ORIGINAL PAPER Indirect Measurement of Suction Rifat Bulut Æ Eng Choon Leong Received: 6 October 2005 / Accepted: 1 December 2007 / Published online: 15 May 2008 Ó Springer Science+Business Media B.V. 2008 Abstract This paper reports on indirect soil suction measurement methods. Indirect suction measurement techniques measure the moisture equilibrium condi- tion of the soil instead of suction. The moisture equilibrium condition of the soil can be determined by primary means as in vapor pressure, secondary means as through another porous medium or tertiary means as in measuring other physical properties of the porous medium that indicates its moisture equi- librium condition. Indirect suction measurement techniques employing primary means include ther- mocouple psychrometers, transistor psychrometer and chilled-mirror psychrometer. Indirect suction measurement techniques employing secondary means includes the filter paper method and indirect suction measurement techniques employing tertiary means include thermal conductivity sensors and electrical conductivity sensors. These techniques have been widely used in engineering practice and in research laboratories. However, each of these techniques has its own limitations and capabilities, and active research into improving these techniques is ongoing in universities, research laboratories, and private sector. This paper outlines working principles, cali- bration, measurement, and application areas of these methods based on recent literature and practice. Keywords Soil suction Á Thermocouple Á Transistor Á Chilled-mirror Á Psychrometer Á Filter paper Á Thermal conductivity Á Electrical conductivity 1 Introduction The understanding and wide acceptance of unsatu- rated soil mechanics principles has produced a gradual change in geotechnical engineering practice. There is more than ever a greater need for reliable soil suction measurement techniques as soil suction becomes an integral part of engineering practice in many situations involving unsaturated soils. Soil suction is a result of capillary action, surface energy properties of soil particles, and ionic concentration of the pore water. Total suction results when both mechanisms are active. Matric suction results when only capillary action and surface energy properties are active. Significant contributions have been made by geotechnical engineers in the measurement of soil suction. However, there is still need for research into the measurement of both matric and total suction in R. Bulut (&) School of Civil & Environmental Engineering, Oklahoma State University, 207 Engineering South, Stillwater, OK 74078, USA e-mail: rifat.bulut@okstate.edu E. C. Leong School of Civil and Environmental Engineering, Nanyang Technological University, Singapore, Singapore 123 Geotech Geol Eng (2008) 26:633–644 DOI 10.1007/s10706-008-9197-0