Indian Geotechnical Conference – 2010, GEOtrendz December 16–18, 2010 IGS Mumbai Chapter & IIT Bombay Effect of Sample Preparation on Strength of Sands Juneja, A. Raghunandan, M.E. Assistant Professor e-mail: [email protected] Research Scholar e-mail: [email protected] Department of Civil Engineering, IIT Bombay, Mumbai ABSTRACT Selection of the most suitable method of sand sample preparation becomes difficult because all available methods affect the fabric and dry density of samples, and none of the available methods are shown to be unique. The objective of this paper is to address some of these issues. A series of consolidated drained (CD) and consolidated undrained (CU) triaxial compression tests were conducted on sand samples prepared using pluviation and tamping techniques, under both dry and moist conditions. The standard triaxial test setup at IITB is described first. Stressstrain behaviour for samples prepared with different sample preparation methods showed considerable difference in the peak stress and dilation, whilst all sample reached the peak stress at 5 to 10% axial strains. Results showed that samples prepared using tamping technique usually strain softens, whilst samples prepared by pluviation technique may harden or soften with strain depending up on the sample relative density and confining pressures applied during testing. Hence, pluviation technique proves to be the more reliable technique to prepare samples for triaxial testing. 1. INTRODUCTION Number of techniques to obtain high quality cohesive samples for laboratory testing has been developed, whilst procedure used to obtain undisturbed cohesionless samples are still very few. The cost to obtain high quality undisturbed cohesionless samples by ground freezing is prohibitive (Yoshimi et al., 1994), hence many researchers rely on preparing remoulded and reconstituted representative samples of sandy soils by dry or wet pluviation, slurry deposition, vibrations, or moist-tamping in layers by undercompacting each layer to its succeeding layer (Ladd, 1978, Amini and Qi, 2000). The structural arrangement of soil grains remains the most important criteria influencing the stress-strain behaviour of reconstituted cohesionless soil samples in the laboratory tests. Samples prepared using moist tamping (MT) technique, usually exhibits strain softening behaviour because of their inherent high void ratios (Vaid et al., 1999). Some studies on samples prepared using MT seems to suggest that the sample are not homogeneous and are less suitable for triaxial testing (Vaid et al., 1999, Frost and Park, 2003, DeGregorio, 1990, Vaid and Sivathayalan, 2000). Ladd (1974) proposed the method of undercompaction in which includes each layer is undercompacted to its successive layer. Air pluviation technique is shown to produce reconstituted sand specimens with least soil degradation (Cresswell et al., 1999). In this method, a wide range of initial void ratio can be achieved by controlling the drop height and pouring rate (Vaid and Negussey, 1988). Air pluviated samples strain softens to a lesser extent when compared to moist tamped specimens (DeGregorio, 1990, Vaid and Sivathayalan, 2000). Amini and Chakravrty (2004) prepared homogeneous sand samples using dry pluviation technique. However, when the soil contained silt in excess of 20 %, air pluviation resulted in soil segregation because, the fines lagged behind on account of their lower velocities within the fixed height. Cresswell et al. (1999) observed that compaction during pluviation reached peak efficiency when a continuous energetic layer was formed. Within the energetic layer, the grain displacement and grain hammering operated to their greatest effectiveness. Sand samples prepared using wet pluviation are initially saturated (Chaney and Mulilis, 1978, Vaid and Negussey, 1988). In wet pluviation, preferred fabric can develop which behaves similar to that of natural alluvial deposits (Ghionna and Porcino, 2006, Oda et al., 1978). Vaid et al. (1999) showed that these samples are uniform with depth and have small deviation in the relative density. Terminal velocity was reached at a very small drop 328 A. Juneja and M.E. Raghunandan height in water, irrespective of total drop height. Pluviation technique fails when used for sands containing fines because of particle segregation (Carraro and Prezzi, 2007). 