Charles Heron

Physical modeling for the evaluation of the seismic behavior of square tunnels

The Chapter summarizes results from dynamic centrifuge tests performed on a rectangular tunnel model embedded in dry sand. The tests were carried out at the geotechnical centrifuge facility of the University of Cambridge, within the Transnational Access Task of the SERIES Research Project (Project: TUNNELSEIS). The experimental data is presented in terms of acceleration and displacement-time histories in the soil and on the tunnel, soil surface settlements, earth pressures on the side walls of the tunnel and internal forces of the tunnel lining. The goal of the experiment is twofold: to better understand the seismic behavior of these types of structures, and to use the high quality and perfectly constrained data to validate the numerical models which are commonly used for the design of rectangular embedded structures. The interpretation of the results reveals (i) rocking response of the tunnel model, (ii) existence of residual values on the earth pressures on the side walls and on the internal forces and (iii) important influence of the tunnel on the shear wave field. These issues are not well understood and are usually not taken into account in the simplified seismic analysis methods.

EXPERIMENTAL AND NUMERICAL INVESTIGATION OF THE SEISMIC BEHAVIOR OF RECTANGULAR TUNNELS IN SOFT SOILS

Underground structures constitute crucial components of the transportation networks. Considering their significance for modern societies, their proper seismic design is of great importance. However, this design may become very tricky, accounting of the lack of knowledge regarding their seismic behavior. Several issues that are significantly affecting this behavior (i.e. earth pressures on the structure, seismic shear stresses around the structure, complex deformation modes for rectangular structures during shaking etc.) are still open. The problem is wider for the non-circular (i.e. rectangular) structures, were the soilstructure interaction effects are expected to be maximized. The paper presents representative experimental results from a test case of a series of dynamic centrifuge tests that were performed on rectangular tunnels embedded in dry sand. The tests were carried out at the centrifuge facility of the University of Cambridge, within the Transnational Task of the SERIES EU research program. The presented test case is also numerically simulated and studied. Preliminary full dynamic time history analyses of the coupled soil-tunnel system are performed, using ABAQUS. Soil non-linearity and soil-structure interaction are modeled, following relevant specifications for underground structures and tunnels. Numerical predictions are compared to experimental results and discussed. Based on this comprehensive experimental and numerical study, the seismic behavior of rectangular embedded structures is better understood and modeled, consisting an important step in the development of appropriate specifications for the seismic design of rectangular shallow tunnels.

Centrifuge model study of thresholds for rainfall-induced landslides in sandy slopes

Rainfall-induced landslides are very common natural disasters which cause damage to properties and infrastructure and may result in the loss of human life. These phenomena often take place in unsaturated soil slopes and are triggered by the saturation of the soil profile due to rain infiltration which leads to the decrease of effective stresses and loss of shear strength. The aim of this study is to determine rainfall thresholds for the initiation of landslides under different initial conditions. Model tests of rainfall-induced landslides were conducted on the Nottingham Centre for Geomechanics geotechnical centrifuge. Initially unsaturated plane-strain slope models made with fine silica sand were prepared at varying densities at 1g and accommodated within a centrifuge container with rainfall simulator. During the centrifuge flight at 60g, rainfall events of varying intensity and duration, as well as variation of groundwater conditions, were applied to the slope models with the aim of initiating slope failure. This paper presents a discussion on the impact of soil state properties, rainfall characteristics, and groundwater conditions on slope behaviour and the initiation of slope instability.

Calibration of strain gauged square tunnels for centrifuge testing

Abstract 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.

Centrifuge Modelling of the Dynamic Behavior of Square Tunnels in Sand

The Chapter summarizes representative experimental results from dynamic centrifuge tests that were performed on square model-tunnels embedded in dry sand. Two model-tunnels were used, a rigid and a flexible one, with the latter model collapsing during the test. The tests were carried out at the geotechnical centrifuge facility of the University of Cambridge, within the Transnational Access action of the SERIES Research Project (TA Project: TUNNELSEIS). The experimental data is presented in terms of acceleration in the soil and on the tunnel, earth pressures on the side walls of the tunnel and internal forces of the tunnel lining. The collapse mechanism of the flexible tunnel is presented and discussed based on the recorded response. The goal of this program is two fold: to better understand the seismic behaviour of this type of structures, and to use the high quality data to validate the numerical models, which should be used for the design of rectangular embedded structures. The interpretation of the results reveals (i) “rocking” response of the tunnels in addition to racking, (ii) existence of residual values on the earth pressures on the side walls and on the internal forces after shaking, affected significantly by the flexibility of the tunnels, and (iii) modification of the induced shear wave field from the presence of the shallow tunnel, which in turn is affecting its seismic response. These issues are not well understood and often are not considered by simplified seismic analysis methods.

Susceptibility of shallow foundation to rocking and sliding movements during seismic loading

Current design codes prevent the rocking and sliding of shallow foundations during seismic loading despite much research indicating the beneficial nature of allowing such movements. The primary benefit is the partial isolation of the structure from the soil beneath and subsequently the reduced ductility demands on the superstructure, saving money and reducing the risk of collapse. However, further research is required in order to be able to fully model and predict the behaviour of the soil-foundation interface when sliding and rocking is permitted. The results presented in this chapter examine how several different parameters including structural stiffness, aspect ratio, soil relative density and earthquake magnitude affect the level of rotation and sliding experienced by the foundation. Six centrifuge tests were performed to examine how these parameters affected the response of the structure and high speed photography was used to track the movements of the foundation precisely. It was found that structures with a high centre of gravity slid more than structures with a low centre of gravity. Also, stiff structures were found to rotate more than flexible structures and structures located on dense sand rotate more than those located on loose sand.

Cross-Facility Validation of Dynamic Centrifuge Testing

This paper compares the results of dynamic centrifuge tests on shallow foundations conducted at two different geotechnical facilities, IFSTTAR (Institut francais des sciences et technologies des transports, de l’amenagement et des reseaux, formerly LCPC), France and Cambridge University, U.K. Both facilities ran tests on a single degree of freedom model structure with its shallow foundation located on dry sand and subjected to dynamic shaking. Measurements were taken at both facilities allowing direct comparisons to be made. Fundamentally the results obtained were found to agree well via comparison of soil amplification profiles and moment-rotation cycles. However, higher frequency components agreed less favourably. This variation is thought to be due to a mismatch between the dynamic properties of the model containers. In this series of tests the higher frequency components are not of great importance and therefore the variation is insignificant, however, in future tests when earthquake signals with higher frequency components are input it may become an issue requiring further investigation.

Interrogation of fibre Bragg gratings through a fibre optic rotary joint on a geotechnical centrifuge

The monitoring of an array of fibre Bragg gratings (FBGs) strain sensors was performed through a single channel, single mode fibre optic rotary joint (FORJ) mounted on a geotechnical centrifuge. The array of three FBGs was attached to an aluminum plate that was anchored at the ends and placed on the model platform of the centrifuge. Acceleration forces of up to 50g were applied and the reflection signal of the monitored FBGs recorded dynamically using a 2.5kHz FBG interrogator placed outside the centrifuge. The use of a FORJ allowed the monitoring of the FBGs without submitting the FBG interrogator to the high g-forces experienced in the centrifuge.