D602F is the 500-KV AC transmission line that connects Manitoba hydro to its export customers. This transmission line originates just north of Winnipeg (Manitoba, Canada) and travels in a southeasterly direction, terminating at Forbes Station in Minnesota.

The line was built in 1979 and is approximately 540 km (336 miles) long, about 210 km (130 miles) of which is in Manitoba, with the rest in Minnesota. Within the Manitoba Hydro zone, the line contains approximately 220 self-supporting structures and 270 guyed structures. The guyed towers are used in swampy regions of southeastern Manitoba. Footing types include both cast-in-place and precast concrete piles.

THE DISCOVERY

In February 2003, during an insulator replacement program on the line, a section of concrete was found some distance from the pile foundation. Closer inspection revealed a section of the pile cap had been damaged. This discovery led to further inspections of other guyed tower foundations in the immediate vicinity. Several more damaged foundations were uncovered.

The first few damaged foundations that were found appeared to have been caused by some form of mechanical damage. With most of the pile caps under several feet of snow, Manitoba Hydro decided to complete a quick ground patrol on portions of the line during the summer months. Access to these guyed structures is always a challenge; consequently, summer ground inspections were not part of the regular inspection program. The extra inspection revealed problems far worse than initially thought. Dozens of foundations appeared to be in an advanced state of concrete decay.

In the fall of the same year, Manitoba Hydro completed site tests on some of the worst locations. The guyed towers have a 450-mm (18-inch) base pin that extends only 300 mm (12 inches) into the concrete footing. Manitoba Hydro excavated six locations to determine the depth of decay. The location of the top rebar ring was also a concern. Several foundations had severe crumbling, which should have revealed the top ring of the rebar cage, but no reinforcing rebar was visible. This caused additional concern, because the base pin that held the tower on the foundation was only 300 mm into the concrete. If the first horizontal rebar ring were more than 300 mm (12 inches) below the top of the foundation, there would be nothing preventing the base of the tower from kicking out. The foundations were severely crumbling. Using your finger, it was possible to remove pieces of the concrete pile.

EXTENT OF DECAY

In the summer of 2004, the utility initiated a thorough inspection to catalogue the condition of all 490 foundations on the transmission line. The results showed that 35 foundations had mechanical damage in need of repair; 57 foundations were crumbling and needed to be replaced immediately; 45 foundations on guyed towers and 12 foundations on self-supporting structures were in critical condition.

A possible root cause of the decay is the freezing of the concrete during curing and batching problems during construction. Some of the foundations appeared to be in good condition, but with the gentle tap of a heeled boot, the foundation cap would crack and reveal crumbling concrete hidden beneath.

FACTORS TO CONSIDER

With 45 foundations on the guyed towers considered to be in a critical condition, it was necessary to implement a remedial plan as soon as possible. Manitoba Hydro outlined that an optimum solution would include the following:

1. No structure relocation
2. Minimal guy wire adjustment
3. No line outage
4. Cost effective
5. Account for a short winter construction window.

Foundations in the poorest condition resided in swampy areas. Since the work would be carried out during the winter season, roads were built with sufficient frost depths to carry the required equipment.

Manitoba Hydro decided early in the project to replace the existing foundations. The depth of concrete decay was unknown for each foundation and there was no guarantee that sound concrete would be found. Putting additional concrete piles in the vicinity of the original foundation would be challenging from a construction standpoint. It would also be time consuming due to the necessary curing time before load transfer could be achieved.

THE SOLUTION

The solution Manitoba Hydro selected was a design variation that had been used on another transmission line with similar ground conditions.

Step 1

A series of four helical screw piles were installed around each of the existing foundations. Almita Manufacturing Ltd. (Panoka, Alberta, Canada) provided the screw pile design. Original construction records provided only a description of the soil at each site. The decision was made to pursue a conservative design based on this data, rather than waiting for a thorough soil investigation to be completed.

The design was based on setting the helical piles to a predetermined depth, rather than using installation torque values. Although the torque readings were monitored to confirm the soil data, they were not the criteria for installation. Two different installation depths were used over the course of the work.

Almita Manufacturing also provided the installation equipment. Its modified Excavator, with a torque head capable of delivering 90,000 ft-lb torque, had little difficulty in driving the helical screw piles through several feet of frost. Once on-site, the installation unit could easily handle and maneuver the 20-ft screw piles. The installation time of four helical screw piles averaged no more than 30 minutes per site.

Step 2

A new design was created for the base of the tower. Four new steel angle legs were installed inside the existing frame. They were tapered to terminate on a new box beam approximately 1.8 m (6 ft) higher than the existing tower base.

Step 3

Depending on the desired setting depth, pipe extensions could be added or cut in order to obtain the desired pipe lengths.

Step 4

The new support frame was maneuvered into position using an all-terrain forklift. Blocking was placed under the frame and raised into position using hydraulic jacks. The frame was raised just high enough to transfer the load from the existing foundation onto the box beam and new frame. Predrilled holes on the frame were used to locate and drill bolt holes on the piles. This then tied the new tower base to the new foundation piles, via the box beam and channels.

Step 5

The old steel tower angles below the new frame were cut away in order to prevent any potential damage from movement of the old footings.

Step 6

Using a similar design from another transmission line, threaded rods connecting a hanger bracket to the steel pile cap were added as necessary in order to allow fine elevation adjustments to be made, should any further movement occur.

TIME AND SECURITY

In total, 45 footings were replaced in 30 days, thus accommodating the short construction window. The approximate cost for each footing was CDN$22,250. In addition to the obvious benefits of time and cost, there was a third benefit: Line security was maintained because the energized line's structures were not on temporary supports for an extended period of time.

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Wes Mueller is currently the transmission line services engineer for Manitoba Hydro. During his 15 years with Manitoba Hydro, he has held a variety of positions dealing with the maintenance and design of transmission lines and structures. Mueller received his bachelor's degree in civil engineering from the University of Manitoba in 1991 and is a registered professional engineer in the Province of Manitoba. wmueller@hydro.mb.ca

Scott Campbell received his bachelor's degree in civil engineering from the University of Manitoba in 2002. He joined Manitoba Hydro that same year. In 2005, he accepted a permanent position in Line Maintenance Services. Campbell recently received his professional registration for the Province of Manitoba. sicampbell@hydro.mb.ca