This paper presents an active control system for the modiËed pressure plate apparatus for matric suction measurement. The experimental results obtained from ...
SOILS AND FOUNDATIONS Japanese Geotechnical Society
Vol. 49, No. 5, 807–811, Oct. 2009
AN ACTIVE CONTROL SYSTEM FOR MATRIC SUCTION MEASUREMENT E. C. LEONGi), C. C. LEEii) and K. S. LOWiii) ABSTRACT Matric suction is an important stress state parameter in unsaturated soil mechanics. Many studies have been carried out in the past to determine the matric suction through direct and indirect methods. Direct measurement of matric suction has been proven possible with high-suction tensiometer; however high-suction tensiometers are still susceptible to cavitation. The axis translation technique developed by Hilf (1956) has been employed in many laboratory tests for unsaturated soils to avoid problem of cavitation in the water pressure measurement system. However in laboratory testing of unsaturated soils, air and water pressures are usually independently controlled and there is no need for a feedback control. The matric suction of soil can be measured using a modiˆed pressure plate apparatus by actively changing the air pressure to maintain the water pressure to be close to zero thus imposing negligible water content change in the soil. A major setback of the existing practice is the need to manually adjust the air pressure of the modiˆed pressure plate in response to the changes in the water pressure. This paper presents an active control system for the modiˆed pressure plate apparatus for matric suction measurement. The experimental results obtained from modiˆed pressure plate apparatus with active control system show good performance as compared to the high suction tensiometer. Key words: active control, axis translation, matric suction, null-type, unsaturated soils (IGC: A1/D4/T12)
many unsaturated soil tests to apply matric suction on the soil specimen. In these tests, typically air and water pressures are independently controlled to allow the soil specimen to equilibrate at the speciˆed matric suction (e.g., Wheeler and Sivakumar, 1995; Nishimura, 2000; Rahardjo et al., 2004). A special application of the axis translation technique is for the measurement of matric suction (Olson and Langfelder, 1965; Fredlund and Rahardjo, 1993). The advantages of using the axis translation technique for measuring matric suction is that the possibility of cavitations in the pore-water pressure measurement system can be eliminated and that there is no net movement of water from soil specimen into the pore-water pressure measurement system or vice versa. By ensuring that there is virtually no net movement of water into or out of the soil specimen, changes to the soil structure due to a change in water content of the soil specimen are minimized. This method of measuring matric suction can be performed using a modiˆed pressure plate apparatus where a pressure transducer is attached to the water chamber beneath the high air entry ceramic disk to read the water pressure. The air pressure supplied to the pressure plate apparatus is then adjusted accordingly to maintain the water pressure close to zero. A typical test will take from an hour to several tens of hours depending on the air entry value of the ceramic disk, the contact condition between the soil
INTRODUCTION Matric suction is given by the diŠerence between poreair pressure and pore-water pressure in a soil. Direct and indirect methods have been used to measure matric suction in unsaturated soils (Bulut and Leong, 2008; Rahardjo and Leong, 2006). Indirect matric suction measurement methods measure the moisture equilibrium condition of the soil instead of matric suction e.g., the ˆlter paper method. Direct matric suction measurement method measures the pore-water pressure e.g., the tensiometer. For direct matric suction measurement using the tensiometer, the practical maximum matric suction measured is about 90 kPa (Stannard, 1992) as the water in the pore-water pressure measurement system will cavitate, rendering the pore-water pressure reading meaningless. In the past decade, high suction tensiometers have been developed that can measure matric suction above 1000 kPa (e.g., Ridley and Burland, 1993; Guan and Fredlund, 1997; He et al., 2006; Lourenco et al., 2008). A key feature of these high suction tensiometers is the very small water volume in the pressure measurement system to reduce the possibility of cavitation. However these high suction tensiometers are still susceptible to cavitation which may be di‹cult to detect during usage. In the laboratory, the axis translation technique developed by Hilf (1956) has been commonly employed in i) ii) iii)
