Vadose Zone Journal | Advancing Critical Zone Science

Water-Entry Pressure and Friction Angle in an Artificially Synthesized Water-Repellent Silty SoilChangho Lee, Heui-Jean Yang, Tae Sup Yun,* Youngmin Choi, and Seongyeong YangWater-repellent soils possess unique hydraulic and mechanical behav-iors that confer large potential for their use in geotechnical applications because particle-scale surface-wettability characteristics significantly influ-ence macroscale manifestations. This study examined the hydraulic and mechanical behavior of an artificially created water-repellent silty soil with four different concentrations of a reactive organo-silane solution. A series of laboratory tests was performed that included measurements of water-droplet penetration time (WDPT), water-entry pressure (WEP), flow rate, and friction angle. Experimental results showed that the artificial treatment pro-duced a unique range of porosity values depending on the concentration and that the WDPT and WEP increased with decreasing porosity and increas-ing concentration. A gravimetric fraction of 40% water-repellent particles was sufficient for bulk soils to exhibit water repellency. The flow rate of speci-mens with a high concentration of reactive organo-silane tended to be high due to the resulting high degree of saturation on water permeation. In con-trast, friction angles tended to decrease with increasing concentration of organo-silane solution under dry conditions and remained quasi-constant on wetting, regardless of the degree of saturation.

Abbreviations: WDPT, water-droplet penetration time; WEP, water-entry pressure.

Water-repellent soils are ubiquitous throughout the world in humid, arid, and semiarid regions (Doerr et al., 2000). Their occurrence is related to natural circumstances, such as the presence of aliphatic compounds released from living plants and leaves, wildfire, the decomposition of litter, humus layers, and fungal hyphae, as well as anthropogenic factors, including hydrocarbon spills, the deposition of combustion products, and the cultivation of crops (DeBano et al., 1967; Doerr et al., 2000; Franco et al., 2000; Horn et al., 1964; Neinhuis and Barthlott, 1997). For in situ soils, water repellency extends from several centimeters to tens of centimeters below the surface with high heterogeneity and tends to decrease with depth (Barrett and Slaymaker, 1989; Dekker and Ritsema, 1994; Jungerius and de Jong, 1989).

Soil water repellency has significant impacts on the following: reduction of soil water infiltration capacity (Bachmann et al., 2007; Deurer and Bachmann, 2007; Hardie et al., 2012; Imeson et al., 1992; Ritsema et al., 1993), enhanced runoff, preferential surface flow, rain-splash detachment (Scott and Van Wyk, 1990; Terry and Shakesby, 1993; Walsh et al., 1994), accelerated soil erosion (Shakesby et al., 1993), and the suppression of water evaporation by reducing the capillary rise of water to the soil surface (DeBano, 1975; Hillel and Berliner, 1974). The methods available for characterizing water repellency include the measurements of water-droplet penetration time (WDPT), the molarity of ethanol droplets (MED), contact angles (e.g., capillary rise and sessile drop methods), and water-entry pres-sure (WEP). The WDPT is the time in which a water droplet fully penetrates into the soil, and the MED is the minimum ethanol concentration at which droplet penetration takes

The degree of water repellency and porosity determines the water-entry pressure and flow rate, and the resultant saturation upon water permeation varies with the initial porosity. The organo-silanes grafted on particle surfaces facili-tates mobilization, resulting in low frictional behavior.

C. Lee, Dep. of Marine and Civil Engi-neering, Chonnam National Univ., 50 Daehak-ro, Yeosu, Jeonnam, 550-749, Korea; H.-J. Yang and T.S. Yun, School of Civil and Environmental Engineering, Yonsei Univ., Yonsei-ro 50, Seodaemun-gu, Seoul, 120-749, Korea; Y. Choi, School of Civil, Environmen-tal, and Architectural Engineering, Korea Univ., 145 Anam-ro, Seong-buk-gu, Seoul, 136-701, Korea; and S. Yang, Samsung C&T Corporation, Daeryung Gangnam Tower, 826-20 Yeoksam1-Dong, Gangnam-Gu, Seoul, 135-935, Korea. *Corresponding author (taesup@yonsei.ac.kr).

Vadose Zone J. doi:10.2136/vzj2014.08.0106Received 13 Aug. 2014.Accepted 9 Jan. 2015.

Original Research

© Soil Science Society of America 5585 Guilford Rd., Madison, WI 53711 USA.

