14-della.indd 163 �AB STRA CT The effects of confi ning pressure, relative density and sample preparation methods on the shearing strength of Chlef sand were studied. Results are presented of drained and undrained monotonic triaxial compression tests, performed on samples with initial densities of 0.29 and 0.80, under initial confi ning pressures ranging from 50 to 200 kPa. Speci- mens were prepared by two depositional methods; dry funnel pluviation and wet deposition. There was a marked dif ference in the undrained behaviour, even though the density and stress conditions were identical. The soil fabric was responsible for this result. The results also indicated that at low confi ning pressures, the specimens reconstituted by the wet deposition method exhibited complete static liquefaction, (zero effective confi ning pressure and zero stress dif ference). As confi ning pressures and densities were increased, the effective stress paths indicated increasing resist- ance to liquefaction by showing increasing dilatant tendencies. The same trends were observed in drained tests results in the form of an increase in the volumetric strain and the rapid transition from the contractancy phase to the dila- tancy phase. Keywords: liquefaction, sand, drained, undrained, dry funnel pluviation, wet deposition, confi nement, density, re- sidual strength, volumetric strain. Drained and undrained shear strength of silty sand: eff ect of reconstitution methods and other parameters � Noureddine Della, Ahmed Arab and Mostefa Belkhatir Laboratory of Material Sciences and Environment, Civil Engineering Department, University of Chlef, Sendjas Street POBox 151 Chlef 02000 – Algeria (nour_della@yahoo.fr) doi: 104154/gc.2011.14 Geologia Croatica 64/2 163–171 18 Figs. 1 Tab. Zagreb 2011 Geologia CroaticaGeologia Croatica European and African plates as shown by Fig. 1, is constantly a very unstable zone subjected to intense seismic activity. On October 10th, 1980 at 13h25 (local time), the region was hit by a disastrous earthquake of magnitude of 7.3 accord- ing to the calculations of PAPASTAMATIOU (1980), followed by strong aftershocks of magnitudes 6 and 6.1 some hours af- terwards, and numerous more aftershocks during se veral sub- sequent months (OUYED, 1981). The main shock generated an important inverse fault, about 40 km long, appearing on the surface (AMBRASEYS, 1981). The epicenter of this earth- quake was localized in the North East of El-Asnam. The disaster of October 10, 1980 resulted in heavy loss of life (about 3000 deaths), the destruction of a large number of buildings, important damage to the linking infrastructures and to public equipment, and generated a certain number of 1. INTRODUCTION During static or cyclic loading, the shaking of the ground may cause saturated cohesionless soils to lose their strength and behave like a liquid. This phenomenon is called soil li- quefaction and will cause settlement or tipping of buildings, failure of earth dams, earth structures and slopes. The mo- dern study of soil liquefaction has been triggered by nume- rous liquefaction-induced failures during the 1964 Niigata, Japan earthquake. Therefore, it is necessary to obtain a pro- per understanding of the effect of parameters such as soil pro perties and the nature of the loading on the severity of soil liquefaction. The region of Chlef situated near the Mediterranean Sea, to the North of Algeria, about 200 km to the west of the cap- ital Algiers, by its proximity to the contact of the continental Geologia Croatica 64/2Geologia Croatica 164 geodynamic phenomena at the surface of the ground: ground movements of variable nature and size, and especially the liquefaction of the sandy soils following a loss of shearing resistance. The phenomenon of liquefaction appeared on a vast alluvial valley crossed by the Chlef River and at the confl uence of this river with the Fodda River as shown in Fig. 2 (DURVILLE & MENEROUD, 1982). 2. PRIOR STUDIES Numerous studies have reported that the behaviour of sands can be greatly infl uenced by the initial state of the soil. PO- LITO & MARTIN (2003) asserted that the relative density and skeleton void ratio were factors that seemed to explain the variation in different experimental results. YAMAMU RO & LADE (1997), YAMAMURO & LADE (1998) and YA- MA MURO & COVERT (2001) concluded that complete sta- tic liquefaction, (zero effective confi ning pressure and zero effective stress difference) in laboratory testing, is most eas- ily achieved in silty sands at very low pressures. KRAMER & SEED (1988) also observed that liquefaction resistance increased with increasing confi ning pressure. Several specimen reconstitution