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57ième CONGRÈS CANADIEN DE GÉOTECHNIQUE 57TH CANADIAN GEOTECHNICAL CONFERENCE 5

57ième CONGRÈS CANADIEN DE GÉOTECHNIQUE 57TH CANADIAN GEOTECHNICAL CONFERENCE 5ième CONGRÈS CONJOINT SCG/AIH-CNN 5TH JOINT CGS/IAH-CNC CONFERENCE USE OF THE GCTS APPARATUS FOR THE MEASUREMENT OF SOIL- WATER CHARACTERISTIC CURVES Hung Q. Pham, Graduate Student, University of Saskatchewan, Saskatoon, Canada Delwyn G. Fredlund, Professor Emeritus, University of Saskatchewan, Saskatoon, Canada J. Manuel Padilla, PhD, president of GCTS, Tempe, Arizona, USA ABSTRACT Details of the new GCTS pressure plate apparatus manufactured by Geotechnical Consulting and Testing Systems, Tempe, Arizona, are presented in this paper. Two series of tests were performed on Botkin silt and Processed silt using the GCTS pressure plate. The experimental results show that the water content versus matric suction measurements of the GCTS pressure plate is comparable to the University of Saskatchewan pressure plate apparatus. The equipment is capable of measuring both volume and water content change and following any stress path under K0 loading condition. Test results are presented for: 1) Botkin silt and 2) Processed silt. The results show that the GCTS pressure plate apparatus is capable of following drying and wetting curves and determine the volume-mass soil properties provided the soil specimen does not separate from the steel confining ring. RÉSUMÉ Cet article présente les détails du nouvel appareil de plaque de pression de GCTS fabriqué par la Geotechnical Consulting and Testing Systems, Tempe, Arizona. Deux séries de tests ont été exécutées sur le limon de Botkin et sur le limon traité utilisant la plaque de pression de GCTS. Les résultats expérimentaux montrent que les mesures de contenu d'eau et de succion matric obtenues avec la plaque de pression de GCTS sont similaires à celles qui ont été obtenues avec l’appareil de plaque de pression de l'Université de la Saskatchewan. L'équipement est capable de mesurer le volume total et le changement de contenu d'eau suivant le chemin de tension sous la condition K0 de chargement. Les résultats sont présentés en considérant les sols suivants: 1) le limon de Botkin et 2) e limon traité. Les résultats montrent que l'appareil de plaque de pression de GCTS est capable de suivre le séchage et le mouillage de sols et de déterminer les propriétés de volume-massifs si le spécimen de sol ne se sépare pas de l’anneau latéral. 1. INTRODUCTION The soil-water characteristic curve represents a plot of the water content versus soil suction for a soil and provides an important unsaturated soil property in that it can be used to estimate the shear strength, permeability and volume change functions of unsaturated soils (Fredlund and Rahardjo, 1993; Vanapalli et al., 1996; Fredlund, 1999; Fredlund et al., 2000). There are three ways in which the soil-water characteristic curves are commonly presented; namely, i) gravimetric water content versus soil suction; ii) volumetric water content versus soil suction; and iii) degree of saturation versus soil suction. The gravimetric soil-water characteristic curve can be measured using conventional pressure plate. The volumetric water content and degree of saturation soil-water characteristic curves require measurements of both water content and volume change of the soil during the test. For a low volume change soil (i.e., sand or compacted silt), it is possible to neglect volume changes during the wetting or drying processes and therefore compute volumetric water content and degree of saturation. Clay and slurry silt can have significant volume changes along the wetting and drying processes; therefore, the measurement of volume change is necessary for the computation of volumetric water content and degree of saturation. Numerous apparatuses for measuring the SWCC have been presented in the research literature (Soilmoisture Equipment Corporation, 1985). During the pre- 1985 period, most apparatuses were designed to measure the SWCC under zero applied total stress. After 1985 and particularly in more recent years, many attempts have been made to measure both water content and volume change as the soil is tested (Jucá and Frydman, 1996; Shimuzu and Nambu, 2003). Diffusion of air through the saturated ceramic stone is one of the problems that has plagued most apparatuses. The GCTS - SWCC apparatus manufactured by Geotechnical Consulting and Testing Systems (GCTS), Tempe, Arizona, is a pressure plate apparatus with some new design features that assist in the accurate measurement of the overall volume and water content of the soil specimen and overcome several limitations of the previous apparatuses. The description of the GCTS pressure plate is described in this paper. Two series of test on two soils are presented. Discussions concerning the application of the apparatus to unsaturated soil testing are presented. 