Use of drainage wicks for an embankment on slickensided clay

Abstract

The 650-ha cooling pond for the Genesee thermal power plant near Edmonton required more than 15 km of earth-containment dyke up to 25 m high to be constructed on a high plastic clay foundation. Other embankment construction activity at the Genesee site indicated that there was very little dissipation of construction pore pressure in the foundation clay and that embankment heights of more than 6 m would require stabilization measures. The high portions of the cooling-pond dyke were built in three stages between 1982 and 1986. An extensive instrumentation monitoring program was established to control the rate of fill placement during construction. Counterbalancing earth berms were used to stabilize most of the high sections of dyke. During the later stages of dyke construction, lack of dissipation of high excess pore pressure in the foundation clay indicated that the sole use of counterbalancing berms might not provide sufficient stabilization. Prefabricated drainage wicks were used as an additional measure to lower the foundation pore pressures under the counterbalancing berms in these critical areas. Spacing of the wick drains was determined by applying Hansbo's wick-drain theory and from data collected from a wick-drain test section. Additional piezometers were installed prior to wick installation to assess the effectiveness of the wicks. Over 14 000 wick drains were installed at spacings of 2.5–2.75 m to a maximum depth of 26 m below the top of the berm. The installation procedures are discussed, along with difficulties of installation. The wicks were very effective in lowering the foundation pore pressures and improving dyke stability. Instrument monitoring data used to assess the wick-drain performance and to control the rate of construction during dyke completion are presented and discussed. Key words: drainage wick, high plastic clay, pore pressure, dissipation, deep, fill, dyke, instrumentation, monitoring.