Ground Penetrating Radar Evaluation of Concrete Tunnel Linings
Abstract - Ground penetrating radar was used to map tunnel lining condition and locate concrete deterioration and voids in a continuous and non destructive manner. GPR surveying of Kapoor Water Supply Tunnel, Victoria, Canada was carried out with a 1000 MHz Sensors and Software Ltd., Conquest system mounted on a custom built cart. This 8.8 km long 2.3 m diameter concrete lined circular tunnel was surveyed in both directions in two days. During this time, major anomalies were drilled to verify interpretations of voids behind the liner. The 30 ns time window and 5 cm sampling interval used provided good data quality and allowed speedy data collection. The processed data provided a wealth of information on the condition of the tunnel lining. Five major types of anomalies were identified. These consisted of variations in water content, void spaces, embedded wood, faults and metallic objects. A guide to interpretation of these anomaly types is presented.
The 17.58 km of GPR data taken in the tunnel showed that GPR continuously mapped concrete liner thickness, presence of reinforcement and delineated zones where mesh roof supports and construction support timbers are embedded in the liner, as well as the locations and orientations of faults that intersect the tunnel. Minor voids, honeycomb sections and areas of rock-liner separation were also detected. Radar responses to voids, zones of slight liner-rock separation, sharp rock pinnacles and hollows under the liner, and embedded wood all had slightly different characters, but were not always uniquely distinguishable from each other.
Keywords - ground penetrating radar (GPR), tunnel inspection, concrete deterioration, tunnel engineering.
Introduction
Tunnel inspections represent some near ideal circumstances for use of Ground Penetrating Radar and also some special challenges. The underground environment is one of the lowest electromagnetic background environments to work in and penetration can be excellent. The relatively constant geometry of concrete lined tunnels reduces clutter in the GPR images often caused by scattering from above ground objects. Operational challenges include internal multiple reflections within the tunnel that require appropriate filtering and the frequent requirement to image upwards which requires special apparatus to keep the antenna in contact with the tunnel roof. The degree of water saturation within the zone surrounding the tunnel may be variable, especially with tunnels that have been drained for inspection.
This paper describes a case history of a tunnel inspection within a concrete lined water supply tunnel conveying municipal water under gravity induced pressure. The tunnel passes under a mountain range and consequently has openings only at each end. As the main water supply to the nearby city of Victoria was interrupted by draining the tunnel, only a short amount of time was available to do the inspection which led to the requirement to conduct drill testing of the liner condition during the radar program instead of afterwards.
The Kapoor Tunnel is located in black slate schists, grey quartzose schists, and green chloritic (volcanic) schists. These rock types exhibit thin foliation and pronounced slaty cleavage or schistosity. Both the strata and schistosity are parallel and dip steeply to vertically. The water supply tunnel was constructed in the 1960’s and it was completed in 1970 using conventional drill and blast techniques, except for an initial problematic section excavated with a tunnel boring machine which was found to be unsuitable for the rock conditions encountered. The 2.3 m diameter, 8.8 km long tunnel has a circular section, except for the downstream 200 m, which is horseshoe shaped. The tunnel is concrete lined but it was not contact grouted during construction to fill voids and separations between the rock and liner which typically form at the crown (top) of the tunnel.
The tunnel was originally a free flow tunnel. To increase flow it was pressurized to a maximum design pressure of about 18 m. The purpose of the radar survey was to map discrete voids and crack, areas of separations of the concrete from the rock (defined as voids over scales greater than a metre with thicknesses less than 3 cm) and areas of poor quality concrete within the liner along survey lines in the crown area of the tunnel.
The drill and blast techniques used along the majority of the tunnel route typically result in rough rock surfaces. A 20 cm nominal thickness liner was poured over the rock to create a smooth inner surface using 24m long tubular forms filled with concrete by pumps. Unless extensive contact grouting is done, air trapped during construction and concrete shrinkage often results in construction voids around the irregularities of the rock, particularly in the crown area. Erosional voids may also develop between the concrete liner and rock surface during service due to water flowing either in or out via construction joints or defects in the liner.
A previous radar survey of the tunnel condition conducted in 1991, used 500 MHz antennas. Areas of major voids identified in this survey were subsequently contact grouted, with several areas of substantial grout takes.
The 1000 MHz frequency of the Sensors and Software Noggin Conquest GPR unit used in the more recent survey were better suited to the conditions in the tunnel. The higher frequency was able to “see” more effectively between closely spaced steel reinforcements. The higher frequency also resulted in higher resolution of any void spaces within the lining. Data taken in the Kapoor Tunnel showed that the GPR continuously mapped liner thickness and quality, the presence of reinforcement and metallic roof supports, and locations where construction support timbers are embedded in the liner, as well as the locations and orientations of faults that intersect the tunnel. Systematic interpretation of the 1000 MHz radar data independently produced a total of 401 anomalies, similar to the number of anomalies detected in the 500 MHz radar program, however areas that were previously contact grouted no longer showed void-like anomalies at the positions of previous voids, indicating that grouting efforts were generally successful in filling these voids. Since complete records of tunnel construction are not available, the radar images are a valuable resource, with much interpretable information regarding construction methods, concrete racter and geological details.
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