Influence of Hydraulic Response Time of Tensiometer in Hydraulic Characteristics Estimation for Riverbank Sand

The knowledge of the continuous evolution of soil hydraulic characteristics is required for several environmental geotechnical applications such as capillary barrier systems and river bank slope stability analysis. Such data are obtained in the field by embedding suction sensors in slopes for stability analysis. The hydraulic equilibrium time and other limitations of sensors in establishing the hydraulic characteristics are not available for the compacted soils. Thus the influence of initial soil moisture content, initial wetting condition of the suction sensor, and soil compaction density on the hydraulic equilibrium time of the tensiometer was studied on riverbank sand. A laboratory method considering the equilibrium data at each saturation/drying state was proposed for establishing the equilibrium-SWCC data. The hydraulic equilibrium time of the tensiometer for the studied soil was found to vary from a few hours to several days depending upon the magnitude of suction and state of the sensor. The conventional method for estimating the instantaneous SWCC severely overestimated the suction values at a given water content. The sensor response time was found to be the single most influencing factor in the estimation of soil hydraulic characteristics. The in-situ tensiometer application should be replaced with the moisture sensor readings combined with the lab-based proposed equilibrium SWCC approach.


Introduction
Geomorphology of riverbanks is an important interdisciplinary subject, which provides the response of riverbanks by dynamic flow currents, sediment load, and climate variations. The deflections of the main river streams towards the banks, especially in braided rivers, contribute to bank erosion and results in a substantial loss of land. The knowledge of bank failure mechanisms is crucial for the identification of suitable stabilization measures and management of bank erosion problems (Osman and Thorne, 1988;Thorne, 1992). Riverbank stability evaluation under dynamic climate variables is necessary for the safety against unexpected bank failures and subsequent consequences in terms of damage and cost-effective countermeasures (Gottardi et al. 2020). The classical limit equilibrium-based approach is not applicable for the riverbank stability analysis under the dynamic wetting-drying conditions due to the limitation in defining the pore water pressure conditions under different flow events (Dapporto et al. 2003).
Many studies have shown that the pore-water pressure (or soil water potential) changes from negative Abstract The knowledge of the continuous evolution of soil hydraulic characteristics is required for several environmental geotechnical applications such as capillary barrier systems and river bank slope stability analysis. Such data are obtained in the field by embedding suction sensors in slopes for stability analysis. The hydraulic equilibrium time and other limitations of sensors in establishing the hydraulic characteristics are not available for the compacted soils. Thus the influence of initial soil moisture content, initial wetting condition of the suction sensor, and soil compaction density on the hydraulic equilibrium time of the tensiometer was studied on riverbank sand. A laboratory method considering the equilibrium data at each saturation/drying state was proposed for establishing the equilibrium-SWCC data. The hydraulic equilibrium time of the tensiometer for the studied soil was found to vary from a few hours to several days depending upon the magnitude of suction and state of the sensor. The conventional method for estimating the instantaneous SWCC severely overestimated the suction values at a given water content. The sensor response time was found to be the single most influencing factor in the estimation of soil hydraulic characteristics. The in-situ tensiometer application should be replaced with the moisture sensor readings 1 3 Vol:. (1234567890) to positive within the riverbank is an important triggering factor for riverbank failure and an important factor for predicting riverbank instability (Wolman, 1959;Hooke, 1979;Thorne 1981;Rinaldi et al. 2004;Das and Bharat, 2020). The instantaneous changes in the pore-pressures in the riverbanks are often monitored (Nam et al. 2010) by placing the suction and moisture sensors and consequently estimating the factor of safety against failure. Further, instantaneous changes in the suction are required to understand the performance of the capillary barrier systems (CBS) in slope protection. The instantaneous suction data are obtained by embedding the tensiometers in the CBS Li et al. 2021). The relationship between the volumetric water content (VWC) and the corresponding soil suction is called the soil-water characteristic curve (SWCC), which is an important constitutive relationship for stability analysis.
