Top Jobs: Overall Winner (Electric Power Category) Waiau-Koolau 138-kV Structure 15 Foundation Installation Project

The workers hand dug a staging area down at the construction site

The workers hand dug a staging area down at the construction site to allow for equipment to be flown in by helicopter.

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Ikaika, a Hawaiian contractor whose name means strength in the native tongue, completed the technically challenging Waiau-Koolau 138-kV Structure 15 Foundation Installation Project for Hawaiian Electric Co. Inc. in Aiea, Hawaii.

The original scope of the project was to manually excavate and install two reinforced concrete anchor bolt foundations for two new steel towers, Structures 15A and 15B, that would be installed by others.

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The construction site location was 300 ft down a steep treacherous slope heavily overgrown with grass, shrubs, and small trees. The slope was greater than a one horizontal to one vertical pitch. This significantly limited access and the type of equipment that could be used.

Two 300-ft ropes were installed by professional climbers for construction workers to access the site. Additionally, the climbers trained the workers to use climbing equipment to get down to the sit and back up the hill safely. All the overgrown vegetation had to be cut down to gain access to the site.

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The workers hand dug a staging area down at the construction site to allow for equipment to be flown in by helicopter. A mini excavator was then flown in to finish the excavation and complete the staging area.

The staging area was intentionally located away from the overhead 138-kV lines so that shut downs and grounding of the lines were not needed when flying in most of the shoring material, equipment and fuel; and when flying the portable toilet in and out for weekly cleaning.

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Two concrete pumps were located at the staging area for concrete pours. One of the concrete pumps was used as a backup.

A McDonnell-Douglass MD 500 was used for all equipment and material that weighed less than 900 lbs. A Sikorsky S61 was used for everything over 900 lbs up to 9,500 lbs.

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The location of each foundation had to be benched level for a work area. This was done by hand using air tools and shovels. A structural engineer designed a plan to use timber shoring with rakers to support the vertical bench cut. This shoring was installed at both locations.

Manual excavation for Structure 15A and 15B foundations was 5 ft in diameter by 26 ft deep including the vertical bench cut. A shoring plan was developed and signed off by a structural engineer to use HDPE pipe for shoring inside the shaft. Each piece of HDPE pipe was 4 ft tall and chained together in sections as the shaft progressed downward.

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Air tools, including mud guns, were used for the excavation. When hard rock was encountered, 60-lb busters had to be used. All excavated material was brought up with the use of buckets and ropes. The shoring was lifted out of the shaft in one piece by helicopter.

The 6,000-lb anchor bolt cage was flown in and set by helicopter at each structure. The overhead 138 kV lines were shut down and the lines grounded. Each anchor bolt cage was dropped in between the 138 kV lines and into each shaft.

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The anchor bolt cages were then raised to the correct elevation and the bolts correctly orientated using chain hoists attached to a small I-beam gantry. The I-beam gantry was flown to the staging area in sections and assembled on site.

The concrete forms were constructed for each foundation and assembled on site. The forms were then set to grade.

The new steel structure is more hurricane resistant

The new steel structure is more hurricane resistant, requires less maintenance, and is less visually intrusive than the old wooden structure.

For the foundation concrete pours, the staging area was cleared of all other equipment so that both concrete pumps could be set up. The concrete was flown in via half cubic yard buckets. Each foundation took approximately 32 cubic yards. The concrete pumps were flown to the helicopter landing zone off site along with the hoses so they could be cleaned and washed.

An issue with Structure 15B further complicated the project. Water seepage was encountered into the shaft at 8 ft down, and the shaft eventually collapsed against the shoring at a depth of 15 ft. The shaft had to be abandoned because the shoring couldn’t be moved and the inside diameter of the shoring was less than the design diameter of the foundation. Nevertheless, the shoring worked per engineered design. The shaft was then filled with grout and abandoned, and the hillside around the abandoned shaft was then jet grouted to stabilize the area.

At that point, Ikaika initiated and developed a new plan to use different sized shoring that would be grouted in place in case water seepage was encountered again. This plan was approved by the soils engineer. Three different sized diameter shoring would be used to accomplish the plan:

  • The first 10 ft of the shaft would be 7-ft diameter steel shoring that was custom made for use on another project and would be grouted in place.
  • The next 9 ft would be 5-ft inside diameter HDPE pipe and would be grouted in place.
  • The last 4 ft of the shaft would be 4-ft inside diameter HDPE pipe, and it would be removed after the excavation was completed.

This approach allowed the shaft to be a minimum of 5’ in diameter as originally designed. The anchor bolt cage was also modified for length in the new location, dropped into place by the helicopter and set to the correct grade and orientation. The 4-ft tall concrete form was made and set to grade. Then 32 cubic yards of concrete was flown in and pumped into the foundation.

For hillside restoration, 123.5 cubic yards of select borrow was flown in to restore the hillside. This took 494 trips by helicopter in increments of 0.25 cubic yard bags that the men could handle on the hillside. The select borrow was compacted in 6-in. lifts using air compacters. After that, 6,500 sq ft of erosion control blankets was installed.

The project will allow replacement of the old wooden structure was at the end of its useful life. The new steel structure provides a new life expectancy of 50-plus years. The installation of the new structure presented the opportunity to upgrade the infrastructure to meet the current building electrical load requirements. Additionally, the new structure is more hurricane resistant, requires less maintenance, and is less visually intrusive due to the elimination of the guy anchors and horizontal cross arms required for the old wooden structure.

The addition of the new structure will help to fortify the reliability of Hawaiian Electric Co.’s grid, thus improving the service to its customers. Tags: ,