Sinkholes at Glan Llyn on the A55 North Wales Coast Road, UK

At Glan Llyn an unusual cluster of sinkholes in the glacial drift which 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 concern about the possible existence of larger, deeper cave systems and cavities lacking surface expression that might cause a potential collapse problem within the areas of highway construction. Comprehensive geological investigations carried out to characterize the foundation conditions included boreholes, geophysics, probe drillholes, trial pits and borehole photography. Subsurface studies failed to disclose cavernous openings in the limestone but revealed instead dissolution-enlarged joints and fractures forming an integrated network and generally infilled with loose clay. The glacial sediment cover consists of stiff clays and medium dense sand and gravel, however, in the vicinity of sinkholes, much weaker strata have developed due to disturbance effects. Highway design and construction incorporated a number of mitigating measures including geological surveillance of earthworks, excavation and infilling of identified sinkholes and geotextile reinforcement of the affected 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 appropriate action taken.