Evaluation of suction measurement by the tensiometer and the axis translation technique S.D.N. Lourenço, D.G. Toll, C.E. Augarde, School of Engineering, Durham University, Durham, UK D. Gallipoli Department of Civil Engineering, University of Glasgow, Glasgow, UK F.D. Evans Controls Testing Equipment Ltd, Wykeham Farrance Division, Tring, Hertfordshire, UK G.M. Medero Department of Civil Engineering, Heriot-Watt University, Edinburgh, UK ABSTRACT: The axis translation technique is a well-established method for imposing values of suction in unsaturated soil samples. High-suction tensiometers are more recently developed devices used for measuring pore water pressures in soils, including negative pore water pressures (i.e. suctions) below absolute zero. Both these techniques are comparable in terms of the suction range in which they operate. In this work a tensiometer has been used to measure suction values imposed by the axis translation technique in kaolin samples inside a pressure plate and a triaxial cell. The tensiometer has been kept in contact with the soil sample to track pore water pressure variations throughout the duration of the tests. The suctions measured by the tensiometer have been compared to those imposed by the axis translation technique and it was found that the suction measured by the tensiometer was always smaller than that imposed. Two scenarios are proposed to explain this. The first scenario considers the presence of water inside and below the high air entry value ceramic plate whereas the second one hypothesises the lack of equilibrium in terms of soil water content when suction is measured. The latter scenario seems to be supported by the evidence in the literature of equilibration times for pressure plate tests that are significantly longer than those reported for the present testing programme. Implications of both scenarios for laboratory testing are discussed. 1 INTRODUCTION The axis translation technique (Hilf, 1956) is commonly used in unsaturated soil mechanics for imposing matric suctions in samples. In this technique, the pore air pressure and the pore water pressure are raised by the same amount so that the matric suction (given by their difference) is kept constant. In this way, the pore water pressure can become positive, thus avoiding water cavitation inside the experimental set up. The technique is employed in the pressure plate device, which consists of a high air entry value ceramic plate saturated by water and acting as a separation filter between the air above and the water below. Soil samples are placed on the ceramic plate and the suction is imposed by controlling independently both the air pressure and the water pressure on the two sides of the plate. The range of suction over which the technique can be applied depends on the air entry value of the ceramic plate (usually between 5 kpa and 15 kpa) and the capacity of the compressor controlling the air pressure. High-suction tensiometers (Ridley and Burland, 1993) are relatively new devices used for the direct measurement of pore water pressures in soils, including negative pore water pressures below absolute zero. Tensiometers are usually employed under conditions where the air pressure is atmospheric and the matric suction is given by pore water under tensile stress, which is directly measured by the tensiometers. High suction tensiometers can be schematically divided into three parts (Figure 1): a miniature water reservoir, a pressure transducer measuring the water pressure inside the reservoir and a high air entry value porous stone acting as a separation filter between the reservoir on one side and the soil on the other side. reservoir porous stone transducer Figure 1. Schematic of the tensiometer used for this research (Lourenço et al., 26).
