Are Soil Moisture Characteristic Curves Developed Under Pressure the Same As Those Developed Under Suction?.

For more than 50 years, soil scientists and more recently geotechnical engineers have used pressure plate extractors to generate soil moisture characteristic curves via the axis translation technique.  With this technique, positive pressure is applied to soil placed on a porous ceramic plate with its outflow at atmospheric pressure.  Recent work described in the geotechnical engineering literature has called into question effectiveness of this technique to generate such curves.  Baker and Frydman (2009) present a theoretical analysis of soil pores draining under pressure (pressure plates) vs. soil pores draining under suction (natural conditions) and find significant differences in the two processes mainly having to do with the lack of cavitation in soil pores under pressure and the assertion that non-capillary water associated with particle surfaces is not affected by elevated air pressure.  The aim of this study is to determine experimentally whether the issues elucidated by Baker and Frydman cause fundamental differences in moisture characteristic curves created under pressure and suction.  To that end, moisture characteristic curves have been developed for several soil types using pressure plate extractors and suction extraction techniques.  Results and practical implications will be discussed.