Primary imbibition curve measurement using large ...

Proceedings of the 19th International Conference on Soil Mechanics and Geotechnical Engineering, Seoul 2017

Primary imbibition curve measurement using large soil column test Mesures de la courbe d imbibition primaire de sols. Application aux colonnes de grandes dimensions. Guanxi Yan, Thierry Bore, Sergio Galindo-Torres & Alexander Scheuermann Geotechnical Engineering Centre, School of Civil Engineering, The University of Queensland, Australia, [email protected]

Zi Li & Ling Li National Centre for Groundwater Research and Training, School of Civil Engineering, The University of Queensland, Australia

ABSTRACT: For analyzing unsaturated conditions of hydraulic and mechanic tests, the Soil Water Retention Curve (SWRC) is the most important information which should be experimentally measured. Therefore, the measurement of SWRC becomes the vital puzzle in the entire map for predicting unsaturated soil behavior regarding seepage, failure, and deformation. Standard SWRC test employs the standard Axis Translation Technique (ATT). However, beyond the primary draining path, it is extremely time-consuming to measure the primary imbibition curves with this device. In order to provide an alternative way for measuring the primary imbibition curve, two 2 m long sandy soil columns were set up with an advanced measuring technique for measuring water content profile. The results of this method are compared with measurements using the hanging column method providing observations in the lower suction range (0~20 kPa). The performance of this experimental setup is demonstrated with a discussion of advantages and disadvantages. RÉSUMÉ : Pour l'analyse des conditions insaturées des essais hydrauliques et mécaniques, la courbe de retention d eau est generallement la seule grandeur mesuree experiementallement. Ainsi, la mesure de la courbe de retention d’eau d un materiau est fondamentale pour prevoir le comportement d un sol en cas d infiltration, deformation ou rupture. Le test SWRC standard utilise la technique de traduction Axis (ATT) standard. Cependant, au-delà de la voie de drainage primaire, il est extrêmement long de mesurer les courbes d'imbibition primaires avec ce dispositif. Afin de fournir une méthode alternative pour mesurer la courbe d'imbibition primaire, deux colonnes de sol sablonneux de 2 m de long ont été mises en place avec une technique de mesure avancée pour mesurer le profil de teneur en eau. Les résultats de cette méthode sont comparés avec des mesures utilisant la méthode de colonne suspendue fournissant des observations dans la plage d'aspiration inférieure (0 ~ 20 kPa). La performance de cette installation expérimentale est démontrée avec une discussion des avantages et des inconvénients. KEYWORDS: SWRC, Primary imbibition, Large soil column test, IPM. 1

INTRODUCTION

The increasing involvement of environmental processes, such as precipitation, evaporation, evapotranspiration and groundwater recharge, in geotechnical problems, required the further development of conventional soil mechanics to take into account unsaturated conditions. The conventional description of hydrostatic pressure and soil effective stress fail for unsaturated soils. In unsaturated soils, the negative pore water pressure only decreases linearly with distance to the water table at equilibrium conditions. Water infiltration or evaporation lead to non-linear distributions of the pore water pressure even at steady state conditions. Moreover, the consideration of negative pore water pressures in the effective stress concept requires the introduction of the saturation since negative pore water pressures usually appear at partly saturated conditions. In order to fill this gap, the concept of soil water retention behavior from agriculture was introduced into soil mechanics (Fredlund and Rahardjo, 1993). Based on early experimental findings from Buckingham (Narasimhan, 2005), the correlation between soil suction and soil moisture content was defined as a constitutive relationship. Further, an instantaneous steady-state assumption was adopted, so that for each moisture content, the effective permeability can be estimated based on the fraction of water in void space. Having both theoretical foundations for two-phase seepage (Richards, 1931) and unsaturated soil stress (Bishop, 1960), the hydromechanical behavior of unsaturated soil is becoming more applicable in geotechnical engineering as the