Work done by Guan (1996) and reported in Guan and Fredlund (1997) used a modified pressure plate to perform a standard drying test by increasing pore air pressure in an initially saturated sample to impose a given value of suction (water pressure was at atmospheric pressure). Subsequently, the air pressure was instantaneously released to atmospheric pressure while a high suction tensiometer, placed in contact with the soil, simultaneously measured the corresponding drop in pore water pressure. This procedure was used to assess the accuracy of the tensiometer calibration over the negative range of pressures. The authors observed that the suction measured by the tensiometer was less than the suction imposed via the pressure plate. The same was observed by Lourenco et al. (26) in similar tests. The tests conducted by Guan and Fredlund (1997) had a unique feature: water in the compartment below the porous stone of the pressure plate was flushed out before the air pressure decrease. No details are provided but it is believed that this was to avoid water flowing from the water compartment to the sample and therefore to avoid a further decrease of suction during the air pressure decrease. It is the purpose of this paper to evaluate the suction measurement of samples prepared at the same initial conditions by the tensiometer and the pressure plate. In this work, a conventional pressure plate sold commercially by Soil Moisture Corporation as well as a triaxial cell, whose pedestal was fitted with a high air entry value ceramic plate, were used. In the pressure plate the compartment below the ceramic plates was always full of water. In the triaxial cell, the compartment could be full or empty of water (but with the ceramic plates always saturated) (Figure 2). Then the procedures and results of the testing programme will be shown and the implications for laboratory testing of unsaturated soils will be discussed. 2 DIRECT SUCTION MEASUREMENT VERSUS AXIS TRANSLATION TECHNIQUE The axis translation technique imposes a value of suction by raising the air pressure above atmospheric value with the water phase kept either at atmospheric or at a given positive value smaller than the air pressure. This forces water to move from (or into) the soil through the ceramic plate. Water will move to or from the compartment below the plate depending on whether the imposed suction is smaller or higher than that initially present in the soil sample. Once equalisation is achieved, no more transfer should occur and the water content in the sample should remain constant at the equilibrium value corresponding to the imposed suction (Figure 3). Figure 3. Working principle for the tensiometer (above) and pressure plate (below). Figure 2. Experimental set-up. In the following part of this paper, the working principle of both the tensiometer probe and the axis translation technique will be initially reviewed in more detail including limitations and terminology. The tensiometer measures directly the water tensile stress existing in the soil pores. After the porous stone of a tensiometer is placed in contact with a soil sample with a negative pore water pressure, an initial equilibration phase takes place whereby a small volume of water is sucked out from the reservoir through the porous stone into the soil producing a deformation of the transducer diaphragm in the di-
rection of the soil. This deformation is transferred to a strain gauged diaphragm from which pressure can be measured. Once this transfer ends, all water inside the tensiometer as well as in the soil will have the same value of negative pressure. The volume of water transferred from the reservoir to the soil is small enough so that it can be considered negligible, and therefore the water content of the sample is not affected. The working principle for both the axis translation technique and the tensiometer are schematically illustrated in Figure 3. One of the main limitations of the pressure plate device is related to the presence of air diffusion through the ceramic plate (e.g. Padilla et al., 26), which needs to be accounted for when the change in water content of the sample is measured by means of volume gauges connected to the water compartment below the ceramic plate. For the tensiometer, the range of measurable suctions is primarily limited by the occurrence of cavitation inside the probe, which is in turn governed by the degree of saturation of the porous stone and reservoir (e.g. Guan and Fredlund, 1997; Lourenco et al., 26). Suction measurements by the tensiometer also appear to be sensitive to temperature as shown by Toker et al. (24). maintained at the atmospheric value. As soon as the air pressure was raised, the tensiometer (placed on the top of the sample) recorded a positive excess pore water pressure, which subsequently started to dissipate. Once the pore water pressure read by the tensiometer dropped back to zero, it was assumed that equilibrium was achieved throughout the sample. The air pressure was then reduced to the atmospheric value and the corresponding negative pore water pressure generated inside the sample was measured by the tensiometer. Increasing values of suction were applied and measured on the sample in a sequence up to a maximum value of 5kPa corresponding to the air entry value of the ceramic plates in both the triaxial cell and the pressure plate. The tests performed in the pressure plate and in the triaxial cell differed in one respect. In the triaxial cell, after suction equalized