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place within a certain time. The WEP is the critical pressure at which water starts to displace air. These methods indirectly indicate repellency in terms of the persistence of hydrophobicity and appar-ent surface tension on porous surfaces. The WDPT tends to be high for soils with a high contact angle (e.g., water-repellent soil), while clay and organic matter contents also affect the measured WDPT. Yet relatively clean silty sand and silt, whose hydraulic conductivity is not as low as clay, exhibit considerable WDPTs once they become water repellent such that the WDPT can serve as an indirect indica-tor of the degree of water repellency (Leelamanie et al., 2008, 2010; Oostindie et al., 2013; Wessel, 1988). The roughness of soil particle surfaces and the pore size distribution are also important influences on the WDPT (Graber et al., 2006). In terms of the critical pressure to initiate water infiltration, the WEP becomes positive in water-repellent soil and its value depends on the porosity and bulk density of the soil due to changes in pore sizes (Carrillo et al., 1999; Letey, 1968; Wang et al., 2000).

Previous studies have focused on phenomenological investigations of natural water-repellent soils, whereas synthetic soils treated with artificial additives have been used to modulate hydromechanical soil properties in engineering practices, such as dust control, soil stabili-zation, erosion control, and capillary barriers (Bardet et al., 2011; Dell’Avanzi et al., 2010; Dong and Pamukcu, 2012; Subedi et al., 2013; Zhang et al., 2008). However, several dominant factors that influence the relevant phenomena have been less well investigated. Given the described uniqueness of water-repellent soils, we made a water-repellent silty soil with different concentrations of an artificial additive, and the samples were subjected to a series of hydraulic and geomechanical measurements. The characterization of the specimens created, with vary-ing concentration and porosity, allowed evaluation of the effect of artificial additives on in situ soil in terms of water repellency by measurements of the WDPT, WEP, flow rate, and friction angle.

6Materials and MethodsSynthesis of Water-Repellent SiltA sample of weathered granitic soil was collected from a near-surface horizon (<0.5-m depth) at An-Mountain, Seoul, Korea, in an area where rainfall-induced erosion is common. The grain-size distribution curve of the sample, constructed using the results of sieve and hydrometer tests, indicated that the sample was a well-graded silt, with a median grain size (d50) of 0.074 mm. The soil was classified as silt with low plasticity (ML), according to the Unified Soil Classification System (ASTM, 2011a).

The natural silt was submerged and washed in deionized water to remove organic matter such as plant roots and

leaves, followed by oven drying for 24 h. The dried sample was again subjected to mechanical stirring for 24 h to ensure sufficient dispersion in deionized water. The sample was then treated with different amounts of the reactive compound n-octyltriethoxysilane (Zycosoil, Zydex Industry) at concentrations of 0.01, 0.1, 1, 2.5, 5, and 10% (wsilane/wwater) for 48 h under continuous stirring. This allowed H bonding between the soil particle surface and the hydroxyl group of silane during treatment. Afterward, the treated sample was washed with deionized water to remove any remaining reactive compound. This process imparts water repellency and has been successfully used in other studies that have characterized the mechanical, thermal, and electrical properties of water-repellent soils (Kim et al., 2011, 2013; Truong et al., 2011).

The soil samples were oven dried at 100°C for 24 h before each experiment. Note that the water repellency of oven-dried soils tends to be greater than that of air-dried and field-moist soils while the same drying condition within the convection oven can mini-mize biased outcomes due to variations in laboratory humidity conditions (Dekker et al., 2009; Doerr, 1998). The effect of the additive concentrations is visually highlighted in Fig. 1, showing scanning electron microscopic images of glass slides simultaneously

Fig. 1. Scanning electron microscopic images (plan and side views) of treated glass slides with different concentrations (C) of n-octyltriethoxysilane.

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treated with soil specimens at the given concentrations. As the concentration increases, silanes form amalgamated structures extending the height from tens of nanometers to hundreds. Note that the grafted pattern on the glass slide may not be identical to that on the silt specimen, while the concentration-dependent morphology of addition seems apparent.

MethodsFour experiments were conducted to investigate the degree of the water-repellency effect by measuring the WDPT, WEP, flow rate, and friction angle, as described below.