formulation of stress-suction-strain involve physical concepts and eliminate previous assumptions. With a great appreciation of the development of unsaturated soil mechanics (Fredlund and Rahardjo, 1993; Lu and Likos, 2004), the prediction of natural soil and its hydro-mechanical behavior has been improved in the last few decades. Although the scientific pursuit of natural soil already provides geotechnical engineers a broad view of the application of unsaturated soil mechanics, there are still uncertainties and knowledge gaps in the existing theoretical framework, which consists of both, conceptual ideas, experimental exploration and mathematical formulation. Unfortunately, there are still several unsolved questions hidden in the concepts of soil mechanics for unsaturated soils: just to name a few, the hysteresis effect (Chen, 2006), dynamic effect in transient flow (Hassanizadeh et al., 2002), scale issue of moisture/suction distribution in test (Kang et al., 2014), deformation inducing multi-SWRCs (Hu et al., 2013), stress-path dependent effective stress (Sheng et al., 2013), inconsistency between unsaturated and saturated soil stress-suction-strain relationship (Sheng et al., 2013), and even the composition and definition of soil suction (Baker and Frydman, 2009), etc. In this study, one of the issues related to hysteresis and experimental determination of the soil water retention curve is investigated. An imbibition test from the fully dry initial condition is implemented using two experimental methods: the standard hanging column test and the Instantaneous Profile Method (IPM) in a 2.4 m long soil column. This wetting path is usually defined as the primary or principle imbibition curve

Proceedings of the 19th International Conference on Soil Mechanics and Geotechnical Engineering, Seoul 2017

which can rarely be measured using the conventional Axis Translation Technique (ATT), because the soil suction is controlled by a gas pressure regulator, which does not allow the application of very low soil suction. Therefore, the fully dry sample was loaded in a hanging column device and a 2.4 m long acrylic column to provide a comparison between the resulting soil water retention curves from these two methods. In principal, the primary imbibition curves should represent the moisture uptake from the unconfined aquifer to the upper vadose zone. Whether or not the REV-sized tests truly replicate the primary imbibing moisture/suction profile is investigated as well at the preliminary stage of this experimental study. 2

EXPERIMENTAL METHODS

2 .1

Hanging Column Method

and the VT is the total specimen volume. The precision of suction head measurements is ± 1 mm, and the accuracy of the measurements of the water volume is ± 0.5 ml. 2 .2 Instantaneous Profile Method The set-up for using the Instantaneous Profile Method (IPM) for determining the soil water retention curve is shown in Figure 2. Four columns with 2.4 meter length filled with two soils are used for this test. The two columns at the left and right were used to carry out drainage tests (Yan et al., 2017; Yan et al., 2016), while the two in the center contained dry soils for imbibition tests. A detailed sketch of the experimental setup can be found in the literature (Yan et al., 2017). This experimental setup uses Spatial Time Domain Reflectometry (Spatial TDR), high precision tensiometer (UMS T5 sensor) and electrical bench scales for continually logging in/outflow.

The hanging column method was set up based on ASTM D6836-02 (2003). Figure 1 shows the fridge containing four socalled Buchner funnels, in which the sand sample with a thickness of 3 cm have been separately loaded. With this arrangement, four tests can be carried out at the same time.

Figure 2. IPM test: Drainage test on left and right; Spontaneous Imbibition test for sand and loam in the center.

(a)

(b)

Figure 1. Hanging Column Method (a) suction control (± 0.1 kPa) and water variation measurement (± 1 ml) (b) Buchner funnels.

First, the sintered glass disk (high air entry ceramic disk) within the funnel had to be saturated using a vacuum in deionized water for 48 hours. A closed hose connected to the funnel kept the disk saturated. The dry soil was then compacted in the funnels to a targeted volume to achieve a desired dry density. Each funnel was connected to a measuring cylinder via the hose, and a gas free hydraulic connection was created between the measuring cylinder and the fully saturated sintered glass disk. Through increasing and decreasing of the elevation of the measuring cylinder, the suction could be applied. The suction head (hc) was given by the height difference between the water table within the measuring cylinder and the glass disk. Once a lower suction value was applied, water imbibition occurred in the soil sample. According to ASTM D6836-02 (2003), it usually takes 48 hours to approach equilibrium for draining tests. However, no recommended equilibrium duration is suggested for wetting tests. For the presented tests, every two weeks a data point was adopted. The variation of water was measured to calculate the volumetric water content (ϴi) for each suction value at hydraulic equilibrium with

i  i 1  Vi / VT

(1)

for imbibition, where ϴi-1 is the previous moisture content; VΔi is the water variation in the cylinder after elevation changed,

