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.