2. DESCRIPTION OF THE APPARATUS The GCTS pressure plate uses the axis-translation technique to control matric suction in the soil specimen (Hilf, 1956; Fredlund and Rahardjo, 1993). The GCTS pressure plate consists of two main parts (Figure 1): i) a Session 5E Page 1 pressure chamber and ii) a loading system. The pressure chamber was designed for measuring soil-water characteristic curves. The pressure chamber is stainless steel and can be subjected to extremely high air pressures. The soil specimen is trimmed into a stainless steel ring and placed on top of the high air entry ceramic stone. During the test, the soil specimen is subjected to a vertical load provided through the loading ram. Various soil ring diameters can be used (i.e., from 25mm up to 75 mm). Different high air entry ceramic stones can be inserted into the base of the apparatus and used for different soil types. It is also possible to use a range of ceramic stones on one soil that is tested over a wide range of matric suctions (i.e., 1 bar, 3 bars, 5 bars and 15 bars). The apparatus is capable of testing the soil specimen over a wide range of matric suctions from 0.1 kPa to 1500 kPa. At a low soil suction (i.e., from 0.1 to 10 kPa), a hanging burette can be attached to provide an accurate value of soil suction to the soil specimen. The hanging column is designed similar to that commonly used for conventional pressure plate apparatuses (not shown in figures). At higher soil suctions (i.e., from 3 kPa to 1500 kPa) the axis-translation technique is used. Dual pressure gauges and regulators are designed to accurately control the applied soil suction over the entire range. Figure 1. Schematic of the GCTS pressure plate apparatus for the measurement of water and volume changes in unsaturated soils. The bottom of the pressure cell (i.e., below the ceramic stone) is connected to two burettes with reading accurate up to 0.07 (ml) of water. The amount of water drained out or absorbed into the soil specimen can be measured using the two burettes. At high soil suctions (i.e., close to the air entry value of the ceramic disk), the amount of the air diffusing through the ceramic stone becomes more significant. The burettes can be connected to a squeezer that is used to flush diffused air from the bottom of the pressure chamber (i.e., below the ceramic stone). In order to flush the diffused air from the bottom of the pressure cell, the following procedure is used: • Open the two valves at the base of the pressure cell (i.e., V1 and V2) that are connect to the two burettes. • Read each of the burettes to obtain the initial readings. • Connect the squeezer to the top of one burette. • Apply pressure to the squeezer and both water and diffused air in the bottom of pressure cell will move to the other burette. Hold the squeezer for about 15 seconds for the diffused air to be escaped from the water (in the other burette). • After flushing air, take the final readings on the burettes and then cover the top of the two burettes to avoid evaporation of water from the burettes. Transducer Vertical load (using a loading system) Air pressure Burette V1 regulator High air entry Soil specimen Valve Legend: - Rubber ring Pressure chamber loading ram Dial Gauge Air Valve V2 Compensator Pressure compensated Pressure Porous stone Stone holder stainless steel plumbing value disk Squeezer Transducer Vertical load (using a loading system) Air pressure Burette V1 regulator High air entry Soil specimen Valve Legend: - Rubber ring Pressure chamber loading ram Dial Gauge Air Valve V2 Compensator Pressure compensated Pressure Porous stone Stone holder stainless steel plumbing value disk Squeezer Figure 2. Photo of the GCTS pressure plate apparatus for measuring water content and volume changes in unsaturated soils under simultaneous control of total stress and matric suction. A loading ram through the top of the pressure chamber is used to apply vertical load to the soil specimen. The loading ram is designed with a pressure compensating system. The pressure compensator is used to counter- balance the air pressure that acts on the loading ram in the pressure chamber. The design of the pressure compensator allows an independent control of matric suction and net vertical stress. A displacement dial gauge is attached to the loading ram to measure vertical displacement of the soil specimen. The design of the Session 5E Page 2 GCTS apparatus allows the uploads/Litterature/ use-of-the-gcts-apparatus-for-the-measurement-of-soil-water-characteristic-curves.pdf