Moisture logging was implemented using Spatial TDR technique (Scheuermann et al., 2009). A 2 m long flat ribbon sensor was buried in the center of soil specimen without creating a preferential flow path. The sensor is connected to a conventional TDR device via a multiplexer. TDR traces are recorded as a normalized reflection over time. Based on assumed distributions for the capacitance and conductance, a TDR trace is calculated using the telegraph equation and compared with the measured TDR trace. In a two-parameter inversion, the capacitance and conductance are varied until a satisfactory agreement between simulated and measured TDR trace is reached. This procedure is implemented in an inversion algorithm developed by Schlaeger (2005). The resulting capacitance profile is then calculated into permittivity using the so-called capacitance model Huebner et al. (2005). Finally, the permittivity profile is transferred into volumetric moisture profile using Topp’s model (Topp et al., 1980). This system enables the measurement of moisture profiles with a temporal resolution of 1~3 mins, depending on the numbers of sensor used, allowing the investigation of dynamic effects in the SWRC (A Scheuermann et al., 2014). The volumetric moisture content is determined with an accuracy of ± 3%. UMS T5 tensiometers with high precision of ± 0.1 kPa were used to observe suction at selected locations. Each sensor was connected to a datalogger to enable recording every 30 seconds (Yan et al., 2017). The T5 tensiometer measures soil pore water pressure from -85 kPa up to 100 kPa. Six tensiometers were inserted along soil column at 0.2, 0.4, 0.6, 1.0, 1.4 and 1.8 m. The outflow and inflow provided by a constant head tank were measured using an electrical bench scale also every 30 seconds. The water table was kept constant at 0.36 ± 0.01 m above the column bottom. The evaporation was prevented by cling wrap fully covering the column top, any other holes and the opening of the constant head tank.

Proceedings of the 19th International Conference on Soil Mechanics and Geotechnical Engineering, Seoul 2017

2 .3 Testing sample Two poorly graded sandy soils were chosen to conduct this test. The specification of the soils and achieved packing conditions are listed in Table 1. It should be noted that compared to beach sand having no fine content (particle size < 75 μm), the loam contains 4.6% fine content which leads to slightly cohesive and adsorptive behavior. Table 1. Initial conditions and parameters of the used soils. Bribe Island Beach Sand

Bricky’s Loam

340

270

SP

SP

2.655

2.655

1.61

1.43

e (Vv/Vs)

0.64 ± 0.03

0.88 ± 0.06

n (Vv/VT)

39% ± 1%

47% ± 2%

Sample options d50 (μm) USCS Soil Classification Gs (ρs/ρw) ρdry (g/cm3)

3

higher than the moisture content calculated from the inflow logging. This is due to the saturated gravel filter in the bottom of soil column which was placed for fixing the bottom terminal of Spatial TDR sensor. This saturated filter results in an initial moisture content which cannot be accurately determined as the porosity and grain size differ from the testing sample. However, after a few months, the moisture front climbed to around 1 m, which is at approximately half the height of the column. This means that the moisture within the soil sample dominates the overall moisture and a representative mean moisture content is determined. Figure 4 shows a satisfactory comparison between Spatial TDR measured and the inflow determined mean moisture contents.