Instantaneous measurement of VWC, in full range, is achieved by the sensors that measure the dielectric constant of a soil based on the capacitance/frequency domain technique. However, the major challenge in developing the SWCC of a given soil is the accurate measurement of the soil matric suction in the field or in the laboratory. Techniques such as osmotic method (Delage et al. 1998;Gapak et al. 2017;Gapak and Tadikonda, 2018), chilled mirror hygrometer (Leong et al. 2003), vapor equilibrium technique (Delage et al. 1998;Tang & Cui, 2005;Sun et al. 2014;Tripathy et al. 2014;Gapak et al. 2017;Gapak, 2018, 2021;Gapak and Tadikonda, 2018), axis-translation technique (Tripathy et al. 2014;Sobti and Singh, 2017), relative humidity (RH) sensors (Agus and Schanz, 2005), tensiometer (Take and Bolton, 2003), and fixedmatrix porous ceramic discs (Tripathy et al. 2016) are widely used to control or measure the soil suction. Only limited techniques, viz., tensiometers and fixed-matrix porous ceramic discs, are feasible for the estimation of continuous suction data while water content changes for slope monitoring applications and in the laboratory. These techniques are most often used in combination with the moisture sensors to record continuous data of the VWC and suction to establish the SWCC when the water content of the soil is allowed to change by ponded water for wetting SWCC (Yang et al. 2004;Li et al. 2005;Cui et al. 2008;Lins et al. 2009;Hou et al. 2019) and water drainage/evaporation for drying SWCC (Rassam and Williams, 2000;Li et al. 2005;Lins et al. 2009;Deka and Sreedeep, 2015;Chetia and Sekharan 2016). The estimated SWCC data by this methodology is termed as instantaneous-SWCC data in this work, as this methodology estimates the temporal response of the sensors without establishing the hydraulic equilibrium with the wet soil.
Recent studies showed that the continuous evaluation of transient suction data by fixed-matrix porous ceramic disc sensor in a loose soil state is erroneous (Tripathy et al. 2016;Karagoly et al. 2018) due to insufficient hydraulic equilibrium time between the sensor disc and the soil. Thus, the stability analysis based on the instantaneous measurement of suction by tensiometer, a commonly used field technique for riverbanks and slopes (Nam et al. 2010), requires reevaluation of the applicability in a compacted state. The influence of hydraulic equilibrium time on the soil hydraulic characteristics in the compacted state, as presented in the field, is required for assessing the stability evaluations. However, there is limited literature available on the hydraulic equilibrium time of suction sensors in soil (Tripathy et al., 2016). Therefore, the objective of the current study is defined as 1. To investigate the influence of tensiometer response time by varying the initial moisture content of the river bank sand, the initial condition of the sensor, and the soil compaction density to explore the cause of incorrect suction measurement. 2. Development of SWCC estimated by establishing the hydraulic equilibrium between the sensor and river bank sand, termed equilibrium-SWCC data and compared with instantaneous -SWCC data.
The sensor response time was found to be the single most influencing factor in the accurate SWCC estimation, and a significant deviation was found between the instantaneous-SWCC data and the equilibrium-SWCC data. The influence of hydraulic equilibrium time on the predicted hydraulic conductivity function was also significant.

Materials
River bank sand (RBS) was procured from the Brahmaputra river bank of Goalpara, Assam, India, for the present study. Air-dried RBS passing through a 2 mm sieve was used in the experiments. The particle size analysis of RBS was determined by the standard test method (ASTM D422-63, 2007). The studied RBS was classified as poorly graded sand (SP) as per the standards. The specific gravity (G s ) was determined by the density bottle method (ASTM D854-92, 2014). The minimum and maximum density, as determined by the relative density test, were 1.34 Mg/m 3 and 1.60 Mg/m 3 , respectively (IS: 2720 Part 14, 1983). The basic geotechnical properties are summarized in Table 1.
A commercial suction sensor, T8-tensiometer (Decagon Devices, Inc.), was used to study the influencing parameters on the suction estimation due to changes in the water contents. The tensiometer estimates the suction in the range of + 100 to − 85 kPa with an accuracy of ± 0.5 kPa. A small ceramic cup is attached to a tube filled with de-aired water, which is connected to a piezoelectric pressure measuring device. When a tensiometer is placed in the soil sample, a small amount of water is exchanged between the soil sample and the high air entry (HAE) ceramic cup of the tensiometer to transmit the negative pore water pressure in the soil sample to the water reservoir inside the tensiometer through a pressure transducer (Lu and Likos, 2004;Oliveira and Marinho, 2008;Wijaya and Leong 2016). The continuity of water between the ceramic cup and soil-water becomes crucial for precise measurement. A commercial moisture sensor, 5TM (Decagon Devices, Inc.), was used along with the suction sensor for measuring the volumetric water content (VWC) of the studied soils. The moisture sensor was calibrated (Topp et al. 1980) on the RBS. A detailed working mechanism of the tensiometer is found elsewhere (Lu and Likos, 2004;Fredlund et al. 2012). The influence of different parameters on the suction estimation was investigated using the procedure outlined in the following sections.

