A Transparent Sand for Geotechnical Laboratory Modeling - CiteSeerX

Geotechnical Testing Journal, Vol. 34, No. 6 Paper ID GTJ103808 Available online at www.astm.org

Fawzy M. Ezzein1 and Richard J. Bathurst2

A Transparent Sand for Geotechnical Laboratory Modeling

ABSTRACT: The paper describes a new transparent granular soil that can be used for laboratory geotechnical modeling purposes. The transparent soil consists of fused quartz particles in combination with a mixture of two mineral oils as pore fluid. The solid particles and the matching liquid have the same refractive index. The soil has important advantages with respect to transparency, stability, health safety, and utility over glass and silica gel materials. The transparent soil is also inexpensive compared to silica gel-fluid materials that have been used in the past. Conventional laboratory shear box, triaxial compression, and permeability tests were carried out to demonstrate that the mechanical properties and hydraulic permeability of the transparent soil are typical of granular soils with angular particles. KEYWORDS: transparent soil, fused quartz, refractive index, granular soil

Introduction Transparent soils hold promise for the visualization of a range of model-scale geotechnical soil-structure interaction problems and for laboratory studies of porous media fluid flow and contaminant transport (e.g., Welker et al. 1999; Iskander 2010). Early attempts at transparent soils used crushed glass or glass beads in combination with a matching fluid with the same refractive index (Wakabayashi 1950; Drescher 1976; Allersma 1982). However, porous media using glass beads are translucent rather than transparent and they do not represent the geotechnical properties of natural granular soil (Mannheimer and Oswald 1993; Sadek et al. 2002). Better success has been achieved by matching transparent solid silica powder or a silica gel medium with a colorless pore fluid having the same refractive index. The use of precipitated and flumed silica powder and silica gel beads to manufacture transparent model clay and sand has been described by Iskander et al. (1994, 2002a, 2002b), Gill and Lehane (2001), Sadek et al. (2002), Liu et al. (2003), Zhao and Ge (2007), and Hird and Stanier (2010). However, the use of transparent silica gel beads to manufacture simulated granular soils has the following limitations to different degrees depending on the actual gel material:

(c) Silica gel particles are hydroscopic and thus affected by high humidity and water which can cause the particles to break and become colored. (d) There may be other chemical processes that cause changes in color and hence reduce transparency with time (Iskander 2010). (e) The clear visible depth into the transparent soil is limited to about 50 mm.

(a) The particles deform plastically even under low confining pressure (Iskander 1998; Iskander et al. 2003; Zhao and Ge 2007). (b) De-airing the internal pores that are part of the silica gel particles is difficult and impractical for quantities that are required for large scale tests (Iskander et al. 2002b, 2003).

Sadek et al. (2002) concluded that there is no silica gel material available that can satisfactorily simulate the mechanical properties of fine sand. The writers are currently engaged in research related to granular soil-structure interaction problems using metallic mesh and (polymeric) geogrid soil reinforcement systems. In order to visually observe the reinforcement elements during load transfer a transparent soil was required. The writers initially examined six different candidate granular particle types and 20 different clear liquids having matching refractive indices. The particle materials were glass beads, crushed glass, silica gel beads, fused silica, clear plastic beads, and granulated magnesium fluoride. The limitation of glass materials noted earlier was independently confirmed by the writers as was the breakage of silica gel beads in the presence of water and high humidity. The magnesium fluoride particles available to the writers proved not to be transparent. Fused silica particles were also examined but were found to contain very small air bubbles trapped within the particles which rendered the particles translucent when saturated with a matching fluid with the same refractive index. Candidate fluid materials were eliminated for laboratory modeling for one or more of the following reasons:

Manuscript received February 10, 2011; accepted for publication June 27, 2011; published online August 2011. 1 Ph.D. Candidate, GeoEngineering Centre at Queen’s-RMC, Dept. of Civil Engineering, Royal Military College of Canada, Kingston, Ontario, K7K 7B4 Canada, e-mail: [email protected] 2 Professor and Research Director, GeoEngineering Centre at Queen’s-RMC, Dept. of Civil Engineering, Royal Military College of Canada, Kingston, Ontario, K7K 7B4 Canada. (Corresponding author), e-mail: [email protected]

(a) Excessive fluid volatility leading to changes in composition and thus changes in refractive index with time. (b) Potential health risk due to inhalation of toxic fumes and high flammability rating in excess of safety regulations for unvented laboratory environments. (c) High viscosity of the liquid. (d) High cost.

C 2011 by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. V Copyright Copyright by ASTM Int'l (all rights reserved); Fri Aug 12 15:02:03 EDT 2011 Downloaded/printed by Royal Military Coll of Canada pursuant to License Agreement. No further reproductions authorized.

1

2

GEOTECHNICAL TESTING JOURNAL

The plastic beads were very expensive and the matching fluids required to make the particles transparent had some of the deficiencies noted above. Regardless of fluid type, a common concern for transparent soils is change in refractive index of the fluid due to changes in temperature (e.g., Ni et al. 2010). However, this problem can be prevented by keeping the fluid and model at a constant temperature. The search for a new transparent soil resulted in the discovery of a hard fused quartz material which when combined with a benign liquid mixture of two white mineral oils creates a material with properties similar to granular soil media. This paper describes the quartz material and fluid components, and the geotechnical properties of the resulting transparent soil based on conventional geotechnical laboratory tests. To the best knowledge of the writers this material and matching fluid recipe is the best transparent material available to reproduce the mechanical behavior of coarse- to fine-size granular soils in laboratory-scale geotechnical models without the disadvantages noted earlier.