techniques, tamping and pluviation being the most common, are in use in current prac- tice. The objective in all of these is to replicate a uniform sand Figure 1: Movement of the Europe-African plate (from the CTC institution – Controle Technique de Construction). Figure 2: Valley of the Chlef River and localization of sand boils due to liquefaction (from the CTC institution – Controle Technique de Construction). Della et al.: Drained and undrained shear strength of silty sand: eff ect of reconstitution methods and other parameters Geologia Croatica 165 specimen at the desired void ratio and effective stresses to simulate the sand mass in-situ. However the effect of the prep- aration method of the samples has been subject to controver- sial research. Many studies have reported that the resistance to liquefaction is higher for samples prepared by the method of sedimentation than for samples prepared by dry funnel plu- viation and wet deposition (ZLATOVIC & ISHIHARA, 1997); other studies have found that the specimens prepared by the dry funnel pluviation method tend to be less resistant than those reconstituted by the wet deposition meth od (MU- LILIS et al., 1977; YAMAMURO & WOOD, 2004). Other researchers indicated that the tests prepared by dry funnel plu- viation are more stable and dilatant than those prepared by wet deposition (BENAHMED et al., 2004; CA NOU, 1989; ISHI- HARA, 1993). VAID et al. (1999) confi rm this result while showing that wet deposition encourages the initiation of li- quefaction in relation to a setting up by pluviation under water. YAMAMURO et al. (2008) concluded after their laboratory investigation, that the method of dry pluviation supports the instability of the samples contrary to the method of sedimen- tation. WOOD et al. (2008) found that the effect of the method of deposition on the undrained behaviour decreases, when the density increases. They also found that this infl uence decreases with the increase in fi nes content, particularly at lower densi- ties. The focus of this study is to identify the differences in drained and undrained triaxial compression behaviour that can result from using different reconstitution techniques to create silty sand specimens. 3. EXPERIMENTAL METHODS An experimental study of the behaviour of loose and dense sand under static loading conditions is presented below. Both drained and undrained tests were performed. 3.1. Sand tested Silty sand samples were collected from the liquefi ed layer of the study areas at a depth of 6.0 m (Fig. 3) close to the Chlef earthquake epicentre (October 10th, 1980). Figs. 4A,B show craters of liquefi ed ground on the banks of the Chlef Figure 3: Geotechnical profi le of the soil deposit at the site. Figure 4: Craters of liquefi ed soil on banks of the River Chlef (a and b). River. Fig. 5 illustrates a typical subsidence location of the liquefi ed soil and sample collection. All tests in the present study were performed on sand from Chlef (Algeria). The sand contains 0.5% silt of the River Chlef. The grain size distri- bution curve of this sand is given in Fig. 6. It is medium sand with rounded grains of medium diameter D50 = 0.45 mm and the predominant minerals are feldspar and quartz. The silt component is non plastic with a plasticity index of 5.81 %. The index properties of the sand used in this laboratory re- search work are presented in Table 1. The specimens were reconstituted at two densities (ID = 0.29 and 0.80) represent- ing the loose and dense states. Figure 5: Subsidence of the Chlef River Banks due to liquefaction. a) b) Geologia Croatica 64/2Geologia Croatica 166 3.2. Testing equipment An advanced automated triaxial testing apparatus, type Bish op and Wesley (BISHOP & WESLEY, 1975) was used to con- duct the monotonic drained and undrained tests (Fig. 7). 3.3. Specimen preparation In this study, two methods of sample preparation, which in- cluded dry funnel pluviation (DFP) and wet deposition (WD), were utilized, and as briefl y described below. In the fi rst method, dry soil is deposited in the mould with the help of a funnel by controlling the height; this meth od con- sists of fi lling the mould by tipping in a rain of dry sand. To have loose samples, it is necessary that the fall height is almost nil. The second method consists of mixing the previously dried sand with a small quantity of water (3 %) and then de- positing the humid soil in the mould in a manner that is as homogenous as possible. The soil was placed in successive layers. A constant number of strokes were applied to get a homogeneous and isotropic structure. Triaxial tests were performed on cylindrical specimens measuring 70 mm in diameter and 140 mm in height (H/D = 2.0). The mass of sand to put in place is determined accord- ing to the desired density (the initial volume of the sample is known). The density state of the sample was defi ned by the density index: ID = (emax – e) / (emax – emin) [1] Where emin and emax indicate the minimum and maximum void ratios, respectively; e is the target void ratio and ID the density index. After the specimen has been formed, the specimen cap is placed and sealed with O-rings, and a partial vacuum of 15 to 25 kPa is applied to the specimen to reduce the disturbances. 