Shallow and deep-seated regolith slides on deforested slopes in Çanakkale, NW Turkey

- Removal of regolith from sub-vertical joint planes causes shallow slides.
- Slip surface material on joint planes consists of mainly Fe-rich oxyhydroxides.
- The destructive deep-seated slide occurred on 17 February 2003.
- 2D ERT image showed the position of the slip surface of the deep-seated slide.
- 1D inversion of VES data revealed the depth of the bedrock.

This study deals with the stripping of regolith on a steep slope by surface wash and shallow landslides and a deep-seated landslide at a lower slope that took place on 17 February 2003 at the village of Mazılık, east of Çanakkale, Turkey. Soil loss and shallow slides dominate on the deforested steep slopes in the study area and occur preferentially along slope-parallel sub-horizontal joint planes with clay coatings, particularly oxyhydroxides that are rich in Fe but poor in Mn as a result of weathering under well-drained conditions. Gully erosion also occurs where the regolith cover is relatively thick (up to ~ 4 m). The area of the deep-seated landslide, however, is dominated by silty clay (46%). A geoelectrical resistivity survey revealed a clay-rich zone at depths of ~ 3-10 m, corresponding to the slip surface of the slide, which was associated with excessive water content after the snowy day of 14 February 2003 with a daily precipitation of ~ 16.4 mm. Based on Thornthwaite's water budget analysis, the study area has a slide-prone condition with excess soil-moisture content, heavy rainfall events, snow accumulation and snow melting in winter months, and low soil permeability also favouring slope instability.