Transparent Soil Components Granular Particles The granular particles in this investigation are fused quartz which is a noncrystalline (glass) form of silicon dioxide (SiO2) quartz sand. The material is manufactured by melting natural quartz crystals present in quartzite sand at approximately 2000 C, and then cooling. Heating causes the crystals within the quartz to become fused together and to be non-porous. Fused quartz is widely used in industrial applications such as semiconductors, solar cells, telescope and microscope lenses, telecommunication equipment and glass chemical containers. The fused quartz particles are impermeable and non-absorbing to the candidate fluids investigated in this study. The particles are hard, fracture and chemically resistant, and have good optical transmission. The physical properties of the fused quartz are shown in Table 1. In this project, the crushed fused quartz particles were manufactured by Mineral Technology Corporation3 in the USA. The source material was supplied in separate screened fine and coarse particle sizes (Fig. 1). The individual (coarse size) particles are designated as angular according to ASTM D2488. The screenings are a byproduct of the manufacturing process according to the company.

TABLE 1—Properties of fused quartz (Weast and Astle, 1981). Property

Value 3

Density (Mg/m )

2.2

Specific Gravity

2.24a

Mohs Hardness Modulus of Elasticity (GPa) Appearance Poisson’s Ratio Refractive Index

4.9 72 Clear 0.16 1.4585b

a

Measured by the writers (ASTM D854). 1.4586 for fused quartz used in this study based on measured refractive index of transparent fluid. b

These materials are inexpensive and are used in a wide variety of applications including personal care products, cosmetics, pharmaceuticals, food processing and plastics. In fact, Puretol 7 is the primary ingredient in baby oil. Table 2 summarizes the properties of these two mineral oils. The volume mixture to match the refractive index of the fused quartz was 68 % Puretol 7 and 32 % Krystol 40 at 22 C. To evaluate the stability of the mixture, an open container was left exposed to the atmosphere for six months without any detectable changes in refractive index value using a digital hand-held refractometer apparatus (“Pocket” Refractometer PAL made by Atago Co., Ltd., Japan) or color. The properties of the mixed transparent liquid are summarized in Table 3. The final mixture of oil has a viscosity of 10 cS which means that this fluid is 10 times more viscous than water. The refractive index of the oil mixture varied over the range 1.4586 6 0.0003 for temperature in the range 22 6 3 C (Fig. 2).

Testing Program General A laboratory testing program was carried out to quantify the geotechnical mechanical and hydraulic properties of the mineral oilfused quartz mixture and demonstrate that the transparent soil has properties that are typical of naturally occurring granular soils.

Liquid The matching liquid must be safe (low toxicity), colorless, stable and odorless. It must also have low volatility, low viscosity and preferably (if applicable) neutral pH. As noted earlier, a large number of candidate fluids were investigated. This involved the trialand-error mixing of two miscible liquids having higher and lower refractive index than the fused quartz particles until the refractive index of the particles (1.4586) was achieved. The writers found that the ideal fluid was two white mineral oils with the trade names “Krystol 40” and “Puretol 7” manufactured by Petro-Canada.4 3 4

Mineral Technology Corporation, PO Box 872, Custer, SD 57730-0872. Petro-Canada Lubricants, 2310 Lakeshore Rd. West, Mississauga, Ontario, L5J 1K2, Canada. http://lubricants.petro-canada.ca

FIG. 1—Coarse (D50 ¼ 1.68 mm) and fine (D50 ¼ 0.33 mm) fused quartz particles.

Copyright by ASTM Int'l (all rights reserved); Fri Aug 12 15:02:03 EDT 2011 Downloaded/printed by Royal Military Coll of Canada pursuant to License Agreement. No further reproductions authorized.

EZZEIN AND BATHURST ON TRANSPARENT SAND FOR LABORATORY MODELING

3

TABLE 2—Properties of Krystol 40 and Puretol 7. Property 3

Density @ 15 C (Mg/m ) Viscosity @ 40 C (cSt) Appearance Odor

Krystol 40

Puretol 7

0.823

0.859

3.4

12.2

Clear and bright No odor

Clear and bright No odor

Flash point, PM, C Refractive index at 22 C

135

170

1.4505a

1.4635a

Source: Petro-Canada Lubricants. Note: that cSt ¼ centistokes ¼ 106 m2/s and PM ¼ Pensky–Martens method (ASTM D93). a Determined by writers using digital hand-held “Pocket” Refractometer PAL made by Atago Co., Ltd., Japan.

Mechanical properties are a necessary precursor to the use of these materials for laboratory modeling of soil-structure interaction problems such as model footings and pilings, or in the case of the writers, physical modeling of embedded metallic mesh and polymeric geogrid soil systems. Particle size analysis, compaction testing, direct shear box testing, triaxial compression testing, onedimensional compression tests, and permeability tests were carried out. Unless noted otherwise, all test specimens were prepared to 95 % relative compaction regardless of pore fluid. Here, relative compaction is taken with respect to the standard Proctor maximum dry density. The relative compaction values were controlled to 60.5 %. Tamping was carried out using a 20 mm diameter rod.