Influencing Parameters on Suction Measurement
The hydraulic equilibrium time of the tensiometer in RBS was determined in both loose and compacted conditions using an initially-wet and initially-dry tensiometer for the suction measurement. The tensiometer was immersed in distilled water in the initiallywet method before placing it in the soil. The suction reading of the initially-wet tensiometer was 0.0 kPa. In the initially-dry test, the air-dried tensiometer in the ambient laboratory conditions was used, where the suction reading was more than the soil suction.
Further, the hydraulic equilibrium time in the loose and compacted soil conditions was estimated. In the loose condition, the tensiometer was inserted in a soil-water mixture having a predetermined weight of soil and water, which in turn was wrapped in a sealed plastic bag (Fig. 1a), similar to the recent study by Tripathy et al. (2016). Tensiometer was secured in place to ensure proper contact with the soil. On the other hand, compacted soil in a cylindrical mold of diameter 140 mm and a height of 150 mm was used for studying the hydraulic equilibrium time in the compacted soil. The reconstituted RBS at the field compaction state was prepared by thoroughly mixing the oven-dried soil with a predetermined amount of water to obtain the targeted water content and then compacted in a cylindrical polyvinyl chloride (PVC) mold to ρ d = 1.5 Mg/m 3 (Fig. 1b). A sharp-edged dummy rod with a diameter slightly less than the tensiometer was used to facilitate the easy insertion of the tensiometer into the compacted soil sample. The soil sample was covered with aluminum foil to avoid evaporation loss of water, as shown in Fig. 1b. The corresponding suction with the elapsed time was recorded continuously using an EM50 data logger (Decagon Devices, Inc.) in both loose and compacted conditions. The suction reading was recorded until the monitored suction showed a negligible variation with time. A similar procedure was adapted with different initial soil moisture contents to find the effect of water content on the hydraulic equilibrium time of the suction sensor.

Instantaneous SWCC
The wetting and drying SWCC experiments by the conventional method employed in the literature were conducted in the PVC column ( Fig. 2a), as mentioned earlier. The RBS was compacted in the column at a dry density of 1.5 Mg/m 3 at air-dry water content. The tensiometer and moisture sensors were kept in the soil column for continuous suction and water content measurement, respectively, as adopted by many researchers for field monitoring and lab studies. The tensiometer was inserted such that its ceramic tip was at a depth equal to the mid-height of the moisture sensor. The water in the standpipe was allowed to flow into the soil column and pond the water by keeping the valve open for wetting the soil, which simulated the complete wetting condition of a river bank due to flooding. The complete wetting of the soil was ensured when the sensors readings were constant and the water level reached about 10 mm above the soil column. On the contrary, the water was allowed to drain out from the soil column for the drying SWCC tests to simulate the rapid drawdown condition. Further, it was kept below the fan for continuous drying. The suction and moisture content data were recorded continuously to obtain the instantaneous-wetting and drying SWCC data of the soil, as followed conventionally, without establishing the hydraulic equilibrium between the sensor and the soil. Schematic of a measurement of instantaneous and equilibrium SWCC using T8 tensiometer b hanging column apparatus for hysteresis behavior of SWCC As this method determines the instantaneous data without establishing the hydraulic equilibrium between the RBS and the suction sensor, the following procedure was proposed to achieve the hydraulic equilibrium in the SWCC estimation in the laboratory.

