This study conducted by University of North Carolina, Charlotte, used WaterRepel 201 as the Organo-Silane product. WaterRepel 201 is a product of L&Q International 703-299-9575 www.l-q-international.com Geotechnical Controls on Organo-Silane M Modification of Soils Matthew Keatts 1, John Daniels 2 1 Graduate Research Assistant, UNC Charlotte, Dept. of Civil and Environmental Engineering, 9201 University City Blvd, Charlotte, NC 28223 2 Associate Professor, UNC Charlotte, Dept. of Civil and Environmental Engineering, 9201 University City Blvd, Charlotte, NC 28223 Introduction Infiltration of water into soil, coal ash and solid waste landfills can be the root cause of many geotechnical and geoenvironmental engineering problems including loss of soil strength, leachate generation, groundwater contamination, and slope stability failure. By rendering a soil hydrophobic (or water-repellent ) through Organo-Silane (OS) surface modification these problems and others can be addressed. Hydrophobic soil could be used in a similar fashion to geosynthetic clay liners or geomembranes as an infiltration barrier, but data are needed to support geotechnical engineering design with this technology. Purpose The goal of this research is to develop the relationship between soil density (compaction), OS treated soil hydrophobicity (contact angle), and water infiltration pressure resistance (water entry pressure). Establishing this relationship will permit engineering design using the three parameters in question.
Materials Organfunctional Silanes (OS) Based on Potassium Methyl Siliconate Clear and initially water soluble Does not form film; permanently modifies surface (Daniels, 2009) Does not bind particles together Covalent bonding mechanism (grafting) Renders surface water repellent (hydrophobic) Unmodified Surface OS Modified Surface Hydrophobic Barrier Hydrophilic Surface OH- OH- OH- O Si O Si O Si O O Si O Si O Si O Silica Substrate Particle Surface Organo Silane Treatment C n H n+1 C n H n+1 C n H n+1 Si Si Si O Si O Si O Si O O Si O Si O Si O Silica Substrate Ottawa Sand Selected for consistent particle morphology Larger particle size (d 10 = 0.44mm) 100% silica composition Coal Fly Ash (CFA) Selected as a test material to address current beneficial reuse challenges Smaller particle size (d 10 = 0.0025mm) Approximately 57% silica composition from x-ray fluorescence (XRF) analysis
Methods Sample Preparation Samples were prepared by mixing soil with OS solution Varying degrees of hydrophobicity (contact angles) were achieved by adjusting treatment levels Samples oven dried for 48 hours at 60 C (140 F) Samples were processed by passing through #20 sieve to remove aggregations Contact Angle (θ) Measurement Contact Angle measurements were performed using a goniometer and the sessile drop technique (Bachmann, 2000) Sample materials were applied to double sided tape affixed to glass slides to form a monolayer of treated soil particles Slide was placed on level goniometer stand Drop of water was carefully applied to monolayer soil surface Contact angle was measured where soil surface and drop surface meet using a digital microscope with 200x magnification and image analysis software A minimum of 10 contact angle measurements were performed per slide Water Entry Pressure (WEP) Procedure Soil samples were prepared for WEP testing by compacting to a target density within a custom infiltration cell Three different target densities were used for each material The water entry pressure of hydrophobic soils was measured using a standpipe capable of SOIL applying a positive infiltration pressure to the compacted samples Initiation of infiltration was determined by visual inspection of the sample, a decrease in pressure head, or both WATER h w (WEP)
Results Contact Angle (θ) Measurement The contact angle measures surface energies at interface of three phase system with solid, liquid and gas (vapor) Idealized Contact Angles Hydrophilic Hydrophobic θ < 90 θ > 90 Vapor Vapor Liquid θ Liquid θ Solid Solid Contact angles for Ottawa sand ranged from 90 to 125 Contact Angles for CFA ranged from 90 to 145
Results Contact Angle (θ) vs. Treatment Ratios 135 θ vs. Treatment Ratio (Sand) Contact Angle, θ (deg) 115 95 75 150 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 Treatment Ratio (mg OS 2 /kg Sand) θ vs. Treatment Ratio (CFA) Contact Angle, θ (deg) 130 110 90 20000 30000 40000 50000 60000 70000 Treatment Ratio (mg OS 1 /kg CFA)
Water Entry Pressure (WEP) vs. Contact Angle (θ) vs. Density (ρ) WEP (cm of H 2 0) 16 12 8 4 WEP vs. θ vs. ρ (Sand) ρ = 1.76 g/cm 3 ρ = 1.66 g/cm 3 ρ = 1.61 g/cm 3 0 90 100 110 120 Contact Angle, θ (deg.) 600 WEP vs. θ vs. ρ (CFA) ρ = 1.22 g/cm 3 WEP (cm of H 2 0) 400 200 ρ = 1.12 g/cm 3 ρ = 1.02 g/cm 3 0 100 110 120 130 140 150 160 Contact Angle, θ (deg.)
Results Predicted Water Entry Pressures Used a variety of estimated pore diameters to predict WEP A pore size distribution was developed from the Soil Water Characteristic Curve (SWCC) (Lu and Likos, 2004) D LRG Largest pore diameter from SWCC estimation Two estimated pore diameters were developed using simple cubic and tetrahedral packing of spheres D SC Loosest possible arrangement of spheres (simple cubic) D TH Tightest possible arrangement of spheres (tetrahedral) Washburn equation for capillary rise used to predict WEP 4 TTcos θθ WWWWWW = dd where, d = estimated pore diameter and T = surface tension of wetting liquid 800 Predicted WEP WEP (cm of H 2 0) 600 400 200 0 90 110 130 150 Contact Angle, ϴ (deg.) DTH DLRG DSC WEP (Dense) WEP (Med) WEP (Loose)
Conclusions Treatment ratios can be optimized to achieve water repellency (θ) with the least amount of OS necessary to achieve maximum hydrophobicity Unsaturated hydraulic conductivity at infiltration pressures less than the WEP is equal to zero If the WEP is exceeded and the hydrophobic soil is saturated hydraulic conductivity of treated material is similar to that of untreated Hydrophobic sand can withstand greater than 12cm of water pressure before infiltration is initiated Hydrophobic CFA can withstand greater than 500cm of water pressure before infiltration is initiated Both the degree of hydrophobicity (contact angle) and the level of compaction (density) have a positive correlation with increasing WEP Washburn model accurately predicts WEP of hydrophobic soil for maximum treatment level using a pore diameter equal to 0.41*D 10 for simple cubic packing and 0.15*D 10 for tetrahedral packing References Bachmann et. al. (2000). Development and application of a new sessile drop contact angle method to assess soil water repellency. Journal of Hydrology, 231-232, 66-75 Daniels et. al. (2009). Nano-scale organo-silane applications in geotechnical and geoenvironmental engineering. International Journal of Terraspace Science and Engineering 1, 1, 19-27 Lu, N. and Likos, W.J. (2004). Unsaturated Soil Mechanics. John Wiley & Sons, Inc., New Jersey, 556p. Acknowledgements I would like to thank the Energy Production & Infrastructure Center (EPIC) at UNC Charlotte for funding this research. For more information on this project please contact: Dr. John Daniels jodaniel@uncc.edu