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Error correction of water evaporation loss on the unsaturated hydraulic conductivity
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ScienceDirect Soils and Foundations 59 (2019) 2341–2347 www.elsevier.com/locate/sandf
Error correction of water evaporation loss on the unsaturated hydraulic conductivity Tiande Wen a, Longtan Shao b,⇑, Xiaoxia Guo b, Yanru Zhao a, Liping Huang c, Xiangsheng Chen a,c b
a College of Civil Engineering, Shenzhen University, Shenzhen 518060, China State Key Laboratory of Structural Analysis for Industrial Equipment, Department of Engineering Mechanics, Dalian University of Technology, Dalian 116024, China c Shenzhen Metro Group Co., Ltd., Shenzhen 518026, China
Received 5 December 2018; received in revised form 21 July 2019; accepted 22 August 2019 Available online 1 November 2019
Abstract Water evaporation loss occurs during multistep outflow experiments from the soil surface and the outflow vessel. The main focus of this paper is to introduce an improved pressure plate instrument that can correct for the error of water evaporation loss on the unsaturated hydraulic conductivity. Experiments using silicon micropowder (SMP) and Guangxi Guiping clay (GGC) were conducted with an improved instrument. The results showed that the measured value of the water content will be overestimated and the unsaturated hydraulic conductivity will be underestimated without considering the water evaporation loss. Ó 2019 Production and hosting by Elsevier B.V. on behalf of The Japanese Geotechnical Society.
Keywords: Unsaturated hydraulic conductivity; Water evaporation loss; Multistep outflow experiment
1. Introduction The hydraulic conductivity of unsaturated soil is a function of the matric suction or water content. Several standard test methods for measuring the unsaturated hydraulic conductivity are described in ASTM 7664 (2010). The unsaturated hydraulic conductivity is generally measured experimentally, or it is estimated from the soil water retention curve (SWRC). The original outflow method for measuring the hydraulic conductivity using a pressure plate instrument was proposed by Gardner (1956). The multistep outflow method is a transient measurement method for estimating the hydraulic conductivity in the laboratory. The major advanPeer review under responsibility of The Japanese Geotechnical Society. ⇑ Corresponding author. E-mail address: [email protected] (L. Shao).
tage of this method is that the matric suction can be precisely controlled, and the hydraulic conductivity and SWRC can be derived simultaneously. Kool et al. (1985) developed an inverse method to estimate the parameters of the SWRC from the multistep outflow experiment and to predict the hydraulic conductivity. Benson and Gribb (1997) presented a comprehensive review of the inverse modeling approaches used to estimate the hydraulic conductivity, including one-step and multistep outflow methods. The multistep outflow method has become one of the most promising and practical methods for estimating the hydraulic conductivity of unsaturated soil (Mualem, 1976; van Genuchten, 1980; Vereecken et al., 1997; Breitmeyer and Benson, 2011; Wayllace and Lu, 2012; Sadeghi et al., 2014; Chen et al., 2016; Shao et al., 2017; Zhang et al., 2017; Wen et al., 2018; Zhao et al., 2019). However, direct measurements of the unsaturated hydraulic conductivity and SWRC in the laboratory are
https://doi.org/10.1016/j.sandf.2019.08.015 0038-0806/Ó 2019 Production and hosting by Elsevier B.V. on behalf of The Japanese Geotechnical Society.
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time-consuming (short one week, long half year), since required time is different depending on the focusing matric suction range for the multistep outflow experiments (Nu¨tzmann et al., 1998; Chen et al., 1999; Gnatowski et al., 2010; Mboh et al., 2012; Lazrag et al., 2013; Wen et al., 2019). Leong and Rahardjo (1997) summarized that the change in water volume is difficult to measure accurately because of the water evaporation loss, and this loss affects the ability to accurately measure hydraulic conductivity. During multistep outflow experiments, water evaporation loss from the soil surface and the outflow vessel creates problems in measuring and predicting unsaturated hydraulic conductivity. This paper introduces an improved pressure plate instrument. With this instrument, water evaporation from the unsaturated soil surface in the pressure chamber can be ignored, and the water evaporation loss from the water outflow vessel can be compensated. The error correction of
water evaporation loss on hydraulic conductivity for silicon micropowder (SMP) and Guangxi Guiping clay (GGC) are also discussed. 2. Apparatus, materials and testing procedures 2.1. Apparatus The improved pressure plate instrument is composed of a pressure chamber, a ring cutter, a gas pressure controlling system, an exhaust gas flushing system, a water outflow vessel, a contrastive vessel, two balances and a computer (see Fig. 1). During the experiment, the top and the bottom of the soil specimen in the ring cutter are tightly attached to two ceramic disks. Increasing the gas pressure leads to elevated gas pressure in the system while maintaining a constant pore water pressure; air holes around the ring cutter guarantee that the gas pressure is applied around the soil
Fig. 1. A diagram of the experimental setup (Wen et al., 2019).
