Peat Mass Movement: Investigating Peatland Stability and Peat-Clay Boundary Interactions. A case study of Skagershultsmossen

Sammanfattning

The large mass movement of 60 hectar peatland at Skagershultsmossen in 2023 made evident that with a changing climate of more frequent extreme weather events, the need to understand peatland dynamics and peat failure mechanisms has become more relevant. In instances where peatland is underlain by marine clay sensitive to becoming quick, the acidification due to peat porewater percolating and interacting with the basal clay could effectively mobilize stabilizing cations. The near-instant precipitation-induced loading of the peat and the preceding leaching and post-depositional alteration of the underlying clay sediments critically impacts local peat stability and may lead to complex, combined failures. This study applied geophysical methods such as ground-penetrating radar (GPR) and continuous vertical electrical sounding (CVES) to investigate the post-landslide peatland stratigraphy along the peat debris fan in Skagershultsmossen, in combination with analysis of peatland hydrochemistry and clay porewater. Cone penetration tests (CPT) and soil samples applied by hand and drill rigs were used to model peat stability in GeoStudio Slope/W to assess the factor of safety (FoS) for planar and circular slip surfaces occurring along the concession boundary.This study proposes a general bog stratigraphy of a massive peat deposit ranging from H1-H7 on the Von Post humification scale on top of a silty quick clay that overlies silty sand. The stratigraphy at the large peat debris fan shows resistivity values indicating a leached clay at depths ranging from 5-6 meters, with a clear GPR-reflector at 8-9 meters interpreted to be the silty sand layers observed in the CPT drillholes, showing signs of subsidence underneath the main debris body. The clay porewater shows extreme signs of alteration, having been completely leached from expected cations instead consisting predominantly of ammonium (NH4+) and ammonium nitrogen (NH4+-N). The physical properties of the NH4+ -saturated clay show remoulded shear strengths <0.4 kPa with varying sensitivity values ranging from St=11-43, and a general liquid index Il >1,6, indicating quick clay formation at the peat-soil boundary. Based on the lack of low FoS in slope/w planar and circular slip surfaces, the morphology of the debris fans, the impermeable clay layer and the precursory precipitation event, the main failure most likely occurred as a “bog burst” due to build-up of pore pressure within the basal peat.