RESULT AND DISCUSSION

The experimental setup was validated with drainage tests for the soils given in Table 1 (Yan et al., 2017). It could be shown that the resulting primary drainage curves are realistic and that transient flow conditions can be observed. According to the objective of this paper, the static primary imbibition curves are presented, compared and discussed for bricky’s loam only. However, first, the challenges of the experimental setup for measuring the primary imbibition curve are discussed. 3 .1 The validation of large soil column test for spontaneous imbibition Figure 3 shows the comparison between TDR trace measured by the device and the TDR trace simulated by forward modeling with the telegraph equations using capacitance and conductance reconstruction method. Two TDR traces measured from both, the top and the bottom end of the sensor in the loam column are fitted well by the simulating data. Therefore, the moisture profile derived from the simulated TDR trace is a good estimate of the actual moisture profile.

Figure 4. Inflow and mean volumetric moisture contents (ϴ) determined using Spatial TDR and from inflow.

In Figure 5, the soil suction logging measurements are shown at six different heights along the loam sample. The sensor at 0.2 m is below the constant water table indicating its height at 0.37 m above the column bottom. Accordingly, a pore water pressure of 1.7 kPa is measured. The sensor at 0.4 m measured at the beginning a positive pore water pressure, which is not reasonable. After one month, the pressure dropped to a suction value of approximately -0.3 kPa. The possible reason for the problem at the beginning is the existence of a bubble trapped inside the shaft of the tensiometer. The Tensiometer at 0.6 cm responded faster from the beginning, and showed a reasonable value of around -4 kPa. The other sensors at 1, 1.4, 1.8 m failed to measure realistic pressures due to drying of the ceramic tip. In future tests, tensiometers should be placed in the soil after reaching the final equilibrium. Nevertheless, the first three tensiometers show a quasi-hydrostatic pore water pressure distribution, which is expected to be reached at equilibrium. Hence, the resulting water content profile at equilibrium can be considered as a realistic primary imbibition curve.

Figure 3. TDR traces inverse analysis using two way capacitance-conductance reconstruction.

Figure 4 demonstrates the performance of Spatial TDR technique based on the mean volumetric moisture content (ϴ) measured for the entire loam column. The moisture content measured by Spatial TDR was from the beginning slightly

Figure 5. Suction logging on soil column: two sensors under 1 m function properly; one sensor at 40cm shows delay in responding and three sensors malfunction due to ceramic tip dried out.

Proceedings of the 19th International Conference on Soil Mechanics and Geotechnical Engineering, Seoul 2017

3 .2 Primary imbibition curves for loamy sand Figure 6 shows the resulting static primary imbibition curves for bricky’s loam. The results from the Instantaneous Profile Method (IPM) in the column are shown as a solid line for the initial and as dashed line for the final water content distribution. The primary drainage curve resulting from the standard hanging column method is shown with open circles. For comparison, the primary drainage curve measured with the hanging column method is also included in Figure 6 with stars as symbols and a parameterization model fitted. As can be seen, the primary imbibition curves from both methods deviate significantly from the drainage curve. This observation highlights the fact that the usage of the primary drainage curve for simulating infiltration and imbibition processes can lead to false predictions. 2 Hanging column test IPM Intial IPM Final Primary Drainage

1.8 1.6

Suction head (m)

1.4 1.2 1 0.8 0.6 0.4 0.2 0 0

10

20



30

40

50

Figure 6. Comparison of primary imbibition curves

The dark solid line shows the initial moisture distribution within the soil column when imbibition test was started. The higher moisture readings from 0 to 60 cm indicate water uptake from the gravel filter into the sample due to adsorption process before constant water pressure was applied. After three months of imbibition, the moisture profile shows the distribution given with a dashed line (IPM final). The comparison between the results from the IPM and the hanging column method shows a significant deviation. In both methods, imbibition reached equilibrium. While only one imbibition step was applied in the IPM, several steps with a period of approximately 2 weeks for each step were required to produce the complete curve using the hanging column method. The large deviation between both curves is surprising. Evaporation was prevented in both experiments, and equilibrium was reached as well. Most probably, the reasons for this observation can be found in the dimensions of the sample. However, further investigations are required to provide answers. 4

