Bardet, JP

Jabbari, N

Jesmani, M

Nunes Lourenco, SD

Géotechnique

HKU Scholars Hub

Title Permeability and compressibility of wax-coated sandsAuthor(s) Bardet, JP; Jesmani, M; Jabbari, N; Nunes Lourenco, SDCitation Géotechnique, 2014, v. 64 n. 9, p. 752-755Issued Date 2014URL http://hdl.handle.net/10722/208203RightsPermission is granted by ICE Publishing to print one copy forpersonal use. Any other use of these PDF files is subject toreprint fees752Bardet, J.-P. et al. (2014). Ge´otechnique 64, No. 9, 752–755 [http://dx.doi.org/10.1680/geot.14.D.002]DISCUSSIONPermeability and compressibility of wax-coated sandsJ.-P. BARDET, M. JESMANI†, N. JABBARI‡ and S. D. N. LOURENC¸O§Contribution by S. D. N. Lourenc¸oThe authors are to be congratulated for advancing thestudy of water-repellent soils in geotechnical engineering(Bardet et al., 2014). As mentioned, the use of water-repellent soils offers a series of new opportunities for groundimprovement, as well as contributing to elucidating thenatural processes linked to these soils. However, despite theirnovelty to geotechnical engineering, soils coated with naturalwaxes have been widely studied in soil science and hydrol-ogy. The discusser has five comments, as listed below.(a) In the schematic diagram of Fig. 1, the contact angleshould refer to the wet side of the water drop. Wettablesoils have contact angles of 08 and water-repellent soilshave contact angles .908. Super-hydrophobic surfacesare non-wettable (contact angle ,1808) (Butt et al.,2006).(b) The assumption of uniform wax coatings in Fig. 4 isquestionable, in particular for the samples with reducedwax content. In naturally occurring water-repellent soils,including burned soils, it has been shown that coatingsare uneven, with a patchy distribution of organic matterleading to wettable and water-repellent regions. Evidenceis from particle- and field-scale studies (Dekker &Ritsema, 2000; Cheng et al., 2009).(c) How water repellent are the samples? Do the authorshave a direct or indirect measure of this? For example,contact angles can be measured with methods from soilscience such as the sessile drop method (Bachmann et al.,2000). This would assist in the interpretation of thepermeability and compressibility data.(d ) How stable is the water repellency induced by theSasolwax? Soil water repellency, as induced by naturalwaxes, is a very unstable condition: (i) it depends on thenatural water content, (ii) it can switch from waterrepellent to wettable and vice-versa, and (iii) prolongedcontact time with water renders the soils wettable (e.g.Doerr et al., 2000).(e) What is the purpose of determining the saturatedpermeability if the soil is water repellent? It is now wellaccepted that naturally occurring water-repellent soils arehighly impervious, such as the case of wildfire-relatedsoils which enhance surface runoff and trigger debrisflows (Moody & Ebel, 2014). The photographs in Fig. 1show a still water drop on top of sands coated with 1%and 6% wax, which suggests that the contact angles are.908. However, the wax-coated sands were flushed withwater, presumably to overcome the water-repellent effect,in order to determine the saturated permeability by fallinghead tests.Authors’ replyThe discusser commented upon and raised questions on thetopic of (a) contact angle of water-repellent soils, (b)uniformity of wax coating, (c) degree of water repellency,(d ) stability of water repellency and (e) permeability ofwater-repellent soils. These specific questions are best an-swered after sketching a background that frames the perspec-tives of the present study and those in soil science andhydrology.BACKGROUNDThe fields of soil science and hydrology abound withworks on soil hydrophobicity in natural environments, forexample, Doerr et al. (2000), Doerr & Moody (2004) andLeighton-Boyce et al. (2007). Hydrophobic soils are waterrepellent; they do not wet readily under rainfall or irrigation.Infiltration and wetting can be delayed from as little as afew seconds to in excess of weeks. In natural environments,soils are understood to be hydrophobic when individual soilparticles are coated by hydrophobic organic molecules,which may be released from a variety of plants, decayingorganic matter, soil fauna and micro-organisms either natu-rally or during fires. Hydrophobicity is often noticeable afterlengthy dry spells and usually disappears after extendedcontact with water. In the field of hydrology, soil hydropho-bicity is documented to increase water runoff and soilerosion, to be distributed in uneven spatial and verticalwetting patterns, to decrease