The Fort Hall Indian Irrigation Project (FHIIP), located in southeast Idaho, is dealing with problems common to many irrigation systems across the western U.S.: The project faces ever-growing rehabilitation costs while operating within an extremely limited budget. The aging infrastructure of the project includes several individual facilities that each carry multimillion-dollar price tags 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 originally was constructed in the early 1900s as a 78-in. 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 cu ft per second of irrigation water to roughly 11,500 acres of farmland. At some point in time after initial construction, about 900 ft of open canal leading to the siphon were replaced with similar cast-in-place concrete pipe, increasing the total length to roughly 5,400 ft. 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 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 was severely corroded, had no remaining structural competency, and was 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 expresses itself along the length of the siphon in a variety of ways: Chronic potholes develop in the overlying road surface, sand and water boil from the sides of the roadway embankment, and saturated soils created marsh areas in an otherwise arid environment. In the case of Tyhee Siphon, leakage from the siphon directly impacts traffic safety and leads 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 were concerns that the surrounding utilities may be compromised in the event of a catastrophic collapse of a soil bridge.
Replacement of the siphon was 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 (typically April 15 to Oct. 15); consequently, all repair and rehabilitation activities must occur in late fall and winter. Replacement of such a large-diameter structure with so many significant potential impacts is a risky and expensive proposition.
Trenchless Construction
Given the many challenges associated with traditional “cut and cover” replacement methods, in-situ (in place) rehabilitation is a desirable alternative. However, while in-situ rehabilitation is significantly less expensive in this case than complete replacement, the multimillion dollar cost to rehabilitate 5,400 ft of 78-in.-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 each phase may be many years as the FHIIP sets aside enough money to rehabilitate the next reach.
DOWL HKM, a multi-disciplined consulting firm, was selected by the U.S. Bureau of Indian Affairs through the request for proposals process to evaluate how to best return the siphon to optimal performance, including complete reconstruction of the utility.
The contract included a study phase to identify and evaluate alternatives, a design phase to develop plans and specifications to construct the selected alternative, and construction phase services to represent the owner during construction.
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, a woman-owned business that provides environmental and natural resources, engineering and construction services, 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 poor 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% of the total siphon length in three slipline segments and two direct replacement segments using 72-in.-diameter Hobas centrifugally cast fiberglass reinforced polymer mortar. 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. Other than the first 900 ft of the siphon barrel, which were in extremely poor condition, despite the severely deteriorated condition of the liner, much of the host pipe was found to be in excellent condition, requiring 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, and 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 lb per cu ft), 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 gauge was installed in the grout feed line to monitor the grout application pressure; the cellular concrete mix design resulted in a grout mixture that was so fluid that the grout pressure did not exceed 3 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 successful. The actual construction duration was approximately four months, and was completed between irrigation seasons. Phase 2 of the rehabilitation is scheduled for the fall and winter of 2014-2015, and will incorporate similar sliplining techniques to limit impacts to the traveling public and risk to adjacent utilities. Although the project is many years from completion and several construction phases still are required, the FHIIP has made significant progress toward the complete rehabilitation of this vital project component.
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