1,721,011 research outputs found
On the apparent viscosity of granular soils during liquefaction tests
No full textLiquefaction is a phenomenon marked by a rapid loss of soil strength and stiffness, which generally occurs in loose saturated sandy deposit during earthquake because of the generation of excess pore water pressure. Several experimental researches concluded that liquefied soil behaves as a fluid during ground movement, but after the earthquake motion ceases, due to the dissipation of excess pore water pressure and soil dilatancy, the liquefied soil recovers its initial stiffness and returns to behave as a solid. Such a change of state can be analysed by considering the soil as an equivalent visco-plastic material, characterized by an apparent viscosity (η) that changes during the cyclic loading. Following this approach, the authors analysed the results of some cyclic undrained triaxial tests carried out on reconstituted and undisturbed (frozen) specimens of sandy and gravelly soils in terms of apparent viscosity decay law (η-Ncyc), highlighting the relevance of η as physically based parameter for the correct identification of the liquefaction triggering. The experimental results confirm that the apparent viscosity decreases with the increase of the shear strain rate and highlight that the flow characteristics of liquefied soils (consistency coefficient and liquidity index) are affected by both grain size distributions and soil state conditions (relative density and confining stress)
Soil Liquefaction: From mechanisms to effects on the built environment
No full textThe analysis of soil liquefaction phenomenon and its consequences on built environment remains one of the more active research areas in geotechnical engineering around the world. In many major earthquakes, liquefaction induced ground failures (sand boils, ground settlements, cracks and lateral spreading, flow failure) caused extensive damages to shallow-founded buildings and other engineering facilities. An accurate liquefaction hazard analysis can be done if all factors governing the liquefaction triggering and its effects are included, in a consistent way, into the assessment procedure. Given the complexity of the phenomenon itself and the variety of ground failure mechanisms, the evaluation of liquefaction hazard at large scale is necessarily done by means of simplified procedures that in some major earthquakes gave a misprediction of liquefaction effects on the built environment, highlighting the limits of their predictive capability. The development of more accurate methods to quantify liquefaction hazard and the associated consequences for buildings and infrastructures is the challenge on which the research is ongoing
Liquefaction triggering of non-saturated sandy soils
No full textIn saturated sandy soils liquefaction triggering is generally well-identified according to stress or strain criteria. On the contrary, the attainment of liquefaction in non-saturated sandy soils is still a cause of discussion in the scientific community. Even if the liquefaction resistance of non-saturated soils is higher than that of saturated ones, these soils may liquefy, as well. The increasing interest for cyclically mechanical behaviour of non-saturated sandy soils is due to the fact that desaturation or induced partial saturation can be used as useful mitigation techniques against soil liquefaction. Therefore, it is important to define, as accurately as possible, the attainment of liquefaction, on which depends the estimation of liquefaction resistance. The relevance of the apparent viscosity as a physically based parameter for the correct identification of the liquefaction triggering for fully saturated soils has been already demonstrated. In this paper, the viscous triggering approach has been used for non-saturated soils, processing some cyclic triaxial tests carried out on different sandy soils. The results confirm the reliability of the apparent viscosity as a liquefaction triggering parameter, showing a tight correlation with the strain liquefaction triggering criterion. Therefore, strain criterion should be preferred in non-saturated sandy soils
Simple approach to evaluate the influence of seismic residual displacements on post-liquefaction settlements of RC-frames