Particle Size Distribution The results of particle size analysis using dry sieving (ASTM C92) are shown in Fig. 3 and in Table 4. Both fine and coarse sample materials have narrow particle size ranges and are classified as SP (poorly graded sand) according to the Unified Soil Classification System.

Compaction Testing Standard Proctor compaction tests were carried out on virgin specimens of the coarse and fine fused quartz in accordance with the ASTM D698 method of test. Dry density versus fluid content plots are presented in Fig. 4. As a result of the uniform particle size of the material, the water content had little effect on the compacted dry density of the material. A single compaction test was done at 100 % oil saturation for each size of fused quartz material. Tests at lower oil contents are not required since the fused quartz is only used with 100 % oil saturation in order to make the mixture transparent. The mineral oil content required to create the transparent soil was 23 and 26 % by total mass for the coarse and

FIG. 2—Refractive index versus temperature for transparent fluid.

fine fused quartz materials, respectively. The dry density of the fully oil saturated fused quartz was 1.31 Mg/m3 for the coarse material and 1.23 Mg/m3 for the fine material. These numbers are only slightly higher than the mean value of 1.27 and 1.17 Mg/m3 for the same materials prepared with water. The dry density value from the compaction test with oil for each particle size was used as the reference value to calculate the target 90, 95, and 98 % relative compaction during preparation of the specimens in oil described in the tests to follow.

Direct Shear Box Tests Direct shear box tests were carried out on coarse and fine fused quartz specimens using the ASTM D3080 test method. The tests were performed in a shear box with dimensions of 100 mm  100 mm  50 mm (length  width  height) and constant rate of deformation equal to 1 mm/min. The specimens were prepared dry and with water and mineral oil. For the mineral oil tests the fused quartz particles were pluviated in oil in three layers. After the placement of each quartz layer, any air bubbles were allowed to

TABLE 3—Transparent fluid properties (volume ratio of 0.32 Krystol 40 to 0.68 Puretol 7). Property

Test Method

Apparatus

Value

Density @ 20 C (Mg/m3)

ASTM D1475

Pycnometer

0.838

Viscosity @ 22 C, (cSt) Refractive index @ 22 C

ASTM D445 ASTM D1218

Digital viscometer Refractometera

10 1.4586

a

“Pocket” Refractometer PAL made by Atago, Co., Ltd., Japan.

FIG. 3—Particle size distributions for coarse and fine fused quartz samples.

Copyright by ASTM Int'l (all rights reserved); Fri Aug 12 15:02:03 EDT 2011 Downloaded/printed by Royal Military Coll of Canada pursuant to License Agreement. No further reproductions authorized.

4

GEOTECHNICAL TESTING JOURNAL TABLE 4—Particle size analysis of coarse and fine fused quartz samples.

Property

Coarse Quartz

Fine Quartz

D60

2.10

0.39

D50

1.68

0.33

D30 D10

1.21 1.03

0.22 0.11

Coefficient of Curvature

Cc

0.68

1.16

Coefficient of Uniformity

Cu

2.04

3.65

Particle size (mm)

rise to the surface until the mixture was transparent (100 % saturation). Example data for fused quartz specimens in mineral oil are presented in Figs. 5 and 6. Load oscillations were observed at postpeak strength for mineral oil specimens (Fig. 5(a)) but not for dry and water fluid specimens (Fig. 6(a)). Skinner (1969) reported similar load oscillations at post-peak strength from direct shear tests on 1-mm diameter glass ballotini (beads) and noted that the magnitude of these oscillations was larger when the pores were

FIG. 5—Direct shear box test results for fine fused quartz specimens with mineral oil.

FIG. 4—Dry density versus fluid content from standard Proctor compaction tests.

filled with water compared to dry specimens. In the current investigation, the post-peak oscillations (for oil fluid specimens) diminished in amplitude as normal stress decreased (Fig. 5(a)). For brevity the entire suite of direct shear box tests is not presented here. However, the performance of mineral oil specimens with the exception of post-peak strength oscillations was qualitatively similar to nominal identical tests done dry and with water as the pore fluid (Fig. 6). Quantitatively, the magnitude of dilation was less for the mineral oil specimens when all other conditions were the same, which is believed to be the result of particle lubrication. Linear regressed lines forced through the origin and peak shear strength data points are plotted on Figs. 7(a) and 7(b). Ranges for peak and residual friction angles computed for tests carried out dry, with water and oil are summarized in Table 5. The residual friction angles were computed from shear loads recorded at the end of the tests (i.e., 10 mm of shear displacement). For practical purposes the peak friction angle is judged to be independent of pore fluid type (i.e., the maximum difference is three degrees based on single failure envelopes and average secant values for the same normal stress).

Copyright by ASTM Int'l (all rights reserved); Fri Aug 12 15:02:03 EDT 2011 Downloaded/printed by Royal Military Coll of Canada pursuant to License Agreement. No further reproductions authorized.

EZZEIN AND BATHURST ON TRANSPARENT SAND FOR LABORATORY MODELING

FIG. 6—Influence of fluid type on direct shear box test results using coarse fused quartz specimens.

5

FIG. 7—Peak shear strength envelopes for coarse and fine fused quartz specimens from direct shear box testing.

