SINKHOLES AT GLAN LLYN ON THE A55 NORTH-WALES-COAST-ROAD, UK

At Glan Llyn an unusual cluster of sinkholes in the glacial drift whic h overlies Lower Carboniferous Limestone poses recurring difficulties for design, construction and maintenance of the A55 North Wales Coast Road. The cluster comprises 51 sinkholes within 35 ha and prompted con cern about the possible existence of larger, deeper cave systems and c avities lacking surface expression that might cause a potential collap se problem within the areas of highway construction. Comprehensive geo logical investigations carried out to characterize the foundation cond itions included boreholes, geophysics, probe drillholes, trial pits an d borehole photography. Subsurface studies failed to disclose cavernou s openings in the limestone but revealed instead dissolution-enlarged joints and fractures forming an integrated network and generally infil led with loose clay. The glacial sediment cover consists of stiff clay s and medium dense sand and gravel, however, in the vicinity of sinkho les, much weaker strata have developed due to disturbance effects. Hig hway design and construction incorporated a number of mitigating measu res including geological surveillance of earthworks, excavation and in filling of identified sinkholes and geotextile reinforcement of the af fected stretch of highway. Regular monitoring is carried out to check the structural integrity of the carriageway and ensure public safety. Early warning signs of failure have been detected at two sites and app ropriate action taken. (C) 1998 Elsevier Science B.V. All rights reser ved.