3.4. Saturation and consolidation Saturation of the specimens was accomplished by fl ushing the specimen with carbon dioxide for approximately 20 min (LADE & DUNCAN, 1973), after which de-aerated water was slowly added from the bottom of the specimen. Appli- cation of a back pressure improves the degree of saturation which was estimated by calculating Skempton’s B-parame- ter as the ratio of measured pore water pressure increase, in- duced by an increase in cell pressure in undrained conditions, and the corresponding increase in cell pressure. The B value was measured to test specimen saturation, and a minimum value greater than 0.96 was obtained for all tests. The triax- ial test samples were isotropically consolidated under con- fi ning pressures ranging from 50 to 200 kPa prior to static loading. 3.5. Shear loading All drained and undrained triaxial tests for this study were carried out at a constant strain of 0.167 % per minute, which was slow enough to allow pore pressure change to equalize throughout the sample with the pore pressure measured at the base of sample. All the tests were continued up to 20 % axial strain. 4. RESULTS OF UNDRAINED TRIAXIAL COMPRESSION TESTS 4.1. Eff ect of confi ning pressure and density For the purpose of studying the effect of variation of effec- tive confi ning pressure on liquefaction resistance, a series of Figure 6: Grain-size distribution curve of tested material. Figure 7: The triaxial system set up. Table 1: Index properties of the used sand. Material emin emax gdmin gdmax gs Cu D50 D10 Grains shape O/Chlef 0.54 0.99 13.4 17.3 26.7 3.2 0.45 0.15 Rounded Della et al.: Drained and undrained shear strength of silty sand: eff ect of reconstitution methods and other parameters Geologia Croatica 167 tests were conducted. Figs. 8 and 9 show the results of the undrained triaxial compression tests performed. All tests were performed on specimens composed of Chlef sand, and each specimen was monotonically loaded in compression under undrained conditions. Figs. 8a and 9a present the undrained stress-strain curves, while Figs. 8b and 9b show the effec- tive stress paths on the Cambridge p’-q diagram in which p’=(σ’1+2σ’3)/3 and q= σ’1- σ’3. It is noticed that as the con- fi ning pressures increased, the liquefaction resistance, (de- viatoric stress), increased for both dry funnel pluviation and wet deposition methods. As can be seen for the samples re- constituted by the wet deposition method, complete static liquefaction occurred in two tests at the lowest confi ning pres- sure (50 kPa) irrespective of sand densities. Static liquefac- tion was coincidental with the formation of large wrinkles in the membranes surrounding the specimens. At a confi ning pressure of 100 kPa the specimens undergo temporary liq- uefaction characterized by the condition where the undrain ed stress difference fi rst achieves an initial peak, after which it declines to a minimum value. Finally, at a confi ning pressure of 200 kPa the resistance to liquefaction increases for both loose and dense samples. In Figures 8 and 9 for the dry funnel pluviation method, it is clear that when the initial confi ning pressure is increased from 50 kPa to 200 kPa, specimens with a density index of either 0.29 (loose) or 0.80 (dense) exhibit behaviour that is characterised by increasing stability or increasing resistance to liquefaction. The effect of increasing confi ning pressure is to increase the dilatant tendencies in the soil. Temporary liquefaction is described as the condition where the undrained stress difference fi rst achieves an initial peak, after which it declines to a minimum value. This is caused by rapidly rising pore pressure which decreases the effective stress. Increasing dilatancy or resistance to liquefaction can also be observed by examining the ratio of the minimum stress difference to the initial peak stress difference, (q(min)/q(peak)) shown in Fig. 10 for the wet deposition method. A q(min)/ q(peak) ratio of zero indicates complete liquefaction, and a q(min)/q(peak) ratio of unity represents completely stable behaviour. The inset diagrams in Figs. 