Chilled Mirror Measurements of Soil Water Retention Curves: Fid-Bau Portal

Chilled Mirror Measurements of Soil Water Retention Curves

Olsen, Harold W. / Repola, Kerry / Wienecke, Christopher J.

The capability of a chilled mirror device for measuring soil water retention curves was investigated on fine grained soils having plasticity indices ranging from about 15 to 80. The measurements were obtained over a wide range of moisture contents on each test specimen to avoid data scatter from material variability. The response times for equilibrating a soil specimen in the sealed chamber of the chilled mirror device varied from about 70 to 300 minutes, increasing with higher plasticities and lower water contents. Chilled mirror measurements of soil water retention curves on fine grained soils were obtained in days rather than the six to eight weeks required with conventional pressure plate methods. The semi-logarithmic display of soil water retention curves on low plasticity clays includes one linear segment between suction levels of 0.01 to 100 MPa and water contents ranging from air dry near the liquid limit of the material. The semi-logarithmic display of soil water retention curves on high plasticity clays can be approximated with a bi-linear relationship consisting of one relatively steep linear segment in the high suction range, and a second relatively flat linear segment in the low suction range. Some characteristics of these linear and bi-linear relationships are inconsistent with the assumptions underlying McKeen's expansive soil classification system. All of the samples tested show zero-water-content intercepts that are higher than McKeen's benchmark intercept of 5.25 log kPa. Two of the three samples exhibiting bi-linear soil water retention curves have slopes that fall in negligible categories for swelling potential, even though they have high plasticity indices, high activities that suggest the clay fraction is smectite, and substantial values of percent swell in the ASTM Swell Consolidation Test.