Soft-sediment deformation during thawing of ice-rich frozen soils: resultsof scaled centrifuge modelling experiments

The origin of periglacial involutions remains uncertain, largely because of the difficulties of field monitoring in modern permafrost regions. This pa per describes an alternative approach, in which process studies are based o n scaled centrifuge modelling of thawing ice-rich soils. Centrifuge scaling laws allow similitude in self-weight stresses between the model scale and the prototype (field) scale to be achieved. In these experiments, 120- to 1 30-mm-thick frozen models comprising a sand unit overlying ice-rich kaolini te clay (three models) or ice-rich silt (one model) were thawed under an ac celeration of 20 gravities. The models were therefore equivalent to 2.4-2.6 m of frozen sediments (permafrost) at the prototype scale. Temperature pro files and porewater pressures during the thawing of each model are describe d. Porewater pressures significantly in excess of hydrostatic were not obse rved in the sand/silt model. In the sand/clay models, however, excess press ures developed rapidly after thawing, and observed fluctuations in pressure were interpreted as water-escape events. After thawing, careful sectioning of the models revealed small-scale deformation structures at the clay-sand interface, resulting from loading of the upper sand layer into very soft f luid-like clay and injection of clay upwards into the base of the sand. It is concluded that these experiments provide analogues for some Pleistocene involutions. Such involutions therefore mark phases of permafrost degradati on when high porewater pressures caused loading and injection along sedimen tary boundaries.