At 7900 ft (1589 km), Hells Canyon on the Idaho-Oregon border is the deepest gorge in North America, carved over the millennia by the raging waters of the Snake River. More than 652,000 acres (263,856 hectares) of the landscape are set aside as a national recreation area of high mountain peaks, untamed waters and solitude.

A few miles upstream, at the canyon's southern end, a series of three hydroelectric dams wring power from the river to help serve Idaho Power Co.'s (IPC; Boise, Idaho, U.S.) approximately 883,000 customers spread over a 20,000-sq-mile (51,800-sq-km) service area that spans southern Idaho and eastern Oregon.

In order to keep pace with increasing load growth, IPC turned to its transmission grid within Hells Canyon. There, construction of a 10.5-mile (16.9-km) long 230-kV transmission line between switchyards at Brownlee and Oxbow, two hydroelectric dams at the southern end of Hells Canyon, allowed the company to transport power from its hydroelectric facilities in combination with additional imported power from neighboring grids.

Engineering and construction of this transmission line presented IPC with a unique set of challenges in each project phase — from routing and structure selection to wire stringing and restoration — that required unconventional approaches and coordinated teamwork.

Background

Because of the aesthetic and recreational values in Hells Canyon, IPC performed a rigorous environmental analysis of the proposed project for the local BLM district, the lead governmental agency for the project. Through this permitting process, the BLM evaluated three alternative routes. The district considered routes on the Idaho and Oregon sides of the river as well as an Oregon inland route. The Oregon river route was selected primarily because the existing 69-kV transmission line in place along the proposed route would be relocated as a second circuit along with the new line. This route would traverse the canyon wall at distances from 20 to 500 ft (6 to 152 m) from an existing paved road along the river edge.

One of the project's primary environmental findings was the presence of bald eagles in the area of the line route. To comply with BLM requirements, IPC would have to schedule the construction phase of the project to avoid winter and spring because of eagle nesting periods. Therefore, IPC would have to squeeze construction into a window between the months of July and December, giving the utility just five months to mobilize equipment and construct the new line.

In addition to a tight schedule, difficult access and severe terrain, the project was remote from existing construction support (two hours from the nearest concrete batch plant). To further complicate the project, the existing 69-kV transmission line was the sole source feed for a small town in Oregon, and either needed to be kept in service or use local diesel generators to power the town for line outages.

Engineering Begins

The structure type to be used for the project was a single shaft tubular steel pole. Both circuits of the line (new 230 kV and existing 69 kV) would be designed for 230-kV operation with the potential for voltage upgrade of the 69-kV line.

It became apparent during the preliminary engineering phase of the project that anchoring the steel poles into the canyon walls was going to be the most difficult and costly portion of the project. To assist in this critical design, IPC selected POWER Engineers Inc. (POWER; Hailey, Idaho) to provide the geotechnical investigation and foundation design for the project. In turn, POWER selected a team of Klein-felder (Boise) and Crux Subsurface Inc. (Spokane Valley, Washington, U.S.) to support the geologic reconnaissance and geotechnical investigation.

Geotechnical Investigation

Upon consideration of structural loads, access, soil/rock conditions and site topography, POWER recognized the uniqueness of each site and required a site-specific foundation design and construction plan. Because terrain and access made a detailed geotechnical investigation at each structure site impractical, POWER chose a program that combined soil/rock borings at representative structure locations with a geophysical survey at other intermediate locations.

The key component of the geotechnical investigation was a series of 40 soil and rock borings. Because the power line traversed topography ranging from vehicle-accessible flat terrain to severe side slopes and cliffs accessible only by foot or helicopter, both rubber-tired truck-mounted and helicopter-transported equipment were used for these geotechnical borings.

The drilling at each site consisted of a soil boring, or a combination soil boring through soil overburden and rock coring once the soil boring encountered rock. The soil boring was a standard 6-inch (15-cm) diameter hole, 40-ft (12-m) deep or until rock was encountered. Soil samples were collected and logged every 5 ft (1.5 m) for laboratory analysis. The rock coring consisted of a 2.4-inch (6-cm) core drilled up to 20 ft (6 m) into bedrock, depending on soil overburden. Like the soil, the rock samples were collected and logged every 5 ft.