TABLE 5—Peak and residual friction angles for fused quartz materials. Direct Shear Tests Coarse Fused Quartz Peak Friction Angle (degrees)

Fine Fused Quartz

Residual Friction Angle (degrees)

Peak Friction Angle (degrees)

Residual Friction Angle (degrees)

c ¼ 0a

Secantb

c ¼ 0a

Secantb

c ¼ 0a

Secantb

c ¼ 0a

Secantb

Dry Water

43 45

41–45 42–47

36 40

35–43 35–42

42 44

41–46 42–46

37 39

35–41 37–42

Oil

42

42–45

39

37–40

43

42–47

40

38–44

Triaxial Tests Dry

44

43–49

42

41–45

50

54–48

45

44–49

Water

45

43–50

41

40–45

51

55–49

45

43–46

Oil

44

43–46

42

41–43

49

54–47

43

42–43

Pore fluid

a

Single linear failure envelope forced through origin. Linear failure envelope from origin to each data point.

b

Copyright by ASTM Int'l (all rights reserved); Fri Aug 12 15:02:03 EDT 2011 Downloaded/printed by Royal Military Coll of Canada pursuant to License Agreement. No further reproductions authorized.

6

GEOTECHNICAL TESTING JOURNAL

Triaxial Testing Consolidated drained triaxial compression tests were carried out on coarse and fine particle specimens. For each particle size the following test parameters were varied: (a) Confining pressure (25, 50, and 100 kPa). (b) Dry (with no pore fluid), fully saturated with de-aired water and mineral oil. (c) Specimens prepared to 90, 95, and 98 % of standard Proctor density. All tests were conducted on specimens 76 mm in diameter and with height to diameter ratio of two. The specimens were prepared and saturated according to standard practice (e.g., Bardet 1997; Bishop and Henkel 1969; Head 1998). The fused quartz specimens were confined within a double latex membrane to avoid membrane puncture by the angular particles and to minimize membrane degradation by the mineral oil pore fluid (Iskander 2010; Zhao et al. 2010). In fact, mineral oil absorption led to swelling and softening of the membranes so that membranes could only be used once. Dry specimens were first saturated with carbon dioxide to facilitate subsequent saturation with water or oil (Chaney et al. 1979). A pore pressure coefficient B value of 98 % was achieved for both water and mineral oil specimens. The specimens were loaded at a constant axial strain rate up to a maximum strain of 20 %. Unless noted otherwise, the rate of loading was 0.5 % strain/minute. During the test, the axial displacement, volume change, axial load, confining pressure, and pore pressure (back pressure) were continuously recorded. The influence of loading rate on stress-strain response of fine quartz specimens under a constant confining pressure of 50 kPa is illustrated in Fig. 8. The data plots show a detectable but very small increase in stiffness (prior to peak shear resistance) and strength of the specimens with increasing rate of loading. However, the quantitative differences are judged to be negligible for practical purposes. Stress-strain curves from conventional triaxial compression tests on coarse and fine particle specimens with water and mineral oil pore fluid are presented in Fig. 9. The data sets for fused quartz with water and oil pore fluid are judged to be typical for

FIG. 8—Influence of loading rate on stress-strain response of triaxial compression tests on fine fused quartz-mineral oil fluid specimens.

granular soils. There is evidence of strain softening at post-peak shear strength and decreasing dilatancy with increasing confining pressure. The latter is consistent with lower dilation measured for the mineral oil specimens in the direct shear tests discussed earlier (Fig. 5(b)). The peak and residual friction angle ranges computed from triaxial tests are summarized in Table 5. Figure 10 shows a comparison of stress-strain behavior for tests at the same confining pressure but prepared in a dry state and with water and mineral oil. The stress-volumetric strain curves for dry specimens must be viewed with caution since changes in specimen volume were computed from changes in triaxial cell fluid volume which is less accurate than conventional measurements using expelled pore fluid in the companion water and mineral oil specimens. There are detectable decreases in shear strength and dilatancy for specimens prepared with mineral oil compared to specimens prepared with water. Example stress-strain data are plotted in Fig. 11 demonstrating that soil shear strength and soil dilatancy increases with increasing density which is expected for granular soils. Similar to the direct shear box tests described earlier, a characteristic feature of the stress-strain response of triaxial specimens with mineral oil at post-peak strength is repeated load loss and recovery cycles (load oscillations) (Figs. 10 and 11). This stick-slip behavior at macro-scale is believed to be due to inter-particle load transmission mechanisms at micro-scale that are amplified for dilatant granular particle assemblies in the presence of a viscous pore fluid. The micromechanical explanation for this behavior follows from the experience of the second author and colleagues as part of numerical simulation work on idealized discrete particle systems (e.g., Rothenburg and Bathurst 1992). At the micro-mechanical scale, boundary shear loads are transmitted through chains of contact forces aligned in the principal stress direction. As the granular assembly dilates during shear, the number of inter-particle contacts becomes less and chains of aligned particles are formed. These chains are meta-stable and they continually collapse and reform randomly during shear. For dry and water fluid specimens these mechanisms are not detectable at the macro-scale due to the large number of load chains that are continually collapsing and re-forming. However, it is posited that for the same assembly with a more viscous pore fluid (compared to water) there are fewer but longer load chains that are created as a result of the support offered by the viscous mineral oil pore fluid. The collapse and reforming of these longer but less stable load chains is detectable at macro-scale as stick-slip cycles during shear of the dilated assemblies. It is interesting to note that stick-slip oscillations in our triaxial tests with angular particles and a viscous pore fluid are qualitatively similar to the behavior of assemblages of spherical glass beads with no pore fluid reported by Alshibli and Roussel (2006). They also identify the collapse of chains of load-carrying particles as a micro-mechanical explanation for the load oscillations observed at macro-scale. Finally, it is reasonable to expect that load oscillations will decrease as the specimen size (i.e., number of particles) increases. Hence, for geotechnical models with transparent soil volumes larger than the laboratory triaxial and direct shear tests reported here, load oscillations may not be detectable. Figure 12 presents peak shear strength data from triaxial testing. The linear failure envelopes are regressed against all data

