Mostefa Belkhatir

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.

The undrained shear strength characteristics of silty sand: an experimental study of the effect of fines

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.

Experimental Investigation on Undrained Shear Behavior of Overconsolidated Sand-Silt Mixtures: Effect of Sample Reconstitution

Liquefaction stability analysis using the undrained shear strength characteristics of sandy soils that are prone to liquefaction is a major challenge in geotechnical earthquake engineering. The objective of this laboratory research work was to study the combined effect of the sample reconstitution (dry funnel pluviation and wet deposition) and overconsolidation ratio (OCR = 1, 2, 4, and 8) on the undrained shear strength of medium dense (D_r = 52 %) sand-silt mixtures under undrained monotonic loading conditions. For this purpose, a series of triaxial tests were carried out on reconstituted saturated silty sand samples with fines content ranging from 0 to 40 %. The confining pressure was kept constant to 100 kPa in all tests. The obtained data showed that the dry funnel pluviated samples were more resistant than the wet deposited samples and complete static liquefaction of samples reconstituted with wet deposition method was observed for the lower overconsolidation ratios (OCR = 1, 2, and 4). The undrained shear strength decreases with the increase of fines content for dry funnel pluviation (DFP) and the inverse tendency was observed in the case of wet deposition (WD) for the range of the overconsolidation ratio under consideration (1 ≤ OCR ≤ 8).

Undrained Monotonic Response and Instability of Medium-Dense Sandy Soil

A series of undrained monotonic triaxial compression tests were performed on natural, medium-dense (relative density (RD) = 50%) Chlef sand containing 0.5% of non-plastic fines, under different confining pressures of 50 kPa, 100 kPa, and 200 kPa. This article focuses on distinctive states of the monotonic undrained response of sands, namely the critical state, the phase transformation state, the quasi-steady state, and the state of undrained instability (onset of flow liquefaction). Specimens were prepared using dry funnel pluviation and wet deposition to investigate the effect of the initial sand fabric on these states. The present data suggest that the initial fabric of the sand appears to have a significant effect on the undrained behavior in the early stages of shearing, with its influence vanishing at large strains. Wet deposition specimens demonstrate considerably larger undrained instability state strength than their dry funnel pluviation counterparts, and a unique critical state locus is reached by both dry funnel pluviation and wet deposition.

Laboratory assessment of saturation and sample molding effects on shear resistance and mechanical characteristics of sandy soil

In order to evaluate shear resistance characteristics of sand, which has previously experienced liquefaction, two series of drained and undrained monotonic triaxial compression tests on medium dense sand were carried out. In the first test series, the influence of the specimen preparation method and confining pressure has been studied. It was found that there was a marked difference in the behavior even though the density and stress conditions were identical. The conclusion was that the soil fabric was responsible for this result. In the second series of tests, the saturation influence on the shear resistance of the sand was examined. The results showed that the decrease in Skemptons pore pressure coefficient B improves the shear resistance and increases the friction angle of the sand.

Laboratory Investigation on the Effects of Overconsolidation and Saturation on Undrained Monotonic Shear Behavior of Granular Material

In order to study pore water response and static liquefaction characteristics of silty sand, which has previously experienced liquefaction, two series of monotonic triaxial tests were run on medium dense sand specimens (RD = 50%) at confining pressure of 100 kPa. In the first test series, the influence of the soil saturation under undrained static loading has been studied. It summarizes results of monotonic tests performed on Chlef sand at various values of the Skemptons pore pressure coefficient. Analysis of experimental results gives valuable insights on the effect of soil saturation on sand response to undrained monotonic paths. In the second series of tests, the overconsolidation influence on the resistance to the sands liquefaction has been realized on samples at various values of overconsolidation ratios (OCR). It was found that the increase of overconsolidation ratio (OCR) increases the resistance of sands to liquefaction.

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

The post-liquefaction stability analysis of silty sand soil deposits that are prone to liquefaction becomes a major challenge in Geotechnical Earthquake Engineering. The objective of this laboratory investigation is to study the combined effect of the confining pressure and the low plastic fines (Ip = 5%) on the liquefaction resistance (undrained shear strength) of medium dense (Dr = 52%) overconsolidated sand–silt mixtures under monotonic loading conditions. In this context, a series of undrained monotonic triaxial tests were carried out on reconstituted saturated silty sand samples with fines content ranging from 0 to 40%. The different samples were prepared with dry funnel pluviation technique and subjected to different overconsolidation ratios (OCR = 1, 2 and 4). The results of this laboratory investigation indicate that the influence of low plastic fines on the undrained shear strength (liquefaction resistance) is clearly observed for the lower overconsolidation ratios (OCR = 1 and 2). This effect becomes insignificant for higher overconsolidation ratio (OCR = 4) for both confining pressures under consideration. The obtained data confirm the existence of a linear relationship between the undrained peak shear strength of overconsolidated Chlef sand samples (qpeak_OC) and undrained shear strength of normally consolidated Chlef sand (qpeak_NC) for the two selected confining pressures ( = 100 and 200 kPa) and initial relative density (Dr = 52%) under consideration.

Experimental characterization of the undrained instability and steady state of silty sand soils under monotonic loading conditions

Abstract This laboratory study aims to analyse the effects of gradation on the undrained instability using the undrained instability stress and steady-state ratios of silty sand soils. In this context, series of undrained compression triaxial tests were carried out on Fontainebleau (France) sand mixed with low plastic fines (I p = 5%) range F c = 0–30%. The samples were prepared at an initial relative density (D r = 52%) and subjected to three different confining pressures ( = 100, 200 and 300 kPa). The test results indicate that the gradation has a significant influence on instability stress and steady-state ratios of sand–silt mixtures. Moreover, the obtained data confirm the existence of simple correlations between undrained instability stress, steady-state ratios, fines content, void ratios and grading characteristics (D 10 , D 30 , D 50 , D 60 and C u ) of the materials under consideration. The introduced grading characteristics ratios (D 10R , D 50R and C UR ) and the equivalent state parameter appear as pertinent parameters for the prediction of the undrained instability and steady-state of the sand–silt mixtures under study.