T. Wen et al. / Soils and Foundations 59 (2019) 2341–2347
specimen. Water is trapped in the soil using a breathable waterproof film. The chemical composition of this film is mainly polytetrafluoroethylene (PTFE), the thickness is 0.07 mm, which can block the passage of external water, and can also prevent the discharge of pore water in the soil inside the ring cutter. Therefore, the water evaporation loss from the soil in the pressure chamber can be ignored. The outflow vessel (30 mm in height and 80 mm in diameter) is covered except for a small hole (2 mm in diameter) to allow for the insertion of a needle. The real-time cumulative water volume of the outflow vessel is measured with a high-precision balance (accuracy: 0.001 g). Despite the presence of only a small hole, water evaporation loss from the outflow vessel will occur. Therefore, we used a contrastive vessel to measure the water evaporation loss at the same time as the water volume measurements from the outflow vessel using the second high-precision balance (see Fig. 1). The improved pressure chamber and the contrastive vessel constitute an evaporation compensation system. The water evaporation from the unsaturated soil surface in the pressure chamber can be ignored, and mass measurements from the contrastive vessel compensated for the water evaporation loss from the outflow vessel during the experimental period.
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Fig. 2. Grain size distribution of the tested soils.
rated with distilled water after vacuum seeding for approximately 2 h and then submerged for approximately 24 h. To prevent any disturbance to the samples during this process, the samples were covered both on the top and on the bottom with filter paper and water-permeable stone.
2.2. Soil specimens
2.3. Testing procedures
Silicon micropowder (SMP) is an artificial soil in China that is widely used in construction. The Guangxi Guiping clay (GGC) used in this study was taken from a deep excavation in Guiping City, Guangxi Province, China. The basic material properties of the soils were measured in the laboratory and are given in Table 1, and their grain size distributions are shown in Fig. 2. The soils were oven-dried for 48 h prior to the preparation of the recompacted soil specimens. According to the dry density required by the test scheme, a certain amount of sieved soil is weighted in the enamel pot, an appropriate amount of distilled water is sprayed and stirred evenly on the soil, and finally the water is evenly distributed into the soil sample. At the same time, a layer of vaseline should be evenly applied on the inner wall of the ring cutter, and it should be adopted to ensure uniform compaction of the soil sample. The samples for the pressure plate test were 20 mm in height and 54.7 mm in diameter. The samples were satu-
The pressure plate instrument is preprocessed in the following manner: (1) flush the bubbles from the exhaust tube and the water tube, and fill the tubes with air-free water; (2) saturate the ceramic disk (its air-entry value is 500 kPa), and measure the saturated hydraulic conductivity of the ceramic disk (1.20e-7 m/s); and (3) install the saturated soil specimen. The specimen was subjected to increasing matric suction in a series of steps to measure the gravimetric water content during drying. The matric suction path of SMP is [10 kPa, 20 kPa, 40 kPa, 60 kPa, 100 kPa, 160 kPa, 300 kPa]; the matric suction path of GGC is [10 kPa, 20 kPa, 40 kPa, 80 kPa, 110 kPa, 170 kPa, 250 kPa, 350 kPa, 450 kPa]. The saturated gravimetric water contents for SMP and GGC are 0.26 and 0.38, respectively. According to the SWRC, the residual gravimetric water content is 0.05 for SMP and 0.18 for GGC (Shao et al., 2017). Finally, the error correction of water evaporation loss on hydraulic conductivity for SMP and GGC are discussed. 3. Method
Table 1 Parameters of the tested soils. Soil
SMP
GGC
Specific gravity Initial dry density (kg/m3) Saturated hydraulic conductivity (m/s) Liquid limit Plastic limit Plasticity index
2.68 1600 1.09e-7 32.5% 20.3% 12.2%
2.74 1350 4.72e-9 51.8% 22.7% 29.1%
The measurement method for hydraulic conductivity presented by Shao et al. (2017) shows better agreement with the test data compared to the Gardner outflow analysis method and the van Genuchten-Mualem prediction model (Gardner, 1956; Mualem, 1976; van Genuchten, 1980). In this method, the transient water content curve and SWRC can be obtained in multistep outflow experi-
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T. Wen et al. / Soils and Foundations 59 (2019) 2341–2347
ments. As a gas pressure is exerted, the difference of the matric potential between the transient water content curve (TWCC) and the SWRC at the same water content is calculated. This difference is considered to have the potential to drive the flow of pore water. Then, the hydraulic conductivity is calculated by using Darcy’s law, as shown in Fig. 3. The equation is 1 DV Du1a 1þ ku ¼ ð1Þ ADt cw Dz where DV is the cumulative outflow volume at the time interval Dt, A is the cross-sectional area of the specimen, Dz is the thickness of the specimen and cw is the unit weight of watert, Dua 1 is the part of the gas pressure potential that overcomes the osmotic resistance and drives pore water. The measurement error e of the hydraulic conductivity can be calculated by Eq. (2) as follows, e¼
ðk u Þ
with
ðk u Þ
ðk u Þ
without
ð2Þ
without
with
Fig. 4. Relationships between cumulative evaporation loss and time for contrastive vessel and outflow vessel.
