Environmental changes within kettle holes at Borras Bog triggered by construction of the A5156 Llanypwll Link Road, North Wales

Abstract

An unusual cluster of 41 kettle holes occupies 3km2 on the surface of the Wrexham delta terrace, a glacial platform underlain by the Singret Member of the Wrexham Formation of Pleistocene (Late Devensian) age. Two coalescent kettle holes at Borras Bog constitute a natural obstacle on the route of the A5156 Llanypwll Link Road and posed difficulties for highway design and construction. Localised deposits of peat infill the kettle holes at Borras Bog and prompted concern about a potential subsidence problem within the area of highway construction. The method adopted to cross the kettle holes involved driving sheet piling along the boundaries of the construction corridor, excavating the peat from between the sheet pile walls and then infilling with granular material. Post-construction monitoring revealed two changes within the mire; first, the level of the bog has subsided at about 10mm/y and secondly, birch woodland has invaded the central part of the mire. These changes indicate progressive dewatering of the kettle holes basin. Breaching of the clay floor of the basin is suspected to have induced drainage of the bog through the core of granular material. A remedial strategy is recommended to encourage the bog to recover. The main plank of the strategy entails grouting of the granular fill with bentonite slurry to seal the leakage points. In addition, the birch woodland growth should be cleared. However, the local community prefer the visual appearance of the flourishing new woodland and vigorously oppose the conservation measures. Deliberations continue on the optimum way forward.