Case study on a deep excavation in soft ground by centrifuge model tests

This paper provides a case study on the deepest excavation carried out so far in the construction of the metro network in Shanghai, which typically features soft ground. The excavation is 38 m deep with retaining walls 65 m deep braced by 9 levels of concrete props. To obtain a quick and rough prediction, two centrifuge model tests were conducted, in which one is for the 'standard' section with green field surrounding and the other with an adjacent piled building. The tests were carried out in a run-stop-excavation-run style, in which excavation was conducted manually. By analyzing the lateral wall displacement, ground deformation, bending moment and earth pressure, the test results are shown to be reasonably convincing and the design and construction were validated. Such industry orientated centrifuge modeling was shown to be useful in understanding the performance of geotechnical processes, especially when engineers lack relevant field experience. © 2010 Taylor & Francis Group, London.

On the use of densification as a liquefaction resistance measure

The use of densification to improve the performance of shallow foundations during the centrifuge modeling of earthquake-induced liquefaction on level sand deposits is discussed. The densification of liquefiable ground provided protection against or significantly reduces liquefaction-related damage. Propagation of accelerations in the deposit exhibited considerable distinct features according to the relative density of the sand in the model. It was found that during the first couple of cycles, the dense soil amplifies the fundamental frequency component of the earthquake and preserves the higher frequency components.

离心机中动冰荷载的模拟

前言吸力式基础的应用起源于欧洲,最初用作浮动结构的稳固基础，1994年欧洲Europipe16/-11E第一次将海洋采油平台建在吸力式基础上[1].……

海洋平台吸力式基础的土工离心模拟研究与分析

综述了国内外已开展的海洋平台吸力式基础的离心模型试验研究，重点讨论了应用离心机模拟吸力式基础承载特性的研究现状。

Dimensionless Analysis of the Simulation of Bucket Foundations under Dynamic Load

Firstly, the main factors are obtained by use of dimensionless analysis. Secondly, the time scaling factors in centrifuge modeling of bucket foundations under dynamic load are analyzed based on dimensionless analysis and control- ling equation. A simplified method for dealing with the conflict of scaling factors of the inertial and the percolation in sand foundation is presented. The presented method is that the material for experiments is not changed while the effects are modified by perturbation method. Thirdly, the characteristic time of liquefaction state and the characteristic scale of affected zone are analyzed.

Foundations for offshore wind turbines

Offshore wind turbines impose unique combinations of loads on their foundations. They impose large lateral loads in relation to vertical loading which must be resisted, but are also subject to approximately a million cycles of loading through their design life. As the performance of these systems is dominated by their dynamic response, the stiffness of the foundations becomes critical in design. Conventional design codes which are conservative by virtue of predicting a lower stiffness than might be observed in practice may not be conservative for these problems. By utilizing centrifuge modeling the behaviour of monopile foundations in both sands and clays under cyclic loading can be investigated in order to predict the dynamic behaviour of these systems. © 2010 Taylor & Francis Group, London.

Development of a new apparatus for modeling deep excavation related problems in geotechnical centrifuge

Underground space is commonly exploited both to maximise the utility of costly land in urban development and to reduce the vertical load acting on the ground. Deep excavations are carried out to construct various types of underground infrastructure such as deep basements, subways and service tunnels. Although the soil response to excavation is known in principle, designers lack practical calculation methods for predicting both short- and long-term ground movements. As the understanding of how soil behaves around an excavation in both the short and long term is insufficient and usually empirical, the judgements used in design are also empirical and serious accidents are common. To gain a better understanding of the mechanisms involved in soil excavation, a new apparatus for the centrifuge model testing of deep excavations in soft clay has been developed. This apparatus simulates the field construction sequence of a multi-propped retaining wall during centrifuge flight. A comparison is given between the new technique and the previously used method of draining heavy fluid to simulate excavation in a centrifuge model. The new system has the benefit of giving the correct initial ground conditions before excavation and the proper earth pressure distribution on the retaining structures during excavation, whereas heavy fluid only gives an earth pressure coefficient of unity and is unable to capture any changes in the earth pressure coefficient of soil inside the zone of excavation, for example owing to wall movements. Settlements of the ground surface, changes in pore water pressure, variations in earth pressure, prop forces and bending moments in the retaining wall are all monitored during excavation. Furthermore, digital images taken of a cross-section during the test are analysed using particle image velocimetry to illustrate ground deformation and soil–structure interaction mechanisms. The significance of these observations is discussed.