The existing siphon could not be removed from service during the irrigation season; consequently, all sliplining activities must occur throughout the late fall and winter.

Tyhee Siphon Rehabilitation: Meeting the Challenge of Aging Infrastructure on a Limited Budget

Repair of this facility presents many challenges, making the total project a seemingly insurmountable obstacle. But the FHIIP is completing rehabilitation of this facility in phases over several years and recently completed the first phase of construction.

The Fort Hall Indian Irrigation Project (FHIIP), located in southeast Idaho, is currently dealing with problems common to many irrigation systems across the western United States; the project faces ever growing rehabilitation costs, while operating within an extremely limited budget. The aging infrastructure of the project includes several individual facilities each of which carries a multimillion dollar price tag for rehabilitation/replacement. One such facility is the Tyhee Siphon. Repair of this facility presents many challenges, making the total project a seemingly insurmountable obstacle. In line with the old adage of how to eat an elephant (one bite at a time), the FHIIP is completing rehabilitation of this facility in phases over several years and recently completed the first phase of construction.

The Tyhee Siphon was originally constructed in the early 1900s as a 78-inch concrete pipe and serves as a primary water conveyance structure of the FHIIP. The siphon is located about two miles south of the Fort Hall Indian Reservation in Bannock County, Idaho, within the city of Chubbuck, and delivers 230 cubic feet per second of irrigation water to roughly 11,500 acres of farmland. At some point in time after initial construction, about 900 feet of open canal leading to the siphon was replaced with similar cast-in-place concrete pipe, increasing the total length to roughly 5,400 feet. The siphon was rehabilitated in the 1940s by installing a steel lining sleeve inside the concrete host pipe to seal the leakage from the concrete pipe. Appurtenant structures to the siphon include a wasteway/drain, an air vent, and one irrigation delivery.

The Growing Problem

Over time, the steel liner and the concrete pipe have deteriorated to the point that leaks are now common. Previous inspections have revealed that the steel liner was not firmly anchored to the host pipe, except at the inlet and outlet portals, resulting in a void between the liner and the host concrete structure. The liner is severely corroded, has no remaining structural integrity, and is sagging under its own weight and flaking off into the water flow. Because of the gap between the steel liner and the concrete, it was not possible to inspect or obtain measurements of the concrete host pipe without creating more leaks in the steel liner. This made it difficult to determine whether the concrete pipe was failing and deforming along with the liner. The siphon is located adjacent to, and partially under, Siphon Road, which is a city street adjacent to businesses, a residential area, and an elementary school.

Leakage expressed itself along the length of the siphon in a variety of ways: Chronic potholes developed in the overlying road surface, sand boil and water came from the sides of the roadway embankment, and saturated soils created marsh areas in an otherwise arid environment. Leakage from the siphon directly impacted traffic safety and lead to regular closures of the westbound lane. Further, because pothole formation and sand boils are indicators of water movement along the outside of the pipe, there are concerns that the surrounding utilities may be compromised in the event of a catastrophic collapse of a soil bridge. 

Replacement of the siphon is further complicated by crossings under both a Union Pacific railroad line and the Yellowstone Highway, in addition to the many municipal utilities that surround the siphon, including both gravity and force main sanitary sewers, water mains, storm drain systems, natural gas, fiber optics, and overhead power. The siphon cannot be removed from service during the duration of the irrigation season, which is typically mid-April through mid-October. Consequently, all repair and rehabilitation activities must occur during the late fall and winter. Replacement of such a large diameter structure with so many significant potential impacts is an extremely risky and expensive proposition. 

Trenchless Construction

Given the many challenges associated with traditional "cut and cover" replacement methods, rehabilitation in place is a very desirable alternative. However, while rehabilitation in place is significantly less expensive in this case than complete replacement, the multimillion-dollar cost to rehabilitate 5,400 feet of 78-inch diameter pipe is still an expense that greatly exceeds most irrigation district capital improvement and maintenance fund budgets. To make this project viable, the siphon had to be rehabilitated in phases in which the time span between phases may be many years as the FHIIP sets aside enough money to rehabilitate the next reach. 