without
and ðk u Þ are the hydraulic conductivity where ðk u Þ for the same time period with and without evaporation compensation, respectively. 4. Experiment and discussion 4.1. Calibration The validity of the instrument improvements was verified using two methods as follows: (1) The outflow vessel and the contrastive vessel held a specific amount of water, and the water evaporation losses from these two vessels were separately measured using the same type of balance during the same
Fig. 3. A diagram of the direct measurement method (Shao et al., 2017).
Fig. 5. Relationships between cumulative outflow and matric suction for (a) SMP (b) GGC.
T. Wen et al. / Soils and Foundations 59 (2019) 2341–2347
time period in the laboratory. The results showed that the water evaporation losses from these two vessels are similar as show as Fig. 4. Therefore, it is reasonable to prove the feasibility of adding the contrastive vessel. (2) The saturated water content is known. The samples can be removed from the pressure chamber at the end of the matric suction, and the gravimetric water content of the sample was measured using the balance, then the outflow pore water volume could be calculated (weighing method). On the other hand, the outflow pore water volume of the sample can also be calculated from the outflow vessel and the contrastive vessel during each step of the matric suction (outflow method). As shown in the Fig. 5, the water content of the soil samples calculated by this two methods is almost the same using the improved instrument, which indicates that there is no evaporation of water from the soil sample inside the pressure chamber.
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In summary, it is reasonable to assume that the water evaporation from the unsaturated soil surface in the pressure chamber can be ignored, and taking mass measurements from the contrastive vessel can compensate for the water evaporation loss from the outflow vessel. 4.2. Multistep outflow experiments The experimental data for SMP and GGC obtained from the multistep outflow experiments were conducted at 27 °C. The cumulative outflow volume can be obtained by measuring the water outflow from the outflow vessel and the change in mass of the contrastive vessel during the measurement period. Fig. 6 shows that the cumulative evaporation loss is 2.281 ml and 2.702 ml for SMP and GGC, respectively. This is remarkable since the cumulative outflow volume is 14.254 ml and 14.165 ml for SMP and GGC, respectively, and the total measurement error is 16% and 19%, respectively, without evaporation compen-
Fig. 6. Relationships between cumulative evaporation loss and time for (a) SMP and (b) GGC; relationships between cumulative outflow volume and time for (c) SMP and (d) GGC.
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T. Wen et al. / Soils and Foundations 59 (2019) 2341–2347
sation. The change in mass of the contrastive vessel within each measurement period reflects the water evaporation loss from the outflow vessel. Water evaporation is a complicated process and is usually related to factors such as temperature, humidity, and wind speed. Only the temperature is controlled (at 27 °C) in these experiments, and from Fig. 6, the evaporation rate can be calculated in a stable area within 10e-8 m/s to 10e-9 m/s for SMP and GGC. Gravimetric water content-time curves and matric suction-time curves for SMP and GGC are shown in Fig. 7. The curve with evaporation compensation is always below the curve without evaporation compensation during each step of the matric suction since water evaporation loss causes the calculated cumulative outflow volume from only the outflow vessel to be less. In addition, the difference in gravimetric water content between that with and without evaporation compensation will be larger as the cumulative water evaporation loss increases during multistep outflow experiments. It can also be concluded that the gravimetric water content is overestimated without considering the water evaporation loss.
