Proceedings of Indian Geotechnical Conference December 22-24,2013, Roorkee
PORTABLE TRAVELING PLUVIATOR TO RECONSTITUTE SPECIMENS OF COHESIONLESS SOILS V. K.* Gade, Research Scholar, IIT Bombay,
[email protected] T. N. Dave, Assistant Professor, PDPU Gandhinagar,
[email protected] V. B. Chauhan, Research Scholar, IIT Bombay,
[email protected] S. M. Dasaka, Assistant Professor, IIT Bombay,
[email protected] ABSTRACT: Preparation of uniform and repeatable sand beds of required density is a prerequisite for obtaining reliable results from laboratory tests on reconstituted sand specimen. A portable traveling pluviator (PTP), working on the principle of air pluviation, is used in the present study to achieve the above objectives. PTP is a simple device which is widely adopted for preparation of large size specimens of cohesionless soils. The PTP essentially consisted of a hopper, orifice plate for varying deposition intensity, combination of flexible and rigid tubes for smooth travel of material, and a set of diffuser sieves to obtain uniformity of pluviated sand bed. Effect of height of fall, deposition intensity and number of diffuser sieves on the uniformity, and density of sand specimen are studied. From the preliminary studies it is noticed that sand beds with a wide range of relative densities, in the range of 41.2%-100%, can be achieved using PTP. It is also observed that denser sand beds can be achieved by controlling deposition intensity, whereas, lower density samples could be obtained by controlling height of fall.
INTRODUCTION In the past sand specimens were prepared for laboratory model testing by using Tamping, vibration and pluviation techniques [2, 13]. Among these methods pluviation method is widely adopted by various researchers because of its unique advantage, wide range of density of sand bed can be achieved compared to other techniques and there is no possibility of particle breakage during preparation of sand specimen. It is easy to prepare the sand samples in stages, which facilitate placement of instrumentation, such as load cells, pressure cells or accelerometers, etc., at various locations within the specimen during the sample preparation process. The method employed to prepare the reconstituted sand specimen has to fulfill the following criteria, as suggested by Kuerbis and Vaid [8]: 1) the method must be able to produce loose to dense sand beds in the unit weight range expected within an in-situ soil deposit; 2) the sand bed must have a uniform void ratio throughout; 3) the samples should be well mixed without particle segregation, regardless of particle gradation or fines content; 4) sample preparation method should simulate the mode of soil deposition commonly found in the soil deposit being modeled.
LITERATURE REVIEW For the last 4 decades air pluviation techniques have been used to prepare large and small sand specimens to conduct model foundation testing [21], calibration chamber testing [6, 9], centrifuge model tests [15, 19, 21], model tests using shaking table [8], and triaxial tests [12, 17]. A wide range of densities were achieved by controlling the sand flow from hopper by using roller and deflector [21], shutter and diffuser [18], nozzle and diffuser [22] orifice, rigid tube and diffuser [7]. EXPERIMENTAL SETUP Present study uses portable travelling pluviator (PTP) device, as shown in Figs.1, 2 and 3 developed by Dave and Dasaka [7] for sand bed preparation. PTP device was designed on the basis of simultaneous control of number of sieves, height of fall (distance between the lowermost diffuser sieve to the top of the sand bed) and deposition intensity (mass of soil falling in the chamber per unit effective area of diffuser per unit time) to achieve a wider range of RD. This device has an advantage of preparing large size sand specimens for laboratory model testing.
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kPa. From the analysis of direct shear test results, angle of internal friction (φ) of the sand is obtained as 34.680.
