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The fast growth of the built environment in liquefaction-prone areas requires new design challenges to be overcome. A significant soil shear strength and stiffness loss associated to soil liquefaction, possibly occurring during severe earthquakes, may induce effects on buildings and on shallow and light structures such as urban tunnels. In densely urbanised areas, interaction between underground and aboveground structures may results in a detrimental combination of such effects. The paper discusses the results of a series of centrifuge tests carried out at the Schofield Centre (Cambridge, UK) modelling the dynamic interaction between an urban tunnel and a building in sand undergoing cyclic liquefaction. Four reduced-scale models were tested at increased gravity to model a prototype 60 times larger. A rectangular aluminium frame embedded in a layer of homogeneous Hostun sand represented a section of a metro tunnel. An aluminium sway frame represented a simple building founded in the same layer, at a side of the tunnel. The experimental results show that in typical urban spacing, sand liquefaction in the foundation ground affects the reciprocal interaction between the kinematic of the tunnel and the building. Centrifuge modelling investigated whether it is possible to mitigate or prevent such interference by improving ground through the addition of hydrated laponite beneath the tunnel floor or as lateral vertical barrier between the building foundation and the tunnel. The results also show the importance of temporary structures – often used to support soil excavation during cut-and-cover tunnel construction – in reducing the effects of interaction between the tunnel and the building in the liquefying soil.

This paper describes the small scale centrifuge modelling activities performed at ISMGEO (Istituto Sperimentale Modelli Geotecnici, formerly ISMES – Italy) laboratory in the frame of the H2020 LIQUEFACT project (http://www.liquefact.eu/). The main aim of the experimentation was to analyze the seismic behavior of loose, saturated, sandy deposits, homogeneous or stratified, subjected to increasing seismic excitations up to liquefaction and to verify the effectiveness of different liquefaction mitigation techniques. Some tests were carried out under free field condition, in some others a simple structure based on shallow foundations was modelled. Thirty-seven centrifuge tests were carried out to this aim, organized in three series: the first one aimed at investigating the liquefaction triggering conditions, the second and third ones devoted at analyzing the effectiveness of three selected liquefaction mitigation techniques: vertical drains, horizontal drains and “Induced Partial Saturation” (IPS). Some of the most significant results are presented in this paper.

In the framework of the LIQUEFACT project a series of centrifuge tests were conducted at ISMGEO (Italy) to verify the effectiveness of three mitigation techniques against soil liquefaction. This paper is focused on the study of horizontal drains as mitigation measure against liquefaction. The centrifuge layout with horizontal drains was numerically modelled and the change of boundary conditions reproduced by the FE code Plaxis 2D. Additionally, slightly different layouts from those tested in the centrifuge were also modelled to expand the scope of the experimental work. Horizontal drains were installed in the model, to analyse their effectiveness in reducing the pore pressure build up as a function of their spacing. The soil was characterized by an advanced coupled soils constitutive model implemented in the Plaxis code: PM4sand. The input parameters for the model are evaluated on the basis of both laboratory element tests and centrifuge tests. &nbsp

Passive structural systems installed at the foundation level of existing buildings are a common solution to protect them against earthquakes. However, installation procedures are expensive and not always feasible, as for instance in the case of valuable buildings. In this paper, an alternative and innovative solution for the seismic isolation of existing buildings is presented: the idea is to create a barrier buried in the ground, made of grouted soil, able to properly filter the seismic input. This can be obtained by using soft grouts, having a dynamic impedance much lower than that of the natural soil. Previous papers published by some of the authors have shown that the soft barrier can be effective, tending to filter the high frequency components of the amplitude spectrum without modifying in a significant way the low frequency ones. This implies that ground treatment has to be tuned on the characteristics of the structure to be protected, above all its natural frequencies. In this paper, the case of a simple four-story building with a reinforced concrete structure and a shallow slab foundation resting on a sandy soil is considered. Analyses are carried out with the commercial codes PLAXIS 2D and SAP2000, considering a fixed base, the foundation on the soil and finally the existence of a soft barrier. It is shown that the natural frequencies of the structure are largely influenced by the characteristics of the subsoil. Once the soft barrier is created in the soil, a sharp decrease of the first natural frequency of the structure is obtained, similarly to the effect of traditional isolators. In the paper, dynamic analyses with different input motions are considered. It is confirmed that the soft barrier, whose dynamic impedance was assigned considering some experimental results obtained by the authors and previously published, is able to filter higher frequencies, while it has a minimum or even nil effect in the case of low frequency input signals.

