Variability of mechanical properties in Bolders Bank till

Bolders Bank till is one of the most widespread glacial seabed soils in the UK North Sea and is the foundation material for many offshore oil, gas and windfarm structures. It also dominated ground conditions at the PISA JIP’s Cowden monopile test site, where it appeared principally as a low-to-medium plasticity, high yield stress ratio (YSR), well-graded stiff clay. The till’s glacio-tectonic genesis and subsequent history of periglacial and temperate climate process all impacted its profile and properties. Careful logging and suites of laboratory tests ranging from index properties to advanced cyclic triaxial and Hollow Cylinder Apparatus (HCA) experiments confirm variability, marked non-linearity and anisotropy. Nom inally identical samples from the same depths demonstrate scatter in shear strengths and small strain stiffness due to the till’s variable gravel/cobble contents and fissuring profile. The effects of sampling disturbance, testing style and specimen size are also influential. This paper highlights and quantifies the scatter in properties that can be expected from this and similar glacial tills; the conclusions drawn should help to improve site characterisation and test specification in such strata