Equilibrium SWCC
The wetting and drying SWCC data, while allowing complete hydraulic equilibrium between the soil and suction sensor at controlled and small variations in the water content, were also established in a similar set-up used for the instantaneous data. The water in the standpipe was released into the soil column by opening the valve only intermittently in this method to allow only a small amount of water to wet the soil in a stepwise manner. The valve was closed, and sufficient time was allowed for the water to distribute in the soil and bring the sensor to the hydraulic equilibrium with the soil. A hydraulic equilibrium was assumed to have been reached when the measured suction variation remained within about ± 0.5 kPa for 6 hours. All tests were run for at least 24 h or until hydraulic equilibrium was achieved. The valve was opened to allow more water after the sensor readings were constant, and the process was repeated until full wetting of the soil. To estimate the equilibrium drying SWCC, the ponded water in the soil column was allowed to drain out in a stepwise process while allowing equilibration of the suction sensor in each step until achieving the complete drying.

Hysteretic Behavior of SWCC
A hanging column method (ASTM D6838-02 2008) was used to establish the hysteretic SWCC in the same PVC pipe used for the other experiments. The soil column was compacted dynamically in different layers of 50 mm thickness. The moisture sensors 1, 2, 3, 4, 5 & 6 were embedded in the soil column at the heights of 250, 350, 400, 450, 500, and 550 mm above the base plate, respectively, as shown in Fig. 2b during the soil preparation in the column. The standpipe was placed at the base level of the soil column, and the water in the standpipe was released into the soil column by opening the valve. The water level increased into the soil column by capillary action and became constant after some time. Further, sufficient time was allowed for water to uniformly distribute in the soil and bring the sensor to equilibrium with the wet soil. The difference between the water level in the standpipe and the moisture sensors level in the soil column was measured as negative head, h, as shown in Fig. 2b. The corresponding suction was obtained by multiplying the negative head with acceleration due to gravity (g) (Buckingham, 1907). Subsequently, the standpipe was brought to an increased elevation, the water level further increased in the soil column, and the corresponding suction was measured as explained above. This process was repeated in a stepwise manner until the complete wetting of the soil took place. Further, the standpipe was brought down to some decreased elevation; the water will drain out from the soil column and reach a certain equilibrium water content. The suction corresponding to this water level difference was measured as explained above. This process was repeated in a stepwise manner until the standpipe was again returned back to the base level of the soil column. In this way, one wetting and drying cycle was accomplished to obtain initial wetting and main drying curves due to hysteresis. Similarly, one more wetting and drying cycle was undertaken to obtain the main wetting and primary drying curves.