10a and 10b show that this ratio is zero at initial confi ning pressure of 50 kPa, indi- cating complete static liquefaction. The ratio then increases at initial confi ning pressures from 100 to 200 kPa, indicat- ing that the specimen exhibits more dilatancy and, thereby, is more resistant to liquefaction. Figure 11 illustrates the variation of the maximum und- rained shear strength (qmax) with the initial density (ID) at various confi ning pressures. It is clear from this fi gure that an increase in the relative density results in an increase in the maximum strength at a given confi ning pressure for both dry funnel pluviation and wet deposition, with a more pro- nounced increase for the method of dry funnel pluviation (Fig. 11a), contrary to the case of wet deposited samples where the evolution of the resistance is less pronounced (Fig. 11b). THEVANAYAGAM et al. (1997) and SITHARAM et al. (2004) report similar behaviour of increasing undrained shear strength with increasing relative density. Figure 8: Undrained tests on loose sand: (a) deviator stress-strain curve, (b) stress path; DFP= Dry Funnel Pluviation, WD= Wet Deposition Figure 9: Undrained tests on dense sand: (a) deviator stress-strain curve, (b) stress path; DFP= Dry Funnel Pluviation, WD= Wet Deposition a) b) a) b) Geologia Croatica 64/2Geologia Croatica 168 Figure 10: Resistance to liquefaction for the wet deposition method: (a) Loose state, (b) Dense state Figure 11: Eff ect of the relative density on the undrained response of sand. 4.2. Eff ect of the method of deposition Figure 12 shows the variation of the undrained shear strength at the peak (qpeak) with the effective confi ning pressures using two methods of deposition. It can be seen from this fi gure that the dry funnel pluviation method shows higher values of the deviator at peak strain, therefore a much higher resistance to liquefaction, contrary to the wet deposition method where we note some lower values of the deviator at peak for low densities (loose state for ID = 0.29), with pro- gressive stabilization around a very small or nil ultimate sta- tionary value representing liquefaction of the sample. When loose and medium dense sandy soils are subjected to undrained loading beyond the point of peak strength, the undrained shear strength declines to a near constant value over large deformation. Conventionally, this shear strength is called the undrained steady-state shear strength or residual shear strength. However, if the shear strength increases after passing through a minimum value, the phenomenon is called limited or quasi-liquefaction. Even limited liquefaction may result in a signifi cant strain and associated drop in resistance. ISHIHARA (1993) defi ned the residual shear strength Sus as: Sus = (qs/2)cosφs = (M/2) cosφs(ps’) [2] M = (6 sinφs)/(3- sinφs) [3] Where qs, ps’ and φs indicate the deviator stress (σ1’- σ3’), the effective mean principal stress (σ1’+ 2σ3’)/3, and the mo- Figure 12: Eff ect of the deposition methods on the undrained shear strength at the peak. a) b) a) b) a) b) Della et al.: Drained and undrained shear strength of silty sand: eff ect of reconstitution methods and other parameters Geologia Croatica 169 bilized angle of inter-particle friction at the quasi-steady state (QSS) respectively. For the undrained tests conducted at a constant confi ning pressure and various initial relative densi- ties and fi nes content, the deviatoric stress (qs) was estimated at a quasi-steady state point along with the mobilized internal friction angle (Fig. 13). Furthermore, the residual shear strength was calculated according to the relationship [2]. Figure 14 shows the evaluated undrained residual shear strength (Sus) and its variation with confi ning pressures and the reconstitution methods. It is clear from this fi gure that the sample preparation method considerably affects the evo- lution of the residual strength. Indeed this residual strength is nil for the samples prepared by wet deposition to a con- fi nement of 50 kPa because of the collapse of samples, but for confi nements of 100 and 200 kPa, the samples prepared by dry funnel pluviation mobilize a more signifi cant residual strength than those prepared by wet deposition. 