The geophysical survey was a non-invasive subsurface characterization of the subsurface conditions consisting of a series of 20 refraction surveys — a measurement and characterization of sound waves reflected off of the various rock formations. This geophysical survey was calibrated with the drilled-hole data and used to prepare a comprehensive estimated depth-to-bedrock map for each structure location. As a result of combining these two investigative techniques, POWER developed an estimated depth-to-rock for each structure on the project, the key parameter required for foundation design. The design and eventual construction of the foundations allowed for site-specific adjustments for the actual rock profile encountered at each hole.

Foundation Type Selection

Prior to detailed foundation design, IPC and POWER performed a foundation feasibility study to identify and define the preferred foundation type for the project. POWER reviewed the physical data, queried prior projects and consulted line foundation contractors to support this decision. Due to the widely varying soil conditions (sandy soil to solid rock) and differing access restrictions (two-wheel drive to helicopter/foot access), the project team considered five unique foundation types. Each of the foundation types was evaluated for constructability, flexibility, material availability and required construction expertise.

• Direct Embedded Steel Poles (Rejected): This foundation type offered a low concrete solution, a key factor in the remote area of the project. However, because this type required poles to be pre-engineered and ordered in advance, it offered no flexibility in the field to vary the embedment depth, a necessary requirement for the project due to the variability of the soils.

• Grouted Anchor Bolts (Rejected): This option considered grouting anchor bolts directly into existing rock formations. It also offered a low concrete solution. However, because of the inconsistent quality of the rock, and concerns for the precision and consequent quality control required for this type, it was rejected.

• Spread Footings (Rejected): This foundation type offered the possibility of smaller construction equipment such as backhoes or track-mounted vehicles that could more easily access the terrain. However, the high ground-line moments of the foundations required an excessive volume of concrete over more traditional foundation designs.

• Concrete Pad with Grouted Anchors (Optional): This option required the construction of a small concrete pad 5- to 7-ft (1.5- to 2-m) thick on the upper portion of the pier with grouted anchoring rods into the bedrock below the concrete pad. This foundation offered advantages of minimal rock excavation and concrete requirements with the ability to perform bolt grouting with more precision because of the concrete pad from which to work. POWER created a design that was offered as an alternate for contractors to consider. The eventual construction contractor did not choose to use this option due to concerns with rock quality, rock anchoring material availability and testing requirements.

• Poured-in-Place Concrete Caisson (Preferred): By using minimum-length anchor bolts and a separate steel rebar cage, a conventional drilled pier could be customized, by length, to the soil/rock condition at each of the sites. The reinforcing cage was custom built for the site after a rock exploratory boring was performed and the foundation design assigned at the structure site.

Foundation Design

Because the project alignment was adjusted several times after the geotechnical soil borings, only 25 of the 103 foundations had direct geotechnical information from which to design. The design and construction of the remaining foundations required an approach that was flexible enough to be modified once excavation started. In its design approach, POWER chose to group the structure foundations by their loadings (primarily ground line moments) and then size the pier depths for a series of soil/rock profiles with varying depth-to-rock. To perform these designs, POWER used the LPILE foundation design program for the foundation designs. A top-of-pier deflection limit of 1 inch was the limiting design criteria for determining the foundation depth.

POWER compiled soil/rock profiles that defined the structural characteristics of the soils and rock and site conditions, such as depth to solid rock, slope of rock, competency of the rock and side-hill characteristics. To minimize the excavation and concrete required, POWER calculated the diameters of the concrete piers with code minimum interface dimensions. Each of the foundations was then placed in one of 24 groups based upon the foundation load and pier diameter. The total foundation depth was determined from the design tables from the foundation grouping and the depth-to-rock encountered in the field. This approach enabled the minimum excavation for each site. Pier diameters ranged from 5.5 to 9 ft (1.7 to 2.7 m) and depths from 11 to 41 ft (3.4 to 12.5 m). A total of 3400 yards (2195 m) of concrete were required for the foundations on the project.

Foundation Construction

Foundation construction proved as difficult as anticipated. Because of the aggressive schedule requirements, the construction contractor needed to average approximately two concrete foundations per day.

To facilitate this, the construction contractor, Great Southwestern Construction (GSW; Castle Rock, Colorado, U.S.) and its foundation subcontractor, A.K. Early (Redding, California, U.S.), had at least three and up to five separate large bore caisson drill rigs in operation. In many locations, a Lo-Dril hole excavator was required for structures in close proximity to the existing 69-kV transmission line. This machine was able to excavate with a maximum height from ground of 20 ft (6 m) as compared to the 40-ft (12-m) masts typical to this equipment.

