To start the ArrowBore CTHDD process, vertical site relief holes were auger drilled, then 18 in. diameter pipe was inserted vertically at 20 ft. intervals. The vertical relief holes are critical to the process and are used to sight the drill stem for on-grade measurements as well as reduce drilling fluid pressure build-up when inserting the pipe at such a close tolerance to the surrounding soil. Engineers were able to verify proper grade during installation with the stem depth verified at each relief hole location using a laser sight and measuring rod in the hole. If the drilling head is off grade, it can be adjusted and realigned with a rod hooked to the pilot drill stem. The continual measurement ensures the accuracy of the gravity sewer grade. While the pipe is being pulled into place, the drilling fluid slurry is relieved through the vertical relief holes allowing the contractor to have control of the drilling slurry flow and fluid pressure. This prevents collapse of the bore hole, as well as surface and utility damage. By reducing the drilling fluid volume, the amount of soil removed is only the amount needed to displace the pipe being installed. The excess slurry is then removed using a vacuum truck.

The close tolerance pilot hole was drilled and the location of the drill stem approved before beginning installation of the pipe. The recommended carrier pipe, 16 in. DR26 Certa-Lok® Yelomine® RJ PVC pipe, in 20 ft. lengths, from Westlake Pipe & Fittings, was installed segmentally in the launch pit as the ArrowBore process progressed and run at a 0.60 percent grade at an average depth of 13 ft. below ground. The outer diameter of the coupling was 17.40 in. and the reamer hole was 17.75 in. The close tolerance back reamer is only 0.25 in.to 0.50 in. larger than the greatest outer diameter of the pipe being installed, which prevents pipe flotation and soil settlement; standard HDD would have used at least a 26 in. bore hole. Once the length of pipe, with the coupling attached, was dropped into the pit, it was placed in a PVC channel. This channel helped ensure the pipe stayed clean for connection to the pipe being pulled and was also used as a guide to assist with pipe alignment. The Certa-Lok cartridge-style assembly measurably reduces the staging area so there is no need to string out lengths of pipe and, as a result, eliminates the need for several hundred feet of working space.

“I have been a proponent of using the ArrowBore technology provided by Trenchless Flowline for many years, and as a result that also means that I’m a proponent of using Certa-Lok PVC pipe.” said Charlie Conklin, P.E., project manager/design engineer for the City of Orlando. “For those not familiar with the ArrowBore process, its technical description is Close Tolerance Horizontal Directional Drilling, which is a controlled grade HDD that can be used for gravity or force main sewer construction. We at the City of Orlando have a strong affinity for PVC pipe, so Certa-Lok pipe is our first choice whenever we need restrained joint pipe, such as required for Close Tolerance HDD.”

The top few feet of soil was loose sand, the remaining 13 ft. was packed clay and very saturated. In the water table with clay and sand mixture, dewatering was necessary only around the manholes, not for the boring process.

The Certa-Lok restrained joints can be assembled in less than one minute per joint. Since this project was such close tolerance, the sub-contractor felt that cutting the spline would reduce drag. In most cases, the spline does not need to be cut.