5. RESULTS OF DRAINED TRIAXIAL COMPRESSION TESTS Figures 15 and 16 show the results of the drained tests on samples prepared by the method of dry funnel pluviation with two densities (ID=0.29 and 0.80). Fig. 15 shows that the resis- tance to liquefaction represented by deviatoric stress increases with an increase in the confi ning pressure and density. Fig. 16 shows the evolution of the volumetric strain ver sus the axial strain. We note that the increase in the density accelerates the transition from the contractancy phase to the dilatancy phase. The same tendencies can be observed in Figs. 17 and 18 which show the results of the drained tests on specimens re- constituted by the wet deposition method. As can be seen from Fig. 17 the resistance to liquefaction represented by the deviatoric stress, increases with an increase in the confi ning pressure and density. Fig. 18 shows the evolution of the vol- umetric strain versus axial strain. It can be noticed that the method of wet deposition increases the phase of contractancy. This increase in the phase of contractancy is highly marked for the loose specimens (Fig. 18a). By comparing the results of Figs. 15−18, we concluded that the specimens reconstituted by the dry funnel pluviation method were more dilatant than those prepared by the wet deposition method. The results of the drained and undrained tests are in per- fect agreement with those given by BENAHMED et al. (2004) and ISHIHARA (1993) who discovered that samples pre- pared by dry funnel pluviation have a resistance to liquefac- tion higher than those prepared by wet deposition. ZLATO- VIC & ISHIHARA (1997) discovered that the resistance of the samples prepared by the method of dry funnel pluviation decreases with the increase in the fraction of fi nes, while the samples prepared by sedimentation showed a reduction in resistance until a fi nes content of Fc=30%, then increase. MULILIS et al. (1977) concluded from their study, that the samples prepared by wet tamping present a resistance higher than those prepared by dry funnel pluviation. These differences of behaviour noted between the two methods of deposition, can be explained by the fact that the Figure 13: Determination of the phase transition point. Figure 14: Eff ect of the deposition method on the undrained residual shear strength. a) b) Geologia Croatica 64/2Geologia Croatica 170 Figure 15: Dry funnel pluviation − evolution of the deviatoric stress versus axial strain: (a) Loose state (ID =0.29), (b) Dense state (ID =0.80). Figure 17: Wet deposition − evolution of the deviatoric stress versus axial strain: (a) Loose state (ID =0.29); (b) Dense state (ID =0.80). Figure 16: Dry funnel pluviation – evolution of the volumetric strain versus axial strain: (a) Loose state (ID =0.29), (b) Dense state (ID =0.80). Figure 18: Wet deposition – evolution of the volumetric strain versus axial strain: (a) Loose state (Dr =0.29), (b) Dense state (Dr=0.80). a) b) a) b) a) b) a) b) Della et al.: Drained and undrained shear strength of silty sand: eff ect of reconstitution methods and other parameters Geologia Croatica 171 molecules of water contained in the structures prepared by wet deposition method prevent grain-grain adhesion. This trend accelerates the instability of the samples which show a very weak resistance and even provokes the phenomenon of liquefaction of the sand for low densities and low confi ne- ments leading to the collapse of the sample. This is contrary to the structures of samples prepared by the method of dry funnel pluviation that show a more dilatory behaviour. 6. CONCLUSION A series of drained and undrained triaxial compression tests in monotonic loading conditions were performed on silty sand samples retrieved from liquefi ed sites on the Chlef River banks (Algeria). The effects of sample preparation methods and other parameters were studied. The study included drain ed and un- drained triaxial tests that have been prepared at den sities of 0.29 and 0.80 for confi nements of 50,100 and 200 kPa. Bas ed on the experimental results presented, the following conclu- sions can be drawn: 1. Complete static liquefaction occurred at low confi n- ing pressure (50 kPa) for the wet deposition method. 2. As the confi ning pressure increased, the liquefaction resistance of the sand increased for both dry funnel pluvia- tion and wet deposition. This observation correlates with most historic cases of apparent static and earthquake-induced liq- uefaction. 3. An increase in the density resulted in an increase in the maximum undrained shear strength of the sand in und- rained tests, and accelerates the transition from the contrac- tancy phase to the dilatancy phase in drained tests. 4. The peak and residual shear strengths of sand are sen- sitive to the sample preparation methods. The dry funnel plu- viation method gives higher values of the peak and residual shear strengths than the wet deposition method. 5. The results also reveal that the method of reconsti- tution has a detectable effect on the drained behaviour of the sand in terms of volumetric strains. 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