Emilio Bilotta

Centrifuge Modeling of Seismic Loading on Tunnels in Sand

The purpose of the work is to provide an experimental benchmark on the seismic behavior of tunnels, with the final aim of calibrating numerical and analytical design methods. A series of plane-strain centrifuge tests with dynamic loading on a model tunnel was, therefore, carried out at the Schofield Centre of the Cambridge University Engineering Department (CUED). Four samples of dry uniform fine sand were prepared at two different densities, in which an aluminum-alloy tube was installed at two different depths. The tube was instrumented with strain gauges to measure hoop forces and bending moments at significant locations. To monitor the amplification of ground motion from the base to the surface, vertical arrays of accelerometers were placed in the soil model and along the box. The instrumentation also included linear variable differential transformers (LVDTs) that measured the soil surface settlement during all test phases. The test procedure and the results are described in this paper, showing the evolution of both accelerations and internal forces along the tunnel lining during the model earthquakes.

Experimental and numerical study on circular tunnels under seismic loading

This paper compares the experimental results of a set of centrifuge models of tunnels in sand under seismic loadings with the predictions of finite element dynamic analyses and of simplified methods. In order to characterise the soil behaviour, mobilised shear stiffness and damping ratio of the sand model have been back-calculated from the experimental results according to two different procedures. Starting from the accelerometer measurements, one was based on the transfer functions from surface to base and the other one on the average shear stress–strain cycles along the sand layer. A series of viscoelastic 2D dynamic analyses were performed to simulate the model tests by a linear equivalent approach. The equivalent shear stiffness and damping ratio determined from stress–strain cycles were used as input values for the analyses. The shear stress transfer at the ground-lining interface was back-analysed to calibrate the interface elements used in the numerical code, in order to improve the assessment of the transient changes of hoop force. Finally, the numerical results have been compared to analytical solutions, widely adopted in the design, and to the experimental data in terms of transient increments of internal forces in the lining. Such a comparison indicates that the analytical formulations give a good estimation of the seismic increment of bending moment in the lining and a reasonable lower bound for the transient changes of hoop forces, provided that cyclic shear strains are correctly evaluated.

Internal Forces Arising in the Segmental Lining of an Earth Pressure Balance-Bored Tunnel

This paper presents experimental data obtained by long-term monitoring of the strains occurring in several segments of a precast RC segmental lining adopted to support a bored tunnel. Careful and tight control of the strains was also carried out during the early installation stages, which only lasted a few hours. The strains were measured using vibrating wire gauges embedded in the segments during construction at the manufacturer’s plant. The measurements were recorded using a wireless data logger, which allowed accurate follow up of the strain changes in the segments beginning with the concreting stage and for a long time after the tunnel construction. With reference to the various construction stages, the measured strains are presented and discussed. Their maximum values are just a few hundred microstrains, making the thermal effects on the gauge readings significant. For this reason the problem of their elimination is briefly addressed in the paper. The process used to derive the internal forces in the lining is not at all straightforward and the paper describes the process and reports the obtained internal forces both in short- and long-term conditions.

Use of a Line of Piles to Prevent Damages Induced by Tunnel Excavation

Buildings founded in proximity to shallow tunnels under construction may be damaged by the ground displacements induced by tunneling. This is a matter of concern for design, and a variety of protective interventions are currently adopted to prevent such damages. Among these, rows of piles or jet-grouting columns are widely diffused. In this paper, the effectiveness of a simple row of piles is computed by means of three-dimensional (3D) finite-element (FE) analyses, thus allowing the investigation of the relationship between performance and some simple geometrical parameters, such as the spacing among the piles. The results of centrifuge tests are reported and used as a benchmark. The potential damage has been quantified in this work, taking into account both the settlement profile and the horizontal strain induced at the ground surface by tunneling. It is shown that although the settlement reduction is significant only for very small spacing (s=2–3 pile diameters), even largely spaced piles (s=5–6 pile di...

Seismic analyses of shallow tunnels by dynamic centrifuge tests and finite elements

The increments of internal forces induced in a tunnel lining during earthquakes can be assessed with several procedures at different levels of complexity. However, the substantial lack of well-documented case histories still represents a difficulty in order to validate any of the methods proposed in literature. To bridge this gap, centrifuge model tests were carried out on a circular aluminium tunnel located at two different depths in dense and loose dry sand. Each model has been instrumented for measuring soil motion and internal loads in the lining and tested under several dynamic input signals. The tests performed represented an experimental benchmark to calibrate dynamic analyses with different approaches to account for soil-tunnel kinematic interaction.