Six structure locations (eight foundations) had no access for conventional drilling equipment. These foundations ranged in size from 66-inch diameter by 11-ft deep (168 cm by 3.4 m) to 90-inch diameter by 20-ft deep (2.3-m by 6 m). They were excavated in solid rock with air-powered hand tools. Each of these excavations averaged three weeks for a crew of three men to excavate.

Five specific foundation construction mobilizations were performed on each structure site:

• Construction Access. For the majority of the sites, a new access road was constructed to the structure site. If a site had an existing access road, it was cleaned and a flattened area created for hole drilling rigs and concrete trucks.

• Foundation Boring/Rock Blasting Operation. GSW contracted with Rhino Works of McCall, Idaho, to drill exploration holes at each structure site in order to define the soil/rock profile for each excavation as required by the foundation design. Once the depth-to-rock was determined, they drilled and shot the rock portion of the hole (fractured the rock) to the required depth for the particular site as required by the design tables.

• Hole Drilling/Excavation. Large bore caisson drill rigs were mobilized to excavate the hole to the design depth and diameter.

• Reinforcing Placement. Steel reinforcement and anchor bolts were installed in the excavated hole.

• Concreting Operations. Concrete was dispatched, placed and finished in the excavated hole as required by the specification. Each hole required at least one set of concrete tests.

Concrete

Because of the remote location of the project, GSW chose to create an on-site concrete batch plant. Materials from Baker City, Oregon, approximately two hours away, were trucked to the project site to facilitate mixing and batching on an as-needed basis.

Daytime temperatures were consistently above 100°F (38°C), which created a constant battle for the contractor to stay within the specification maximum requirement of 90°F (32°C) concrete. This often meant commencing concrete mixing operations as early as 4 a.m. to take advantage of cooler temperatures.

In addition, concrete aggregate was constantly watered to keep its temperature down. Afternoon foundation pours focused on minimizing the amount of time the concrete was in the mixing truck (the temperature rise in the concrete from batch plant to the hole could increase up to 15 degrees per hour). Several trucks of concrete were rejected because of the heat specification.

At the eight holes inaccessible by overland vehicles, helicopter concreting was performed. A ¾-yard bucket with drop hatch was used to ferry the concrete between the concrete truck and hole. These operations were performed at the rate of approximately 8 to 10 yards (7 to 9 m) per hour of concrete placed.

Completion

The foundation construction of 103 foundations in 84 separate locations was completed in approximately 10 weeks. This allowed the remainder of the line construction to be performed and completed on time. This construction included the following:

• Setting 103 steel poles, including 90 pieces of steel being picked and placed in one and a half days with an Erickson Skycrane helicopter.

• Stringing 10 miles (16 km) of bundled conductor for the new 230-kV circuit.

• Stringing, cutting over and energizing 10 miles of conductor of the existing 69-kV line.

• Installing and cutting over approximately 4.5 miles (7 km) of distribution underbuilt conductor on the new line.

• Installing approximately 10 miles of fiber-optic shield wire.

On Dec. 15, 2003, five months from the construction start, the new Brownlee to Oxbow 230-kV transmission line went into service, bringing reliable, efficient power to the Snake River plain.

Darel Tracy, PE, is a senior transmission line project engineer for POWER Engineers. Prior to joining POWER, he spent six years as a member of Idaho Power Co.'s (IPC) Project Management Department, responsible for coordination of design, rights of way, public relations and operational personnel throughout IPC's system in the implementation of electrical facilities.Tracy was IPC's Project Manager for the Brownlee-Oxbow transmission line project. Tracy holds BSCE and MSCE degrees from University of Idaho.
dtracy@powereng.com

Ron Carrington, PE, is a senior project manager with 20 years of experience in transmission line engineering and consulting. He is a recognized expert in line uprating and failure analysis. Carrington's experience includes engineering and managing multiple complex overhead and underground transmission line projects from 69 kV to 500 kV. He has a BSCE degree from the University of Colorado at Boulder.
rcarrington@powereng.com

Todd Adams, PE, is a senior transmission line engineer for Idaho Power Co. In this capacity, Adams is responsible for design of transmission and distribution lines from 12 kV to 345kV. Adams, who has 13 years of experience in wood pole, steel pole, lattice structures and line foundation design, was the design engineer for the Brownlee-Oxbow transmission line project. Adams has a BSCE degree from Montana State University.
tadams@idahopower.com