GPR applications in structural detailing of a major tunnel using different frequency antenna systems

•Non-destructive health monitoring and assessment of an “immersed tube” tunnel type.•Use of two sets of antenna systems with different frequencies (900MHz and 2GHz GPR).•Structural detailing of a tunnel roof at the immersion joints.•Identification of materials, structural design configurations and rebar positions.•Information given about survey planning and site procedure in a complex operation. This paper reports an extended study on the applications of ground penetrating radar (GPR) on the structural detailing of a major tunnel located under the River Medway in north Kent, United Kingdom (the Medway Tunnel). Construction of the tunnel was completed in 1996 and it carries a substantial volume of traffic between two major areas of Medway in the south-east of England. The construction of the tunnel is an “immersed tube” tunnel type that connects a number of segments at immersion joint points. This investigation reports the utilisation of two sets of antenna systems with different frequencies (900MHz and 2GHz GPR) in establishing structural details of the tunnel roof at immersion joints. The processed data compiled as a result of this investigation provided essential information to tunnel engineers for forthcoming maintenance planning purposes. The data also provided ample information confirming rather doubted construction design drawings/plans originally produced. The results obtained were conclusive in terms of construction materials and structural design configurations (shape and dimensions) as well as the identification of rebar positions at all three immersion joint locations. A comparison between the different frequency antenna sets provided invaluable information in confirming the above findings. This paper also provides useful information within the context of survey planning and site procedure in a complex operation in terms of adaptation of the GPR systems used.