evaporation and to increaserisks of groundwater pollution.Bardet et al. (2014) carried out their study with differentperspectives and objectives. They intended to create a scien-tific and engineering knowledge about the synthetic surfacesthat have been proposed to replace natural soil surfaces inhorse racing. Synthetic surfaces have raised controversies;they have been praised to be safer for horses and jockeys, buthave been also intensely criticised for various reasons (Finley,2010). They are manufactured with the intention of replicat-ing the optimal properties of natural soils even during adverseweather conditions such as heavy rains. Unfortunately, syn-thetic surfaces are manufactured without much engineeringby mixing empirically sandy soils with additives such aswaxes, petroleum derivatives and polymers (for example, seeThe Blood Horse (2007) and Bardet et al. (2009)).CONTACT ANGLEBardet et al. (2014) defined the contact angle Ł ofhydrophobic soils as shown in Fig. 15, whereas it is usuallydefined as ¨ ¼ 1808  Ł in soil sciences and the physicsand chemistry of interfaces (e.g. Butt et al., 2003). For wax-coated sands with wax content 1% and above, they measured¨ to be larger than 908; their wax-coated samples werewater repellent. Department of Civil Engineering, University of Texas at Arlington,Arlington, Texas, USA.† Department of Civil Engineering, Faculty of Engineering, ImamKhomeini, International University, Qazvin, Iran; Sonny AstaniDepartment of Civil and Environmental Engineering, University ofSouthern California, Los Angeles, USA.‡ Sonny Astani Department of Civil and Environmental Engineering,University of Southern California, Los Angeles, USA.§ Department of Civil Engineering, The University of Hong Kong,Hong Kong.UNIFORMITY OF WAX COATINGBardet et al. (2014) manufactured wax-coated sand sam-ples in the laboratory under controlled conditions. Theyheated sand that had a uniform grain size distribution on ahot plate at a temperature above the wax melting point,poured melted wax onto the hot sand and mixed the hotsand thoroughly until the melted wax gave the sand particlesan even darker colour. They estimated the wax coatinguniformity by visually inspecting the uniformity of the sandcolour. The most uniform coatings were obtained for sandsthat had been washed clean of silt and clay particles. Basedon this careful manufacturing, Bardet et al. (2014) calculatedthe average thickness of wax coating after assuming that thewax coating was uniform and the sand particles werespherical. Their main objective was to obtain an approximatethickness of wax coatings and to show that wax coatingseven as thin as 0.01 to 0.04 mm were sufficient to influencesignificantly the mechanical and hydraulic properties ofsands.Bardet et al. (2014) selected a few industrial petroleumwaxes for their adhesion properties. Industrial waxes areengineered to meet the demands of various applications(Krupa & Luyt, 2000, 2001; Sasol Wax, 2014). They areused to improve the resistance of inks and coated films torubbing, scuffing and scratching. In the corrugated boardand paper industry, wax coatings prevent the transmission ofmoisture, microorganisms and odours. They are used forpackaging food, for example, fruit, fish, meat, vegetablesand sweets, as well as non-food articles. They protectvarious metals against corrosion, and tyres and rubbersagainst ultraviolet light and ozone. The construction industryutilises waxes to cure concrete surfaces evenly and enhancethe insulation properties of building materials.Compared to natural waxes and organic hydrophobic com-pounds, industrial and polymeric waxes have engineeredproperties that are designed for particular applications. It isreasonable to conclude that the wax-coated sands that aremanufactured in the laboratory with engineered waxes arelikely to have more stable hydrophobic coatings than natu-rally occurring water-repellent soils that result from highlyvariable environmental processes.AMOUNT OF WATER REPELLENCYThe direct measurement of contact angles on an individualsand particle is almost impossible by conventional methods.The wax-coated sand was compressed into a flat cake towhich water drops were applied. However, the contact anglewas not measured precisely using the modified sessile drop-let method (Bachmann et al., 2000), water droplet penetra-tion time method (Washburn, 1921) and other methods(Yuan & Lee, 2013). The effects of water repellency onpermeability and compressibility are definitely worthy offurther investigation.STABILITY OF WATER REPELLENCYStability of water repellency is an issue relevant