The unsaturated hydraulic conductivity can be measured by using Eq. (1). The unsaturated hydraulic conductivities in this test ranged from 1e to 8 m/s to 1e-12 m/s for SMP and from 1e to 9 m/s to 1e-16 m/s for GGC, as illustrated in Fig. 8. The curve without evaporation compensation is below that with evaporation compensation, and the opposite occurs when gravimetric water contents are investigated. Therefore, the change in value without evaporation compensation is smaller than that with compensation. This means that the measured value of hydraulic conductivity will be underestimated without evaporation compensation. The relationship between hydraulic conductivity and measurement error can be obtained based on the same effective saturation by using Eq. (2). For SMP, it can be seen that the range of measurement error falls primarily within 100% to 500%, and the maximum measurement error is approximately 700%. The range of measurement error is primarily from 100% to 1000% for GGC, and the
Fig. 7. Gravimetric water content-time curves and matric suction-time curves for (a) SMP and (b) GGC.
Fig. 8. Relationships between hydraulic conductivity and effective saturation for (a) SMP and (b) GGC.
T. Wen et al. / Soils and Foundations 59 (2019) 2341–2347
maximum measurement error reaches up to 2000%. In the multi-step outflow experiments, the evaporation losses of SMP and GGC are remarkable and will lead to a measurement error in the hydraulic conductivity without compensating for evaporation. 5. Conclusions An improved pressure plate instrument is introduced in this paper. The water evaporation from unsaturated soil surfaces in the pressure chamber can be ignored, and the water evaporation loss from water outflow vessel can be compensated for using a contrastive vessel during multistep outflow experiments. The validity of the instrument improvements has been verified. The error correction of water evaporation loss on hydraulic conductivity for SMP and GGC are discussed. The results showed that the measured value of the gravimetric water content would be overestimated without considering the water evaporation loss. The total water content measurement errors of SMP and GGC are 16% and 19%, respectively. In addition, the unsaturated hydraulic conductivity will be underestimated without considering the water evaporation loss. The main range of measurement error falls within 100% to 500% for SMP, and the maximum measurement error reaches up to 2 orders of magnitude for GGC. Acknowledgement This research was funded by the National Natural Science Foundation of China (Grant Nos. 51978413, 51479023), the Shenzhen Science and Technology Innovation Commission (Grant No. JCYJ20170811160740635). Declaration of Competing Interest There is no conflict of interest. References ASTM D7664-10, 2010. Standard Test Methods for Measurement of Hydraulic Conductivity of Unsaturated Soils, ASTM International, West Conshohocken, PA. Benson, C.H., Gribb, M.M., 1997. Measuring unsaturated hydraulic conductivity in the laboratory and field. Geotech. Spec. Publ. 68, 113– 168. Breitmeyer, R.J., Benson, C.H., 2011. Measurement of unsaturated hydraulic properties of municipal solid waste. Geotech. Spec. Publ. 211, 1433–1442. Chen, P., Wei, C., Yi, P., Ma, T., 2016. Determination of hydraulic properties of unsaturated soils based on non-equilibrium multistep outflow experiments. J. Geotech. Geoenviron. Eng. 143 (1), 0416087.
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Chen, J., Hopmans, J.W., Grismer, M.E., 1999. Parameter estimation of two-fluid capillary pressure–saturation and permeability functions. Adv. Water Resour. 22 (5), 479–493. Gardner, W.R., 1956. Calculation of capillary conductivity from pressure plate outflow data. Soil. Sci. Soc. Am. J. 20 (3), 317–320. Gnatowski, T., Szatyłowicz, J., Brandyk, T., Kechavarzi, C., 2010. Hydraulic properties of fen peat soils in Poland. Geoderma 154 (3), 188–195. Kool, J.B., Parker, J.C., Van Genuchten, M.T., 1985. Determining soil hydraulic properties from one-step outflow experiments by parameter estimation: I. Theory and numerical studies. Soil. Sci. Soc. Am. J. 49 (6), 1348–1354. Lazrag, T., Kacem, M., Sghaier, J., Dubujet, P., Bellagi, A., 2013. Determination of unsaturated hydraulic properties using drainage gravity test and particle swarm optimization algorithm. J. Porous Media 16 (16), 1025–1034. Leong, E.C., Rahardjo, H., 1997. Permeability functions for unsaturated soils. J. Geotech. Geoenviron. Eng. 123 (12), 1118–1126. Mualem, Y., 1976. A new model for predicting the hydraulic conductivity of unsaturated porous media. Water Resour. Res. 12 (3), 513–522. Mboh, C.M., Huisman, J.A., Van, G.N., Rings, J., Vereecken, H., 2012. Coupled hydrogeophysical inversion of elecrical resitances and inflow measurements for topsoil hydraulic properties under constant head infiltration. Near Surf. Geophys. 10 (5), 1–14. Nu¨tzmann, G., Thiele, M., Maciejewski, S., Joswig, K., 1998. Inverse modelling techniques for determining hydraulic properties of coarsetextured porous media by transient outflow methods. Adv. Water Resour. 