The 330-lf crossing of 86-in steel casing was originally designed to be constructed by either hand-mine operations or slurry microtunneling, due to the geotechnical report anticipating 15,000-psi rock including clasts up to 35,000-psi. Since no ground water was present in the formation, Horizontal Boring LLC selected an Akkerman TBM with a disc cutterhead designed for the alignment. This method would prove to be safer and more efficient than hand-mining, while more cost-effective than slurry microtunneling.
Pipe jacking is done from a launch shaft to a reception shaft with advancement provided by a hydraulic jacking frame located in the launch shaft. Excavation is controlled at the face by the operator as the TBM is advanced by thrust forces transmitted from the jacking frame through the product pipe. Excavated material is transferred into an electrically powered haul system that carries the material back to the launch shaft for removal. At the end of each section of pipe, a new section of pipe along with tunnel provisions are connected for continued advancement.
An Akkerman 5200 pipe jacking system was used in the launch shaft with 37.5-ft of rail. This configuration
allowed Horizontal Boring LLC to setup and launch the TBM in one section and accommodate the 20-ft long pipe. The pump unit serves as the jacking frame and hydraulic power supply for the pipe jacking sequence and can accommodate TBMs ranging from 48-in. to over 100-in. OD with proper setup.
Each Akkerman TBM is shipped with a standard dirt bar and carbide bar cutterhead to suit ground conditions ranging from soft ground to weathered rock (UCS < 4000-psi). These standard cutterheads can be changed underground if necessary. Closed face cutterhead attachments that can mechanically control unstable ground conditions with hydraulically closeable doors require installation prior to the tunnel construction. Due to the rock conditions on the SR-51 crossing, a disc cutterhead was required to complete the alignment, and installed at the factory prior to shipping.
To efficiently fracture the breccia and schist rock formation, the cutterhead was designed with 11.5-in. single disc cutters as well as bolt-on carbide tipped scrapers to remove soil and other debris. For maximize disc cutter life, the overall thrust applied was monitored and regulated through the load applied to the articulation joint of the TBM. Removable grizzly bars enabled the operator to adjust the cutterhead opening ratio (COR) or allowed access the face for potential obstruction removal. Since no pressurized ground conditions were exhibited in the geotechnical report, closed mode tunneling or costly compressed air interventions, would not be required on the SR-51 crossing.
Tunnel guidance is provided by a simplified laser-to-target system. The TBM operator assesses the position of the tunnel laser at the cutterhead every 10-14 inches of advancement. Based on the position of the laser, the operator makes steering adjustments with the three-point steering system to maintain the desired line and grade. While enhanced guidance systems can be added to Akkerman TBMs to meet engineering specifications such as electronic data logging or remote monitoring, the conventional laser-to-target guidance system has been proven for decades, and effective at distances exceeding 1,000 ft.
In order to maintain efficient pipe jacking operations in the hard ground conditions displayed on the SR-51 crossing, the overcut was designed to 1-in. (per radius). This design provides clearance on the annulus of the pipe to maintain low thrust forces while allowing steering corrections along the alignment in hard ground conditions. If the overcut diameter is too small in hard ground, the tolerances of steel casing will stack-up, causing high jacking loads along the pipe string where steering corrections are required to maintain line and grade. This can potentially lead to sudden spikes in jacking forces, pipe failure, or a seized tunnel.
Bentonite injection was used to fill the annulus between the steel casing and rock to support the pipe jacking process. Lubrication is critical in all trenchless pipe jacking methods, as the proper engineered mix maintains the annulus, reduces risk of settlement, and lowers overall thrust requirement of the installation. The proper lubrication mix should always be engineered based on the actual ground conditions and adjusted accordingly if ground conditions change along the alignment. A distinct advantage to conventional pipe jacking is that the operator can monitor the interaction of the cutterhead, so changes in ground conditions are evident in real time.
The overcut and bentonite proved to be an important factor as Horizontal Boring LLC was required to stop tunneling for approximately 60-days, awaiting the completion of the settlement monitoring system along the crossing’s right-of-way. With the TBM advanced nearly 50-ft into the alignment, crews monitored the shafts for the risk of flooding as Arizona was inundated by early season rain events. Once green lighted to commence operating again, Horizontal Boring LLC experienced a break-out jacking force of around 350-ton but dropped to around 100-ton after pipe advancement and additional bentonite injection.
Horizontal Boring LLC achieved around 15 to 20 ft of 86-in. OD steel casing per shift. A second shift was deployed to set and weld the 20-ft sections of steel casing in the launch shaft which can often require 8 hours to complete.
Akkerman Inc. is proud to have been a part of the infrastructure solution that is essential to the economic health and vitality of Phoenix and wants to congratulate Horizontal Boring LLC, The City of Phoenix, and all others involved in the Phoenix Drought Pipeline Project.