Consulting firm DOWL HKM  was selected by the BIA through the RFP process to evaluate how to best return the siphon to optimal performance, including complete reconstruction of the utility. DOWL HKM is a multi-discipline firm owned by senior managers of the company. It has 400 employees and 23 offices throughout Alaska and seven other states.

The contract included a study phase to identify and evaluate alternatives, a design phase to develop plans, specifications to construct the selected alternative, and services to represent the owner while the work was under way. DOWL HKM performed a detailed field investigation that included internal survey and dimension measurements, as well as external test pits and concrete coring. The results of these investigations were used to develop a phased rehabilitation plan for construction.

Long Term Solution

McMillen of Boise, Idaho, was awarded the project. McMillen is a woman-owned business founded for the purpose of providing environmental and natural resource engineering and construction services. It was awarded the project through the U.S. Small Business Administration 8(a) Business Development Program, which helps small, disadvantaged businesses compete in the marketplace.

The phasing sequence began at the diversion into the siphon and progressed toward the siphon outlet. This most upstream reach was chosen as the first phase of construction, as it was known to be in terrible condition and it was the only place where conventional replacement would not be complicated by adjacent utilities and would have a minimal impact on the traveling public. The ability to fully excavate the siphon allowed for large launch/receiving pits from which to conduct "pilot" slipline operations, which provided the first opportunity to evaluate the host pipe for deformation. 

During the fall and winter of 2012-2013, Phase 1 was completed, including rehabilitation of roughly 20 percent of the total siphon length in three slipline segments and two direct replacement segments using 72-inch diameter Hobas centrifugally cast fiberglass-reinforced polymer mortar (CCFRPM) pipe. Slipline preparation involved removing the deteriorated liner and measuring the roundness of the host pipe to develop a slipline plan that included installation of EPDM runners as low friction guides and maintain an annular space around the full circumference of the new carrier pipe. The first 900 feet of the siphon barrel was in extremely poor condition, but despite the severely deteriorated condition of the liner, much of the host pipe was found to be in excellent condition. It required only minor reshaping of the concrete pipe to successfully slipline the new carrier pipe. Once the slipline was complete for each segment, a bulkhead was constructed of dry pack grout around the circumference of the carrier pipe at each end of the slipline. Then Cellular Concrete was pumped in to fill the annular space between the new carrier pipe and the host pipe. 

Cellular Concrete is a lightweight (approximately 40 pounds per cubic foot), high flow, low strength cementitious grout. It incorporates a foaming agent into a mix of neat cement and water to bulk up the mixture and produce a highly fluid, fresh grout that can be used to fill the annular space of several hundred feet of sliplined host pipe. The highly fluid nature of the grout is an important design characteristic as it allows the grout to flow freely out through cracks and open joints in the host pipe, filling soil voids around the exterior of the pipe. A gage was installed in the grout feed line to monitor the application pressure; the Cellular Concrete mix design resulted in a mixture that was so fluid that the grout pressure did not exceed three psi. Compressive strength testing on samples cast in the field resulted in 28-day compressive strength results of approximately 300 psi. 

The Phase 1 Tyhee Siphon rehabilitation project was extremely successful. The actual construction duration was approximately four months and was completed between irrigation seasons. Phase 2 of the rehabilitation was scheduled for the fall and winter of 2014-2015 and incorporates similar sliplining techniques to limit impacts on the traveling public and risk to adjacent utilities. Although the project is many years from completion and several construction phases are still required, the FHIIP has made significant progress toward the complete rehabilitation of this vital project component.

Hobas manufactures pipe in sizes from 18 inches to 126 inches in pressure and non-pressure classes. For more information, visit

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