Freeze-thaw experiments on some British soils
Relic periglacial solifluction features cover most of Britain, particularly the Midlands and the South, causing serious and continuing earthwork and construction problems. Although the engineering significance of the presence of periglacial solifluction deposits is widely appreciated, the mechanics of emplacement of these deposits has received surprisingly little attention. The principal objective of the research has been to investigate the freeze-thaw behaviour of some clay soils in Britain which have been exposed to periglacial conditions. The behaviour was examined using a Permafrost Oedometer or Permode, based on the apparatus used by Morgenstern and Smith, (1973). Tests have been carried out on Lias Clay, Weald Clay and Oxford Clay. In each test, a specimen was placed under an applied stress and subjected to 24 hour cycles of freezing and thawing. Freezing was mainly imposed from the top of the specimen downwards. The tests were carried out undrained. The pore water pressures were found to increase as a result of cyclic freeze-thaw conditions. Greater pore water pressures were generally recorded at the top than at the base of the specimen. This was considered to be due to moisture migration towards the freezing front during freezing, and impeded filtration, (due to underlying frozen material), during thaw. The significance of the additional pore water pressures in terms of slope stability was considered. A semi-infinite slope analysis model was used primarily, but reference was made to other, similar research. The analysis found that the predicted slope angle of failure is reduced significantly by relatively low increases in pore water pressure. Greater increases in pore water pressure are gained for soils of lower residual shear strength/higher plasticity. It was determined that the generation of excess pore water pressure and corresponding decrease in residual strength have considerable implications for the stability of slopes previously exposed to periglacial conditions.