1,721,164 research outputs found
Seismic Behaviour of Urban Underground Structures in Liquefiable Soil
No full textSoil liquefaction has been often one of the most significant causes of damage to aboveground structures in urban areas during recent earthquakes, e.g. 2012 Emilia (northern Italy), 2011 Tohoku Oki (Japan) and particularly 2011 Canterbury-Christchurch (New Zealand), where about half of the e25 billion loss was directly caused by such a phenomenon. In some cases, sewer pipes or open-cut tunnels in liquefied deposits have been affected by floatation and large uplift. The current and future construction of relatively shallow and light underground structures in seismic regions may involve areas that are exposed to the risk of liquefaction thus increasing possible associated damages. This paper investigates the behaviour of a tunnel during soil liquefaction, from an experimental and numerical point of view, focusing on the combined effects of soil liquefaction in urban areas, where underground structures are likely to interfere with buildings. Such an aspect is rather unexplored, and the research in this field may contribute to the performance-based design of urban underground facilities
Seismic microzonation by means of finite element approaches
No full textWhen an earthquake occurs, seismic waves radiate away from the source and travel rapidly through the earth's crust. The motion recorded at the ground surface of an area could be really different, in terms of duration and frequency content, from the reference outcrop motion due to the following site conditions: sequence of soil layers, velocity contrast between soil layers, thickness of each layer, dynamic behaviour of the soils, topography and geometry of the sub-interface.
Concerning the site-effects, it is possible to discuss about one-dimensional (1D) effects, two-dimensional (2D) and three-dimensional (3D) ones. One-dimensional ef-fects are induced in case of horizontally layered deposits with a horizontal ground surface (vertically heterogeneous media). Two-dimensional site-effects are generated in case of a complex stratigraphic sequence (vertically and laterally heterogeneous media) and/or in case of an uneven ground surface. In presence of stratigraphic and topographical surface varying in any direction (vertically, laterally and transversally heterogeneous media), it is necessary to refer to three-dimensional site-effects.
Seismic microzonation (SM) studies are used to assess local geological and ge-otechnical site conditions and to identify earthquake characteristics. A SM study can be undertaken according to three different levels of details, as reported in the Italian guidelines. In particular, numerical analyses are requested for a level III SM to quantify the reference motion modification.
This work aims to evaluate complex site-effects for a real case study, i.e. the Bo-vino village, located in South of Italy. This case study has been chosen due to the presence of a soft soil valley surrounded by rock outcrop hills. As a consequence complex site-effects are expected.
Essential ingredients for predicting site effects are: topography, stratigraphy, interface between soil layers and dynamic soil behaviour. Moreover it is necessary to select the reference seismic event and to define properly the input motion used in the numerical analyses.
The present thesis addresses how to perform a site response analysis using the finite element (FE) method. Two different FE codes, in the time domain, have been used: QUAKE/W, based on the equivalent-linear approach, and PLAXIS 3D, which adopts a non-linear Hardening Soil model with small strain stiffness (HSs).
At first, some numerical approaches to simulate 1D site response are defined with reference to ideal case studies. These approaches have been validated by compari-son with results obtained with the code EERA, which is based on an equivalent-linear approach in the frequency domain. 1D schemes have also been used to clarify the definition of reference motion and numerical input motion.
The numerical simulations of seismic site response for the Bovino case study are then presented. Before discussing the results of these analyses, local geology and topography are described. The geotechnical model is subsequently defined, based on field investigations and laboratory data. The reference outcrop motion is then selected, according to the Italian probabilistic seismic hazard maps. Seven real accelerograms have been selected as reference outcrop motions.
The results of the 2D analyses performed using QUAKE/W, with reference to 22 sections, are presented first. The results of the 2D analyses are compared with those of 1D analyses performed with reference to 42 soil columns, extracted along two sections.
Finally, results of 1D, 2D and 3D analysis performed with PLAXIS 3D, assuming two reference motions, are discussed. The analyses allowed to identify the effects of dimensional scheme to seismic site response, the dependency of each amplification pattern to the selected reference motion, the ground motion modification due to dif-ferent topography and soil layers interface, the comparison between different consti-tutive approaches to the same problem (i.e. linear equivalent and non-linear)
Preliminary Numerical Simulation of Centrifuge Tests on Tunnel-Building Interaction in Liquefiable Soil
No full textThis paper shows the results of preliminary numerical analyses performed to simulate centrifuge tests that will be 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. Two models will be tested in centrifuge, in the first one only the tunnel is modelled while in the second one a simplified structure on strip foundations is added. In order to analyze a cut-and-cover tunnel in an urban environment a rectangular section has been assumed. The tests were simulated by means of Plaxis 2D and the soil was characterized by PM4Sand model. The results were evaluated in terms of pore pressure build up, accelerations and settlements. The distribution of excess pore pressure induced around the tunnel and the building basement may also be useful to identify where mitigation techniques that may locally reduce pore-pressure build-up would be most effective against the effects of soil liquefaction and should be implemented in the forthcoming tests
Experimental and Numerical Investigation on the Effectiveness of Polymeric Barriers to Mitigate Vibrations
No full textThe 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
The ecology of perception of safety in urban environment: Assessing cumulative risk exposure in the city of Rome.
No full textS. Kabisch, A. Kunath, H. Feldmann (Eds.
Geotechnical properties and slope stability in structurally complex clay soils. Chapter 3: Physical and mechanical properties
No full text- …
