In the spring of 2001, Sierra Pacific Power Co. began the design phase of the Falcon Project, a 180-mile-long, 345-kV transmission line that connects Falcon Substation northwest of Carlin, Nevada, with Gonder Substation near Ely, Nevada. The new line was a vitally needed link that improved Sierra's import capacity by an additional 250 MW. After an extensive study process, the final line route is located entirely in the rural mountains and valleys of central Nevada.
During the pre-design process, Sierra identified five separate segments of the route, totaling more than 51 miles, that would require helicopter construction. The new towers would have to be as lightweight and helicopter friendly as possible, especially since the high point of the project was more than 8100 ft, greatly reducing the lifting capacity of the helicopter. In addition, the design had to account for a wide range of soil conditions, ranging from saturated valley silts with high ground water to solid rock outcrops.
In the early stages of the permitting process, tubular steel H-frames were chosen for the tower configuration (lattice tower designs were ruled out due to bird perching concerns). Fortunately, Sierra had gained a good working experience installing tubular steel H-frames on a previous project (see sidebar). The drawback to an H-frame design, however, was that they are typically heavier than their lattice counterparts, and the direct buried tower legs do not allow much flexibility for variable soil conditions.
To address the challenges of helicopter construction and multiple soil types, the Falcon Project design team came up with a unique idea. The base sections were designed separately from the towers and a bolted flange connection was placed 3 ft above ground line on the tower legs.
This new design had a number of benefits over the conventional H-frames. First, separating the base sections eliminated between 13 ft and 25 ft from the overall tower length, reducing the total tower weight by 4600 to 6000 lbs. With this weight savings, even the tallest tower could be lifted in one piece by the helicopter. In the past, heavy H-frame structures typically required lifting in two pieces.
Second, the interchangeable bases of various lengths allowed foundation depths to be designed for each individual tower. Following an extensive geotechnical investigation, soil types and loading capabilities were determined for every area of the project. After the towers were spotted and actual foundation loads were determined, the base sections were designed to meet the required depth.
Third, the separate bases gave the contractor more flexibility in non-helicopter construction zones. Towers could be pre-assembled with the foundations and erected conventionally by crane. Where muddy winter conditions prevented access with the crane, foundations could be placed separately to maintain productivity, and the crane could return later to set the tower when the soils had dried out.
Finally, with interchangeable base sections, the contractor was able to use longer or shorter foundations where soil conditions were not as expected (as approved by the engineer).
When initially approached with the separate-base design, the tower manufacturer, Thomas and Betts (Memphis, Tennessee), had some reservations. Unlike typical steel poles with baseplates, the flange-to-flange connection for this project would not use leveling nuts, and there would be no way to adjust the tower alignment once it was set in place (short of re-excavating the foundations). If the bases were out of plumb by only 1 degree, the tallest tower would be almost 2 ft out of plumb at the top. Furthermore, manufacturing tolerances would have to be near perfect since the stub sections were designed to be interchangeable. Typically, flange plates are bolted together in the factory before the shafts are welded on to ensure proper alignment. The piece-marked segments then must be matched in the field. For this project, we wanted complete interchangeability.
After a meeting with the T&B plant managers, Sierra was assured that a jig could be constructed to precisely align the flange plates. To field test the constructability of the design, a tower was fabricated and the contractors that bid on the project were invited to witness a test installation at the T&B plant in Hager City, Wisconsin.
The field test provided an excellent example of what would be involved during helicopter installation, and allowed the contractors to feel comfortable that the base sections could be installed and aligned with a high degree of accuracy. The manufacturer modified the design of the spreader bar to eliminate deflection during setting of the foundations and addressed other minor concerns raised during the demonstration.
To the credit of everyone involved, the new design paid off. After completing a competitive bid process, Sierra selected Irby Construction (Jackson, Mississippi) and work began in March 2003.
Irby's construction crews proved to be extremely efficient at catching and placing the towers, setting an average tower in under 4 minutes. A long steel cable catch line was placed on the tower legs and fed through a specific bolt hole. As the structure was flown to the tower site, the dangling cable was caught, fed through the matching hole on the base-section flange and pulled through a pulley attached to the base section. This helped to guide the hovering tower legs into place. Once the tower was set on the foundations, crews hammered in drift pins to align the plates, then inserted and hand-tightened four bolts per tower leg. The helicopter then let go and proceeded back to the fly yard for the next tower while the catch crew finished installing the remaining bolts.