Associate Professor, School of Civil & Environmental Engineering, Nanyang Technological University, Singapore (cecleong＠ntu.edu.sg). Formerly Project O‹cer, ditto. Associate Professor, School of Electrical & Electronic Engineering, ditto. The manuscript for this paper was received for review on November 17, 2008; approved on June 8, 2009. Written discussions on this paper should be submitted before May 1, 2010 to the Japanese Geotechnical Society, 4-38-2, Sengoku, Bunkyo-ku, Tokyo 112-0011, Japan. Upon request the closing date may be extended one month. 807
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specimen and the ceramic disk, and the soil. To an inexperienced operator, the test is tedious and daunting as the long equilibration time may lead the operator to stop the test prematurely. It is possible to automate the process using an active control system. The purpose of this paper is to describe an active control system for the measurement of matric suction using a modiˆed pressure plate apparatus. MODIFIED PRESSURE PLATE APPARATUS FOR MATRIC SUCTION MEASUREMENT A typical modiˆed pressure plate apparatus for matric suction measurement is shown in Fig. 1. It consists of three main parts: the cell, the air pressure supply system and the water pressure measurement system. The cell shown in Fig. 1 (195 mm in height, 160 mm in internal diameter and 15 mm in wall thickness) is made of stainless steel and consists of three sections: the cell body, the base plate and the lid. The cell body is attached to the base plate and the lid is secured to the cell body using six numbers of wing-nuts each. The wing-nuts enable the lid to be fastened to the cell body quickly during testing. Good sealing is ensured by means of O-ring in a groove between the base plate and the cell body, and between the lid and the cell body. The base plate contains a 6 mm recess and a grooved water reservoir in a circular area of 68 mm in diameter as shown in Fig. 2. The grooved water reservoir consists of a spiral water channel that is 3 mm wide and 2 mm deep. A 15-bar ceramic disk, 68 mm in diameter and 6 mm in thickness, was epoxy glued into the recess of the
Fig. 1. Modiˆed pressure plate apparatus for matric suction measurement
Fig. 2.
Base plate of modiˆed pressure plate apparatus
base plate such that the top surface of the ceramic disk is ‰ushed with the surface of the cell base. The spiral water channel facilitates ‰ushing of the water reservoir by allowing water from the ‰ushing pot to ‰ow in one direction from the center of the water reservoir to the periphery of the water reservoir and out of the cell through the outlet at the side of the base plate. This spiral design minimizes the chances of air bubbles being trapped in the water reservoir. The air pressure supply system consists of a source of air pressure (an air compressor), a pressure regulator and a pressure gage. The water pressure measurement system consists of a de-airing block, a pressure transducer and a pressure readout unit as shown schematically in Fig. 1. The test procedure consists of placing the soil specimen onto the ceramic disk, placing a dead-weight on the soil specimen to ensure good contact between the soil specimen and the ceramic disk, replacing the lid quickly and increasing the air pressure before the water pressure below the ceramic disk becomes highly negative. The air pressure regulator is manually adjusted to ensure that the water pressure below the ceramic disk is close to zero. At equilibrium, the water pressure below the ceramic disk remains close to zero and the matric suction of the soil specimen is given by the diŠerence between the air pressure and the water pressure. The test may take about an hour to several tens of hours. ACTIVE CONTROL SYSTEM TO AUTOMATE MATRIC SUCTION MEASUREMENT The active control system consists of a feedback control board that is connected to the water pressure transducer and an electro-pneumatic pressure regulator. The system block diagram is shown in Fig. 3. The corresponding schematic of the modiˆed automated pressure plate apparatus with active control system for measuring matric suction is shown in Fig. 4. The electro-pneumatic pressure regulator supplies air pressure ranging from 0 to 900 kPa depending on the input signal voltage (0–10 Vdc). The feedback control board is based on an 8-bit embedded processor that has a high performance RISC CPU (Central Processing Unit) with many peripherals integrated within a chip. The processor has 32 kB of ‰ash memory, 1536B of RAM and 256B of EEPROM (Electrical Erasable Programmable Read Only Memory) on the chip. It operates at 40 MHz and can support up to 10
Fig. 3.