Water-Droplet Penetration TimeThe specimens were placed in cylindrical containers (35 mm in diameter and 17 mm in height) by tamping to achieve a range of porosity from loose to dense conditions (Fig. 2a). A 50-mL water droplet was placed on the specimen surface by a micropipette (Axygen, AXAP-100). The containers were then placed inside a sealed container containing 26% NaCl solution at the bottom to maintain a relatively constant humidity of ?75% and to avoid evaporation of the water droplet (Wexler and Hasegawa, 1954). Two-dimensional images of the specimens were captured every 2 min until all of the water droplet had completely penetrated into the soil; the elapsed time was taken as the WDPT. In addition, we mixed the artificially created soil with untreated soil in vary-ing weight fractions and repeated the WDPT test to explore the critical fraction that imposes overall water repellency in mixtures.

Water-Entry PressureThe WEP was measured by placing the soil samples in cylindrical cells made of polytetrafluoroethylene (PTFE); the PTFE walls prevent preferential water infiltration between the specimen and the wall (Carrillo et al., 1999). The cells (30 mm in inner diameter and 120 mm in height) contained two electrodes and a filter paper at the bottom (Fig. 2b). The cells were filled with soil and were then tapped to achieve a range of porosity from loose to dense conditions with a constant soil height of ?60 mm (Wang et al., 2000). The resul-tant porosity values for each concentration were not purposefully targeted, although similar tapping energies were applied during sample preparation to manipulate the poros-ity of the specimens. Hydrostatic pressure was then applied from the top at a constant rate (?0.2 kPa/s) by using a pressure panel (Trautwein, M100000) with a compressor. Carrillo et al. (1999) suggested that the WEP be measured at the moment water begins permeating into the soil, although the moment of initial penetration was dif-ficult to capture. Therefore, the WEP was

determined as the corresponding hydrostatic pressure when the sharp decrease in resistance was captured, attributed to the perme-ated water wetting the filter paper resting on the electrodes. Note that the measured WEP values in this study were systematically higher than the actual pressure necessary to induce water entry and served as qualitative indicators to capture porosity and con-centration effects rather than quantitative values. The tests were repeated for specimens with varying porosities and concentrations of reactive solution.

Flow RateThe flow continued for a while after the WEP was reached, and the collected volume of effluent was computed with time. This allowed the flow rate to be computed for the given specimen condi-tions once a constant flow rate was achieved (i.e., the flow became equilibrated). The specimen was then disassembled to measure the resulting degree of saturation.

Friction AngleThe shear strength of the treated specimens was measured by using a direct shear device (KD Precision Co. Ltd.) following the desig-nated testing procedure (ASTM, 2011b). Specimens were placed in cylindrical shear boxes (60 mm in inner diameter and 20 mm in height). For a given solute concentration, the porosity of the dry specimen was controlled by tamping to achieve a constant value. Specimens with the same solute concentration were independently compressed at four different effective stresses (100, 200, 300, and 400 kPa) for consolidation, followed by horizontal shearing at a rate of 1 mm/min. The friction angles at peak stress and large strain (16% in this study) were obtained from horizontal stress displacement. For partially saturated specimens, a predetermined weight of water was thoroughly mixed with the dry soil to vary

Fig. 2. Experimental configurations to determine (a) water droplet penetration time (WDPT), where NaCl solution resides in the sealed container to avoid evaporation of the water drop placed on the soil surface during the test; and (b) water-entry pressure (WEP) measurement, in which the hydrostatic pressure is gradually increased from the top until the resistance measured by electrodes at the bottom sharply decreases.

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the degree of saturation. The mixture was then placed within the shear box. An accurate saturation value was calculated again after the test by measuring the water weight. Note that friction angles serve as important design parameters for geotechnical structures such as retaining walls and in assessments of the bearing capacities of foundations, embankments, and slopes.

6Results and DiscussionWater-Droplet Penetration TimeFigure 3 presents sequential images of a water droplet placed on a specimen with a porosity of 0.476 and treated with a reactant solution at a concentration of 2.5%. The red dotted line denotes the initial boundary of the droplet. The size of the water droplet decreased in the early stages but retained its original shape until ?180 min; subsequently, both the contact angle and contact area gradually decreased. The contact angles of each specimen at the ini-tial stage were obtained from the two-dimensional snapshots. Figure 4 shows that the average contact angle values tended to increase with increasing reactive solution concentration in spite of the variance and that all obtained contact angles were >90°. Note that the error bars indicate the range of porosity for the given concentration ratios.

