23-25 September 2013
GEOMAR East shore
Europe/Berlin timezone
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The influence of excess pore pressure, fluid flow and depositional patterns on subaquatic slope instability: a detailed case study of Lake Villarrica (South-Central Chile)

Presented by Dr. Jasper MOERNAUT on 23 Sep 2013 from 17:15 to 19:15
Type: Poster presentation
Session: Poster session

Content

One of the key challenges in submarine landslide research is to understand and quantify the role of different preconditioning factors that affect subaquatic slope stability. Excess pore pressure in a sedimentary sequence reduces effective stress and lowers the factor of safety. In-situ measurements and observations of fluid escape features at the sea floor commonly provide evidence for such overpressure. However, a detailed study of the spatial relationship of fluid flow indicators and landslides, and the origin and timing of the fluid flow is often lacking. In the present study, we characterize a failed and remnant sublacustrine slope in a seismically-active region, based on high-resolution seismic profiling, side-scan sonar, multibeam bathymetry and geotechnical lab tests on piston cores. Moreover, free-fall CPTU measurements allow characterizing the in-situ shear strength and ambient pore pressure state of the sedimentary column. Composite pockmarks (up to 80 m wide), acoustic turbidity and wipe-outs spatially correlate with the headwall scarp of several large subaquatic landslides. This suggests that focused fluid escape locally influenced the stability of the sedimentary column. Where no fluid escape features were found, the headwall scarp developed above a distinct downward-steepening slope break. The occurrence of isolated pockmarks which do not relate to slide scars implies that fluid escape is not triggered by sudden unloading due to slope failure, but is a persistent phenomenon. The location of the pockmarks is strongly controlled by morphological highs in the buried glacial landscape. Thick sequences of older lake sediments and glacial till may provide the source and pathway for excess fluids, respectively. Fluid flow from these sequences can also be interpreted from in-situ dissipation tests showing increasing values of excess pore pressures with stratigraphic position (i.e. largest values in lowermost stratigraphic units), but no correlation with overburden. The basal shear surface of the slope failures developed on top of a prominent sitly-sand tephra layer and retrogressive failure took place in a unit of quick clay. Quick clay development in fresh water is rare and we speculate that persistent upwards fluid flow maintained high pore pressures in the clay unit. This prevented normal consolidation, and retained the flocculated framework of the rapidly-deposited fine-grained proglacial sediments. This study confirms that pore fluid pressure and focused fluid escape can be major preconditioning factors for subaquatic slope failure in formerly glaciated areas. Moreover it reveals that the relative position of the headscarp may be determined by these factors. However, more research is needed to unravel the respective role of excess pore water pressure and free gas in the pore spaces on effective stress conditions.

Place

Location: GEOMAR East shore
Address: Wischhofstr. 1-3 / D-24148 Kiel
Room: Lithothek

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