Compression of Unsaturated Clay under High Stresses
Publication: Journal of Geotechnical and Geoenvironmental Engineering
Volume 143, Issue 7
The isotropic compression response of compacted, low-plasticity clay specimens having various initial degrees of saturation to high stresses under drained and undrained conditions was investigated using the approach of Mun and McCartney (2015). The compression curves in terms of void ratio versus mean effective stress or mean total stress are shown in Fig. 1(a).
After an initial elastic response, the drained specimens reach an apparent mean effective preconsolidation stress that increases with decreasing . As increases further, the compression curves for the unsaturated specimens converge with the curve for the saturated specimen as air is expulsed. The value of required to reach the point of pressurized saturation increases with decreasing . At higher , the initial soil structure induced by compaction has an effect on the shape of the compression curves, which are also distorted by the logarithmic scale for .
The compression curves for undrained specimens with a lower have a softer response due to the compression of the air-filled voids. With increasing mean total stress, the pore air dissolves into the pore water until reaching the point of pressurized saturation, which depends on . After this point, the specimens are water-saturated and the shapes of the compression curves are dominated by the pore-water compressibility. The point of pressurized saturation can be assessed by revisiting the model of Hilf (1948). Specifically, by considering a pressure-dependent solubility of air in water (, where is the pore-air pressure and is Henry’s constant), the value of for a change in mean total stress can be calculated, as follows:where = initial porosity; ; and = coefficient of volume compressibility of the soil in undrained conditions. The change in mean total stress required to reach pressurized saturation () can then be estimated as follows:
(1)
(2)
The predicted values of for specimens having different values of is shown in Fig. 1(b) along with the experimental points of pressurized saturation from the undrained compression curves in Fig. 1(a). A good match is observed, with differences due to the choice of for different values.
Implications
Although the trends in for the drained curves are well-captured by available suction-hardening models, the process of pressurized saturation and the slope of the compression curve for unsaturated soils need to be better characterized. The shapes of the drained curves at high indicate that a bi-log-linear compression curve should not be used for values of greater than 10 MPa. Instead, an exponential decay model may better capture the transition to void closure. The transition point at which the undrained compression curve is dominated by the air-filled or water-filled voids can be better captured using the modified analysis of Hilf (1948).
Acknowledgments
Funding from ONR grant N00014-11-1-0691 is acknowledged.
References
Hilf, J. W. (1948). “Estimating construction pore pressures in rolled earth dams.” Proc., 2nd Int. Conf. on Soil Mechanics and Foundation Engineering, Vol. 3, Rotterdam, Netherlands, 230–240.
Mun, W., and McCartney, J. S. (2015). “Compression mechanisms of unsaturated clay under high stress levels.” Can. Geotech. J., 52(12), 2099–2112.
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©2017 American Society of Civil Engineers.
History
Received: Oct 18, 2016
Accepted: Oct 27, 2016
Published ahead of print: Feb 20, 2017
Published online: Feb 21, 2017
Published in print: Jul 1, 2017
Discussion open until: Jul 21, 2017
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