GPR investigations to characterize Medieval and Roman foundations under existing shop premises: a case study from Chester, Cheshire, UK

Abstract

ABSTRACTA high‐resolution, ground‐penetrating radar (GPR) investigation was performed on existing commercial shop premises within The Rows, Chester, UK to detect near‐surface features, including a suspected Medieval undercroft (cellar), the location of which was unknown. GPR 2D profiles were acquired on a 0.5 m square grid‐pattern both within the premises and on an exterior, Medieval raised walkway. Multi‐frequency (250, 500 and 800 MHz dominant frequency) repeat‐surveys were acquired in selected areas over the grid to resolve progressively deeper near‐surface structures and used to generate 500 and 800 MHz horizontal ‘time‐slices’.Floor‐supporting wooden beams with steel supports on the exterior walkway, the Medieval undercroft, associated passageways and a hitherto unexpected access trap‐door were all successfully located. The undercroft was found to be air‐filled. Three near‐surface rectangular areas were identified and found not to be associated with the undercroft; these were interpreted to represent heterogeneous, rubble back‐fill. Separate, isolated, raised areas were also identified ~1.5 m below present floor level; these were interpreted as relict brick column supports from a long‐demolished, Roman building. GPR data acquired on the exterior raised walkway also found evidence for relict support structures from a former shopfront.All of the targeted anomalies were intrusively investigated and provided positive confirmation of the geophysical interpretations. Archaeological investigations of trial pits confirmed the site’s varied history and depth to bedrock. GPR was therefore effective in this study to locate and characterize the modern, Medieval and Roman features without extensive intrusive investigation, whilst causing minimal disruption to commercial operations. These results demonstrate the effectiveness of carefully acquired and processed, high‐resolution, closely‐spaced GPR survey data in near‐surface characterization for geotechnical investigations.