Cyclic seasonal effects on infrastructure earthworks

Abstract

Slope failures may cause substantial loss of life and damage to infrastructure. In UK much of the rail network was constructed over 150 years ago, and the earth structures were not built to modern standards. As these structures get older, they have become a potential threat to the safety of transport operations. Climate conditions directly influence the behaviour and failure of slopes. The presence of trees increases the depth and extent of desiccation and cracking and may increase the permeability to greater depths, leading to greater changes in seasonal pore water pressures. Seasonal pore water pressure changes lead to corresponding cyclic changes in effective stress. The fatigue of the clay brought about by seasonal effective stress cycles is a possibility though not well established. Climate change is expected to bring extreme weather patterns to the UK in which wetter winters and drier summers will prevail, potentially giving larger cycles of stress.

Field investigations of cracks and other macro pores are carried out as part of the current study. To establish appropriate values for the change in near surface permeability of infrastructure cut slopes caused by opening and closing of cracks, field permeability experiments on a cut slope in Newbury were carried out during different seasons of the year. Appropriate near surface permeability was established based on this. A mathematical and numerical study of the influence of a single crack on permeability is presented.

To investigate the influence of cycles of effective stress associated with pore pressure changes brought about by seasonal variations in climate on the development of accumulated strain and on the strength of stiff over consolidated clay materials, a series of cyclic triaxial experiments were carried out on soil samples from railway embankments. The results from cyclic triaxial tests on undisturbed and reconstituted Gault and Lias Clay embankment fill materials are presented and analysed.

The near surface vertical permeability of the cut slope in Newbury (10^-8 m/s — 10^-6 m/s) is found to be at least two orders of magnitude higher than that of intact London clay (10^-10 m/s) and the end of summer vertical permeability (~10^-6 m/s) two orders of magnitude higher than that of end of winter (~10^-8 m/s). Near surface vertical permeability varies between summer and winter possibly mainly due to cracks opening and closing. The undrained shear strength of the reconstituted Lias Clay is consistent between 42.5 kPa to 47.5 kPa. Critical friction angle of reconstituted Lias Clay varies between 24° and 27°. For Lias Clay triaxial samples, permanent axial strains are accumulated with cycles of pore water pressure. The rate of accumulated strain decreases with the number of cycles when the effective stress ratio is below that corresponding to critical state line. The stress conditions under which the sample failed (ϕ’mob = 340) when pore pressure is cycled is with in the range of strength obtained for monotonic tests on the undisturbed fill material (which was between 28° and 37°).

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ORCID for William Powrie: orcid.org/0000-0002-2271-0826

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