Stress-temperature effects on peat compression

Laboratory and field tests have shown that accelerated compression of peat soils occurs in response to moderate heating and an effective overconsolidation is produced on subsequent cooling. This phenomenon has been termed thermal precompression. In this study, long-term laboratory and field tests were conducted to investigate secondary compression behavior of peat as a function of stress and temperature. Step-stress and step-temperature compression tests were conducted on two peat soils. Dramatic effects were observed upon heating and cooling. A field test site in Middleton, Wisconsin, USA was constructed to test the thermal precompression concept for in situ peats. The site contained two instrumented embankments. The soils under one test embankment were left at ambient temperature conditions. The soils beneath the second embankment were heated for a one-year period using a circulating hot water system. An increase in temperature up to 20°C over ambient conditions was achieved at depths extending to 5 m. Secondary compression rates for the heated embankment were up to 4 times greater than the corresponding rates for the unheated embankment. After the soils under the second embankment cooled for one year, a second lift of soil was applied to both embankments and subsequent settlement was monitored over a ten-year period. The relationship of rate of void ratio change during secondary compression as a result of temperature and stress changes is described. This is provided as a function of void ratio for the peat soils using laboratory tests and for in situ compression using the field test site data.