The WDPT values obtained for specimens with different solution concentrations and porosities are plotted in Fig. 5. Because the specimens with concentrations of 0.01 and 0.1% exhibited <1 to 2 s of penetration time, they were not included. The measured values were subjected to multiple linear regression analysis with variables of concentration C and porosity n, and the estimated values were

plotted with open symbols. It would be reasonable to conclude that the WDPT tended to increase with decreasing porosity, with a less pronounced effect of concentration. Note that the WDPT values in this study are an order of magnitude higher than those categorized as extremely water repellent (e.g., WDPT >60 min) for in situ soil in the literature (Bisdom et al., 1993; Doerr et al., 2000).

The WDPT values for the mixture specimens (i.e., artificially cre-ated silt mixed with untreated silt, C = 5%, n = 0.409) are shown in Fig. 6. The shadowed area indicates the “extremely water-repel-lent” region. As the fraction of water-repellent soil decreased, the WDPT gradually decreased, and at least 40% water-repellent soil was required for the bulk soil to behave as water-repellent soil. The specimen with 20% water-repellent soil shows <1 min of WDPT

Fig. 3. Optical observation of water droplet penetration into soil with time (n-octyltriethoxysilane concentration C = 2.5%, porosity n = 0.476).

Fig. 4. Contact angles measured on the soil surface for different n-octyltriethoxysilane concentration treatments. Error bars indicate the range of porosity for each concentration. As the concentration increases, the contact angle tends to increase.

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and behaved similarly to a wettable soil. Thus, the critical fraction of 40% water-repellent soil determined the overall response of the bulk soil to the wetting condition.

Water-Entry PressureFigure 7a shows typical results of the pressure and resistance response with time for a soil treated at a concentration of 2.5% with porosity of n = 0.558. The hydrostatic pressure applied at the soil surface gradually increased (?0.2 kPa/s), whereas the resis-tance measured at the bottom of the soil remained constant at the initial stage, showing that the bottom soil was dry. A sudden decrease in resistance occurred as the water permeated into the filter paper in contact with the electrodes; the corresponding pres-sure was determined as the WEP. The pressure obtained in this way does not necessarily correspond to the pressure at which the water began penetrating the soil surface. As expected, the WEP values linearly increased with the specimen height, yet this systematic procedure (i.e., the same specimen height and pressure increment rate) for measuring the WEP ensures consistency among results

for each test. The data collected for varying porosities and reac-tive concentrations are plotted in Fig. 7b. Specimens treated with the same concentration of solution are closely clustered within a similar range of porosity, and overall WEP values are inversely pro-portional to porosity on a semi-logarithmic plot. It appears that specimens subjected to high concentrations of reactive solution exhibit high WEP values, whereas those subjected to low concen-trations exhibit low WEP values. Furthermore, all measured values tend to follow a single trend line regardless of concentration in Fig. 7b, similar to the observation from the WDPT.

Figure 8 presents the measured WEP values with the concen-trations of reactive solution. Although the WEP values at each concentration show a range of data with the porosity of the specimen, the average WEP values increase with increasing concen-tration. Note that the error bars denote the range of porosity. This observation leads to the conclusion that the additive concentration naturally groups the range of porosity and induces the increase of the WEP. It is corroborated by previous studies showing that WEP

Fig. 5. The measured water-droplet penetration time (WDPT; solid symbols) of water-repellent soil samples with various n-octyl-triethoxysilane concentration (C) and porosity (n). The samples were subjected to multiple linear regression analysis and the estimated val-ues are superimposed (open symbols).

Fig. 6. As the fraction of water-repellent soil decreases in the mixtures with untreated soil (n-octyltriethoxysilane concentration C = 5%, porosity n = 0.409), the water-droplet penetration time (WDPT) decreases. The shadow area indicates the extremely repellent region whose WDPT is >60 min (Bisdom et al. 1993).

Fig. 7. (a) Determination of water entry pressure (n-octyltriethoxysilane concentration C = 2.5%, porosity n = 0.558). The resistance suddenly drops when permeated water wets the filter paper, and the corresponding value of applied pressure is determined as the water-entry pressure (WEP); (b) the WEP linearly increases with the specimen height (C = 10%, n = 0.41).

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is controlled mainly by the contact angle, pore size, porosity, and soil structure (Annaka and Hanayama, 2005; Ritsema and Dekker, 2003; Wang et al., 2000). It is notable that the WDPT is mainly determined by the porosity, while both porosity and concentra-tion govern the WEP. The linearity between WEP and specimen height provides indirect evidence that flow rates in the soils are constant and that no further wetting occurs in a water-repellent soil once water reaches the bottom of the soil column (Dekker and Ritsema, 1994).

