Long Range Sewer Influent Tunnel with Shotcrete Liner

In this case, a metropolitan sewer department had an influent tunnel that had a distance of approximately 8,000 feet between available access points. The tunnel had a poured in place concrete liner for the first 50 feet with shotcrete lining the rock cut tunnel for the remainder of the distance. The tunnel was expected to be 60% full and flowing during the inspection. The customer wanted to assess the tunnel lining condition above the waterline and to assess debris or sedimentation below the waterline. The customer also wanted to look through the lining if possible to detect the presence of any support structures or voids behind the liner.

Float In Flowing Tunnel

Hibbard Inshore has vehicles for fully flooded, partially flooded, and dry scenarios, and in this case chose to use a large floating vehicle due to its ability as a platform for inspection both above and below the waterline. The float was outfitted with 8,000 feet of umbilical tether in order to provide real time data during the inspection along with multiple cameras, lighting, sonar to dimension debris below the water’s surface, and two ground penetrating radar antennas on masts to roll along the tunnel liner. The GPR antennas were of different frequencies to look through the liner to different depths. The inspection showed that the liner was present and still bonded to the rock but had some signs of deterioration while the tunnel was relatively free from major debris or sedimentation.