Copyright by ASTM Int'l (all rights reserved); Fri Aug 12 15:02:03 EDT 2011 Downloaded/printed by Royal Military Coll of Canada pursuant to License Agreement. No further reproductions authorized.

EZZEIN AND BATHURST ON TRANSPARENT SAND FOR LABORATORY MODELING

7

FIG. 9—Influence of particle size, pore fluid type, and confining pressure on stress-strain response of specimens during triaxial compression.

points in a test series and forced through the origin. The peak friction angles for fine and coarse fused quartz materials with mineral oil are 49 and 44 , respectively. These friction angle values are judged to be typical of granular soils comprised of angular particles (Holtz and Kovacs 1981). Importantly, the data show that the friction angle of the materials in a dry state is (for practical purposes) the same as for materials prepared with water and mineral oil (i.e., within one degree). It can be noted that the peak internal friction angle of the coarse and fine quartz specimens obtained from triaxial tests are generally greater than corresponding values deduced from direct shear tests (Fig. 7). This difference is consistent with observations made by Kulhawy and Mayne (1990) for sands with high triaxial peak friction angles.

One Dimensional Compression Tests One-dimensional (1-D) compression testing was carried out using conventional laboratory oedometer equipment and in general accordance with the ASTM D2435 method of test. Figure 13(a) shows load and unload curves for the two quartz materials and a local natural silica beach sand that has been used extensively by the writers and co-workers in related reinforced soil research (e.g., Huang et al. 2009). This natural sand has D50 ¼ 0.34 mm, coefficient of curvature Cc ¼ 2.25 and coefficient of uniformity Cu ¼ 1.09. The fines content (particle sizes < 0.075 mm) was less

than 1 %. This sand is also classified as SP according to the Unified Soil Classification System but is more broadly graded than the fused quartz material. Nevertheless, the mean particle size is close to the fine fused quartz material (Table 4). The initial 1-D compression response is very similar for all three soils in a dry state up to about 500 kPa pressure but the more broadly graded natural soil is stiffer thereafter (Fig. 13(a)). The slopes of the unload compression curve for the coarse fused quartz and natural sand are very similar. The fine fused quartz material has slightly greater rebound. The fine quartz material is detectably less stiff when prepared with mineral oil than dry or with water fluid (Fig. 13(b)). The slopes of the unload curves are very similar. Over a vertical stress range of 800 to 1600 MPa the compressibility index for the coarse and fine fused quartz in oil was calculated to be 0.17 and 0.12, respectively. These values fall between values for dense angular to subangular carbonate and quartz sands subjected to the same change in vertical stress (Mesri and Vardhanabhuti 2009). Based on the data presented here the two fused quartz material gradations with a mineral oil pore fluid are judged to be acceptable analogues to natural granular materials under 1-D compression.

Particle Breakage and Sample Preparation A desirable feature of any granular soil analogue for laboratory testing is acceptable resistance to crushing. Clearly, the

Copyright by ASTM Int'l (all rights reserved); Fri Aug 12 15:02:03 EDT 2011 Downloaded/printed by Royal Military Coll of Canada pursuant to License Agreement. No further reproductions authorized.

8

GEOTECHNICAL TESTING JOURNAL

FIG. 10—Example results showing influence of particle size and pore fluid type on stress-strain response of specimens during triaxial compression.

FIG. 11—Example results showing influence of particle size and relative compaction on stress-strain response of specimens with mineral oil fluid during triaxial compression.

interpretation of tests is complicated if the particle size distribution is modified by particle crushing. Particle crushing has been demonstrated to affect sand compressibility and shear strength in laboratory triaxial tests (Lee and Seed 1967). The magnitude of particle crushing of fine and coarse fused quartz specimens in dry and wet conditions was quantified by sieving the soil before and after shear box and 1-D compression testing. Figure 14 shows the particle size distribution curves. For the 1-D compression test taken to 1600 kPa normal stress, the D50 of the coarse sand particles was reduced from 1.68 to 1.51 mm (10 % decrease in the D50 size) (Fig. 14(a)). The reduction in particle sizes was less (2 % decrease in the D50 size) for the same test conditions and the fine quartz material (Fig. 14(b)). The fused quartz specimens prepared with mineral oil could not be sieved because it was not possible to remove residual traces of oil coating the particles. In practice, the best way to prepare the transparent soil for geotechnical laboratory modeling is to pluviate the particles into the oil and then tamp the material to 95 % relative compaction as was done for the 1-D compression tests. This ensures 100 % oil saturation and minimizes air bubbles. It is reasonable to assume that particle breakage is even less when prepared in this manner compared to wet specimens (i.e., negligible). Hence, particle breakage is not expected to be a concern for the transparent soil investigated in

this study. Finally, it is recommended that the as-received fused quartz materials be washed with water to remove particle dust and then dried. This initial preparation step was shown to improve the transparency of the particle-oil mixtures.

