Stamatopoulos and Associates Co

Athens, Greece

Stamatopoulos and Associates Co

Athens, Greece
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Lopez-Caballero F.,French National Center for Scientific Research | Modaressi-Farahmand-Razavi A.,French National Center for Scientific Research | Stamatopoulos C.A.,Stamatopoulos and Associates Co. | Stamatopoulos C.A.,Hellenic Open University
International Journal of Geomechanics | Year: 2016

This article presents an assessment of the effects of pore water pressure generation of the soil foundation on the seismic road embankment response. Numerical simulations were carried out to study the preloading technique as an improvement method for reducing the liquefaction potential and the induced settlements in a sandy soil profile. The analyses showed that the use of preloading reduces the induced settlements mostly because of the increase in lateral confinement in the superficial soil layers that results from an increase of the coefficient of lateral earth pressure at rest (ko). The research also showed that the efficiency of the countermeasure method was limited to cases in which earthquakes produced a liquefaction zone lower than the depth of the overconsolidated soil. © 2015 American Society of Civil Engineers.


Stamatopoulos C.A.,Hellenic Open University | Stamatopoulos C.A.,Stamatopoulos and Associates Co | Lopez-Caballero F.,École Centrale Paris | Modaressi-Farahmand-Razavi A.,École 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.


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.


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.


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.


Foumelis M.,European Space Agency | Papageorgiou E.,CNR Institute of Acoustics and Sensors Orso Mario Corbino | Stamatopoulos C.,Stamatopoulos and Associates Co Ltd
International Journal of Remote Sensing | Year: 2016

The Eastern Thessaly Plain presents an area of severe settlement phenomena, owing to the over-exploitation of the underground aquifer systems, causing significant damages to national infrastructures and private properties annually. Herein, both Persistent Scatterers (PS) and Small Baselines (SB) interferometric techniques were applied to study the history of ground deformation along the entire plain. Although the area consisted mostly of agricultural land, a sufficient number of point targets was obtained, well-distributed over the entire plain, permitting the recognition of spatial variations of the displacement field in addition to temporal trends. Our findings outline the southern part of the basin as the mostly affected area, whereas local subsidence patterns of lower magnitude were also recognized elsewhere. Episodes of significant ground subsidence, reaching several centimetres within a few months, characterize the deformation pattern of the area. Although average ground deformation rates do not exceed 2 cm year−1, line-of-sight (LOS) displacements of up to 13 cm were observed, occurring during the summer–autumn periods. A geographic information system (GIS)-based post-processing approach for the analysis of synthetic aperture radar (SAR) time series is presented, by which these abrupt settlement episodes can be identified in both temporal and spatial domains. The analysis allows the separation between rapid subsidence phenomena during the summer–fall season and annual deformation rates, thereby providing valuable information regarding the actual deformation pattern of the area. The results confirm in situ geological observations, highlighting the unique behaviour of the area due to intense water pumping. The study underlines that average SAR displacement rate maps might be inadequate to describe complex deformation scenarios and could lead to misinterpretations. Exploitation of the full capacity of SAR time series by detailed examination of the displacement histories, through a tailored data-mining strategy, could provide valuable information to geotechnical engineers and planners. © 2016 Informa UK Limited, trading as Taylor & Francis Group.


Stamatopoulos C.,Stamatopoulos and Associates Co. | Balla L.,Stamatopoulos and Associates Co.
International Conference on Engineering Mechanics, Structures, Engineering Geology, International Conference on Geography and Geology - Proceedings | Year: 2010

The paper proposes and validates a constitutive model simulating the change of resistance along clay slip surfaces under both undrained and drained conditions. The proposed model is based on (a) the critical state theory and (b) the assumption that the critical state changes once failure is reached, in terms of the further shear displacement. Under undrained conditions, the proposed model simulates the excess pore pressure generation and subsequently the change of clay resistance along the slip surface from its initial value to the peak strength and then at large displacement the residual value measured in constant-volume ring shear tests. Under drained conditions, the model simulates the normal displacement change and subsequently the change of clay resistance along the slip surface in clays as measured in drained ring shear tests.


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.

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