Geotechnical controls on a steep lateral moraine undergoing paraglacial slope adjustment

Abstract

Sustained post ‘Little Ice Age’ retreat of the northern lobe of the Feegletscher, Valais, Switzerland, has exposed lateral moraines that show pronounced oversteepening on the upper proximal slopes, with upper-slope segments displaying angles of up to about 70°. Paraglacial processes have eroded gullies into the upper-slope segments, and associated debris-flow deposits result in lower angles of between 34° and 25° in the mid-slope and slope-foot zones, respectively. In order to assess the geotechnical properties of morainic sediments that permit development of quasi-stable, oversteepened slope segments, a standard suite of geotechnical measures was applied to samples of Feegletscher moraine sediments. Shear box testing yielded angles of friction ranging from 35° for loose samples to 52° for dense samples. Although the heterogeneous nature of moraine deposits makes laboratory testing of the whole size range of in situ sediments impractical, shear box test results imply that in situ upper-slope angles exceed the angles of friction of moraine sediments by 26°–40°. We are unable to replicate angles of friction in shear box tests that correspond to in situ angles of the upper-slope sections measured in the field. However, we suggest that distal dipping mica-schist clasts may play an important role in permitting high-angle slope stability. Quasi-stable storage of glacigenic sediments in high-angle moraine sequences over decadal timescales has implications for understanding the period following deglaciation over which paraglacial reworking and redistribution of sediments may operate.