Effects of stress and temperature on secondary compression of peat

Abstract

Results from field and laboratory tests illustrate the influence of vertical effective stress and temperature on the secondary compression of peat from Middleton, Wisconsin, U.S.A. A large fraction of the total settlement of the peat is due to secondary compression. The coefficient of secondary compression, Calpha, generally increases with time, and settlement predictions using a constant Calpha may underestimate field settlement. Laboratory step-stress and step-temperature tests indicate that the rate of void ratio change during secondary compression varies exponentially with both temperature and vertical effective stress. A stress coefficient of creep, Ctheta, and a temperature coefficient of creep, CT, are defined to characterize these relationships. For Middleton peat, the logarithm of Ctheta decreases linearly with void ratio and CT is constant. The results suggest that laboratory stress and temperature conditions should match those in situ for accurate determination of secondary compression parameters. Cooling also has an important effect on the secondary compression of peat. For large decreases in temperature, an overconsolidated condition is created for which additional secondary compression is greatly reduced. Settlement data from two test embankments on peat illustrate the effect of staged construction and subsurface heating on secondary compression in the field. The temperature changes achieved in situ were not large enough to cause a strong thermal precompression effect. Key words: peat, organic soil, secondary compression, consolidation, settlement.