Gianpiero Russo

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.

Full Scale Loading Tests on Instrumented CFA Piles

Continuous flight auger piles (CFA) procedure allows the rapid installation of piles with diameters usually ranging from 0,4 to 1,0 m and a length up to 30-35 m, free from the vibration and noise of driven piles and substantially reducing the ground loosening of bored piles. CFA are becoming increasingly popular in many countries, and sometimes they are claimed to have the advantages of both driven and bored piles, without the related shortcomings. The behaviour of CFA piles, however, is actually affected by technological factors; among them the capacity of the equipment in terms of thrust and torque and the volume of concrete pumped in during auger retrieval. In the paper the results of three failure loading tests on instrumented CFA piles are presented. The analysis of the observed behaviours confirm that the CFA piles are somewhat intermediate between bored and driven piles. Introduction Continuous Flight Auger (CFA) piles are installed by means of an auger with a hollow stem having an inner diameter of 10 to 20 cm, inserted into the soil by the combined action of an axial thrust and a torque. The stem is provided with a temporary closure plate at the bottom; after the auger has reached the desired depth, the plate is pushed out by pumping concrete or mortar through the stem, and the auger is lifted removing from the ground the soil within the screw. The sides of the hole are thus supported at all times by the soil filled auger or by the pumped concrete. The procedure allows a rapid and noiseless installation of piles with diameter of 40 to 100 cm and length up to 30-35 m, and is becoming increasingly popular all over the world. The ratio between the rate of penetration and the rate of revolution of the auger is generally less than the pitch of the screw; the penetration involves thus a lateral compression but also the removal of some soil. Further removal of soil occurs during the extraction stage. If the volume of the removed soil is less than the final volume of the pile, the net resulting effect is a compression of the soil surrounding the pile; the resulting stress state within the soil is somewhat intermediate between that of a bored pile and that of a driven one.

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...

Multitemporal synthetic aperture radar for bridges monitoring

The present work is devoted to analyze the potentiality of satellite-based techniques for structural monitoring of bridges. Specifically, the well-known case study of the cable stayed bridge across the river Garigliano and Ausente stream is presented. The available “in situ” data have been compared and integrated with satellite-based measurements (ERS, ENVISAT satellites) for the common monitoring period (1993-2004); thus, the temporal observation window has been extended until 2010 (ENVISAT satellite). DInSAR represents a consolidated tool for deformation monitoring and its application on man-made structures and infrastructures can make easier the detection of potential problems with a consequent improvement of risks management.

Seismic Vulnerability Reduction for Historical Buildings with Non-Invasive Subsoil Treatments: the Case Study of the Mosaics Palace at Herculaneum

ABSTRACT The possibility to reduce the seismic vulnerability of historical buildings through subsoil treatments aimed to modify the seismic site response is a recent advancement in earthquake engineering: the paper presents the case study of the Mosaics Palace, located close to the archeological site of Herculaneum which is about 10 km south of Naples. The study was developed through uncoupled dynamic analysis of the seismic site response and of the masonry structure. All the available information on the subsoil were collected and integrated with on-purpose in situ tests; a 1-D subsoil model was adopted for linear equivalent seismic response analysis. Several hypotheses of soil treatments, spanning from soft to stiff grouted layers, were taken into account. On the other hand, the capacity of the structure was defined by a pushover analysis and compared to the seismic demand for each soil treatment option, allowing for a final assessment of the effectiveness of the proposed technology on the building performance.