By August, all of the tower base sections were installed in the helicopter zones and their helicopter subcontractor, Columbia Helicopters (Portland, Oregon), mobilized to the project. When completed, Irby Construction and Columbia Helicopters placed 215 towers in just four and a half days. Not a single tower was out of alignment or needed any adjustments. Separating the tower stubs saved money in excavation and backfilling costs, and greatly reduced the required helicopter time.
Overall, the Falcon Project proved to be a phenomenal success, with 180 miles of line in very demanding terrain constructed in just 13 months. The 735 towers installed totaled more than 15 million pounds of steel. The project was completed on time, under budget and with an excellent safety record. The final construction contract had less than one-half of 1% in change orders.
Jim Lehan is a professional engineer responsible for transmission and substation design at Sierra Pacific Power Co., and was the senior project engineer on the Falcon Project. Lehan received a BSCE degree from the University of Nevada Reno in 1991, and is a registered professional engineer in Nevada and California. He has been with Sierra Pacific for 19 years. JLehan@SPPC.com
John Berdrow is a professional engineer and is a senior project manager at Sierra Pacific Power Co., and served as project manager on the Falcon Project. Berdrow received a BSCE degree from the University of Nevada Reno in 1984, and is a registered professional engineer in Nevada and California. He has been with Sierra Pacific for 21 years. JBerdrow@SPPC.com
MAKING THE MOST OF A NEW DESIGN
The innovative design of separating the foundations from the tower legs contributed significantly to the success of the Falcon Project. Other design modifications also resulted in considerable time and money savings:
Spreader bars were designed and tested to ensure accurate placement of the foundations. A flange plate on the spreader bar ensured that all bolt holes were in proper alignment. Once bolted together, the spreader bar and bases were then lifted and placed in one piece. The assembly was checked for elevation, alignment and levelness before being backfilled. As a result, every tower that was placed by helicopter fit perfectly.
The bottoms of the foundations were fitted with an uplift/bearing plate that was approximately 15 inches in diameter larger than the shaft. On many of the towers, foundation depth was governed by pull-out of one tower leg in the extreme wind case. Calculations showed that the additional 7.5-inch lip around the base of the stub reduced embedment depths by 12 to 18 inches. This reduced total excavation depths on the project by approximately 1500 ft.
An 18-inch-diameter hole was cut out in the bottom flange of the tower legs. This allowed the contractor to use pyramid-shaped guides on the bases to make helicopter placement more efficient, and also saved approximately 250 lbs per tower.
Along with the tower bases, the contractor was supplied with a small number of 5-ft tower extensions that can be bolted between the tower leg and the base. These were included in the event that a tower needed additional clearance for any reason. It turned out to be money well spent when, during the course of construction, a 10-ft elevation bust in the profile, due to an error in the aerial survey, was discovered. The addition of two 5-ft tower extensions quickly solved the problem.
LESSONS LEARNED ON A 160-MILE LINE
In the mid-1990s, Sierra Pacific Power Co. constructed 160 miles of 345-kV line from Reno, Nevada, to Alturas, California, using a very similar tubular steel H-frame design. The towers were not designed specifically for helicopter construction, but helicopters were eventually used to construct approximately half of the line due to permitting difficulties and time constraints.
In soils that could be augured, the contractor planned to drill an approximately 13-ft-deep by 5-ft-diameter hole for the tower legs and have the helicopter set the tower into the holes. However, in most locations, sandy soils or a high water table prevented the vertical cut face of the augured hole to remain stable long enough for the helicopter to complete installation. In rocky soils, the holes had to be dug with an excavator and blasted, leaving a large uneven hole that a tower could not be safely placed in without temporary support.
In these difficult soil areas, the contractor chose to dig an oversized hole and install a 5-ft-diameter corrugated metal pipe (CMP). The outside of the CMP was backfilled and compacted, and the towers were placed by helicopter into the CMP. Later, the contractor returned to each tower site to plumb the tower with a crane, and backfill between the CMP and the tower leg with concrete. The procedure proved to be very costly and time-consuming.