The Effect of Compaction on Moisture Characteristic Curves of Compactible Soils Measured in a UFAT

The objective of this study was to develop and test methods to allow the use of the Unsaturated Flow Apparatus (UFAT) for characterization of hydraulic properties of compactable soils often encountered in vadose zone environments. Use of the UFA in this application is limited by compaction of the soil under the applied centrifugal force. The UFA significantly reduces the time required to reach moisture equilibrium by applying driving forces thousands of times greater than natural driving forces for unsaturated flow through sample cores. However, the centrifugal force will also cause some soils to compress in the instrument, significantly changing the macropore volume distribution and thus the moisture characteristic curve. Moisture characteristic curves of undisturbed soil cores were measured both by traditional methods and in the UFA. Changes in pore volume distributions were estimated using X-ray micro-focus tomography (XMT) both before and after adjustment of the moisture content. Using a mathematical model, compaction of the pores at each UFA rotational speed can be accounted for and an original uncompacted macropore volume distribution can be estimated. This uncompacted macropore volume distribution can then be used to predict the moisture characteristic curve of the original soil, greatly shortening the time necessary to complete these measurements.