying moment–rotation backbone shape and energy dissipation during cyclic loading. Utilising this model within a finite-element model of the structure could potentially allow a coupled analysis of the full soil–foundation–structure system's seismic response in a simplified manner compared to other methods proposed in literature. This permits the beneficial soil–structure interaction characteristics, such as the dissipation of seismic energy, to be reliably included in the design process, resulting in more efficient, cost-effective and safe designs. In this paper the derivation of the model is presented, including details of the calibration process. In addition, an appraisal of the likely resultant error of the model prediction is presented and visual examples of how well the model mimics the experimental data are provided. The authors would like acknowledge the collaborative and financial support received through the
European Community’s Seventh Framework programme (FP7/2007-2013) under grant agreement
number 227887 (SERIES – Seismic Engineering Research Infrastructures for European Synergies).This is the accepted manuscript. The final published version is available at http://www.icevirtuallibrary.com/content/article/10.1680/geot.SIP.15.P.020
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Designing urban deep basements in South East England for future ground movement: Progress and opportunities for experimental simulation of long-term heave
In recent years, there has been a boom in urban infrastructure projects in and around London that require deep basements to be excavated, such as underground railway stations and shopping malls. The permanent removal of topsoil due to basement construction inevitably causes upward movement of the remaining soil. In London clay and other over-consolidated clay strata, this upward movement continues over many years after the basement structure’s completion, a process known as long-term heave.
Urbanisation causes more and more of such deep basements to be constructed to greater depths and sizes than before. This has renewed interest in research on the long-term behaviour of base slabs in over-consolidated clay, because the basement structure must be designed to accommodate these long-term heave movements. The drive towards green construction techniques in next-generation infrastructure will require the methods of design need to be updated to allow more efficient use of material.
This paper reviews a range of current techniques used in the design of deep basement slabs where significant long-term heave deformations are expected. While current design guidance is sufficient in ensuring the safety of construction and operation of underground urban spaces, there is a strong feeling within the construction industry that the design criteria are inefficient and need to be improved with the help of experimental data.
Geotechnical centrifuge simulation is the main technique for physical modelling of long-term heave behaviour, as artificial gravity allows a year of real-life movements of soil to be replicated in a small-scale model in an hour of laboratory time. This paper reviews recent research in geotechnical centrifuge simulations on heave behaviour of deep excavations in over-consolidated clay, identifying key findings and pointing out areas that will require further research. These experimental simulations will allow the effect of long-term heave to be quantified more accurately in future design guidance, thereby addressing the need to conserve construction material as the requirement for urban underground space increases
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Influence of air injection on the liquefaction-induced deformation mechanisms beneath shallow foundations
Earthquake-induced liquefaction of soils frequently causes serious damage to structures with shallow foundations. Reducing the degree of saturation of liquefiable soils by air injection is offered as a cost-effective and reliable method of mitigating liquefaction hazards. Nevertheless, very little experimental research is available on the performance of this method. Particularly, the way that air injection influences the deformation mechanisms beneath shallow foundations is not well defined. Gaining a deeper insight into soil displacements during and after air injection can pave the way for developing effective guidelines for the use of this particular technique. For this purpose, a series of dynamic centrifuge tests are presented in this paper. The prevailing deformation mechanisms are identified in a novel way using displacement vector fields. The results indicate that air injection alters the deformation mechanisms that develop underneath and in the ground surrounding a shallow foundation, substantially reducing the average settlements.The first author wishes to acknowledge the financial assistance provided during the course of this study by the Ministry of National Education (M.E.B.) of Turkey
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