Hydraulic Equilibrium Time
When a tensiometer is placed in the soil sample, a small amount of water is exchanged between the soil sample and the high air entry (HAE) ceramic tip of the tensiometer. The recorded suction data using tensiometer with the elapsed time using initially-wet and initially-dry tensiometer in the loose and compacted states at different initial water content conditions of RBS were presented in Fig. 3a-d. The suction values increased with time with the initially-wet tensiometer and decreased with the initially-dry sensor before reaching the equilibrium. The required hydraulic equilibrium time and the corresponding measured suction for different initial water contents of RBS using an initially-wet and initially-dry sensor conditions are shown in Table 2. The hydraulic equilibrium time for the tensiometer varied from a few hours to several days depending upon the magnitude of suction and initial condition of the sensor (i.e., wet or dry). Further, a higher hydraulic equilibrium time was observed for the sample having a higher initial suction value. A similar finding was also reported by (Oliveira and Marinho, 2008).
Moreover, it is noteworthy to point out from Fig. 3 that the suction equilibrium time was found to be in the range of 0.2-4 days with initially-dry sensors and 0.6-14.58 days with initially-wet sensors at all different moisture conditions under loose conditions. Similarly, it was observed to be in the range of 0.05-0.4 days with initially-dry sensors and 0.4-1 day with initially-wet sensors at all different moisture conditions under compacted conditions (refer to Table 2). The suction equilibrium time was found to be smaller with initially-dry sensors as compared to initially-wet sensors at all different moisture conditions under both loose and compacted conditions. This can be attributed to the higher hydraulic conductivity of RBS than the HAE ceramic tip, which allows an easy flow of water from the saturated soil to the HAE tip when the sensor is dry. Moreover, the compacted soil sample took less time to achieve hydraulic equilibrium than the sample in loose conditions with both initially-wet and dry sensors (Table 2). This may be due to the lesser tortuous pathways and availability of continuous flow channels in an adequately compacted soil compared to loose soil. In the case of loose soil, the tortuosity is more due to the presence of more air in the soil, and hence more time is required to reach the equilibrium. Figure 4 shows the initial water content and equilibrium suction value obtained using both initiallywet and dry sensors in loose and compacted conditions. The measured suction under equilibrium conditions was found to be slightly higher for initially-dry sensors than that of initially-wet sensors. A   Fig. 3 Hydraulic equilibration time of T8 tensiometer at different soil water contents by a initially wet sensor in loose condition, b initially dry sensor in loose condition, c initially wet compacted condition, and d initially dry in compacted condition slight difference between the measured suction value with initially-wet and dry sensors can be described based on water exchange between the soil and the HAE ceramic tip leading to the hydraulic equilibrium. When an initially-wet tensiometer was used for suction measurement of a soil sample, a small amount of water was withdrawn through the saturated HAE ceramic tip to the soil sample to achieve hydraulic equilibrium. In this case, the saturated HAE ceramic tip of the tensiometer undergoes a marginally drying process, whereas the soil sample undergoes a slightly wetting process. In contrast, when an initially-dry tensiometer was used for measuring the soil suction, a marginal amount of water flows from the soil sample to the HAE ceramic tip of the tensiometer to attain the hydraulic equilibrium. In this case, the HAE ceramic tip of the tensiometer undergoes a slight wetting process, whereas the soil undergoes a slightly drying process. Therefore, the final equilibrium suction value was observed to be less for the initially-wet tensiometer as compared to the initially-dry sensor due to the hysteretic effect. The hysteretic effect of the HAE ceramic tip of the tensiometer on the SWCC can be seen in Fig. 4. In both loose and compacted conditions, the drying SWCC (i.e., the soil is drying by initially-dry sensors) remains above the wetting SWCC (i.e., the soil is wetting by initially-wet sensors), which is consistent with the hysteretic trend of the other soils, but the effect is marginal. Therefore, for an accurate estimation of the SWCC of soils utilizing the tensiometer, sufficient equilibration time needs to be provided based on whether the sensor is initially-dry or wet.

