NDT Assessment of Concrete Tunnel Lining

An underground tunnel buried 40 ft below grade had been in full service for 80 years carrying potable water supply for the local region. The concrete tunnel lining has 8 ft internal diameter and 14 in wall thickness. The upper portion of the lining is covered with formed #12 U.S. gage steel plates at the exterior face. Each steel plate is approximately 3 ft x 1-1/2 ft.

A structural evaluation was needed to respond to reported water inflow from the tunnel into a nearby excavation site. Because any investigation would require flow stoppage and service interruption, the most expedient, least invasive methods were essential.

Visual inspection revealed a large circumferential crack crossing the tunnel lining, allowing inflow of ground water from its upper portion. There were concerns regarding separation and voiding behind the lining. A nondestructive testing (NDT) program was implemented to determine the presence of a cavity and its approximate magnitude.

The NDT method used was Impulse Response (IR), assisted by Impulse Radar. This method is capable of evaluating structural support conditions, especially for plate-like elements like a tunnel lining wall. A concrete thickness survey also addressed the possibility of internal defects. Based upon NDT data, the contractor probed a bore hole above the major crack. The significant quantity of grout material injected indicated the volume of void present. Length of testing: One day.

The relatively short downtime for this NDT investigation and the successful repair will encourage inclusion of these techniques in future scheduled maintenance.