Tensiometer Development for High Suction Analysis in Laboratory Lysimeters

This paper presents results when using a tensiometer designed in the Soil Mechanics Laboratory at COPPE/UFRJ, and several other instruments. This new instrument measures suction values up to 1500 kPa. In more typical tensiometers, the cavitation of the system hinders the measurement of suction with values over 100 kPa. Ridley and Burland (1993) designed a new model of tensiometer to measure suction of more than 100 kPa. Mahler et al. (2002) presented a new tensiometer with which suction values up to 350 kPa were measured. This paper introduces an updated model of a tensiometer, using a variation of the equipment proposed by Mahler et al. (2002). The updated model worked extremely well and cost little to build. The equipment used allows suction to be measured relatively quickly and, as previously stated, is economical to produce (about US$300). To date, the range of suction levels that can be measured reaches 1500 kPa with no difficulty. The tests were carried out in two boxes of 160× 50× 60 cm under laboratory conditions. The equipment used included a 15 m.c.a. ceramic block pressure sensor, de-aerated water and a special acrylic tube specifically designed for this instrument. The results were compared to two simple automated tensiometers and equivalent tensiometers. In general very good results were obtained. The main final remarks so far are as follows:

• the high bubble air entry of the ceramic block inhibits the presence of air bubbles, but the response time is slower for suction values of more than 200 kPa;

• the saturation process used for the ceramic stone worked very well;

• as expected, the position of the equivalence tensiometer influences the value measured;

• the mini-lysimeter system proved to be a very good alternative for laboratory tests and for the development of instruments that measure suction;

• the new instrument presented herein proved to be a good and an economical alternative for measuring matrix suction in the soil.