FIG. 12—Peak shear strength envelopes for coarse and fine particle specimens from triaxial testing.

Copyright by ASTM Int'l (all rights reserved); Fri Aug 12 15:02:03 EDT 2011 Downloaded/printed by Royal Military Coll of Canada pursuant to License Agreement. No further reproductions authorized.

EZZEIN AND BATHURST ON TRANSPARENT SAND FOR LABORATORY MODELING

FIG. 13—One-dimensional load and unload response of granular materials.

Hydraulic Conductivity Constant head permeability tests were performed to determine the water and mineral oil coefficient of permeability for coarse and fine fused quartz materials. The tests were carried out at 20 C and in general accordance with the ASTM D2434 method of test. The test results are shown in Table 6. The permeability coefficient values of the coarse and fine quartz obtained from the tests with oil and water fall in the range of clean sand according to Holtz and Kovacs (1981) and coarse and medium sand according to Das (2008). Due to the viscosity of the mineral oil the permeability coefficient dropped by a factor of 3 and 7.5 compared to water permeability for the coarse and fine fused quartz column tests, respectively. The same coarse size fused quartz sand in combination with the mineral oil mixture recommended in this paper has been used successfully by co-workers to investigate surface fluid infiltration mechanisms and saturated/unsaturated fluctuations at the fluid-air interface in laboratory transparent column tests (Peters et al. 2009, 2011; Siemens et al. 2010). However, the geotechnical properties of this material are reported here for the first time.

9

FIG. 14—Influence of test type on particle crushing of dry fused quartz specimens.

Transparency Degradation Transparency degradation with depth into transparent media of the type investigated in the current study is unavoidable. The transparency of the model (container and transparent soil) is affected by the difference in refractive indices between the container and the transparent soil. Container materials with a refractive index close to the refractive index of the transparent soil are best. For example, the clear visible depth of the coarse transparent soil in this study was found to be 120 mm when the container is made from 25-mm thick Plexiglas (refractive index ¼ 1.488) (Fig. 15). However, the clear depth of view suitable for camera image analysis drops to 50 mm when 25-mm thick tempered glass is used (refractive index ¼ 1.52). Transparency is also affected by TABLE 6—Permeability of coarse and fine fused quartz samples (using ASTM D2434). Pore Fluid Water (m/s) Mineral Oil (m/s)

Copyright by ASTM Int'l (all rights reserved); Fri Aug 12 15:02:03 EDT 2011 Downloaded/printed by Royal Military Coll of Canada pursuant to License Agreement. No further reproductions authorized.

Coarse Quartz 3

3.47  10 1.09  103

Fine Quartz 4.6  104 0.68  104

10

GEOTECHNICAL TESTING JOURNAL

(b) Particles do not contain internal air voids that can make de-airing difficult. (c) Particles do not react chemically with or absorb the oil pore fluid. (d) The refractive index and transparency of the colorless oil is stable with time. (e) The oil fluid is odorless, nontoxic, has low volatility and high ignition temperature which make it an ideal pore fluid in a laboratory environment. (f) There is a clear visible depth of 120 mm (coarse fused quartz) when placed in a container made of 25-mm thick Plexiglas. (g) The fused quartz material and mineral oils are inexpensive.

FIG. 15—Photograph of fused quartz soil in dry and saturated conditions. Note. Geogrid target is located behind 120 mm of soil and front Plexiglas sheet 25 mm thick.

the presence of air bubbles. As noted earlier, pluviation of layers of fused quartz particles into the mineral oil is recommended to allow any air bubbles to rise to the surface and be released. De-airing can be accelerated by applying a vacuum to the surface of the layers but this is not a necessary procedure and may not be practical for large laboratory models.

Conclusions A new transparent sand soil suitable for geotechnical laboratory-scale modeling was developed. The sand particles are fused quartz particles and are rendered transparent by saturation with a mixture of two white mineral oils with the same refractive index as the quartz. A laboratory test program using fine and coarse sand gradations was carried out to classify the soil and to establish the geotechnical properties (behavior) of the transparent soil. Tests were carried out dry, water saturated, and mineral oil saturated to investigate the influence of the oil fluid on mechanical properties. Triaxial and direct shear test results showed that the materials behave similarly to natural sands with angular particle shapes and have similar friction angles even though the viscosity of the oil mixture is 10 times that of water. The particles were shown to be tough and particle breakage in oil-saturated mixtures was judged to be negligible. The coefficient of permeability using the mineral oil pore fluid was smaller than with water due to the difference in viscosity; however, both values are within the range of water permeability values expected for clean sands. The transparent sand soil described in this paper has advantages over candidate mixtures that have been reported in the literature, particularly sands manufactured with silica gel beads. Some of the major advantages include the following: (a) Particles are hard with negligible breakage in oil and do not deform plastically under loading.