to horseracing, reminiscent of the problem that Finley (2010) re-ported for the Santa Anita racetrack in California. In 2007,Santa Anita replaced its one-mile course with a springy newsynthetic surface, specially designed with a new microcrys-talline wax that could withstand the hot temperatures inSouthern California. Four months later, after the first heavyrains in December, the synthetic material had become un-stable owing to drainage problems, and had transformed intoa muddy field inoperable for horse racing.The wax-coated sands that are manufactured with engi-neered waxes may be more stable than naturally occurringwater-repellent soils, but are not without stability problems.The wax coating is beneficial when it adheres to sand grains,but has an adverse effect when it separates from sand grains,migrates downward and clogs drainage pores. Bardet et al.(2009) observed the wax saturation of drainage pores inhorse-racing tracks after the wax had melted down owing tohigh temperatures, migrated down the track profiles andclogged the pores of lower sand layers. This wax saturationis also observed in the laboratory; wax may fill the pores ofporous stones in the consolidation testing apparatus.There is a need for additional research to improve thestability and adhesion of hydrophobic coatings on sandparticles, and the durability of wax coating under repetitiveloading conditions.WATER REPELLENCY AND PERMEABILITYA priori water repellency and permeability appear to beincompatible properties, and one could question the meaningof measuring the saturated permeability of soils that repelwater. However, from another point of view, this question issurprising because synthetic surfaces have been praised forspeedy drainage characteristics, allowing horse races toresume shortly after heavy rains.Figure 16 shows the laboratory experimental set-up thatBardet et al. (2014) used to measure the permeability of wax-coated sands. The specimen is confined between two porousθΘ WaterWax-coated sandFig. 15. The contact angle Ł in Bardet et al. (2014) is related tothe contact angle ¨ used in the physics and chemistry ofinterfaces, for example, Butt et al. (2003)PorousdiscsSoil sampleGraduateBurettestandNo displacementOverflowDatumhFig. 16. Laboratory experimental set-up for measuring per-meability of water-repellent soilsDISCUSSION 753stones in a fixed-ring consolidation cell (e.g. Bardet, 1997)and the upper porous stone is prevented from moving upward.At first, the burette is filled with water and its contentsdischarged several times through the bottom porous stone tosaturate the specimen. Once the specimen is saturated, thepermeability coefficient k is calculated by measuring theheight h of the water column in the burette at several timeintervals. The results obtained from this experiment, which issimilar to the falling head permeability test (e.g. Bardet,1997), were repeated multiple times and found to be reliableand consistent. The measured values of k almost tripled whenthe wax content increased from 0% to 6%.Figure 17 shows the typical profile of a synthetic surface.The top layer that is 15–20 cm thick is made of wax-coatedsand or similar products, and rests on top of a 5–7 cm thicklayer of coarse sand that has no wax coating. A syntheticmesh lies below the sand layers on top of a 15 cm thickcrushed stone layer with excellent drainage properties. Alllayers rest on an impervious membrane connected to adrainage collector system. Fig. 17 also shows the experimen-tal set-up that was designed to measure the permeability ofthe top layer in the field. The field experiment consists of(a) pushing a cylindrical tube downward through the surface,(b) filling it with water and (c) measuring the time requiredto discharge the water column through the synthetic surface.The test estimates the time required for rainwater to infil-trate the layer. The time t required for a water column ofheight h to drain completely ist ¼ L=k log(1þ h=L) (31)where k is the permeability of the top layer and L is itsthickness. For normal ranges of operation, k is larger than0.01 cm/s; it takes less than 6 min for 4 cm of water todrain through a 15 cm thick layer. The speedy drainage ofsynthetic surfaces may be counterintuitive because they aremade of water-repellent materials, but field observationsand laboratory experiments give similar results; permeabil-ity increases, rather than decreases with wax content andwater repellency. In contrast to natural soil surfaces thatmay have low-permeability hydrophobic crusts (e.g. Con-treras et al., 2008), synthetic surfaces have porous surfacesthat