2. EXPERIMENTAL SETUP All tests were conducted in ADsoil laboratory at Indian Institute of Technology Bombay using Gujarat sand. Figure 1 shows the particle size distribution curve of the sample, and the results showed D50 = 0.3 mm, CU = 2.12, CC = 1.47, and specific gravity, GS = 2.63. The maximum and minimum void ratios of the sand sample were 0.795 and 0.492 respectively. Samples were prepared in 100mm diameter and 200mm long split mould which had the facility to attach 80mm long collar at its top. The mould was clamped to firm base during sample preparation. The split mould was provided with a vacuum port to stretch the membrane and prevent necking during sample preparation (after Wijewickreme & Sanin, 2006). 80 Percentage finer (%) Table 1: Details of Funnels and Mesh Used Diffuser D1 D2 D3 D4 D5 Pore Size (mm) 2 2 8 9.4 10 Deposition Intensity x10-3 (g/s/mm2) 2.1 5.1 88.7 121.6 147.8 Note: Diffuser fabricated as per ASTM E 323-80. Center-to-center distance between the pores was varied to control the deposition intensity. 100 60 40 20 0 0.01 diffuser during sample preparation. In this second method, the diffuser was slowly raised concurrently as the sample was formed. The two methods are referred to as fixed diffuser (DF), and rising diffuser (DR) in the text. Detailed procedure and the comparison of both DF and DR are mentioned in Raghunandan & Juneja (2010). In general, sand was rained from 280- to 600 mm height above the base of mould. This height is the usual clearance available between the cell and the cross head in triaxial shear frame. 0.1 1 Sieve size (mm) 10 Fig. 1: Particle Size Distribution for the Sand Sample The procedure to prepare sand samples using different sample preparation technique used in this study are as mentioned in Raghunandan & Juneja (2010). However, to brief with tamped samples were prepared using tamping rod attached to 50 mm diameter circular footing made of aluminium with total weight of less than 250 g. Air dried sand was used in dry tamping. Moist tamped samples were prepared by tamping the sand under submerged conditions. Dry and wet pluviated samples were prepared using the same mould as that used in the tamping technique. In dry pluviation, air dried sand was rained through a diffuser into the mould. When otherwise was filled with water up to the brim in wet pluviation. Five diffusers D1 to D5 were used in the tests. The pore size and deposition intensity of the diffusers are tabulated in Table 1. In the table, deposition intensity is equal to the mass flow of the sand per unit area of the diffuser. Half of the samples were prepared by keeping the diffuser at fixed height above the base of mould. The remaining half of the samples was prepared by raising the Samples prepared using tamping and pluviation techniques under dry and moist conditions were tested in standard triaxial compression. Table 2 shows the experiment program used in this study. The samples were directly prepared on the triaxial base using split mould. Initial height and diameter of the samples were measured at four locations using dial gauge and Pi-tape respectively. Water was percolated within the sample under a head of about 10kN/m2 while a small confining pressure of about 15kN/m 2 maintained to hold the sample. The samples saturated under cell pressure and back pressure increments Table 2: Sample Preparation and Test Conditions Used in CD and CU Tests Sample Preparation Description Initial Void Ratio Dry Tamping Normal compaction; 3 and 5 layers; 25 blows/layer 0.603 – 0.630 Moist Tamping Normal compaction; 3 and 5 layers; 25 blows/layer 0.615 – 0.605 Dry Pluviation Fixed diffuser, and rising diffuser techniques 0.634 – 0.695 Wet Pluviation Fixed diffuser, and rising diffuser techniques 0.681 – 0.687 of not more than 35kN/m2 until B-factor (Skempton, 1954) of about 0.97 to 0.99 was achieved. The samples were then consolidated isotropically under effective confining pressures and sheared to failure with drainage valve open or closed