Instantaneous and Equilibrium SWCC
The temporal variation of volumetric water content (VWC) and suction during the wetting and drying processes for establishing instantaneous-SWCC data under the ponding condition were shown in Fig. 5a, b. During the wetting process, the VWC of the soil, as recorded by the 5TM moisture sensor, increased rapidly, but a change in suction was negligible until t = 0.125 h. Moreover, the soil column nearly took 0.25 h for complete saturation, but the tensiometer reading showed a constant suction value (ψ = 0 kPa) after half an hour. This was mainly due to the requirement of a large equilibrium time for the tensiometer in comparison to the moisture sensor. In contrast, the  disparity between the suction and moisture sensors was not observed in the drying process, wherein a decrease in moisture content and increase in suction occurred nearly simultaneously (Fig. 5b). This was attributed to the slower drying rate of evaporation, where the suction sensor got sufficient time to achieve equilibrium with the soil. The drying process was run for nearly 200 h, and during that time interval, the soil suction reached 40 kPa.
The temporal variation of VWC and suction, measured during the stepwise wetting and drying process in the equilibrium-SWCC method estimation for RBS, was presented in Fig. 6a, b. During each stage, sufficient time was allowed for the moisture sensor and suction sensor to achieve hydraulic equilibrium with the soil. Therefore, a steady decrease in suction was observed for the studied soils, unlike in the previous case due to wetting (Fig. 5a). The RBS took 32 days to undergo complete saturation by the proposed method (Fig. 6a), while the soil drying process took 20 days, and during that time interval, soil suction reached nearly 40 kPa, as shown in Fig. 6b. The time required to achieve equilibrium VWC and suction at each stage varied for all the tests, as shown in Fig. 6a, b. At each wetting-drying step, the equilibrium values of suction and VWC were recorded to obtain the equilibrium point on the SWCC. Further, these equilibrium data points were used to establish the equilibrium-SWCC, as shown in Fig. 7. Figure 7 shows the measured wetting and drying SWCC for the studied soil obtained by combining Fig. 6 Temporal variation of VWC and suction measured from equilibrium data during a wetting and b drying (b) (a) the tensiometer and moisture sensors for the instantaneous and equilibrium data. A significant difference in the instantaneous and equilibrium wetting SWCC data was observed. The instantaneous-SWCC data indicated that the suction remained constant for a wide VWC variation during the wetting process, which shows the erroneous SWCC measurement. Thus, instantaneous wetting-SWCC data should not be considered for in-situ stability analysis during rainfall. On the other hand, the equilibrium-SWCC data showed a steady decrease in suction with an increase in VWC as sufficient time was allowed for the tensiometer to reach equilibrium with the soil at any given water content. Moreover, the equilibrium and instantaneous SWCC showed a smaller disparity during the drying process, as the soil sample took considerable time for the natural drying process, which enabled the suction sensors to reach near-equilibrium conditions. Similar trends were also reported in the previous literature for instantaneous drying SWCC, where natural drying of soil occurs by evaporation. The equilibrium-SWCC data represented a typical SWCC trend observed for most of the clays in the literature by different methods (Agus and Schanz, 2005;Tripathy et al. 2014Tripathy et al. , 2016Gapak and Tadikonda, 2018). Therefore, the application of tensiometer for the field applications should be limited as the required equilibration time for suction estimation would be longer than the water content changes in the in-situ soil due to environmental changes, especially during the rainfall events and submerged river bank conditions. Thus the proposed equilibrium SWCC method along with the moisture sensor readings could provide the temporal suction variations for in-situ stability analysis as the moisture sensor's response time is significantly small.

Hysteretic Effect
The SWCC data from the hanging column method were obtained at very small suction values and nearly coincided with the proposed equilibrium approach. The wetting-drying SWCC data obtained from the proposed equilibrium approach and hanging column method were presented together to represent the hysteretic behavior of the RBS, as shown in Fig. 8. The initial wetting and main wetting data were represented by different filled markers with different colors. Similarly, the unfilled markers represent main drying and primary drying. The variation between the water contents at any given suction value was significantly high, but variation decreased in subsequent wetting-drying cycles. Hysteresis in the SWCC indicates that for any specific matric suction value, the VWC in the soil is not unique. For both the wetting and drying cycle, the drying SWCC retains more water content than the wetting SWCC for the same magnitude of suction, which is consistent with the previous  (Lu and Likos, 2004;Fredlund et al. 2012;Gapak and Bharat, 2018). This is attributed to the different contact angles between wetting and drying at the soil particle-pore water interface, non-homogenous pore size distribution, and entrapped air (Lu and Likos, 2004). The hysteretic effect was predominant in the medium suction range, while the wetting and drying curves coincide at the saturation region and near residual water content region. This trend was in good agreement with the earlier work (Pham et al. 2003).
Several advanced models are available to predict the SWCC behavior of soil, considering the compaction rate and microstructure of soil (Kodikara et al. 2020;Ranaivomanana et al. 2022). Since pure sand is used the current study, therefore, the microstructural perspective of the soil has not been considered. It is established that the van-Genuchten equation (vG, 1980) fitted the SWCC data in a satisfactory way for sandy soil. Thus, in this study, the experimental equilibrium SWCC data were best fitted using the van-Genuchten equation (vG, 1980) with retention curve (RETC) software. The model parameters were obtained using the RETC program with m parameter constrained by n parameter as m = 1 − (1/n), and the fitting parameters were presented in Table 3. The SWCC data fitted in a satisfactory way with an empirical Van Genuchten equation, which demonstrates the accuracy of the measurement method.

