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Stamatopoulos C.A.,Hellenic Open University | Stamatopoulos C.A.,Stamatopoulos and Associates Co | Lopez-Caballero F.,Ecole Centrale Paris | Modaressi-Farahmand-Razavi A.,Ecole Centrale Paris
Soil Dynamics and Earthquake Engineering | Year: 2015

The paper studies the effect of preloading on the liquefaction cyclic strength of silty sands in the free field condition. This effect first is investigated by cyclic shear tests where horizontal shear stress oscillated about a zero mean value. Samples with varying fines content and at varying pre-stress ratios, densities and vertical stresses are tested. Test results show a marked increase of the liquefaction cyclic strength with the pre-stress ratio. The effect is more pronounced for tests with less liquefaction cyclic strength without pre-stress. Using critical state soil mechanics concepts, factors simulating the effect of preloading on the liquefaction cyclic strength are identified and based on the results of the laboratory program an empirical expression is proposed predicting the increase in the liquefaction cyclic strength induced by pre-stress. This expression is validated by numerical simulation of the relevant laboratory tests using an elastoplastic multi-mechanism model. In addition, based on the derived expression, a methodology is proposed predicting the increase in liquefaction cyclic strength as a result of preloading in the field in the case of the free field condition. This methodology is validated by the comparison with field measurements on liquefaction-susceptible soils before and after the field application of preloading. Last but not least, the increase in liquefaction cyclic strength which the proposed methodology predicts for typical soil profiles and embankment preloads is predicted and discussed. © 2015 Elsevier Ltd. Source


Stamatopoulos C.A.,Stamatopoulos and Associates Co | Stamatopoulos C.A.,Hellenic Open University
Soil Dynamics and Earthquake Engineering | Year: 2015

Slopes consisting of saturated sand have recently moved rapidly down-slope tens or hundreds of meters as a result of the action of earthquakes. In the seismic risk assessment of such slopes, typically the conventional sliding-block model is utilized. However, this model assumes constant strength along the slip surface and predicts co-seismic displacement, which typically is less than tens of centimeters. The landslide risk described above is associated with post-seismic very large displacement. It occurs when static failure occurs, as a result of loss of soil strength, under the applied earthquake loading. The paper first derives simple analytical expressions predicting when enormous displacement may occur along a planar homogeneous slip surface of saturated sand during earthquakes. For this purpose, the sliding-block model and a recently proposed simple constitutive model simulating saturated sand response along a slip surface are utilized. The paper then validates the proposed analytical expressions by extensive parametric numerical analyzes using the sliding-block model with the proposed constitutive model, and based on these analytical expressions, proposes an easy-to-apply method predicting earthquake-induced landslide triggering of any potentially two-dimensional unstable mass along slip surfaces consisting of saturated sand. Finally, the proposed equations and method are applied (a) to predict the observed triggering of four well-documented earthquake-induced landslides and (b) to establish relations giving characteristics of the seismic motion causing triggering of landslides. © 2015 Elsevier Ltd. Source


Stamatopoulos C.A.,Stamatopoulos and Associates Co
Soil Dynamics and Earthquake Engineering | Year: 2014

Preloading is a temporary loading, usually an embankment, applied to improve subsurface soils by densification. This paper studies the effect of preloading on the amplification characteristics of soft sites with an elaborate parametric analysis. The soil type, the depth of the bedrock, the water table depth, the level of preloading, the applied earthquake, the shear wave velocity of the bedrock and the shear modulus and damping versus shear strain relations were varied in a systematic manner. The analysis was performed by the commonly used one-dimensional equivalent-linear dynamic method. The shear wave velocity versus depth and the effect of preloading on shear velocity are computed with well-established soil mechanics equations. The results illustrated that the seismic response at the top of the profile generally decreases as a result of preloading. A more detailed analysis of results shows that the effect of preloading on the seismic response depends on the soil type and the depth of the bedrock. Based on these results, a method is proposed by which a practicing engineer involved with improvement of soft ground can simulate the effect of preloading on the seismic motion. © 2014 Elsevier Ltd. Source


Stamatopoulos C.A.,Stamatopoulos and Associates Co
Soil Dynamics and Earthquake Engineering | Year: 2010

An elaborate program of monotonic and cyclic triaxial laboratory tests on mixtures of sand and silt with fines content 0%, 15% and 25% was performed to investigate the effect of density, consolidation stress and non-plastic fines on the liquefaction strength. The monotonic tests illustrated that the critical state lines of all mixtures do not cross each other, and are, approximately, parallel to each other. The results of the cyclic tests illustrated that the relationship between the cyclic strength and the state parameter does not depend on the consolidation stress, the soil density and the silt content. Analysis in terms of the state parameter showed that: (i) as the consolidation stress increases, the cyclic strength decreases and this effect is more pronounced as the specimens become denser, especially as the fines content increases and (ii) the cyclic strength decreases as the fines content increases and this effect is more pronounced as the specimens become denser. © 2010 Elsevier Ltd. Source


Stamatopoulos C.A.,Stamatopoulos and Associates Co | Stamatopoulos C.A.,Hellenic Open University | Di B.,University of Sichuan
Soil Dynamics and Earthquake Engineering | Year: 2014

Slopes consisting of saturated sand have recently moved down-slope tens or hundreds of meters under the action of earthquakes. This paper presents a simplified but accurate method predicting the triggering and displacement of such landslides. For this purpose, a simplified constitutive model simulating soil response of saturated sands along slip surfaces is proposed and validated. Then, this constitutive model is coupled with the multi-block sliding system model to predict the triggering and displacement of such slides. The multi-block model considers a general mass sliding on a trajectory which consists of n linear segments. The steps needed to apply this method are described in detail. The method was applied successfully to predict the triggering, the motion and the final configuration of the well-documented (a) Higashi Takezawa, (b) Donghekou and (c) Nikawa earthquake-induced slides. © 2014 Elsevier Ltd. Source

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