CONCLUSION

Primary imbibition curves have been measured using two methods: the standard hanging water column method and the Instantaneous Profile Method. The comparison between both results shows significant differences, which are most probably not caused by evaporation or transient conditions. Further

investigations are required to identify the reasons for this observation. REFERENCES ASTM D6836-02. 2003. Test methods for determination of the soil water characteristic curve for desorption using a hanging column, pressure extractor, chilled mirror hygrometer, and/or centrifuge American Society for Testing and Materials (ASTM). West Conshohocken, Pa. Baker, R., andFrydman, S. 2009. Unsaturated soil mechanics: Critical review of physical foundations. Engineering Geology, 106(1), 2639. Bishop, A.W. 1960. The principles of effective stress: Norges Geotekniske Institutt. Chen, L. 2006. Hysteresis and dynamic effects in the relationship between capillary pressure, saturation, and air-water interfacial area in porous media. Fredlund, D.G., andRahardjo, H. 1993. Soil mechanics for unsaturated soils: John Wiley & Sons. Hassanizadeh, S.M., Celia, M.A., andDahle, H.K. 2002. Dynamic effect in the capillary pressure–saturation relationship and its impacts on unsaturated flow. Vadose Zone Journal, 1(1), 38-57. Hu, R., Chen, Y.-F., Liu, H.-H., andZhou, C.-B. 2013. A water retention curve and unsaturated hydraulic conductivity model for deformable soils: consideration of the change in pore-size distribution. Geotechnique, 63(16), 1389-1405. Huebner, C., Schlaeger, S., Becker, R., Scheuermann, A., Brandelik, A., Schaedel, W., andSchuhmann, R. 2005. Advanced measurement methods in time domain reflectometry for soil moisture determination Electromagnetic Aquametry (pp. 317-347): Springer. Kang, M., Perfect, E., Cheng, C., Bilheux, H., Lee, J., Horita, J., andWarren, J. 2014. Multiple pixel-scale soil water retention curves quantified by neutron radiography. Advances in Water Resources, 65, 1-8. Lu, N., andLikos, W.J. 2004. Unsaturated soil mechanics: J. Wiley. Narasimhan, T. 2005. Buckingham, 1907. Vadose Zone Journal, 4(2), 434-441. Richards, L.A. 1931. Capillary conduction of liquids through porous mediums. Journal of Applied Physics, 1(5), 318-333. Scheuermann, A., Galindo-Torres, S., Pedroso, D., Williams, D., andLi, L. 2014. Dynamics of water movements with reversals in unsaturated soils. Paper presented at the 6th International Conference on Unsaturated Soils, UNSAT 2014. Scheuermann, A., Huebner, C., Schlaeger, S., Wagner, N., Becker, R., andBieberstein, A. 2009. Spatial time domain reflectometry and its application for the measurement of water content distributions along flat ribbon cables in a full‐scale levee model. Water Resources Research, 45(4). Schlaeger, S. 2005. A fast TDR-inversion technique for the reconstruction of spatial soil moisture content. Hydrology and Earth system sciences discussions, 9(5), 481-492. Sheng, D., Zhang, S., andYu, Z. 2013. Unanswered questions in unsaturated soil mechanics. Science China Technological Sciences, 56(5), 1257-1272. Topp, G., Davis, J., andAnnan, A.P. 1980. Electromagnetic determination of soil water content: Measurements in coaxial transmission lines. Water Resources Research, 16(3), 574-582. Yan, G., Li, Z., Bore, T., Galindo-Torres, S., Schlaeger, S., Scheuermann, A., andLi, L. 2017. An Experimental Platform for Measuring Soil Water Characteristic Curve under Transient Flow Conditions Advances in Laboratory Testing and Modelling of Soils and Shales (ATMSS) (pp. 231-238): Springer. Yan, G., Scheuermann, A., Schlaeger, S., Bore, T., andBhuyan, H. 2016. Application of Spatial Time Domain Reflectometry for investigating moisture content dynamics in unsaturated sand. Paper presented at the 11th International Conference on Electromagnetic Wave Interaction with Water and Moist Substances, Florence, Italy.