Twenty-one years of string movements on the Liippasuo aapa mire, Finland

Abstract

The movements of four 80-100 m long strings crossing a 750 m long aapa mire were measured, and two-pools were drained and local snow depth and ground frost conditions were observed in order to test the role of water pressure and ground frost in string motions. Located close to the Arctic Circle, Liippasuo is characterized by a maximal snow depth of about 1 m, a mean annual temperature of -1 °C and a maximal variation in temperature of over 75°C. The strings moved at the rate of 50-150 cm in the course of 21 years. The most active parts were those located in the area of steepest gradient. Periods with normal annual shifts in the range 2-5 cm were interrupted from time to time by moves of abnormally high magnitude. This resulted in swelling of the nearby pools and increased curving of the strings, giving rise to a small-scale zigzag course in the strings. The principal factors responsible for the motion were ice/frozen peat push, solifluction and water pressure. Movements occurred not only during the thawing period but also in summer-time. They were as common in the direction of the contours as when following the slope. The unexpected uphill direction of movements is explained by winter-time freezing and expansion of the whole mire, in the same way as the ice cover on a lake will expand as a result of temperature changes.