The results of an experimental activity aimed at investigating the ability of barriers made of a soft polymer [Super Absorbing Polymer (SAP)] to reduce the impact of ground vibrations on existing structures are described in this paper. A series of field tests was performed to examine the screening effectiveness of such barriers. Perturbing shear waves were generated using a vibrant source with controlled force and frequency. Field measurements of wave propagation with and without the barrier show the reduction of amplitude in several points of interest, caused by the extremely low shear stiffness of SAP. Two different geometrical configurations of the barriers were adopted in the field: one having the classical vertical trench shape, and the other with a less common shape of an upside-down pyramid. A FEM model was built and numerical analyses were performed and validated against the experimental results. After validation, the numerical analyses were also used to add further information on the behaviour of barriers of different shape. © 2019, Springer Nature Switzerland AG.

Relatively shallow and light underground structures, such as urban tunnels, may run through liquefiable sand deposits. In urban area they are likely close to the foundations of buildings and easily interact with them during earthquakes. The reciprocal influence of a tunnel and an adjacent building in presence of soil liquefaction has been investigated in this work. For this purpose, centrifuge tests were carried out at the Schofield Centre of the University of Cambridge (UK) on a reduced scale model of box-type section tunnel. In a first test a model tunnel was embedded in a liquefiable layer of sand. In the second one, a model building was added and founded in the proximity of the tunnel. The study contributes to the wider topic of the resilience of urban environment to natural hazards, and to earthquake-induced soil liquefaction specifically.

The research presented herein was carried out in the framework of the H2020 LIQUEFACT project ( http://www.LIQUEFACT.eu/ ). This paper presents the results of a set of numerical analyses performed to study the soil-structure interaction in presence of a liquefiable soil. In the first part of the paper, the attention was focused on the ability of some advanced soil constitutive models to simulate the centrifuge tests carried out within the project. Two finite element codes, Plaxis 2D and OpenSees, were used for the computations. The results were analysed in terms of accelerations, displacements and pore pressures build up. In the second part of the paper the effects of the introduction of horizontal drains, as mitigation techniques against liquefaction, were shown and discussed in terms of their ability in reducing pore pressure build-up and building settlement. All the numerical predictions confirmed the experimental evidence that horizontal drains are able to reduce the excess pore pressure during shaking and the structural settlement. The amount of reduction depends on the water boundary condition at surface: a free seepage surface generally allowed a larger mitigation than where the liquefiable layer was topped by clay.

The Theodosian walls of Constantinople have suffered many partial destructions in their long-lasting life, often because of strong seismic events, undergoing subsequent reconstructions. With reference to the 1999 Kocaeli earthquake, the article illustrates a procedure implemented to understand the damages suffered by one of the towers (tower 19) of this magnificent piece of heritage, that had just been partially reconstructed in the framework of a large manutention and preservation program of the whole historic defensive system. The seismic action on the tower was calculated considering local seismic site amplification, and the seismic behaviour of the masonry tower was analysed taking into account dynamic soil-structure interaction. Based on the available level of knowledge, the seismic demand of the tower is first evaluated by the classical modal response analysis and then by a linear time domain approach. Under both situations, the comparison between the compliant and the fixed base models allowed to recognize the detrimental effect of Soil Structure Interaction (SSI). Furthermore, the dynamic linear analysis allowed to detect the mechanism of earthquake induced triggering damage into the masonry panels.

This paper shows the results of the preliminary numerical analyses carried out for the project STILUS within the framework of the European funded network SERA (Seismology and Earthquake Engineering Research Infrastructure Alliance for Europe). The aim of the project is to investigate the problem of tunnel-structure interaction in liquefiable soil through a series of centrifuge tests and to assess an effective mitigation technique. A preliminary numerical study was carried out in Plaxis 2D software. The soil was characterized by UBCSAND model imple- mented in the Plaxis code. The input parameters for the models are evaluated on the basis of laboratory element tests. Numerical analyses were also performed to evaluate the efficacy of drains as mitigation techniques. Drains were installed in the models, in order to analyse their ef- fectiveness in reducing the pore pressure build up and then the tunnel uplift and building move- ments as a function of their spacing, length and position.