Specimens disassembled after the WEP test showed that permeat-ing water rarely wet loose soil that had a saturation of ?15% (see Fig. 9a and 9c for the case of C = 2.5%). In loose soil, water forms preferential flow paths, and successive injections of water may not permeate into void spaces, which results in fairly dry and crumbly conditions. In contrast, dense soils became almost fully saturated (Fig. 9b and 9c). This was presumably attributed to dense specimens having a high WEP that may disseminate the water path network while it resists the hydrostatic pressure. Similar observations were

consistently made on other specimens, highlighting porosity as the key parameter determining wetting conditions on water permeation.

Water is supposed to eventually permeate into the soil even below the WEP, in agreement with the WDPT results presented in Fig. 3. Thus, the specimens (C = 10% and n = 0.411) were subjected to hydrostatic pressure lower than the corresponding WEP (e.g., 49.9 kPa) to evaluate how long the soil resists water permeation under a constant pressure. Figure 10 shows that the elapsed time until water began to penetrate and the measured resistance sharply dropped under a constant hydrostatic pressure below the WEP. The elapsed time sharply increased as the applied pressure decreased. This result suggests that water-repellent soils can resist a certain level of hydrostatic pressure for a period of time and indicates the possibility of creating a temporary hydro-barrier by implementing water repellency in a soil.

Flow Rates under Steady-State ConditionsThe initial porosity determines the degree of saturation, as shown in Fig. 9, which in turn determines the permeability in partially saturated soil (i.e., permeability tends to decrease as the degree of saturation and porosity decrease). Therefore, the computed flow rate through water-repellent soil should be influenced by the degree of saturation. Because Darcy’s law is valid in saturated soils, the effec-tive flow rate can be computed from the obtained flow rate divided by the hydraulic gradient and cross-sectional area of a specimen, followed by multiplication by the saturation (Fig. 11). Effective flow rate values for saturated silty soils were estimated using the Kozeny–Carman equation (Carman 1956; Kozeny 1927):

( )

2350

2 1801

dg nKn

r=

m - [1]

where r is a particle density (kg/m3), g is gravitational acceleration (m/s2), m is viscosity (Pa s), n is porosity, and d50 is the median parti-

cle size (e.g., 0.074 mm in this study). These estimated values are shown in Fig. 11 as a dashed line. The measured effective flow rate ranged from 10-5 to 10-3 m/s, regardless of concentra-tion. The f low rates in specimens treated with 10% solute concentra-tion coincide with the value (dashed line) estimated by Eq. [1], presumably because they were close to a fully sat-urated condition (i.e., they are similar to untreated specimens whose initial conditions are fully saturated). In contrast, specimens treated with solution concentrations of 1, 2.5, and 5% plot below the dashed line, as the degree of saturation is in the range of 7 to 80%, regardless of porosity. The

Fig. 8. Increase in water-entry pressure with increasing solute concen-tration. Error bars denote the range of specimen porosity.

Fig. 9. Disassembled specimens for n-octyltriethoxysilane concentration C = 2.5% after the water-entry pressure experiment: (a) loose condition (porosity n = 0.596, saturation S = 15%) and (b) dense condition (n = 0.536, S = 95%); and (c) saturation profiles for the two specimens. The loose specimen exhibits much lower saturation along the height than the dense one.

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increase in solute concentration induces two results: (i) a decrease in porosity; and (ii) an increase in saturation. Therefore, the clustered data points for each concentration should move toward the top left side, getting close to the dashed line, as the concentration increases.

Friction AnglesFigure 12a shows typical shear stress–strain responses from the direct shear test for soils treated with a solution of C = 2.5% under dry conditions. The shear stress t increases with the horizontal strain and reaches a maximum or peak stress, followed by quasi-constant residual states over 10% of horizontal strain. The value of the maximum shear stress allows an estimation to be made of the peak friction angle (f¢), which represents both particle inter-locking (dy/dx) and sliding friction resistance between particles (m) (Taylor, 1948):

peak dtan

dyx

t æ ö÷ç¢= f =m+ ÷ç ÷ç¢ è øs [2]

where s¢ is vertical stress and dx and dy are the incremental hori-zontal and vertical displacements, respectively. Figure 12b presents the relationship between the vertical stress and the shear stress for all tested specimens. No cohesion (i.e., y intercept = 0) was assumed because the specimens were classified as ML (Mitchell and Soga, 2005). The computed values of both the peak and large strain friction angles under a dry condition are plotted in Fig. 12c. The peak friction angle tended to decrease with increasing concentrations of the reactive solution, presumably because the grafted alkyl siloxane facilitates particle mobilization and reduces resistance to internal rolling and sliding among particles. However, this effect diminished as the specimens experienced shear strains larger than peak strains, such that the large strain friction angle, computed at 16% of the horizontal strain, remained quasi-constant regardless of the concentration (i.e., f¢large strain = ?36.5°). Figure 12d underscores the influence of the degree of saturation on both