kpa and before releasing the air pressure to zero, water was flushed out below the ceramic plate by air circulation. Once the air pressure was dropped and the reading from the tensiometer was taken, water was restored below the ceramic for the application of the next suction stage. In the pressure plate, water at atmospheric pressure was present in the compartment below the ceramic plate throughout the entire test. 3 TESTING PROGRAMME, EQUIPMENT AND MATERIAL If a given suction is imposed in a soil sample by using the axis translation technique, one would expect that an equal value of suction would be read by a tensiometer when placed in contact with the same sample. In order to verify this, tests were conducted by imposing given values of suction on Speswhite kaolin samples in the pressure plate while a tensiometer was placed in contact with the sample to track pore water changes throughout the test. A kaolin slurry was prepared at a water content of 2% and was deposited directly on the previously saturated ceramic plates inside the triaxial cell and the pressure plate. In order to avoid spreading, the slurry was placed in a cylindrical mould (diameter 38mm) with open top and bottom ends. The tensiometer was then set directly on the top surface of the kaolin slurry and a plastic mesh was also used to keep the tensiometer in the right position during the test, i.e. to avoid it falling or tilting. The tensiometer used in this work has a nominal measuring capacity of 1 kpa in both the positive and negative ranges. The tensiometer was previously saturated and calibrated according to procedures described in Lourenço et al. (26). Suction was imposed in the sample inside the pressure plate by quickly raising the air pressure to the required value while pore water pressure was 4 RESULTS AND DISCUSSION Figure 4a shows the results for the test performed in the pressure plate. Inspection of Figure 4a indicates that, after the air pressure was increased to 187.6 kpa, the pore water pressure measured by the tensiometer instantaneously increased by 17 kpa and then progressively dissipated back to zero. After equilibrium was achieved, air pressure was reduced to zero and this induced a reduction of the pore water pressure from zero to -149.2 kpa, i.e. a reduction about 2% smaller than the corresponding reduction in air pressure. Subsequently, the air pressure was increased again to a higher level of 396.5 kpa and, after dissipation of the excess pore water pressure from 16 kpa, was reduced again to zero. Also in this case, the corresponding reduction of pore water pressure from zero to -278.5 kpa was about 3 % smaller than the corresponding reduction of air pressure. For the case of Figure 4b, the air pressure was increased in 4 stages to 99.6 kpa, 2 kpa, 299.4 kpa, and 4kPa. The measured water pressures were 95.7 kpa, 191.4 kpa, 284.9 kpa, and 381.6 kpa, respectively. Comparing to Figure 4a, the imposed and measured suctions are smaller. Table 1 shows for each test the difference, in percentage, between the applied suction and measured suction. This difference was seen to vary throughout the air pressure releases. For instance, for test T14 in
Figure 4a for the first air pressure release the ratio 149.2/187.6 gives 2.4%. In the second release this ratio gives 3.5%. Therefore during each air pressure release this difference became greater. a) b) 4 2-2 u a=187.6 kpa u w=-149.2 kpa u w=-278.5 kpa -4 5 1 15 2 6 4 2-2 -4 u a u w u a=396.5 kpa -6 2 4 6 8 Figure 4. Axis translation tests with u w measured with the tensiometer. (a) Test T14 conducted in the pressure plate and, test T4 conducted in the triaxial cell. Results from all tests are shown in Figure 5 where the suction imposed by the axis translation technique is plotted against the suction measured by the tensiometer. The difference between the measured and imposed suction were larger when the pressure plate was used. Previous work by Guan and Fredlund (1997) showed similar results, with the suction measured by the tensiomer smaller than the suction imposed by the axis translation technique by a margin ranging between.5% to 8.5%. Table 1. Difference between the imposed and measured suctions for each test. Test nr Device Difference (%) T14 Pressure plate 2.4-3.5 T17 Pressure plate 12.5-18.2 T31 Triaxial cell 1.5-11. T35 Triaxial cell 4.6-1.5 T36 Triaxial cell 12.5-16.25 T38 Triaxial cell 14.2-17.2 T39 Triaxial cell 4.3-6. T4 Triaxial cell 4.3-4.6 Figure 6a shows an expanded view of the final part of the test shown in Figure 4a. It can be seen that, after the instantaneous initial drop in pore water pressure, the pore water pressure slowly rises over a period of about 2 minutes until it stabilises at a value of approximately -73kPa. A similar result is shown in Figure 6b, which presents part of a test carried out in the triaxial cell where the pore water pressure recorded by the tensiometer, after an initial instantaneous drop, rises under constant air pressure and stabilises at a value of -335kPa.This was a common feature of behaviour observed in all tests where the air pressure was maintained at zero for some time, after reducing it from the imposed value of suction. This result might be a consequence of the availability of free water inside the ceramic plate or below it. This water is sucked into the sample under the action of the negative pore water pressures generated by the air pressure drop, thus increasing water content and reducing soil suction. measured suction (kpa) 4 3 2 1 expected pressure plate