No full textResidual permanent displacements generally characterize the status of structures at the end of severe earthquake events because of large inelastic deformations demand. When the structure is founded on soils susceptible to liquefaction phenomenon, this status could particularly influence the subsequent damages due to the occurrence of liquefaction induced vertical settlements. As a consequence, the correct prediction of the effects of earthquake induced liquefaction on the built environment must consider the residual displacements induced by the ground shaking. To this aim, the paper proposes a simplified approach to evaluate the seismic response of RC frames based on a four-steps procedure composed of a set of nonlinear static analyses reproducing a sequential process of a ground shaking followed by a liquefaction induced ground settlements. The novelty of the proposal is to carry out sequential nonlinear static analyses by preserving the status of the RC frame at the end of the seismic event in terms of residual displacements and then to directly consider their influence on the subsequent analysis simulating the occurrence of post-liquefaction vertical settlements. A parametric study has been developed by means of numerical analyses, referring to two RC frames derived from literature, in order to highlight the relevance of the seismic residual displacements on the damage induced by the subsequent vertical settlements due to soil liquefaction. The proposed simplified approach has then been validated through more sophisticated nonlinear dynamic time-history analyses. The results presented in the paper underline the efficacy of the proposed approach, the simplicity in implementing the four-steps procedure and the lower computational effort with respect to nonlinear dynamic time-history analyses
A liquefaction potential integral index based on pore pressure build-up
No full textThe increased attention paid to the effects of liquefaction on the built environment has led over time to the introduction of vulnerability integral parameters that somehow quantify for a given site and a given seismic shaking the severity of the potential liquefaction damage induced at ground level. If the vulnerability parameter is properly calibrated and effective, it should somehow correlate to the observed damage datasets, increasing as the severity of liquefaction effects increase. In some cases, as for instance for the Canterbury earthquakes (New Zealand, 2010–2011) and for the 2012 Emilia Romagna seismic sequence, the existing vulnerability parameters for the assessment of liquefaction potential were not able to capture the observed damages induced by liquefaction, highlighting that further progress remains to be made in the current state-of-practice. The potentiality of a new index (Induced dAmage paraMeter- IAM), strictly related to the free-field post-volumetric consolidation settlement and therefore based on the estimate of liquefaction induced excess pore pressure, has been verified for a reference site located in the area struck by the 2012 Emilia seismic sequence by means of simplified and 1D effective stress dynamic analyses. The results were compared with those obtained by using one of the most convincing parameters existing in literature (Liquefaction Severity Number, LSN), showing the robustness of IAM, as well as the possibility to calculate it at different levels of complexity (via either coupled, simplified or super simplified and conservative analyses). The outcomes of the analyses allowed to define a simplified procedure for defining liquefaction microzonation maps, then representing a step forward towards the definition of a sound but easy-to-implement procedure to assess the resilience to liquefaction of urban centres
A simple procedure to calibrate a pore pressure energy-based model from in situ tests
No full textThe simultaneous generation, dissipation and redistribution of excess pore pressures within the layers of a soil deposit, due
to a seismic event, can significantly modify the seismic response of the whole deposit. The reliable estimate of the excess
pore pressure induced by shaking within the soil is important to predict the behaviour of the soil at a large scale, and
consequently, earthquake effects on built environment. Recently, pore pressure energy-based models are developing.
