Ridley, A. M. & Burland, J. B. (1993). zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDC Giotechnique 43, No. 2, 321-324 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONML TECHNICAL NOTE A new instrument for the measurement of soil moisture suction A. M. RIDLEY* and J. B. BURLAND* KEYWORDS: field instrumentation; groundwater; laboratory equipment; laboratory tests; partial satura- tion; pore pressures. INTRODUCTION The generally accepted definitions of soil mois- ture suction are those given by Aitchison & Richards (1965). Quantitatively, the soil moisture suction is represented by the difference between the ambient air pressure and the soil pore water pressure. In soils with a measurable suction the latter will always be less than the former; more specifically, when the air pressure is atmospheric the pore water pressure will be negative. Instruments for the measurement of soil mois- ture suction have suffered from a number of dis- advantages. Devices that measure the suction directly do so by actually measuring the pore water pressure, but either they are restricted to measuring a very low suction (e.g. the tensiometer), or to measure a high suction they require the use of a raised air pressure (pressure plate). Devices that are used to measure a high suction at atmospheric air pressure are called indirect because they are calibrated against some other physical property, such as humidity (the psychrometer), absorption (filter paper) or electri- cal resistance (gypsum or thermal block) which is also related to the soil moisture suction. Most of the available methods of suction measurement suffer from a ‘slow’ response time (at best several hours, and often weeks or even months). Also, the accuracy of many of them is not good, particu- larly in the suction range lOC-1000 kPa. Table 1 summarizes the most common techniques of soil suction measurement. METHODS OF DIRECT MEASUREMENT OF SOIL SUCTION Instruments that measure soil suction directly do so by an exchange of water between the instrument and the soil. The driving force that causes the exchange is the negative pore water pressure. The essential features of this group of Discussion on this Technical Note closes 1 October 1993; for further details see p. ii. * Imperial College of Science, Technology and Medi- cine. instruments are shown in Fig. l(a): they comprise a porous disc and a measurement device (usually a manometer or a pressure gauge) separated by a fluid reservoir. The flow of water in a saturated porous medium is from a point of higher pore water pres- sure to a point of lower pore water pressure. The flow will continue until both points are at an equal pore water pressure that lies between the original pressures. In instruments such as the suction plate and tensiometer the water and air pressures are initially both atmospheric. The negative pore water pressure in the soil causes a flow of water from the instrument into the soil when the soil is placed on the porous stone. In all the available versions of this type of equipment the highest suction that can be sustained in the Soil sample-- y”“‘A Water reservoir-. Negative I pressure High air pressure 0 rmg seal Pressure transducer u / 15 bar (air entry) ceramic Water reservoir----. l--F’ushing Pump - W Fig. 1. Direct measurement of soil suction: (a) ten- siometer; (b) pressure plate apparatus 321