triaxial cell 1 2 3 4 imposed suction (kpa) Figure 5: Imposed versus measured suctions for all tests. A consistent result emerging from this work, as well as previous work by Guan (1996), is that the instantaneous pore water pressure decrease recorded by the tensiometer is generally smaller than the imposed drop of air pressure (Figure 6a and Figure 6b). Two possible explanations are provided here to interpret this result. One possibility is that, despite the air pressure drop being applied almost instantaneously, some water is still sucked back into the sample, which limits the magnitude of the measured pore water pressure reduction. This explanation seems consistent with the observation that pore water pressure reductions tending to be proportionally smaller for tests carried out in the pressure plate, where water is permanently present below the ceramic plate, than for tests carried out in the triaxial cell, where water below the ceramic plate is flushed out before each air pressure drop. A second possibility is that the water content in the soil sample had not yet come to equilibrium, despite the pore water pressure having done so. Equilibrium was assumed to be achieved at each imposed value of suction when the tensiometer read a value of zero pore water pressure. After this condition was attained, the air pressure was decreased and the corresponding negative pore water pressure drop was
measured by the tensiometer. However, although the pore water pressure is equal to zero throughout the specimen, it is possible that water content is still reducing inside the sample due to a slow rearrangement of water menisci at the interface between gas and liquid phases inside the pores. Such a hypothesis seems to be supported by the observation that pressure plate tests published in the literature (where the achievement of equilibrium is based on the measurement of the sample mass during equalisation) usually require significantly longer time than the tests reported in this work (where the achievement of equilibrium is based on the dissipation of the excess pore water pressures measured by the tensiometer). For example, the tests shown in Figure 4a and Figure 4b, both which involved imposing more than one suction value to the sample, took overall 4 hours and 5 days respectively. Tinjun et al. (1997) and Vanapalli et al. (1997) reported equalisation times for clay samples of 5-8 days and 6-7 days respectively for each imposed value of suction in the pressure plate. However, both authors did not measure the sample s mass, equilibrium conditions were ensured when the outward flow of water from the sample stopped. a) b) 4 2-2 u a=396.5 kpa u w=-278.5 kpa -4 125 15 175 2 8 4-4 u a=599.4 kpa u w=-473.5 kpa -8 2 4 6 8 1 12 u w=-335 kpa Figure 6: Kaolin response after releasing the air pressure. a) Suction measured by the tensiometer continuously decreasing and, b) stabilizing at a 335 kpa. If the above hypothesis were true, the dissipation of pore water pressure to zero would not be enough to conclude that a given suction is imposed on the sample, as assumed by Guan and Fredlund (1997) and Lourenço et al. (26). Hence, the difference between the measured and imposed suction is simply due to lack of equilibrium in terms of water content. The tensiometer would be expected to measure suctions closer to the imposed ones if longer periods of time are waited during equalisation. A testing program is on the way to confirm this. 5 CONCLUSIONS This paper presents a series of measurements performed by high suction tensiometers on kaolin samples, which were previously subjected to different suction levels by using the axis translation technique. It was found that the suction measured by the tensiometers was always smaller than that imposed by the axis translation technique. Two different hypotheses have been put forward to justify such discrepancy. One possibility is that the smaller measured suctions are due to the absorption of water by the sample from the ceramic plate and/or the compartment below it. However, another possibility is that for each imposed value of suction equilibrium conditions had only been achieved in terms of pore water pressure but not water content. This idea is suggested by comparison with published data on the equilibrium times for pressure plate tests, which have required longer times. This might be explained by considering that water menisci at the interface between the gas and liquid phases inside soil pores take longer to re-arrange in a stable configuration after the pore water pressure has come to equilibrium. Should this hypothesis hold, then it would not be correct to assume achievement of equilibrium based on the pore water pressure read by the tensiometer but equilibrium should be assessed on the basis of subsequent sample mass measurements during the equalisation phase. Further testing is currently being undertaken to confirm or refute such a hypothesis. ACKNOWLEDGEMENTS This research was funded by the Engineering and Physical Sciences Research Council of the United Kingdom through a CASE research grant, with additional financial support from Controls Testing Equipment Ltd. Support from the European Commission via the Marie Curie Research Training Network contract number MRTN-CT-24-56861 is acknowledged. Technical support was given by Mr. C. McEleavy and Mr. S. Richardson. REFERENCES Guan, Y. (1996). The measurement of soil suction, PhD Thesis, University of Saskatchewan, pp. 331
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