Despite several advantages, their calibration is generally complex. The paper aims to provide a simple calibration procedure
of the pore pressure energy-based prediction model developed by Berrill and Davis (1985), in order to make easier
and more common the use to practitioners. The energy-based model of Berrill and Davis (1985) has been calibrated in this
study by means of a dataset of 46 undrained cyclic triaxial and simple shear tests carried out on different sandy soils. The
best fitting procedure with the envelope of the experimental curves has been adopted. The experimental evidences show
that the two parameters (a and b) on which the model depends, are linked and can be related to the results of CPT or SPT
in situ tests. The paper introduces two relationships to compute the calibration parameters from the well-known equivalent
cone tip resistance (qc1Ncs) or the corrected SPT blow count ((N1)60cs). The applicability of the proposed procedure at a
large scale has been discussed interpreting the results of cyclic simple shear tests on undisturbed sandy specimens from an
energetic perspective. The validity of the calibration procedure has been finally verified performing 1D non-linear site
response analyses by means of DEEPSOIL code, reproducing two centrifuge tests and three case histories. The close
matching between the simulations of the excess pore pressure time histories with the experimental data of the centrifuge
tests, together with the simulated excess pore pressure profiles of three case histories compared with the results achieved by
using another 1D non-linear code demonstrates the effectiveness of the simplified procedure to calibrate the pore pressure
energy-based prediction model of Berrill and Davis (1985)
Liquefaction mechanisms and mitigation techniques
No full textSeismic shaking of loose, saturated granular soils causes a transient pore pressure build-up and a progressive reduction of stiffness and shear strength, that may eventually lead to a state transition from solid to fluid when the effective stresses tend to zero. This final stage is called liquefaction. Even before full liquefaction, however, critical mechanisms on structures and infrastructures may be triggered by the reduction of effective stresses. The effects on the built environment differ from the inertial ones typically observed during earthquakes, being basically related to excessive settlements and tilting, rarely resulting in human losses but often producing extremely relevant repair or dismantling-rebuilding costs. The mitigation of liquefaction risk is therefore a relevant issue, also because experimental evidence shows that this mechanism may repeatedly take place on previously affected areas upon new seismic events, which is somehow counterintuitive because of the densification effect of post seismic consolidation. After an introductory section devoted to a description of liquefaction mechanisms and to the effects induced on the built environment, the paper briefly introduces and comments codes rules for liquefaction risk assessment, to finally end up with some design indication for the most popular or promising ground improvement technologies suited to mitigate liquefaction risk. Because of the large and ever-increasing number of technologies and patents, design hints are herein proposed categorizing ground improvement interventions on the base of the modification induced by treatment, without entering in details on the specific technology adopted to the aim
Efficiency of Piles Stabilizing Slopes in Fine-Grained Soils
No full textEqually spaced piles can be designed to ensure the stability of a slope with a desired safety factor or to restrain slow slope movements to prevent damage to existing structures. The piles are usually arranged in lines installed transversally to soil movements or in groups of limited extension in a plan according to their stabilizing or protective function. A numerical study has been performed to analyze the role of some design parameters in the efficiency of equally spaced piles, adopting two-dimensional (2D) and three-dimensional (3D) finite-difference (FD) models. Plane strain models have been used to study the soil arching mechanism and the associated stress transfer from the yielding soil to the stabilizing piles, while 3D numerical analyses have been performed to evaluate the efficiency of pile groups. The effect of varying pile spacing, pile embedded length, soil mechanical properties, and sloping ground conditions have been investigated for two-layer slopes assuming fixed or unconstrained piles. The outcomes of the parametric study made it possible to derive a simple analytical relationship to quantify the resisting contribution provided by the piles in an infinite slope. The paper shows that the proposed approach, even though limited to the ranges of the adopted parameters, can be used for a preliminary design of stabilizing piles to ensure the desired increase in the slope safety factor, as well as to achieve the desired reduction of downhill slope movements
Effect of the pore fluid salinities on the behaviour of an electrokinetic treated soft clayey soil
No full textDredging activities of harbours and rivers are becoming very important in many countries all over the world and, as a consequence, the disposal of dredged sediments is a critical concern from an environmental point of view. In order to facilitate the disposal or the reuse of large volume of dredged soils, usually under-consolidated and with a high water content, an electrokinetic treatment can be adopted with the goal to dewater and strengthen the sediments. This paper presents the results of some electrokinetic tests performed on reconstituted clayey specimens at different pore fluid salinities (0.2 < sc < 30 g/l) treated with electrokinetic (EK) technique. The results indicate that the presence of small quantities of salts in the pore fluid enhances the electro-osmotic consolidation. On the contrary, for high salt concentrations of the pore fluid the electro-osmotic dewatering is significantly reduced. The mechanical behaviour of treated specimens has been investigated at the micro (SEM) and macro scale (triaxial and oedometer tests). The experimental results highlighted the relevant and expected contribution of the pore fluid characteristic on the effectiveness of the treatment as ground improvement technique
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