Numerical Analyses of the Effectiveness of Soft Barriers into the Soil for the Mitigation of Seismic Risk

An approach to mitigate the seismic risk of existing structures by means of the creation of a continuous thin layer of grouted soil at a convenient depth is presented. A parametric numerical analysis is reported using different constitutive models with reference to two geometrical schemes. It is shown that if the grouted layer has a stiffness significantly lower than that of the surrounding soil, it may be effective in reducing the seismic demand. In the parametric analyses, the positive role of yielding is also observed, which indicates that the barrier is more effective with larger input amplitudes.

Centrifuge modelling of the seismic performance of soft buried barriers

The paper presents the results of an experimental work carried out in a geotechnical centrifuge at the Schofield Centre of Cambridge University. Two reduced scale models of soft barriers in a sand layer underwent a series of ground shaking. In the first model a thin horizontal layer made of latex balloons filled with a cross-linked gel was created at about mid-height of the sand layer. In the second, the same balloons were deployed to form a V-shaped barrier aimed at isolating a relatively shallow volume of sand. The aim of the study was to get experimental evidence of the capability of such soft barriers to isolate a volume of soil thus reducing amplification of ground motion during severe seismic events. The experimental results were compared with FE numerical analyses of the same models, carried out also in free field to have a benchmark condition. By validating the FE modelling via the comparison with the experimental results, a robust model has been built, aimed at being used for carrying out a wider parametric numerical testing. The experimental results confirm the effectiveness of such soft barriers to reduce amplification in the isolated volumes during seismic events.

Experimental Assessment of the Stress–Strain Behaviour of Leighton Buzzard Sand for the Calibration of a Constitutive Model

Abstract A number of constitutive models are nowadays implemented in numerical codes which simulate the stress–strain behaviour of soil from very small to large strain. In this paper, the mechanical behaviour of Leighton Buzzard sand (grade E), used worldwide for physical modelling, has been thoroughly characterized by laboratory testing along several stress paths. Tests were aimed at calibrating a constitutive model, that allows considering stiffness nonlinearities in a wide range of strains, in the framework of isotropically hardening plasticity. As a validation, the results of dynamic centrifuge tests on a layer of the same sand were compared with finite element predictions.

Effect of Travelling Waves on Tunnels in Soft Soil

The paper proposes a three-dimensional numerical model able to catch the main deformation patterns of the soil and of an underground tunnel structure subjected to non-uniform seismic shaking. The proposed 3D FE model for instance, investigate the effect of the non-uniform seismic loading condition due to the wave passage along the longitudinal axis of a tunnel and a comparison between the uniform and non-uniform case is proposed both in terms of free field and soil-structure interaction analysis. The numerical results show the non-negligible effect of wave passage on the tunnel structure compared with the uniform case. The dynamic increment of the transversal component of the internal tunnel forces is higher if the asynchronism is considered, in function of the shear waves velocity of propagation in the bedrock. In addition, longitudinal components of the internal forces arise on tunnel, i.e. longitudinal normal force, horizontal bending and shear force acting in the horizontal plane along the structure.

Liquefaction Triggering, Consequences, and Mitigation

The importance of predicting ground deformation in loose, saturated granular soils has been widely recognized for a reliable evaluation of liquefaction damage. A procedure is proposed in this paper for the evaluation of post-cyclic consolidation settlements, as a result of volumetric strains induced by the dissipation of excess pore pressure. A stress-based model is first adopted for generating the excess pore water pressure in 1D free-field conditions, allowing for an effective stress analysis according to a loosely coupled approach. Then, the post-cyclic settlement is simply calculated integrating the vertical strains. To this aim, by considering a welldocumented case history in which an extremely small settlement was observed upon seismic excitation, soil stiffness is estimated on the basis of either CPT data or shear stiffness decay curve, to show the effect of modelling hypothesis on the results. Both approaches result into a value of the settlement close to the observed one and much lower than that calculated using a well-established empirical procedure.