are often raked to erase hoof prints and to loosensynthetic materials.CONCLUSIONThere are similarities and differences between the water-repellent soils that are found in the natural environment andthose that are manufactured with clean sands, industrialwaxes, polymers and petroleum derivatives. Naturally oc-curring soils have hydrophobic properties that are attributedto organic substances released by plants and micro-organ-isms with or without the effects of fires. Natural water-repellent soils contain fine-grained soil particles and canprevent or delay water infiltration. In contrast synthetic soilsare usually made of coarser grains with a uniform grain sizedistribution and have engineered coatings that are more evenand certainly more stable than their natural counterparts.Their inherent porosity and surrounding drainage systemsmake them highly permeable. Additional research at thecrossroads of soil science, hydrology and geotechnical en-gineering is definitely warranted to explore the effects ofhydrophobic coatings on soil behaviours.REFERENCESBachmann, J. R., Horton, R. R., van der Ploeg, R. R. & Woche, S.(2000). Modified sessile drop method for assessing initial soil-water contact angle of sandy soil. Soil Sci. Soc. Am. J. 64, No.2, 564–567.Bardet, J. P. (1997). Experimental soil mechanics. Upper SaddleRiver, NJ, USA: Prentice-Hall.Top synthetic surface(15–20 cm thick)Drainage sand(5–7 cm thick)Crushed rocks(15 cm thick)Drainage pipe collectorImperviousmembranePorous meshCylindrical tubeWatercolumnLhFig. 17. Typical vertical profile of synthetic surfaces of horse-racing tracks and field test formeasuring permeability of synthetic surfaces754 LOURENC¸O, BARDET, JESMANI AND JABBARIBardet, J. P., Benazza, C., Bruchon, J. F. & Mishra, M. (2009).Mechanical and hydraulic properties of wax-coated sands forsport surfaces. In Powders and grains proceedings of the 6thinternational conference on micromechanics of granular media(eds M. Nakagawa and S. Luding), vol. 1145, pp. 792–795.Melville, NY, USA: AIP Publishing.Bardet, J. P., Jesmani, M. & Jabbari, N. (2014). Permeability andcompressibility of wax-coated sands. Ge´otechnique 64, No. 5,341–350, http://dx.doi.org/10.1680/geot.13.P.118.Butt, H.-J., Graf, K. & Kappl, M. (2003). Contact angle phenomenaand wetting. In Physics and chemistry of interfaces, pp. 118–144. Weinheim, Germany: Wiley.Butt, H.-J., Graf, K. & Kappl, M. (2006). Physics and chemistry ofinterfaces, 2nd edn. Weinheim, Germany: Wiley.Cheng, S., Bryant, R., Doerr, S. H., Wright, C. J. & Williams, R.(2009). Investigation of physico-chemical surface properties ofsoil particles and model materials with contrasting hydrophobi-city using atomic force microscopy. Environ. Sci. Technol. 43,No. 17, 6500–6506.Contreras, S., Canto´n, Y. & Sole´-Benet, A. (2008). Sieving crustsand macrofaunal activity control soil water repellency in semiarid environments: Evidences from SE Spain. Geoderma 145,No. 3–4, 252–258.Dekker, L. W. & Ritsema, C. J. (2000). Wetting patterns andmoisture variability in water repellent Dutch soils. J. Hydrology231, No. 232, 148–164.Doerr, S. H. & Moody, J. A. (2004). Hydrological effects of soil waterrepellency: on spatial and temporal uncertainties. Hydrol. Pro-cesses, Special Issue: Hydrogeomorphology 18, No. 4, 829–832.Doerr, S. H. R. A., Shakesby, R. & Walsh, P. D. (2000). Soil waterrepellency: its causes, characteristics and hydro-geomorphologi-cal significance. Earth Sci. Rev. 51, No. 1, 33–65.Finley, B. (2010). Ground control, the (real) truth about syntheticsurfaces. Thoroughbred Daily News (TDN) Magazine, Winterissue, see http://www.thoroughbreddailynews.com/restricted/pdf/magazine/GroundControl.pdf (accessed 26/06/2014).Krupa, I. & Luyt, A. S. (2000). Thermal properties of uncross-linked and cross-linked LLDPE/wax blends. Polymer Degrada-tion Stability 70, No. 1, 111–117.Krupa, I. & Luyt, A. S. (2001). Mechanical properties of uncros-slinked and crosslinked linear low-density polyethylene/waxblends. J. Appl. Polymer Sci. 81, No. 4, 973–980.Leighton-Boyce, G. S., Doerr, H., Shakesby, R. A. & Walsh, R. P.D. (2007). Quantifying the impact of soil water repellency onoverland flow generation and erosion: a new approach usingrainfall simulation and wetting agent on in situ soil. Hydrol.Processes, Special Issue: Soil Water Repellency: Origin, Envir-onmental Controls and Hydrological Impacts 21, No. 17, 2337–2345.Moody, J. & Ebel, B. A. (2014). 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Permeability and compressibility of wax-coated sands

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