One disadvantage of the fused coarse material as supplied by the manufacturer is that the particles are very angular. However, a strategy to make the particles less angular is to autogenously grind the material or use a Los Angeles Abrasion machine with steel balls. This has not been done by the authors at the time of writing but offers the possibility of increasing the range of strength and stiffness of the transparent soil. The viscosity of the transparent oil (10 times that of water) may be a concern in some geotechnical and porous media fluid flow models. However, for sand-structure interaction problems carried out at slow rates of loading the relatively high viscosity is unlikely to modify model results. In summary, the transparent soil described in this study offers a wide range of opportunities as an artificial soil in geotechnical laboratory testing and modeling that is focused on granular soilstructure interaction problems and fluid flow in porous media.

Acknowledgments The work reported in this paper was supported by grants to the second author from the Natural Sciences and Engineering Research Council (NSERC) of Canada and the Department of National Defence (Canada). The writers wish to acknowledge the support of Petro-Canada Lubricants who provided the mineral oils that were used to make the transparent granular soil described in the paper.

References Alshibli, K. and Roussel, L., 2006, “Experimental Investigation of Slip-stick Behaviour in Granular Materials,” Int. J. Numer. Anal. Meth. Geomech., Vol. 30, No. 14, pp. 1391–1407. Allersma, H., 1982, “Photoelastic Investigation of the Stress Distribution During Penetration,” Proceedings of the Second European Symposium on Penetration Testing, Amsterdam, May 24–27, ESOPT-II, Netherlands, pp. 79–83. ASTM C92, 2010, “Standard Test Methods for Sieve Analysis and Water Content of Refractory Materials,” Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA. ASTM D93, 2010, “Standard Test Methods for Flash Point by Pensky-Martens Closed Cup Tester,” Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA. ASTM D445, 2010, “Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (and Calculation of

Copyright by ASTM Int'l (all rights reserved); Fri Aug 12 15:02:03 EDT 2011 Downloaded/printed by Royal Military Coll of Canada pursuant to License Agreement. No further reproductions authorized.

EZZEIN AND BATHURST ON TRANSPARENT SAND FOR LABORATORY MODELING

Dynamic Viscosity),” Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA. ASTM D698, 2007, “Laboratory Compaction Characteristics of Soil Using Standard Effort,” Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA. ASTM D854, 2010, “Standard Test Methods for Specific Gravity of Soil Solids by Water Pycnometer,” Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA. ASTM D1218, 2007, “Standard Test Method for Refractive Index and Refractive Dispersion of Hydrocarbon Liquids,” Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA. ASTM D1475, 2008, “Standard Test Method for Density of Liquid Coatings, Inks, and Related Products,” Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA. ASTM D2434, 2006, “Standard Test Method for Permeability of Granular Soils (Constant Head),” Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA. ASTM D2435, 2004, “Standard Test Method for One-Dimensional Consolidation Properties of Soils,” Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA. ASTM D2488, 2009, “Standard Practice for Description and Identification of Soils (Visual-Manual Procedure),” Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA. ASTM D3080, 2004, “Direct Shear Test of Soils Under Consolidated Drained Conditions,” Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA. Bardet, J. P., 1997, Experimental Soil Mechanics, Prentice Hall, Upper Saddle River, NJ, p. 583. Bishop, A. W. and Henkel, D. J., 1969, The Measurement of Soil Properties in the Triaxial Test, William Clowes and Sons, London, p. 228. Chaney, R. C., Stevens, E., and Sheth, N., 1979, “Suggested Test Method for Determination of Degree of Saturation of Soil Samples by B Value Measurement,” Geotech. Test. J., Vol. 2, No. 3, pp. 158–162. Das, B. M., 2008, Advanced Soil Mechanics, Taylor & Francis, New York, p. 567. Drescher, A., 1976, “An Experimental Investigation of Flow Rules for Granular Materials using Optically Sensitive Glass Particles,” Geotechnique, Vol. 26, No. 4, pp. 591–601. Gill, D. R. and Lehane, B. M., 2001, “An Optical Technique for Investigating Soil Displacement Patterns,” Geotech. Test. J., Vol. 24, No. 3, pp. 324–329. Head, K. H., 1998, Manual of Soil Laboratory Testing, Vol. 3 of Effective Stress Tests, John Wiley & Sons, West Sussex, England, p. 442. Hird, C. C. and Stanier, S. A., 2010, “Modelling Helical Screw Piles in Clay using a Transparent Soil,” Proceedings of the 7th International Conference on Physical Modelling in Geotechnics, Zurich, International Society for Soil Mechanics and Geotechnical Engineering, Jun 28–July 1, Switzerland, p. 6. Holtz, R. D. and Kovacs, W. D., 1981, An Introduction to Geotechnical Engineering, Prentice Hall, Englewood Cliffs, NJ, p. 733. Huang, B., Bathurst, R. J., and Hatami, K., 2009, “Numerical Study of Reinforced Soil Segmental Walls using Three Different Constitutive Soil Models,” J. Geotech. Geoenviron. Eng., Vol. 135, No. 10, pp. 1486–1498.

