ABSTRACT
In 1985, a failure took place during the construction of a UK motorway cutting (Arrowsmith et al., 1985), which was shown to be due to lenses of laminated clay embedded in the glacial till. The slope had been cut to a standard angle specified by the road authority, a typical angle for stiff clays. The laminated clay lenses proved to be nearly parallel to the slope. Ground investigations did indicate that the lenses were there but they were interpreted as pockets of laminated clay or horizontal layers of laminated clays, not unusual and not considered a risk. In 1993, a land fill design was based on the assumption that the underlying glacial till was impermeable. The arguments that a till can contain lenses of more permeable materials led to the proposal being rejected at a public inquiry (Gray, l993). A dredging contractor claim that they did not expect boulders because the borehole logs did not indicate their presence was rejected on the grounds that glacial till can contain boulders. A piling contractor took as read that they would be constructing bored piles in stiff clay because that was the description given in the borehole logs. They chose to use smaller diameter piles than those considered at the design stage, which meant that they were longer and, importantly, extended below the depth of most of the boreholes. This proved to be an expensive mistake as the pile holes had to be cased to full depth to prevent water ingress and collapse of the sides of the boring when they encountered a layer of water-bearing sand. Hand excavation was proposed for a tunnelling scheme on the basis of the soil descriptions; the contractor ended up using explosives because the till was much stronger in situ. Bell and Culshaw (1991) consider the glacial till to be a problematic soil together with collapsible soils, quick sands, peat, expansive clays and frozen soils because of the variability in the composition. These examples highlight issues of misinterpreting borehole logs, the failure to appreciate the fabric and structure of glacial soils and poorly planned ground investigations.