Fig.2 Details of diffuser sieve set schematic diagram (Dave and Dasaka, 2012)
Fig.1 Details of Portable travelling pluviator assembly (Dave and Dasaka, 2012) MATERIAL PROPERTIES Indian standard sand, commercially known as Ennore sand, is used in the present study, hereafter referred to as Grade II sand. Typical particle size distribution curve of Grade II sand is shown in Fig. 4. From the results of the particle size analysis, it can be observed that Grade II sand is uniformly graded medium to fine sands and classified as SP according to the Unified Soil Classification System (USCS). Some of the important physical properties of sand are presented in Table 1. Direct shear tests are performed on Grade II samples placed at 68% relative density, as per IS: 2720-Part 13 [10]. All samples are sheared at a constant rate of displacement of 1.25 mm/min under four normal stresses, viz. 50 kPa, 100 kPa, 150 kPa and 200
All dimensions are in mm Fig.3 Diffuser sieve set assembly (Dave and Dasaka 2012) Page 2 of 6
Portable traveling pluviator to reconstitute specimens of cohesionless soils
Table 1 Properties of sand used in the study Property Value Gs 2.62 D50 (mm) 0.57 Cu 1.36 Cc 0.95 emin 0.538 emax 0.848 3 γd min (kN/m ) 14.18 (ASTM D4254-00) γd max (kN/m3) 17.04 (Pluviator) Gs – Specific gravity of soil solids, D50 – Mean diameter of soil particles, Cu – Coefficient of uniformity, Cc – Coefficient of curvature, emin – Minimum void ratio, emax - Maximum void ratio, γdmin – Minimum dry unit weight, γdmax – Maximum dry unit weight
accordance with the standard procedure of inverting cylinder (ASTM D4254-00)[2] and other procedures, viz., can method and funnel method, suggested by Mehdiratta and Triandafilidis [16], and the results are reported in Table 2. Among all the three methods, more consistent results are achieved by inverting cylinder method. Table 3 Relation between orifice size and deposition intensity Size of orifice (mm) Deposition intensity (g/cm2/sec) 5 0.468 6 0.584 8 1.857 10 3.69 12 4.832 15 11.529
Table 2 Minimum unit weight of sand (kN/m3) by ASTM D4254 and methods suggested by Mehdiratta and Triandafilidis (1978) Funnel method 14.50
Cylinder method 14.28
Can method 14.34
14.60
14.18
14.48
14.39
14.18
14.45
Fig.5 Effect of deposition intensity and height of fall on relative density
Fig.4 Grain size distribution curve of Grade II sand Maximum unit weight is determined by air pluviation, avoiding particle crushing, following the procedure suggested by Lo Presti et al. [13]. The minimum unit weight is obtained in
EXPERIMENTAL RESULTS Tests are performed in order to evaluate effect of HF (varied from 2.5 cm to 30 cm), DI (using orifice diameter of 5 mm, 6 mm, 8 mm, 10 mm and 12 mm), and number of diffuser sieves (varied from 0, 2, 4, 6, 8 and 10) on RD of pluviated specimen. Sieves are rotated 450 horizontally with respect to each other [18]. A cylindrical mould of volume 3213 cm3 is used for preliminary studies on evaluation of DI and density of sand bed. The effect of orifice size on the DI for Grade II sand is presented in Table 3. Observed DI increases with increase in diameter of orifice. Lower density of sand specimens can be achieved at higher DI, Page 3 of 6
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however at higher DI sand specimens may not be uniform. A linear relationship is observed between RD and HF for low DI values. Effect of HF on density of sand bed for constant DI and without diffuser sieves is observed very high, such as 73.4% to 100% as shown in Fig. 5, which is line with observation made by Choi et al. [6].
Fig.7 Effect of number of sieves and height of fall on relative density for orifice dia 10 mm on Grade II sand
Fig.6 Effect of number of sieves and height of fall on relative density for orifice dia 8 mm Similarly, effect of DI, number of diffuser sieves, and HF on unit weight of sand are reported in Figs. 4 to 6. Increase in the unit weight of sand specimen is observed with increase of HF, for a constant DI. As the number of diffuser sieves increases lower density sand specimens can be achieved, and these finds are in agreement with the observations made by Rad and Tumay [18] and Choi et al. [6]. Difference of RD achieved without sieves and with 2 sieves is more pronounced compared to difference of RD achieved with 2 and 4 sieves or 4 and 6 sieves etc., as shown in Fig 6 to Fig 8. Change in RD of sand specimens with the number of diffuser sieves is significant up to 8 sieves and further increase in the diffuser sieves has little or no significance on RD of sand specimen. In conclusion, RD increases with increase in HF, decrease in number of sieves and decrease in DI. From Figs. 4 to 6 it is observed that, irrespective of height of fall, the decrease in RD is more pronounced with increase in number of sieves for lower DI than for higher DI.
