SETTLEMENT OVER PEAT ON THE A5 AT PANT-DEDWYDD NEAR CERRIGYDRUDION, NORTH-WALES

The A5 London to Holyhead trunk road suffers from large and varied set tlements at Pant Dedwydd where it crosses a geotrophic peat bog develo ped on the glacial drift that overlies bedrock of Silurian Denbigh Gri ts Group. Recent carriageway settlements have created an undulating pr ofile over a 100 m stretch, restricting forward visibility and posing recurring and persistent difficulties for highway maintenance. The hig hly compressible peat underlies almost 400 m of highway, ranges up to 7 m deep and a typical profile consists of an upper sphagnum peat laye r and a basal reedy, woody and sedge grass peat. The highway was origi nally constructed by Thomas Telford in 1819 as a low floating embankme nt over the organic terrain. Increases in traffic over the intervening period together with successive asphalt overlays have resulted in a h istory of major repairs and although records are incomplete it is like ly that the carriageway is now over 2 m thick over parts of the bog. C omprehensive geological investigations carried out to characterize the peat deposit included dynamic probe holes, boreholes and a trial pit. No unusual subsurface conditions were identified. However, scrub will ow or grey sallow (Salix cinerea) was observed growing in spiraea thic kets alongside the deepest hollows in the highway. The willows may con tribute to the road subsidence by extraction of water from the peat be neath the carriageway. Accordingly, removal of the scrub willow and co nventional re-grading of the carriageway have been recommended to corr ect the existing problem. (C) 1998 Elsevier Science B.V. All rights re served.