wettable and water-repellent specimens (C = 5%) tested at similar initial porosities (n ? 0.433). Symbols of A and B in Fig. 12c and 12d indicate the same specimens. It is well known that the shear strength of a natural sand or silt depends on its density (i.e., the denser the soils, the higher the shear strength). Because the addition of water into a soil increases its dry unit weight during compaction, the water content tends to be an important factor affecting the shear strength of natural soils. Furthermore, menisci are formed when small amounts of water are present between par-ticles in wettable soils, and these increase the negative effective stress (i.e., the effective stress between particles increases), which results in an increase in the shear strength of wettable soils (Cho and Santamarina 2001). In contrast, Bardet et al. (2011) suggested that the friction angle of water-repellent soil is less susceptible to changes in water content based on experimental results. As shown in Fig. 12d, the peak friction angle for untreated speci-mens decreased with increasing saturation, although specimens of C = 5% show quasi-constant friction angle values regardless of saturation. The presence of water facilitates the mobilization of particles and induces densification during the shearing of wettable soils, whereas its effect on water-repellent soils is nominal due to preexisting silanes that overwhelm the role of water. Kim et al. (2013) also suggested that the effects of the degree of saturation are minimized in hydrophobic sands, which is presumably attributed to the poorly defined menisci at interparticle contacts.

6Summary and ConclusionsEngineered water-repellent soils are potentially useful in geotech-nical engineering. The present study investigated the hydraulic and mechanical behavior of artificially synthesized water-repellent silty soil. Water-repellent silty soils were synthesized in the laboratory using six different concentrations of reactive solution (0.01, 0.1, 1, 2.5, 5, and 10% n-octyltriethoxysilane) and were subjected to a series of characterizations to measure the WDPT, WEP, flow

Fig. 10. Elapsed time in until water began to penetrate and the mea-sured resistance sharply dropped under constant hydrostatic pressure below the water-entry pressure (WEP) (n-octyltriethoxysilane con-centration C = 10%, porosity n = 0.411).

Fig. 11. Permeability as affected by porosity and n-octyltriethoxysilane concentration. Note that the y axis is the permeability k multiplied by the degree of saturation S. Estimation by the Kozeny–Carman equa-tion (dashed line) was made based on a 100% saturated condition so that the highly saturated specimens (e.g., 10% concentration speci-mens in Fig. 8) are close to the dashed line.

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rate, and friction angle. As the concentration of reactive solu-tion increased, the resultant porosity of the specimens tended to decrease. Both concentration and porosity highly influenced the WEP results, while the effect of concentration on the WDPT was less pronounced than the effect of porosity. For mixtures of artificially synthesized and untreated soils, at least 40% water-repellent soil is required to impose overall water repellency in the bulk soil. The porosity becomes critical to the water perme-ation and its consequence on saturation. The denser specimens exhibited a higher degree of saturation than the loose ones that formed preferential flow. This effect was, in turn, reflected by the measured flow rate such that the specimens with high solute con-centrations tended to have high flow rates. The peak friction angle decreased with increasing concentrations of treatment solutions under dry conditions without significantly influencing the large strain friction angle. However, once the permeation of water is initiated, the water works as a lubricant on particle surfaces, result-ing in a decrease in the friction angle. In summary, the combined effect of reactive solution concentration and the resulting poros-ity significantly control the hydraulic and mechanical behaviors of artificially created silty soil. These observations may be useful for predicting or estimating the engineering behavior of naturally occurring water-repellent soils (e.g., organically contaminated soils or shallow fire-exposed soils) and may lead to baseline guidelines for the establishment of engineering design parameters.

AcknowledgmentsThis work was supported by the National Research Foundation of Korea (NRF) and Korea CCS R&D Center (KCRC) grants funded by the Korean government (no. 2011-0030040, 2011-0022883, 2013035972), and was supported by a grant (14-RDRP-B076564-01) from Regional Development Research Program funded by Ministry of Land, Infrastructure and Transport of the Korean government. Changho Lee acknowledges the financial support of Samsung C&T Corporation.

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