Fig.8 Effect of number of sieves and height of fall on relative density for orifice dia 12 mm on Grade II sand CONCLUSIONS Portable travelling pluviator, used in the present study, consists of multiple diffuser sieve arrangement for obtaining uniform sand rain and set of orifice plates for DI control. In this paper pluviation studies are carried out using Indian Standard Sand Grade II, and the following major conclusions are drawn from the study: • Relative density of sand specimens increases with increase of height of fall and decrease with increase of DI. • Without diffuser sieves higher relative densities can be achieved, at the cost compromising on uniformity of sand bed.
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Portable traveling pluviator to reconstitute specimens of cohesionless soils
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•
RD decreases with increase in number of sieves for any particular height of fall and DI. As DI increases effect of number of sieves on RD decreases at higher height of fall. With diffuser sieves arrangement, a range of RD from 41.2% to 100% can be achieved.
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10. IS: 2720-Part 13 (2002). “Method of test for soils Direct shear test.” Bureau of Indian Standards, New Delhi. 11. Kuerbis, R., and Vaid, Y. P. (1988). “Sand sample preparation – The slurry deposition method’, Soils and Foundations, 8(4), 107-118. 12. Lagioia, R., Sanzeni, A., and Colleselli, F. (2006). “Air, Water and Vacuum Pluviation of Sand Specimens for the Triaxial Apparatus.” Soils and Foundations 46(1), (2006)61-67. 13. Lo Presti, D. C. F., Pedroni, S., and Crippa, V. (1992). “Maximum dry density of cohesionless soils by pluviation and by ASTM D 4253-83: A comparative study.” Geotechnical Testing Journal, 15(2), 180-189. 14. Lo Presti, D. C. F., Berardi, R., Pedroni, S., and Crippa, V. (1993). “A new traveling sand pluviator to reconstitute specimens of wellgraded silty sands.” Geotechnical Testing Journal 16(1), 18-26. 15. Madabushi, S. P. G., Houghton, N. E., and Haigh, S. K. (2006). “A new automatic sand pourer for model preparation at Univeristy of Cambridge.” Proc., 6th International Conference on Physical Modeling in Geotechnics, Hong Kong, Taylor & Francis, London, 217-222. 16. Mehdiratta, G. R. and Triandafilidis, G. E. (1978). “Minimum and Maximum densities of granular materials.” Geotechnical Testing Journal, 1(1), 34-40. 17. Miura, S., and Toki, S. (1982). “A Sample Preparation Method and its Effect on Static and Cyclic Deformation Strength Properties of Sand.” Soils and Foundations, 22(1), 61-77. 18. Rad, N. S, and Tumay, M. T. (1987). “Factors Affecting Sand Specimen Preparation by Raining.” Geotechnical Testing Journal, 10(1), 31-37. 19. Stuit, H. G. (1995). ‘Sand in the Geotechnical Centrifuge’, Ph.D. thesis, Tech. Uni. Delft, Netherlands. 20. Vaid, Y. P., and Negussey, D. (1988). “Preparation of Reconstituted Sand Specimens.” Advanced Triaxial Testing of Soil and Rock, ASTM STP 977, ASTM International, West Conshohocken, PA, 405417.
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21. Walker, B. P., and Whitaker, T. (1967). “An apparatus for forming uniform beds of sand for model foundation tests.” Geotechnique, 17, 161-167. 22. Zhao, Y., Gafar, K., Elshafie, M. Z. E. B., Deeks, A. D., Knappett, J. A., and Madabushi, S. P. G. (2006). “Calibration and Use of New Automatic Sand Pourer.” Proc.,6th International Conference on Physical Modeling in Geotechnics, Hong Kong, Taylor & Francis, London, 265-270
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