Design and Performance of an Expressway Constructed Over Peat by Preloading

Abstract

This paper presents the complete record of the laboratory investigations and field observations made in conjunction with the construction of expressway embankments built by preloading over a muskeg. The peat deposit, located on the north shore of the Saint-Laurent River, was initially 10–19 ft (3.0–5.8 m) thick and is underlain by sand. Settlements of the highway fills ranging from 5 to 11 ft (1.5 to 3.3 m) were measured during construction.Laboratory and field tests were performed before and during construction. Field vane tests made at the end of the preloading stage indicated a considerable gain in the shear strength as a result of consolidation. Compressibility characteristics of the peat measured in the laboratory are compared with the field behavior.Periodical field level observations were taken during the 4 years after the removal of the temporary surcharge and have shown that the highway embankments heaved 2–3 in. (5.0–7.5 cm) over a period of 12 months at a gradually decreasing rate. The rebound was followed by long-term secondary settlements totaling 0.5–1.5 in. (1.3–3.8 cm) over the following 3 years of observations. Analysis of the settlement records shows that preloading has been beneficial in reducing the coefficient of secondary compression of the peat. Preloading was simulated in the laboratory by means of oedometer tests on peat samples; both the laboratory results and the field observations revealed a remarkable agreement between the duration of the surcharge stage and the time for rebound to be complete after removal of the surcharge.