22 (3), 273–284. Shao, L.T., Wen, T.D., Guo, X.X., Sun, X., 2017. A method for directly measuring the hydraulic conductivity of unsaturated soil. Geotech. Test. J. 40 (6), 20160197. Sadeghi, M., Tuller, M., Gohardoust, M.R., Jones, S.B., 2014. Columnscale unsaturated hydraulic conductivity estimates in coarse-textured homogeneous and layered soils derived under steady-state evaporation from a water table. J. Hydrol. 519, 1238–1248. Vereecken, H., Kaiser, R., Dust, M., Pu¨tz, T., 1997. Evaluation of the multistep outflow method for the determination of unsaturated hydraulic properties of soils. Soil Sci. 162 (9), 618–631. van Genuchten, M.T., 1980. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil. Sci. Soc. Am. J. 44 (5), 892–898. Wayllace, A., Lu, N., 2012. Atransient water release and imbibitions method for rapidly measuring wetting and drying soil water retention and hydraulic conductivity functions. Geotech. Test. J. 35 (1), 103– 117. Wen, T., Shao, L., Guo, X., 2018. Permeability function for unsaturated soil. Eur. J. Environ. Civ. En., 1–13 Wen, T., Shao, L., Guo, X., 2019. Effect of hysteresis on hydraulic properties of soils under multiple drying and wetting cycles. Eur. J. Environ. Civ. En., 1–13 Zhang, L., Yang, L., Wang, J., Zhao, J., Dong, H., Yang, M., Song, Y., 2017. Enhanced CH4 recovery and CO2 storage via thermal stimulation in the CH4/CO2 replacement of methane hydrate. Chem. Eng. J. 308, 40–49. Zhao, Y., Yang, H., Chen, Z., Chen, X., Huang, L., Liu, S., 2019. Effects of jointed rock mass and mixed ground conditions on the cutting efficiency and cutter wear of tunnel boring machine. Rock Mech. Rock Eng. 52 (5), 1303–1313.
Available online at www.sciencedirect.com
ScienceDirect Soils and Foundations 59 (2019) 2341–2347 www.elsevier.com/locate/sandf
Error correction of water evaporation loss on the unsaturated hydraulic conductivity Tiande Wen a, Longtan Shao b,⇑, Xiaoxia Guo b, Yanru Zhao a, Liping Huang c, Xiangsheng Chen a,c b
a College of Civil Engineering, Shenzhen University, Shenzhen 518060, China State Key Laboratory of Structural Analysis for Industrial Equipment, Department of Engineering Mechanics, Dalian University of Technology, Dalian 116024, China c Shenzhen Metro Group Co., Ltd., Shenzhen 518026, China
Received 5 December 2018; received in revised form 21 July 2019; accepted 22 August 2019 Available online 1 November 2019
Abstract Water evaporation loss occurs during multistep outflow experiments from the soil surface and the outflow vessel. The main focus of this paper is to introduce an improved pressure plate instrument that can correct for the error of water evaporation loss on the unsaturated hydraulic conductivity. Experiments using silicon micropowder (SMP) and Guangxi Guiping clay (GGC) were conducted with an improved instrument. The results showed that the measured value of the water content will be overestimated and the unsaturated hydraulic conductivity will be underestimated without considering the water evaporation loss. Ó 2019 Production and hosting by Elsevier B.V. on behalf of The Japanese Geotechnical Society.
Keywords: Unsaturated hydraulic conductivity; Water evaporation loss; Multistep outflow experiment
1. Introduction The hydraulic conductivity of unsaturated soil is a function of the matric suction or water content. Several standard test methods for measuring the unsaturated hydraulic conductivity are described in ASTM 7664 (2010). The unsaturated hydraulic conductivity is generally measured experimentally, or it is estimated from the soil water retention curve (SWRC). The original outflow method for measuring the hydraulic conductivity using a pressure plate instrument was proposed by Gardner (1956). The multistep outflow method is a transient measurement method for estimating the hydraulic conductivity in the laboratory. The major advanPeer review under responsibility of The Japanese Geotechnical Society. ⇑ Corresponding author. E-mail address: [email protected] (L. Shao).
tage of this method is that the matric suction can be precisely controlled, and the hydraulic conductivity and SWRC can be derived simultaneously. Kool et al. (1985) developed an inverse method to estimate the parameters of the SWRC from the multistep outflow experiment and to predict the hydraulic conductivity. Benson and Gribb (1997) presented a comprehensive review of the inverse modeling approaches used to estimate the hydraulic conductivity, including one-step and multistep outflow methods. The multistep outflow method has become one of the most promising and practical methods for estimating the hydraulic conductivity of unsaturated soil (Mualem, 1976; van Genuchten, 1980; Vereecken et al., 1997; Breitmeyer and Benson, 2011; Wayllace and Lu, 2012; Sadeghi et al., 2014; Chen et al., 2016; Shao et al., 2017; Zhang et al., 2017; Wen et al., 2018; Zhao et al., 2019). However, direct measurements of the unsaturated hydraulic conductivity and SWRC in the laboratory are
https://doi.org/10.1016/j.sandf.2019.08.015 0038-0806/Ó 2019 Production and hosting by Elsevier B.V. on behalf of The Japanese Geotechnical Society.