11

Iskander, M., Lai, J., Oswald, C., and Mannheimer, R., 1994, “Development of a Transparent Material to Model the Geotechnical Properties of Soils,” Geotech. Test. J., Vol. 17, No. 4, pp. 425–433. Iskander, M., 1998, “Transparent Soils to Image 3D Flow and Deformation,” Proceedings of Second International Conference on Imaging Technologies: Techniques and Applications in Civil Engineering, Davos, Switzerland, May 25–30, 1997, ASCE, New York, pp. 255–264. Iskander, M., Liu, J., and Sadek, S., 2002a, “Transparent Amorphous Silica to Model Clay,” J. Geotech. Geoenviron. Eng., Vol. 128, No. 3, pp. 262–273. Iskander, M., Sadek, S., and Liu, J., 2002b, “Optical Measurement of Deformation using Transparent Silica Gel to Model Sand,” Int. J. Phys. Modell. Geotech., Vol. 2, No. 4, pp. 13–26. Iskander, M., Liu, J., and Sadek, S., 2003, “Modeling 3D Flow and Soil Structure Interaction using Optical Tomography,” Final Report, NSF Project No. CMS 9733064, p. 280. Iskander, M., 2010, Modeling with Transparent Soils, Visualizing Soil Structure Interaction and Multiphase Flow, Non intrusively, Springer, New York, p. 331. Kulhawy, F. H. and Mayne, P. W., 1990, “Manual on Estimating Soil Properties for Foundation Design,” EPRI Electric Power Research Institute, Ithaca, NY. Liu, J., Iskander, M., and Sadek, S., 2003, “Consolidation and Permeability of Transparent Amorphous Silica,” Geotech. Test. J., Vol. 26, No. 4, pp. 390–401. Lee, K. and Seed, H. B., 1967, “Drained Strength Characteristics of Sands,” J. Soil Mech. Found. Div., ASCE, Vol. 93, No. SM6, pp. 117–141. Mannheimer, R. and Oswald, C., 1993, “Development of Transparent Porous Media with Permeabilities and Porosities Comparable to Soils, Aquifers, and Petroleum Reservoirs,” Ground Water, Vol. 31, No. 5, pp. 781–788. Mesri, G. and Vardhanabhuti, B., 2009, “Compression of Granular Materials,” Can. Geotech. J., Vol. 46, No. 4, pp. 369–392. Ni, Q., Hird, C. C., and Guymer, I., 2010, “Physical Modelling of Pile Penetration in Clay using Transparent Soil and Particle Image Velocimetry,” Geotechnique, Vol. 60, No. 2, pp. 121–132. Peters, S. B., Siemens, G. A., Take, W. A., and Ezzein, F., 2009, “A Transparent Medium to Provide Visual Interpretations of Saturated/Unsaturated Hydraulic Behaviour,” 62nd Canadian Geotechnical Conference, Halifax, NS, Sep 21–23, Canadian Geotechnical Society, Canada, pp. 59–66. Peters, S. B., Siemens, G. A., and Take, W. A., 2011, “Characterization of Transparent Soil for Unsaturated Applications,” Geotech. Test. J. (in press). Rothenburg, L. and Bathurst, R. J., 1992, “Micromechanical Features of Granular Assemblies with Plane Elliptical Particles,” Geotechnique, Vol. 42, No. 1, pp. 79–95. Sadek, S., Iskander, M., and Liu, J., 2002, “Geotechnical Properties of Transparent Silica,” Can. Geotech. J., Vol. 39, No. 1, pp. 111–124. Siemens, G., Peters, S., and Take, W. A., 2010, “Analysis of a Drawdown Test Displaying the Use of Transparent Soil in Unsaturated Flow Applications,” Proceedings of the 5th International Conference on Unsaturated Soils, Barcelona, Spain, Sep 6–8, pp. 733–738. Skinner, A. E., 1969, “A Note on the Influence of Interparticulate Friction on the Shearing Strength of a Random Assembly of Spherical Particles,” Geotechnique, Vol. 19, No. 1, pp. 150–157.

Copyright by ASTM Int'l (all rights reserved); Fri Aug 12 15:02:03 EDT 2011 Downloaded/printed by Royal Military Coll of Canada pursuant to License Agreement. No further reproductions authorized.

12

GEOTECHNICAL TESTING JOURNAL

Wakabayashi, T., 1950, “Photo-elastic Method for Determination of Stress in Powdered Mass,” J. Phys. Soc. Jpn., Vol. 5, No. 5, pp. 383–385. Weast, R. and Astle, M. J., 1981, CRC Handbook of Chemistry and Physics, 62nd ed., CRC Press, Boca Raton, FL. Welker, A. L., Bowders, J. J., and Gilbert, R. B., 1999, “Applied Research using a Transparent Material with Hydraulic Proper-

ties Similar to Soil,” Geotech. Test. J., Vol. 22, No. 3, pp. 266–270. Zhao, H. and Ge, L., 2007, “Dynamic Properties of Transparent Soil,” Proceeding of GeoDenver 2007, Dynamic Response and Soil Properties (GSP 160), Denver, CO, Feb. 18–21, pp. 1–9. Zhao, H., Ge, L., and Luna, R., 2010, “Low Viscosity Pore Fluid to Manufacture Transparent Soil”, Geotech. Test. J., Vol. 33, No. 6, Paper ID GTJ102607, p. 6.

Copyright by ASTM Int'l (all rights reserved); Fri Aug 12 15:02:03 EDT 2011 Downloaded/printed by Royal Military Coll of Canada pursuant to License Agreement. No further reproductions authorized.