Ahmed Arab

Identification of the behavior of sandy soil to static liquefaction and microtomography

In this paper, undrained triaxial compression tests were performed on specimens obtained in Oued Rass sand (Chlef, Algeria). The objective of this study is to investigate the effects of specimen deposition methods, the initial density, and the confining pressure on the undrained behavior of this sand. The test results show that the initial confining pressure and the relative density affected, in a significant manner, the resistance to liquefaction. However, it increases with confining pressure and relative density. The results also show that the specimens prepared by dry deposition method have a greater resistance to liquefaction than those prepared by moist tamping. A nondestructive comparative analysis of the deposits by X-ray microtomography carried out at the granular scale made it possible to characterize more precisely the difference of the obtained structures for the two deposition modes.

Undrained behavior of silty sand: effect of the overconsolidation ratio

This experimental study deals with the effect of the overconsolidation ratio on the monotonic undrained shear behavior of silty sand. The study is based on the undrained monotonic triaxial tests for the overconsolidation ratios (OCR = 1, 2, 4, and 8), with different silt contents ranging from 0% to 40%. The laboratory tests were carried out at an initial relative density of Dr = 50%. The paper is composed of two parts. The first one presents the tested soils; the second one gives an analysis of the test results and discusses the influence of the overconsolidation ratio on the shear strength of the soil. The test results indicate that the shear strength of the soil increases with the increase of the overconsolidation ratio resulting in an increase of soil dilatancy. The increase in the amount of fines from 0% to 40% increases the phase of the contractancy and consequently reducing the phase of dilatancy of the tested material

Insight into the Effects of Gradation on the Pore Pressure Generation of Sand–Silt Mixtures

Liquefaction of saturated sandy soils has been considered as the main cause of most geotechnical hazards during earthquakes. Generation of excess pore water pressures in saturated silty sands when subjected to monotonic, cyclic, and earthquake loading has been shown to cause the liquefaction, which can be defined as the transformation of stable soil structure into unstable liquid form. The proposed research investigated the effect of grading characteristics on the generated excess pore water pressure of sand–silt mixture samples in loose, medium dense, and dense states. The laboratory investigation aimed at understanding the extent or degree at which excess pore pressure of sand–silt mixture soil is affected by its gradation under static loading conditions. For the purpose of clarifying and evaluating the generated pore pressure characteristics of sandy soils, a series of undrained monotonic triaxial tests were carried out on different reconstituted samples of sand–silt mixtures with various gradations. The soil samples were tested under a constant confining pressure (σ3′ = 100 kPa) and at three relative densities (Dr = 20 %, 53 %, and 91 %). The results from this laboratory investigation were used to develop insight into the pore water pressure response of sand and sand–silt mixtures under monotonic loading conditions. The analysis of the obtained data revealed that the grading characteristics [D10, D50, Cu, effective size ratio (ESR), and mean grain size ratio (MGSR)] have significant influence on the generation of the excess pore water pressure. It was found that maximum positive excess pore water pressure (Δumax) can be correlated to the grading characteristics for the sand–silt mixture. The ESR and MGSR appear as pertinent parameters to predict the excess pore water pressure response of the sand–silt mixtures for soil gradation under study.

Laboratory Study on the Hydraulic Conductivity and Pore Pressure of Sand-Silt Mixtures

The hydraulic conductivity plays a major role on the excess pore pressure generation during monotonic and cyclic loading of granular soils with fines. This paper aims to determine how much the hydraulic conductivity and pore pressure response of the sand-silt mixtures are affected by the percentage of fines and void ratio of the soil. The results of flexible wall permeameter and undrained monotonic triaxial tests performed on samples reconstituted from Chlef River sand with 0, 10, 20, 30, 40, and 50% nonplastic silt at an effective confining stress of 100 kPa and two relative densities (Dr = 20, and 91%) are presented and discussed. It was found that the pore pressure increases linearly with the increase of the fines content and logarithmically with the increase of the intergranular void ratio. The results obtained from this study reveal that the saturated hydraulic conductivity (k) of the sand mixed with 50% low plastic fines can be, on average, four orders of magnitude smaller than that of the clean sand. The results show also that the hydraulic conductivity decreases hyperbolically with the increase of the fines content and the intergranular void ratio.

Shear Strength Response of a Geotextile-Reinforced Chlef Sand: A Laboratory Study

During the last earthquake that occurred in Chlef (El Asnam 1980, Algeria), a significant decrease in the shear strength has caused major damages to several civil and hydraulic structures (earth dams, embankments, bridges, slopes and buildings), especially for the saturated sandy soil of the areas near Chlef valley. This paper presents a laboratory study of drained compression triaxial tests conducted on sandy soil reinforced with horizontal layers of geotextile, in order to study the influence of geotextile layer characteristics both on shear stress–strain and on volumetric change–strain. Tests were carried out on medium and dense sand. The experimental programme includes some drained compression tests performed on reinforced sand samples, for different values of the geotextile layers number (Ng), of confining pressure (

Undrained Monotonic Response and Instability of Medium-Dense Sandy Soil

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Laboratory study on undrained shear behaviour of overconsolidated sand–silt mixtures: effect of the fines content and stress state

σc′) and relative density (Dr). The test results have shown that the contribution of the geotextile at low values of the axial strain (ε1) is negligible, for higher values of (ε1); geotextile induces a quasi-linear increase in the deviator stress (q) and leads to an increase in the volume contractiveness within the reinforced samples. A negligible influence of geotextile layers number (Ng) on the stress–strain behaviour and the volumetric change has been shown, when normalized with Ng. The results indicate that the contribution of geotextile to the stress–strain mobilization increases with increasing confining pressure, while its contribution to the volume contraction decreases with the increase in the confining pressure.

Behavior of Loose Silty Sand of Chlef River: Effect of Low Plastic Fine Contents and Other Parameters

This laboratory investigation has been conducted to elucidate how the fines fraction affects the undrained residual shear strength and liquefaction potential of sand-silt mixtures (Algeria). A series of monotonic and cyclic undrained triaxial tests were carried out on undrained, reconstituted, saturated samples of sand with varying fines content ranging from 0 to 50%. These were undertaken in order to evaluate the effect of the fines fraction on the undrained residual shear strength and liquefaction potential of loose, medium dense, and dense sand-silt mixtures (Dr = 17%, 53%, 62% and 91%), under an initial confining pressure of 100 kPa. The results of the monotonic tests indicate that the stress-strain response and shear strength behaviour is controlled by the percentage of the fines fraction and the samples become contractive for the studied relative density (Dr = 17% and 91%). The undrained residual shear strength decreases as the gross void ratio decreases, and the fines content increases up to 30%. Above this level of fines, it decreases with increasing gross void ratio. Moreover, the undrained residual strength decreases linearly as the fines content and the intergranular void ratio increase. Cyclic test results show that for the studied amplitude, the increase in fines content leads to an acceleration of liquefaction. The liquefaction resistance decreases with the increase in gross void ratio and the loading amplitude.