Schematic drawing of feedback control
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MIPS (Million of Instructions per Second) sustained operation. Though the processor has a 5-channel multiplex 10-bit ADC (Analogue to Digital Conversion), only one channel is needed in this application. As the electropneumatic pressure regulator supplies air pressure from 0 to 900 kPa, a 10-bit ADC will have a resolution of 900/ 210 i.e., 0.9 kPa/bit. Hence it is possible to regulate the air pressure to about 1 kPa but for this application, change in air pressure of 10 kPa is su‹cient. A feedback program in the C programming language is written to read the water pressure transducer using analogue channel 0 every 2 s. The feedback program consists of a system-level task and a control task. After the system initialization of variables and peripheral registers, the interrupt timer is enabled and the system-level task goes into a continuous loop and waits for the interrupt timer. The control task is an interrupt service routine, which is invoked by the timer at 2 s interval. For each interrupt service routine, it begins by reading in the pressure output
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voltage via the ampliˆer and the ADC channel 0 as shown in Fig. 3. The analogue voltage signal is converted to a digital signal for the program to determine if the water pressure is within ±10 kPa. If the permissible water pressure range is exceeded, the PWM (Pulse Wave Modulation) generation module of the processor will generate a 20 kHz switching signal which is low pass ˆltered and ampliˆed to give an analogue signal u for actuating the electro-pneumatic pressure regulator to increase or decrease the air pressure. Using the active control system, the feedback control board is switched on once the soil specimen is placed on the high air entry ceramic disk and the lid is quickly replaced and secured. The air pressure is increased rapidly initially to ensure that the water pressure does not become highly negative to avoid cavitation. The air pressure is automatically adjusted by the feedback control board in steps of 10 kPa to ensure that the water pressure below the ceramic disk is within ±10 kPa. As the water pressure is checked every 2 s, a ˆne control is exercised over the increase in air pressure resulting in a relatively smooth matric suction measurement response. The algorithm for the feedback control program is shown in Fig. 5. PERFORMANCE OF ACTIVE CONTROL SYSTEM
Fig. 4.
Modiˆed pressure plate apparatus with active control system
To evaluate the performance of the active control system for matric suction measurement, compacted samples of kaolin and sand mixtures were prepared. Mixtures of 80z kaolin and 20z sand by weight were used. The basic soil properties of kaolin-sand mixture are summarized in Table 1. The grain size distribution and compaction curve of the kaolin-sand mixture are shown in Figs. 6 and 7, respectively. The soil samples were compacted at water content of approximately 23z using the standard Proctor method (ASTM D698, 1998). The soil samples were then trimmed to retain only the middle portion (specimens of approximately 70 mm diameter by 20 mm height). The soil specimens were then placed in a pressure plate apparatus to achieve various matric suctions. The soil specimens were then used for matric suction measurement in the modiˆed pressure plate apparatus. As it is di‹cult to obtain ``identical'' soil specimens using the pressure plate apparatus, direct measurements using the high suction tensiometer described by He et al. (2006) were also conducted as an independent check. The performance of the active control system can be observed from the output pressure of the electro-pneuTable 1.
Fig. 5.
Feedback control algorithm
Basic properties of kaolin-sand mixture
Soil Properties
K80S20
Liquid limit (z) Plastic limit (z) Plasticity index (z) Speciˆc gravity Max. dry density (Mg/m3) Optimum water content (z)
78 37 41 2.62 1.31 25
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Fig. 6.
Grain size distribution of kaolin-sand mixture
Fig. 8.
Fig. 7.
Test results
Compaction curve for kaolin-sand mixture
matic pressure regulator (air pressure) and the water pressure transducer readings which were recorded by a data acquisition system at intervals of 5 s in Fig. 8. The embedded program in the feedback control board monitors the water pressure increase and adjusts the air pressure via the electro-pneumatic pressure regulator. The feedback algorithm implemented starts with zero air pressure and adjusts the air pressure in steps of 10 kPa if the water pressure exceeds ±10 kPa. The plots on the left column in Fig. 8 show the air pressure and water pressure as a function of time while the plots on the right column show the matric suction (given as the diŠerence between air and water pressures) as a function of time. Several observations can be made from Fig. 8: (1) Air pressure increment was very gradual; (2) Water pressure was kept at nearly zero at all times; (3) Because of the nearly zero water pressure response, the matric suction response was nearly the same as the air pressure increase; (4) The matric suction response curves were more gradual; and (5) Equilibration time ranged from 50 to 150 minutes for the matric suction range measured.
