The Horncliffe landslide, Berwick-upon-Tweed, December 2008

This report describes the geological context, form and composition of a small rotational failure and debris flow in unconsolidated sediments in the bank of the River Tweed at Honey Farm, Horncliffe, approximately six miles west of Berwick on the Union Bridge road. Slippage of the bank resulted in undermining of the road, cracking and subsidence of the tarmac, rupture of a water main, and tension cracking across some of the remaining road adjacent to the failure. Consequently the road was closed, and may need to be rerouted to avoid potential future hazards. The road at this locality was previously re-routed (1954) as a result of a similar failure, thus it seems likely that the sediments here are inherently prone to failure, given a sufficient trigger. This recent slippage probably occurred as a result of saturation of the ground either by seepage from a buried culvert, or from the water main, that locally raised pore water pressures. The slippage was possibly exacerbated by maintenance works that had removed natural materials and backfilled holes with gravel and cobble aggregate, which disrupted natural drainage and increasing normal stresses on the underlying sediments.