based on the type of test. The drained shear strength measured during triaxial shear was corrected for 329 Effect of Sample Preparation on Strength of Sands the membrane stiffness (ASTM D4767-04). In this study, 0.3mm thick rubber membrane of Young’s modulus equal to 1780 kN/m2 was used. 1200 (a) 800 -12 600 DP - DF; e = 0.695 400 Dry pluviation (DP); e = 0.634 Wet pluviation (WP); e = 0.687 Dry tamping (DT); e = 0.630 Moist tamping (MT); e = 0.615 200 0 0 10 20 30 AxialAxial Strainstrain (%) (%) DT; e = 0.603 MT; e = 0.605 DP - DR; e = 0.634 -10 Volumetric strains (%) Deviator stress (kN/m2) 1000 and further followed by shear dilation. The stress-strain plots of all the samples tends to follow same path at axial strains (εa) greater than 30%, showing very less volume change with stress ratio value of about 1.24. Stress ratio is the term for deviator stress normalised with effective confining pressure. Hence at this stage the samples are considered to have reached their critical or steady state. Figure 3 shows the variation of volumetric strains (εv) with εa . Similar trend was observed with a small initial compression followed by shear dilation. WP - DF; e = 0.681 -8 WP - DR; e = 0.683 -6 -4 -2 500 (b) Deviator stress (kN/m2) 400 0 2 0 10 20 30 strain AxialAxial Strains (%)(%) 300 Fig. 3: Variations of Volumetric Strains with Axial Strains DP; Fixed diffuser; e = 0.695 DP; Rising diffuser; e = 0.634 WP; Fixed diffuser; e = 0.681 WP; Rising diffuser; e = 0.683 DT; h = 20mm; e = 0.603 MT; h = 20mm; e = 0.605 200 100 0 0 10 20 30 Axial strain (%) Axial Strain (%) Fig. 2a-b: Stress-Strain Behavior of Sand Samples Tested Under (a) Undrained and (b) Drained Conditions 3. RESULTS AND DISCUSSION In this paper, the behaviour of sand samples prepared using different sample preparation techniques, at void ratios 0.603 to 0.687 to consolidated undrained (CU) and drained (CD) shear is studied. Figure 2a-b show the stress-strain response of the sand samples to drained and undrained shear respectively. In general, all specimens showed initial peak Figure 4 shows the variation in ratio of deviator stress at end state with consolidated relative density. Ratio of deviator stress at end state shall be defined as the ratio of deviator stress at critical to peak state, i.e. qcric/qmax. The qcric/qmax ratio explains the total energy loss or drop in deviator stress as a ratio during continued shearing, hence explains the behaviour of the sample as either strain softening of strain hardening. For the ratio qcric/qmax less than 1 implies drop in the deviator stress between peak and critical states, thus the sample shows strain softening or over-consolidated (OC) behaviour. Similarly, if ratio qcric/ qmax = 1 implies that the sample shows strain hardening or normally-consolidated (NC) behaviour, whilst ratio qcric/ qmax can never be greater than 1. The observations from Figure 4 show that ratio qcric/qmax varies between 0.6 to 1 for all the samples. The samples prepared using dry and wet tamping techniques have ratio qcric/qmax < 0.85, whilst for samples prepared using dry and wet pluviation technique qcric/qmax ratio varied between 0.8 to 1 depending effective confining pressures applied during testing. Based on the above discussions it can be concluded that, samples 330 A. Juneja and M.E. Raghunandan prepared using tamping technique usually strain softens, whilst samples prepared by pluviation technique may harden or soften with strain depending up on the sample relative density and confining pressures applied during testing. qcric/qmax 1 0.8 CU - DP CU - WP CU - DT CU - MT CD; DP-DF CD; DP-DR CD; WP-DF 0.6 CD; WP-DR CD; DT CD; MT 0.4 0.5 0.6 Initial void ratio 0.7 Fig. 4: Variations in Ratio of Deviator Stress at End State with Initial Void Ratio 4. CONCLUSION Discussions in this paper presents data relating to the CU and CD triaxial compression tests on samples prepared using pluviation and tamping techniques, under both dry and moist conditions. All samples showed initial compression followed by shear dilation when sheared at σ×3 = 150kN/m2, with peak stress at εa ranging between 5 to 10%. 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