Beckett, Christopher

Fourie, Andy B.

Toll, David

Velikova, Milena

Ward, Phil

English

Extreme climatic events in the 21st century threaten the resilience of eotechnical engineering structures. Low-permeability barriers are at a particularly high risk of inundation under flooding or cracking during droughts, compromising the barriers and permitting contamination of the surrounding ground.Water repellent (WR) granular soils present an innovative solution to this problem. Hydrophobisation can arise naturally (e.g. due to deposition of organic matter, treatment with wastewater or wildfire), but can also be achieved in laboratory conditions by chemical treatment, typically with dimethyldichlorosilane (DMDCS).Besides being resilient to volumetric change under changes in saturation, water repellent soils have been shown to slow down water and root penetration, thus making them a potential candidate for waste storage liners. However, the effect of water repellency on soil strength and permeability is not well understood

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     Edinburgh Research Explorer                                      Hydromechanical behaviour of water-repellent sandsCitation for published version:Velikova, M, Beckett, C, Toll, D, Fourie, AB & Ward, P 2018, 'Hydromechanical behaviour of water-repellentsands', 7th International Conference on Unsaturated Soils (UNSAT) 2018, Hong Kong, Hong Kong, 3/08/18.Link:Link to publication record in Edinburgh Research ExplorerDocument Version:Publisher's PDF, also known as Version of recordGeneral rightsCopyright for the publications made accessible via the Edinburgh Research Explorer is retained by the author(s)and / or other copyright owners and it is a condition of accessing these publications that users recognise andabide by the legal requirements associated with these rights.Take down policyThe University of Edinburgh has made every reasonable effort to ensure that Edinburgh Research Explorercontent complies with UK legislation. If you believe that the public display of this file breaches copyright pleasecontact openaccess@ed.ac.uk providing details, and we will remove access to the work immediately andinvestigate your claim.Download date: 17. May. 202210 -1 10 0 10 1 10 2 10 3Matric suction,  (kPa)0510152025303540Gravimetric water content, %Hydrophilic (drying)Hydrophilic (wetting)Water repellent (drying)Water repellent (wetting)040 352030 3040Suction (kPa) Normal stress (kPa)Peak shear strength (kPa)20 25602010800 15HydrophilicWater repellentHydrophilic (trend)Water repellent (trend)Hydromechanical behaviour of water-repellent sandsIntroduction: Extreme climatic events in the 21st century threaten the resilience of geotechnical engineering structures. Low-permeabilitybarriers are at a particularly high risk of inundation under flooding or cracking during droughts, compromising the barriers and permittingcontamination of the surrounding ground.Water repellent (WR) granular soils present an innovative solution to this problem. Hydrophobisation can arise naturally (e.g. due todeposition of organic matter, treatment with wastewater or wildfire), but can also be achieved in laboratory conditions by chemicaltreatment, typically with dimethyldichlorosilane (DMDCS).Besides being resilient to volumetric change under changes in saturation, water repellent soils have been shown to slow down water androot penetration, thus making them a potential candidate for waste storage liners. However, the effect of water repellency on soil strengthand permeability is not well understood.Geotechnical challenges• We must understand the hydromechanical behaviour of WRsoils if we are to use them as part of geotechnical structuresfaced with varying degrees of saturation.• Traditional unsaturated soils display wetting properties whereall contact angles are <900; water menisci are concave andsuction is positive (Figure 1a).• Water repellent (WR) surfaces are defined as those withwater-solid contact angles >90⁰. WR soils display convexmenisci: water pressures in such structures must be positive(Figure 1b). This phenomenon has yet to be demonstrated in alaboratory setting.Preliminary studies• We manufactured an artificial WR soil by mixing industrial sand with dimethyldichlorosilane (DMDCS). Drying and wetting retention curves were found for treated and untreated material using pressure plates.• Shear strengths of both materials were examined under a range of suctions and normal stresses• WR soil retention curves resembled hydrophilic counterparts but were displaced to higher suctions• No significant difference was found between the two materials’ behaviour in shear. We believe this to be false.Conclusions and new researchAxis translation techniques used in the preliminary study maskedrepellency’s effects by forcing water menisci to adopt concaveshapes. This new PhD project will examine the fundamentalbehaviour of WR soils and how they can be used in the field:• Understanding the formation and evolution of water meniscibetween WR particles• Understanding the mechanical and chemical properties ofDMDCS-treated sand and how these are affected by testingtechnique• Examining material repellency longevity in situ andenvironmental consequences of DMDCS treatmentA fascinating materialWR soils represent a broad range ofgeotechnical and agricultural challenges andopportunities. However, a myriad of questionssurrounding their use exist which cannot betackled by one group alone.As part of our project, we will offerlaboratories and groups samples of our WRsoil for free. Please contact us by scanning theQR code to the right or via the group emailaddresses given above.particles(b)particlesconcave meniscusconvex meniscus(a)Milena VelikovaPhD studentThe University of Edinburghmilena.velikova@ed.ac.ukDavid TollProfessorDurham Universityd.g.toll@dur.ac.ukAndy FourieProfessorThe University of Western Australiaandy.fourie@uwa.edu.auPhil WardSenior Research ScientistCSIROp.r.ward@csiro.auChris BeckettLecturer in Geotechnical EngineeringThe University of Edinburghchristopher.beckett@ed.ac.ukwater dropletFigure 1. Water menisci between silica spheres a) hydrophilic; b) water repellentFigure 2. Water retention curves (left) and unsaturated shear strengths (right) for treated and untreated granular soil

Hydromechanical behaviour of water-repellent sands

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Hydromechanical behaviour of water-repellent sands

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