Hydraulic Characteristics
The hydraulic conductivity functions (HCFs) from the SWCCs using van-Genucten-Mualem (vGM, 1980). The van-Genuchten-Mualem (vGM, 1980) model is most widely used for sand, silt, silty loam, and clayey soil to predict the hydraulic conductivity function using SWCC data. However, the model shows a large deviation in the predicted hydraulic conductivity for clayey soil (Van Genuchten, 1980). Since pure sand is used in this study, therefore, this model can simulate the hydraulic conductivity very well. The hydraulic conductivity function in terms of normalized volumetric water content is given by is the normalized volumetric water content. is the volumetric water content, s is the saturated water content and r is the residual water content.
In this study, the HCFs is determined using the RETC program with m parameter constrained by n parameter as m = 1 − (1/n), as shown in Fig. 9. A significant difference in the HCF predicted from the transient-SWCC was observed between the wetting and drying paths, as shown in Fig. 9a. Further, a wide variation in the predicted hydraulic conductivity was found for the constant suction during the wetting process, while during drying, the hydraulic conductivity decreased with an increase in the suction. On the other hand, the hydraulic conductivity predicted from equilibrium-SWCC data showed a steady decrease in the conductivity with an increase in the suction during the wetting and drying paths. Similar trends were also reported by various researchers (Singh and Kuriyan, 2003;Zhai and Rahardjo, 2015;Sobti and Singh, 2017). As the water content reduces in the soil with increased suction, the air enters into the largest pore of the soil matrix. The connected paths available for water flow shrink to become smaller and more tortuous with the increase in suction, which causes a reduction in the hydraulic conductivity. Thus, it is concluded from the experimental laboratory results that for studied soil, the instantaneous SWCC varied significantly when compared to the equilibrium SWCC during the wetting process. As the rainfallinduced slope failures require accurate knowledge of the soil hydraulic characteristics during the rainfall infiltration (wetting phenomenon), the applicability of the instantaneous SWCC to the development of early warning facilities remains questionable.

Summary and Conclusions
The hydraulic equilibrium time of the tensiometer for river bank sand was studied using initially-wet or dry sensors for varied moisture contents with both loose and compacted states. Further, the laboratory wetting and drying studies in compacted soil columns with embedded suction and moisture sensors were undertaken to understand the influence of response time on the estimated wetting and drying SWCC of the soil. The instantaneously estimated SWCC was evaluated and compared against the proposed approach, which considers the hydraulic equilibrium between the sensors and wet soil. In addition, the hysteretic behavior of sandy soil was also studied using the hanging column method. Moreover, the hydraulic conductivity of the studied sand was also estimated from transient and equilibrium SWCC. The following conclusions were drawn based on the detailed analysis: • The hydraulic equilibrium time is influenced by the initial state of the tensiometer, i.e., initiallywet or dry, for varied moisture contents and the initial state of the studied sandy soil. Moreover, the equilibrium time is also dependent on whether the soil is in a loose or compacted state. A significant time is required for the tensiometer to read the equilibrated suction data, and much more time might be required for fine-grained soils. • The conventionally adopted laboratory procedure for the SWCC estimation significantly deviated from the proposed procedure, which considered the sufficient equilibration between the soil and the tensiometer sensor. The proposed procedure showed good agreement with the hanging column method in the limited measured suction range. The deviation between the measured SWCC by proposed and conventional methods increased with the wetting/drying rate. Thus disparity was less significant in slow drying cases when compared to quick wetting in the ponding condition. The SWCCs are overpredicted by the conventional procedure. • The estimated drying and wetting hydraulic conductivity functions by the conventional method significantly deviated from the proposed method, which can significantly overestimate the flow rates in the soil. • The application of tensiometer for in-situ application should be limited as the required equilibration time for suction estimation would be much longer than the water content changes in the in-situ slopes due to environmental changes. The measured deviation is significantly higher during rainfall events and submerged river bank conditions. Thus the proposed equilibrium SWCC laboratory method, along with the moisture sensor readings, could provide the temporal suction variations at in-situ for the stability analysis.
To study the mechanism of suction equilibrium time in a simplistic way, coarse grain soil such as river bank sand was selected in this study. This concept can be further extended for other types of soil using different types of sensors.
Funding The authors received no specific funding for carrying out this research work.
Data Availability Some or all data used are available from the corresponding author by request.

Conflicts of Interest
The authors have no conflicts of interest to declare that are relevant to the content of this article.