Transmission Foundations Case History : Helical piles Hydro One Networks Inc.

Transcription

1 Transmission Foundations Case History : Helical piles Hydro One Networks Inc.

2 Late in 2008, an aerial patrol discovered three 500kV towers with partially-collapsed foundations on the power line between the Pinard Transformer Station and Porcupine Transformer Station on the Hydro One system. It was a serious problem. The line transmits power from the Otter Rapids, Harmon and Kipling Generating Stations 100 miles down to Timmins. An example of the grillage buried underneath the towers. Members that were broken off by ice 2

3 HELICAL PILES : New foundation design revives collapsing 500kV towers by Doug Hudspeth Front Line Manager Hydro One Networks Inc. Ontario, Canada Hydro One owns almost 30,000 km of high-voltage transmission lines in Ontario. Those towers are V shaped, supported by a single pedestal foundation and four guy wires. The buried, pedestal foundations are constructed of lattice steel four legs of angle irons that taper out as they extend down until they are 3 feet wide. From there they extend straight down an additional 5 feet. The angles are supported by diagonal members (flat bars and angle irons). The pedestal sits on top of a network of steel angles and timber mats. No concrete is involved. After the helicopter patrol found the problem, a ground-based patrol went out to assess the situation. The site is marshy and during the winter the frost heaves the ground up and down. This movement tore some of the diagonals off of the grillage footing. Without the support of the diagonal members, the main foundation legs bent and the foundation began to give way. Three structures had partially collapsed, but they hadn t completely tipped. A quick repair was required to prevent a complete failure of the foundations, but access to the structures with large equipment/crane was not possible without extensive road construction. Instead, a temporary repair was made. Our travelling line crew out of Sudbury installed blocking to carry the weight of the structures, until permanent repairs could be made. Winter was fast approaching and it was too late to do anything else in Deciding on a Solution Back in the 1980 s, Hydro One had used some Chance piles under similar conditions, but no one who worked on that project was still around and the work was not well documented. But, after reviewing the situation, it was decided that horizontal beams welded onto helical piles would be the best option. With this approach, we would not need to excavate or remove and replace the existing foundations. Therefore, we would not need to bring in big, earthmoving equipment. 3

4 Marshy land and tight clearances made the job more challenging. Note the jig on the right that lets the drill operator visually check the drilling angle. We knew that Hubbell Power Systems had taken over Chance and we have good relationship with our Hubbell representative, Roger Melanson, so we called him and he said he could definitely help us out. He put us in contact with Shawn Downey, a Hubbell helical pile and anchor expert, who came out for a site visit in early For that meeting, we brought in Hydro One s Engineering Design Team. Together, they reviewed the soil characteristics and defined what load the piles would have to support. (Knowing that we would need good load bearing data on the soil at the site, we drilled pins into the ground and measured the torque. What we found was about 7 feet of poor material and below that good, load-bearing clay.) The team also considered the length of the pile sections. There were some concerns about clearance with the tower during installation. Consideration was also given to the torque capacity of our drive motor. After the meeting, Shawn Downey went back and designed an anchoring system that would meet all of our requirements. He was very accommodating. Hydro One also considered all possible environmental issues and concerns. Thankfully, the site was not in a special treatment area. That is something we check anytime we need to enter a right-of way. The Big Fix Permanent repairs were made early in The work was performed by the Sudbury Travelling Line Crew under the direction of Rob Beange. Rob is the Crew Supervisor (or Union Trade Supervisor II as it is referred to at Hydro One). Rob and his crew came up with some innovative ideas that allowed the work to be safely and effectively executed. It was a really good project. It was something interesting and I know the crew enjoyed the work. The crew installed the piles at the three towers in two days. The work would have been finished sooner, but there was some distance between the towers. It took one day per tower to finish the work. To begin, a drive motor was installed on an excavator, which was used to install the helical piles through the weak soil and into the underlying clay. The crew used 4

5 At this point, all four helical piles have been driven into place. The next steps are excavation, cutting and welding the I-beams. an excavator rather than a radial-boom digger because the operator could better control the drive motor and could install the piles at a more precise angle. The crew also built a jig to use as a visual reference. The jig was a tripod with the legs angled at the correct drilling slopes (in two different directions). The drill operator could compare the angle of the piles he was driving to the slope of the jig. Angle was important because the helical piles had to be fairly close to the base of the tower at the top, but they had to taper away from the tower to provide lateral stability and to clear the grillage foundation below it. The operator screwed the piles down until the required torque was reached. That occurred at various depths, but on average were about 20 feet down. The lead section of the helical piles was 10 feet long (including a 7-foot by 2-inch-square solid shaft with helix plates 10, 12 and 14 inches in diameter topped by a 3-foot by 8-inch-diameter pipe/box-coupler section). We then used two extensions, each 10 feet long and 8 inches in diameter. Continued on next page >> 5

6 Jacking up a 500kV tower. Once driven in, workers cut off the piles horizontally at the precise elevation that would allow for the installation of the beams without requiring us to change the elevation of the tower. Once the I-beams were welded in place, they had to be perfectly horizontal and all the I-beams under each tower had to be at exactly the same elevation. To do this, one of our technicians used a laser level. (We ended up raising the towers a bit about half of a 3 4-inch bolt hole. That was just enough room to wiggle the bolts out.) Hydro One also hired a welder to weld the I-beams across the piles perpendicular to the direction of the line. These beams became the platform for the first set of temporary I-beams that were placed parallel to the direction of the line and used as a base to jack up the towers. Then, workers cut the top part of the grillage off below grade (the rest was left in place) and, permanent support beams were slid in, under the tower, and welded in place. The tower was then lowered onto the new beams and the guy wires were re-tensioned. We did not have to do much excavating during the project, nor did we want to. At that site, once you dig a hole, it fills with water. We were lucky at a couple of locations: We dug down about 2 feet and there was not too much water coming in. It was the welder who suffered the most. He had to lie on his back in the hole to weld the beams to the piles after grinding the galvanizing off of the piles. All work was complete by August 2009, and we have not had any more problems with frost heave at those sites. Documenting the Work This project was carefully documented. The engineers determined and recorded all the weights and tensions for the work and then we created a job document a step-by-step guide of what we were going to do. When the work started, a Work Methods Technician came out to the site to document the work on the first structure. We now have a well documented job procedure ready for the next person who 20 years from now has to do the same thing. 6

7 Except for the backfill, all work is complete. Below: The finished product. Doug Hudspeth, the author, was graduated from Mohawk College of Applied Arts and Technology in 1987, as an Electrical Control Engineering Technologist. He worked for a short time at a steel manufacturing plant before joining Ontario Hydro (the predecessor to Hydro One) in He worked on a variety of transmission line maintenance projects as a Technician before becoming Manager of the Transmission Line Technicians. For details on CHANCE Helical Foundations, contact your local Hubbell Power Systems representative or see our Catalog Section 4B at 7

8 About Hubbell Power Systems Hubbell Power Systems (HPS) manufactures a wide variety of transmission, distribution, substation, OEM and telecommunications products used by utilities. HPS products are also used in the civil construction, transportation, gas and water industries. Our product line includes construction and switching products, tools, insulators, arresters, pole line hardware, cable accessories, test equipment, transformer bushings and polymer precast enclosures and equipment pads. Preprinted for a Vol 16, 2012 issue of Hubbell TIPS & NEWS Hubbell has a policy of continuous product improvement. We reserve the right to change design and specifications without notice. Copyright 2012 Hubbell Incorporated Printed in U.S.A. 02/12RGS2M Bulletin