Non-destructive Detection of the Anomalies and Thickness of a Shaft Using GPR

Ground penetrating radar (GPR) has been proven to be one of the most promising non-destructive methods that has been widely used in engineering quality inspections and condition assessments of underground structures. This paper reports an application study of GPR that presents a comprehensive survey of the auxiliary shaft of the Anli coal mine in China. During shaft installation, water gushing with a maximum inflow rate of approximately 16.0 m³/h occurred, and thus, it was urgent to determine the distribution range of anomalies and evaluate the lining thickness of the shaft to provide precise technical guidance for later treatments. A GPR grid scan was carried out by towing the antenna along 6 vertical measuring lines and 11 orthogonal circular measuring lines on the shaft surface. The GPR results show that the anomalies experienced by the shaft are voids, uncompacted areas and water bodies. Most anomalies were located in the outer shaft lining and surrounding soil. A vast majority of the shaft lining thickness reach its designed value, with a qualified rate of 91.2% for the inner shaft lining and 89.9% for the outer shaft lining, and the outer shaft lining exhibited inferior smoothness and uniformity. Under the guidance of the GPR results, a grouting treatment was carried out to water seal and strengthen the shaft lining, which, consequently, reduced the water inflow rate by 90%. The favourable grouting effect demonstrates that the GPR method can feasibly and successfully reflect the shaft lining and anomalies in shaft assessments. This engineering practice could serve as a case study reference for investigating any similar underground structure in the future.