Fig. 9. Comparison of matric suction measurements from modiˆed pressure plate apparatus and high suction tensiometer
COMPARISON OF MATRIC SUCTION MEASUREMENTS A high suction tensiometer capable of measuring matric suction up to 500 kPa directly was used to made independent matric suction measurements for comparison with the modiˆed pressure plate apparatus. Details of the high suction tensiometer can be found in He et al. (2006). The matric suction of several compacted soil samples were ˆrst measured using the high suction tensiometer followed by the modiˆed pressure plate apparatus. The comparison is shown in Fig. 9. Figure 9 shows good
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agreement between the matric suction measurements from the high suction tensiometer and the modiˆed pressure plate apparatus, with the discrepancy being within ±10z. CONCLUSION An active control system to automate the modiˆed pressure plate apparatus for matric suction measurement is described in this paper. The control system employs a feedback control board and an electro-pneumatic pressure regulator for active control of the air pressure to maintain the pore-water pressure of the soil specimen to be close to zero. The test results showed that a very wellbehaved response was obtained from the modiˆed pressure plate apparatus. The matric suction measured using the modiˆed pressure plate apparatus agreed well with the matric suction measured using the high suction tensiometer. The diŠerence is within ±10z. ACKNOWLEDGEMENTS The second author acknowledges the support of RGM8/05. The technical assistances of Ms Win Nu Nu Win of the School of Electrical and Electronic Engineering and Mr. Eugene Tan of the Geotechnics Laboratory, Nanyang Technological University, are gratefully acknowledged. REFERENCES 1) ASTM D698 (1998): Test Method for Laboratory Compaction Characteristics of Soil Using Standard EŠort (12400 ft-lbf/ft3 (600 kN-m/m3)), Annual Book of ASTM Standards, 04.08, Soil and Rock (II), pp. 77–84, West Conshohocken, PA: American Society for Testing and Materials.
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2) Bulut, R. and Leong, E. C. (2008): Indirect measurement of suction, Journal of Geotechnical and Geological Engineering, 26(6), 633–644. 3) Fredlund, D. G. and Rahardjo, H. (1993): Soil Mechanics for Unsaturated Soils, John Wiley & Sons, Inc., New York. 4) Guan, Y. and Fredlund, D. G. (1997): Use of the tensile strength of water for the direct measurement of high soil suction, Canadian Geotechnical Journal, 34(4), 606–614. 5) He, L., Leong, E. C. and Elgamal, A. (2006): A miniature tensiometer for measurement of high matric suction, Unsaturated Soils 2006 (eds. by Miller, G. A., Zapata, C. E., Houston, S. L. and Fredlund, D. G.), Geotechnical Special Publications, 147(2), 1897–1907. 6) Hilf, J. W. (1956): An investigation of pore-water pressure in compacted cohesive soils, Ph.D. Dissertation, Tech. Memo. No. 654, U.S. Dept. of the Interior, Bureau of Reclamation, Design and Construction Div., Denver, CO, 654 pp. 7) Lourenco, S. D. N., Gallipoli, D., Toll, D. G., Augarde, C. E., Evans, F. D. and Medero, G. M. (2008): Calibrations of a high-suction tensiometer, Geotechnique, 58, 659–668. 8) Nishimura, T. (2000): Direct shear properties of a compacted soil with known stress history, Unsaturated Soils for Asia, Proc. Asian Conference on Unsaturated Soils (UNSAT-ASIA 2000) (eds. by Rahardjo, H., Toll, D. G. and Leong, E. C.), Singapore, 317–321. 9) Olson, R. E. and Langfelder, L. J. (1965): Pore-water pressures in unsaturated soils, J. Soil Mech. Found. Div., Proc. Amer. Soc. Civil Eng., 91(SM4), 127–160. 10) Rahardjo, H., Ong, B. H., Leong, E. C. (2004): Shear strength of a compacted residual soil from consolidated drained and constant water content triaxial tests, Canadian Geotechnical Journal, 41(3), 421–436. 11) Rahardjo, H. and Leong, E. C. (2006): Suction measurement, Unsaturated Soils 2006 (eds. by Miller, G. A., Zapata, C. E., Houston, S. L. and Fredlund, D. G.), Geotechnical Special Publications No. 147, 1, 81–104. 12) Ridley, A. M. and Burland, J. B. (1993): A new instrument for measuring soil moisture suction, Geotechnique, 43(2), 321–324. 13) Stannard, D. I. (1992): Tensiometer—Theory, construction, and use, Geotechnical Testing Journal, ASTM, 15(1), 48–58. 14) Wheeler, S. J. and Sivakumar, V. (1995): An elasto-plastic critical state framework for unsaturated soil, Geotechnique, 45(1), 35–53.