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T. Wen et al. / Soils and Foundations 59 (2019) 2341–2347
time-consuming (short one week, long half year), since required time is different depending on the focusing matric suction range for the multistep outflow experiments (Nu¨tzmann et al., 1998; Chen et al., 1999; Gnatowski et al., 2010; Mboh et al., 2012; Lazrag et al., 2013; Wen et al., 2019). Leong and Rahardjo (1997) summarized that the change in water volume is difficult to measure accurately because of the water evaporation loss, and this loss affects the ability to accurately measure hydraulic conductivity. During multistep outflow experiments, water evaporation loss from the soil surface and the outflow vessel creates problems in measuring and predicting unsaturated hydraulic conductivity. This paper introduces an improved pressure plate instrument. With this instrument, water evaporation from the unsaturated soil surface in the pressure chamber can be ignored, and the water evaporation loss from the water outflow vessel can be compensated. The error correction of
water evaporation loss on hydraulic conductivity for silicon micropowder (SMP) and Guangxi Guiping clay (GGC) are also discussed. 2. Apparatus, materials and testing procedures 2.1. Apparatus The improved pressure plate instrument is composed of a pressure chamber, a ring cutter, a gas pressure controlling system, an exhaust gas flushing system, a water outflow vessel, a contrastive vessel, two balances and a computer (see Fig. 1). During the experiment, the top and the bottom of the soil specimen in the ring cutter are tightly attached to two ceramic disks. Increasing the gas pressure leads to elevated gas pressure in the system while maintaining a constant pore water pressure; air holes around the ring cutter guarantee that the gas pressure is applied around the soil
Fig. 1. A diagram of the experimental setup (Wen et al., 2019).
T. Wen et al. / Soils and Foundations 59 (2019) 2341–2347
specimen. Water is trapped in the soil using a breathable waterproof film. The chemical composition of this film is mainly polytetrafluoroethylene (PTFE), the thickness is 0.07 mm, which can block the passage of external water, and can also prevent the discharge of pore water in the soil inside the ring cutter. Therefore, the water evaporation loss from the soil in the pressure chamber can be ignored. The outflow vessel (30 mm in height and 80 mm in diameter) is covered except for a small hole (2 mm in diameter) to allow for the insertion of a needle. The real-time cumulative water volume of the outflow vessel is measured with a high-precision balance (accuracy: 0.001 g). Despite the presence of only a small hole, water evaporation loss from the outflow vessel will occur. Therefore, we used a contrastive vessel to measure the water evaporation loss at the same time as the water volume measurements from the outflow vessel using the second high-precision balance (see Fig. 1). The improved pressure chamber and the contrastive vessel constitute an evaporation compensation system. The water evaporation from the unsaturated soil surface in the pressure chamber can be ignored, and mass measurements from the contrastive vessel compensated for the water evaporation loss from the outflow vessel during the experimental period.
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Fig. 2. Grain size distribution of the tested soils.
rated with distilled water after vacuum seeding for approximately 2 h and then submerged for approximately 24 h. To prevent any disturbance to the samples during this process, the samples were covered both on the top and on the bottom with filter paper and water-permeable stone.
