The Tyhee Siphon is a 5,400-foot long, 78-inch inside diameter concrete pipe which conveys irrigation water between surface ditches as part of the Fort Hall Irrigation Project. Originally constructed in the early 1900s, the siphon was first rehabilitated in the 1940s by installing an interior, sectional steel liner. Over time, both the steel liner and concrete pipe had deteriorated to the point where leakage and exfiltration was causing settlement and damage to Siphon Road, located above and adjacent to the siphon. This caused the Bureau of Indian Affairs to pursue the current rehabilitation effort starting in 2009. American West Construction (AWC) was selected to complete Phase 2 of the rehabilitation project, which was completed ahead of schedule in March, 2015.
The scope of the project included creating excavations to access the existing pipe, removing the existing steel liner, chipping and grinding the existing concrete pipe to fit the proposed liner/carrier pipe, installing 1,200 linear feet of 72-inch glassfiber reinforced (GRP) Hobas pipe via modified sliplining methods, and annular space grouting. Rehabilitation of the siphon was complicated by the fact that it could not be taken out of service during the irrigation season. The work therefore needed to be performed during the winter months between October 15 and April 1. Additionally, because the siphon crosses underneath an active Union Paciffic Railroad right-of-way and falls within a municipal right-of-way, it was necessary for AWC to secure right-of-way access permits and coordinate with both the railroad and City of Chubbuck, Idaho. While there was no feasible way to avoid a long-term road closure, which was coordinated with the City, local schools and emergency services, the project was successfully completed with zero disruptions to Union Pacific Railroad.
Crews began work by accessing the siphon via two excavations within the 1,200-foot Phase 2 area. These excavations were shored using extra-long trench boxes capable of proving an unobstructed span of 30 feet. This span was necessary in order to allow for the existing steel liner to be removed and for the new 20+ foot sections of Hobas pipe to be installed while providing additional space for ventilation equipment and personnel egress.
Due to the age and condition of the existing steel liner, as well as OSHA confined space regulations, removal methods were particularly limited. A mechanized system was implemented to transport each section of removed liner from the removal point to the access excavation and out of the siphon pipe, but the overall removal process and most effective process proved to be exceedingly labor intensive.
Once the existing steel liner was removed, the interior of the existing concrete host pipe was thoroughly cleaned and surveyed in detail. Measurements of interior diameter, roundness, elevation, and horizontal alignment were taken at 5 foot intervals such that the profile of the existing host pipe could be analyzed for acceptance of the new GRP liner. During the survey process, it was determined that the existing host pipe diameter was significantly smaller than anticipated and in some areas was not large enough to fit the proposed liner pipe. Consequently, AWC began the process of chipping and grinding the host pipe concrete as necessary to achieve the necessary inside diameter. After chipping and grinding was complete, a mandrel was used to verify the host pipe diameter and to ensure that the new GRP liner could be successfully installed.
The new GRP liner was then installed by means of a modified sliplining method. Typically, sliplining is accomplished by pushing sections of carrier pipe into the host pipe one section after another. This method, while performed successfully by AWC on previous projects, was not chosen for this particular project; instead, each section of pipe was essentially pulled into place using a system of sheaves, pulleys, and cables. Rather than jointing each new section of pipe in the insertion pits and pushing the pipe string into place, each section was first pulled into place and then jointed with the previous section inside the host pipe. The process and mechanical effort required of pulling each section of pipe into place was made easier by installing low-friction skids on the invert of the host pipe prior to installation of the GRP liner. These skids not only reduced the friction between the host pipe and the slipline pipe, they also served as spacers to ensure adequate encapsulation during annular space grouting.
Once the new GRP liner was installed, each joint was air tested to ensure gaskets were seated properly. A 12-inch service lateral connection was also made between the new carrier pipe and the existing service. Bulkheads were also constructed at each end of the sliplined reach in order to retain the annular space grout, and internal band seals were installed to provide a watertight connection between the new GRP liner and the existing steel liner, which remained in place at the Phase 2 construction limit.
With the sliplining complete and the bulkheads installed, cellular foam grout was pumped into the annular space. As it was critical that the annular space be filled completely with the cellular foam grout, AWC proposed using a 50-60 pcf cellular foam grout mix in conjunction with 2-inch injection tubes stationed every 100 feet along the siphon alignment. These grout tubes were installed between the ground surface and the crown of the host pipe prior to installation of the GRP liner pipe. Grout installation was started at the lowest elevation and, using the head pressure from the grout itself, essentially pushed uphill throughout the annular space under its own weight. This technique ensured that the new liner pipe would be completely encapsulated and that all air within annular space was able to escape through the upstream grout tubes located on top of the pipe.
Another critical factor addressed for the annular space grouting process was the method for preventing pipe floatation, because the buoyant forces acting on the GRP liner pipe when submerged and encapsulated in the annular space grout far exceeded the weight of the pipe, it was necessary to provide additional ballast to prevent the pipe from floating. Pipe floatation, while acceptable in some situations, was undesirable in this circumstance as the service lateral connection and bulkheads could have become damaged were the pipe to float. To address this, AWC installed temporary inflatable plugs inside the 72” GRP liner pipe at each end of the sliplined reach and filled the pipe with approximately 250,000 gallons of water. This additional weight inside the pipe provided sufficient ballast to prevent floatation during grouting.
Finally, after all annular space grout had experienced initial cure, the ballast water was drained, and the plugs were removed. The access excavations were then backfilled, asphalt patching and roadway restoration was completed, and the road was reopened to traffic. Slipline rehabilitation offers a unique advantage to traditional open-cut repairs in that minimum surface disturbance is required.