At the St Asaph Bypass, surface cracking developed along the crest of a modern highway cutting, above a cluster of badger setts excavated in the sandy soils of the cutting slope. This prompted concerns about the extent of the underlying tunnel systems and the possible existence of deep cavities lacking surface expression that might cause a potential slope instability problem. The full threat posed to the integrity of the cutting and to the safety of road users remained uncertain. In addition, intrusive investigations were deemed inappropriate and a non-invasive solution to the problem was required.
Ground-penetrating radar was used along a 50 m stretch of the cutting to determine the full extent of the badger setts beneath the surface. The geophysical survey identified 324 m of tunnels and demonstrated that the tunnel network was generally shallow (c. 0–2 m). Pronounced radar reflections characteristic of air voids were identified in the data and interpreted as the badgers’ access tunnels and living and nursery chambers whereas the more subdued features were interpreted as collapsed and abandoned diggings. This case history illustrates the benefits of using ground-penetrating radar to provide an understanding of slope stability and local ground conditions in areas of environmental sensitivity when non-intrusive investigations are required to provide reassurances in relation to public safety.