2.2. Soil specimens
2.3. Testing procedures
Silicon micropowder (SMP) is an artificial soil in China that is widely used in construction. The Guangxi Guiping clay (GGC) used in this study was taken from a deep excavation in Guiping City, Guangxi Province, China. The basic material properties of the soils were measured in the laboratory and are given in Table 1, and their grain size distributions are shown in Fig. 2. The soils were oven-dried for 48 h prior to the preparation of the recompacted soil specimens. According to the dry density required by the test scheme, a certain amount of sieved soil is weighted in the enamel pot, an appropriate amount of distilled water is sprayed and stirred evenly on the soil, and finally the water is evenly distributed into the soil sample. At the same time, a layer of vaseline should be evenly applied on the inner wall of the ring cutter, and it should be adopted to ensure uniform compaction of the soil sample. The samples for the pressure plate test were 20 mm in height and 54.7 mm in diameter. The samples were satu-
The pressure plate instrument is preprocessed in the following manner: (1) flush the bubbles from the exhaust tube and the water tube, and fill the tubes with air-free water; (2) saturate the ceramic disk (its air-entry value is 500 kPa), and measure the saturated hydraulic conductivity of the ceramic disk (1.20e-7 m/s); and (3) install the saturated soil specimen. The specimen was subjected to increasing matric suction in a series of steps to measure the gravimetric water content during drying. The matric suction path of SMP is [10 kPa, 20 kPa, 40 kPa, 60 kPa, 100 kPa, 160 kPa, 300 kPa]; the matric suction path of GGC is [10 kPa, 20 kPa, 40 kPa, 80 kPa, 110 kPa, 170 kPa, 250 kPa, 350 kPa, 450 kPa]. The saturated gravimetric water contents for SMP and GGC are 0.26 and 0.38, respectively. According to the SWRC, the residual gravimetric water content is 0.05 for SMP and 0.18 for GGC (Shao et al., 2017). Finally, the error correction of water evaporation loss on hydraulic conductivity for SMP and GGC are discussed. 3. Method
Table 1 Parameters of the tested soils. Soil
SMP
GGC
Specific gravity Initial dry density (kg/m3) Saturated hydraulic conductivity (m/s) Liquid limit Plastic limit Plasticity index
2.68 1600 1.09e-7 32.5% 20.3% 12.2%
2.74 1350 4.72e-9 51.8% 22.7% 29.1%
The measurement method for hydraulic conductivity presented by Shao et al. (2017) shows better agreement with the test data compared to the Gardner outflow analysis method and the van Genuchten-Mualem prediction model (Gardner, 1956; Mualem, 1976; van Genuchten, 1980). In this method, the transient water content curve and SWRC can be obtained in multistep outflow experi-
2344
T. Wen et al. / Soils and Foundations 59 (2019) 2341–2347
ments. As a gas pressure is exerted, the difference of the matric potential between the transient water content curve (TWCC) and the SWRC at the same water content is calculated. This difference is considered to have the potential to drive the flow of pore water. Then, the hydraulic conductivity is calculated by using Darcy’s law, as shown in Fig. 3. The equation is 1 DV Du1a 1þ ku ¼ ð1Þ ADt cw Dz where DV is the cumulative outflow volume at the time interval Dt, A is the cross-sectional area of the specimen, Dz is the thickness of the specimen and cw is the unit weight of watert, Dua 1 is the part of the gas pressure potential that overcomes the osmotic resistance and drives pore water. The measurement error e of the hydraulic conductivity can be calculated by Eq. (2) as follows, e¼
ðk u Þ
with
ðk u Þ
ðk u Þ
without
ð2Þ
without
with
Fig. 4. Relationships between cumulative evaporation loss and time for contrastive vessel and outflow vessel.
without
and ðk u Þ are the hydraulic conductivity where ðk u Þ for the same time period with and without evaporation compensation, respectively. 4. Experiment and discussion 4.1. Calibration The validity of the instrument improvements was verified using two methods as follows: (1) The outflow vessel and the contrastive vessel held a specific amount of water, and the water evaporation losses from these two vessels were separately measured using the same type of balance during the same
Fig. 3. A diagram of the direct measurement method (Shao et al., 2017).
Fig. 5. Relationships between cumulative outflow and matric suction for (a) SMP (b) GGC.
T. Wen et al. / Soils and Foundations 59 (2019) 2341–2347
time period in the laboratory. The results showed that the water evaporation losses from these two vessels are similar as show as Fig. 4. Therefore, it is reasonable to prove the feasibility of adding the contrastive vessel. (2) The saturated water content is known. The samples can be removed from the pressure chamber at the end of the matric suction, and the gravimetric water content of the sample was measured using the balance, then the outflow pore water volume could be calculated (weighing method). On the other hand, the outflow pore water volume of the sample can also be calculated from the outflow vessel and the contrastive vessel during each step of the matric suction (outflow method). As shown in the Fig. 5, the water content of the soil samples calculated by this two methods is almost the same using the improved instrument, which indicates that there is no evaporation of water from the soil sample inside the pressure chamber.
