Void Detection behind Tunnel Liner Utilizing Ground Penetrating Radar

"In the early 1920’s, the City of Tulsa, Oklahoma constructed an approximately 8,238-foot long tunnel, the Tiawah Tunnel, through bedrock. Voids behind the concrete tunnel liner were detected via probe drilling in early 2000 as part of an interior visual condition assessment. In 2012 and 2015, conditions along the entire tunnel were efficiently assessed through geophysical investigation using ground penetrating radar (GPR) along the inner tunnel crown and at various select locations within the tunnel. Fifty-seven (57) potential voids, extending up to 19 inches in depth and 12 feet in width, were detected with 1,000 MHz GPR and confirmed through probe drilling with approximately 80% accuracy. GPR proved to be an efficient and reliable technology for void detection behind concrete tunnel lining. This paper highlights the geophysical investigation and how geophysical efforts were combined with other investigation efforts to definitively determine the presence of voids outside of the tunnel lining.INTRODUCTION The city of Tulsa, Oklahoma has a population of about 400,000 people. The original source of water for the city was the Arkansas River. The first pumping operation provided untreated water to homes and businesses in 1904. The residents of Tulsa found this water to be unacceptable due to the high levels of salt and gypsum present. As the city grew, land and storage/treatment facilities were acquired in order to obtain and distribute a more suitable raw water. The raw water needs for the city are currently satisfied by transport and treatment of about 100 million gallons per day (MGD) of water from surface water sources. Water is transported many miles from Spavinaw/Eucha and Oologah lakes to Yahola Lake and the Lynn Lane Reservoir (totaling over 3-billion gallon capacity). From here, the raw water is treated and transported to the various customers (City of Tulsa 2015). The Tulsa Metropolitan Utility Authority (TMUA) is responsible for the operation and maintenance of the raw water transport system."