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In summary, it is reasonable to assume that the water evaporation from the unsaturated soil surface in the pressure chamber can be ignored, and taking mass measurements from the contrastive vessel can compensate for the water evaporation loss from the outflow vessel. 4.2. Multistep outflow experiments The experimental data for SMP and GGC obtained from the multistep outflow experiments were conducted at 27 °C. The cumulative outflow volume can be obtained by measuring the water outflow from the outflow vessel and the change in mass of the contrastive vessel during the measurement period. Fig. 6 shows that the cumulative evaporation loss is 2.281 ml and 2.702 ml for SMP and GGC, respectively. This is remarkable since the cumulative outflow volume is 14.254 ml and 14.165 ml for SMP and GGC, respectively, and the total measurement error is 16% and 19%, respectively, without evaporation compen-
Fig. 6. Relationships between cumulative evaporation loss and time for (a) SMP and (b) GGC; relationships between cumulative outflow volume and time for (c) SMP and (d) GGC.
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sation. The change in mass of the contrastive vessel within each measurement period reflects the water evaporation loss from the outflow vessel. Water evaporation is a complicated process and is usually related to factors such as temperature, humidity, and wind speed. Only the temperature is controlled (at 27 °C) in these experiments, and from Fig. 6, the evaporation rate can be calculated in a stable area within 10e-8 m/s to 10e-9 m/s for SMP and GGC. Gravimetric water content-time curves and matric suction-time curves for SMP and GGC are shown in Fig. 7. The curve with evaporation compensation is always below the curve without evaporation compensation during each step of the matric suction since water evaporation loss causes the calculated cumulative outflow volume from only the outflow vessel to be less. In addition, the difference in gravimetric water content between that with and without evaporation compensation will be larger as the cumulative water evaporation loss increases during multistep outflow experiments. It can also be concluded that the gravimetric water content is overestimated without considering the water evaporation loss.
The unsaturated hydraulic conductivity can be measured by using Eq. (1). The unsaturated hydraulic conductivities in this test ranged from 1e to 8 m/s to 1e-12 m/s for SMP and from 1e to 9 m/s to 1e-16 m/s for GGC, as illustrated in Fig. 8. The curve without evaporation compensation is below that with evaporation compensation, and the opposite occurs when gravimetric water contents are investigated. Therefore, the change in value without evaporation compensation is smaller than that with compensation. This means that the measured value of hydraulic conductivity will be underestimated without evaporation compensation. The relationship between hydraulic conductivity and measurement error can be obtained based on the same effective saturation by using Eq. (2). For SMP, it can be seen that the range of measurement error falls primarily within 100% to 500%, and the maximum measurement error is approximately 700%. The range of measurement error is primarily from 100% to 1000% for GGC, and the
Fig. 7. Gravimetric water content-time curves and matric suction-time curves for (a) SMP and (b) GGC.
Fig. 8. Relationships between hydraulic conductivity and effective saturation for (a) SMP and (b) GGC.
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maximum measurement error reaches up to 2000%. In the multi-step outflow experiments, the evaporation losses of SMP and GGC are remarkable and will lead to a measurement error in the hydraulic conductivity without compensating for evaporation. 5. Conclusions An improved pressure plate instrument is introduced in this paper. The water evaporation from unsaturated soil surfaces in the pressure chamber can be ignored, and the water evaporation loss from water outflow vessel can be compensated for using a contrastive vessel during multistep outflow experiments. The validity of the instrument improvements has been verified. The error correction of water evaporation loss on hydraulic conductivity for SMP and GGC are discussed. The results showed that the measured value of the gravimetric water content would be overestimated without considering the water evaporation loss. The total water content measurement errors of SMP and GGC are 16% and 19%, respectively. In addition, the unsaturated hydraulic conductivity will be underestimated without considering the water evaporation loss. The main range of measurement error falls within 100% to 500% for SMP, and the maximum measurement error reaches up to 2 orders of magnitude for GGC. Acknowledgement This research was funded by the National Natural Science Foundation of China (Grant Nos. 51978413, 51479023), the Shenzhen Science and Technology Innovation Commission (Grant No. JCYJ20170811160740635). Declaration of Competing Interest There is no conflict of interest. References ASTM D7664-10, 2010. Standard Test Methods for Measurement of Hydraulic Conductivity of Unsaturated Soils, ASTM International, West Conshohocken, PA. Benson, C.H., Gribb, M.M., 1997. Measuring unsaturated hydraulic conductivity in the laboratory and field. Geotech. Spec. Publ. 68, 113– 168. Breitmeyer, R.J., Benson, C.H., 2011. Measurement of unsaturated hydraulic properties of municipal solid waste. Geotech. Spec. Publ. 211, 1433–1442. Chen, P., Wei, C., Yi, P., Ma, T., 2016. Determination of hydraulic properties of unsaturated soils based on non-equilibrium multistep outflow experiments. J. Geotech. Geoenviron. Eng. 143 (1), 0416087.
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