ELECTRICAL TRANSMISSION AND SUBSTATION …

Unique Solution for 230kV Transmission Tower Grillage Foundation Corrosion Harry V. Durden Jr., P.E., MASCE1

Jonathan M. Maddox2

Stacy S. Sprayberry, P.E.3 1Civil - Structural Design and Maintenance Support, Southern Company Transmission, PO Box 2625 Birmingham, Alabama, 35291, Ph: (205)-257-4263, email: [email protected] 2Civil - Structural Design and Maintenance Support, Southern Company Transmission, PO Box 2625 Birmingham, Alabama, 35291, Ph: (205)-257-4496, email: [email protected] 3Civil Field Services, Southern Company Services, Highway 54, St. Clair Road, Vincent, Alabama, 35178, Ph: (205)-672-3401, email: [email protected] Abstract This document presents a unique and cost saving solution that was developed and installed to address extremely corroded steel grillage foundations supporting a 3 tower angle structure on a critical Southern Company 230kV line in south Alabama. Description of Problem In July of 2010 a routine field inspection identified significant vertical displacement of a helical anchor grillage foundation supporting an Alabama Power Company transmission tower on a critical 230kV line in south Alabama. A closer below grade inspection revealed severe corrosion of the helical anchors at and slightly below the ground line. The foundation supported the middle tower of a 3 tower guyed aluminum lattice structure on a critical 230kV line in the Mobile, Alabama area. Each tower of the structure was supported on individual helical anchor grillage foundations and each foundation was composed of 3 helical pipe anchors and a wide-flange grillage top. The helical anchors were originally installed to a depth of approximately 40 feet (12.19 meters) and into dense sands. The structure and foundations were installed in 1970. See Photograph 1

239ELECTRICAL TRANSMISSION AND SUBSTATION STRUCTURES 2012 © ASCE 2013

Photograph 1 - 230kV 3 Tower Aluminum Guyed Angle Structure The structure is located in a wetland where water is at or above the ground line during seasonal wet periods, making below grade inspections very difficult. Previous inspections of the tower and foundations had identified below grade corrosion of the pipe anchors, but the severity was not determined. The above ground wide flange sections composing the grillage tops of the foundations had been observed to be in excellent condition with no corrosion and the galvanizing protective coating still evident. See Photographs 2 - 4

Photograph 2 – Helical Anchor Grillage Foundations


Photographs 3 and 4 – Severely Corroded Grillage Pipe Anchors An inspection of the towers and foundations was conducted by the Southern Company Transmission Civil/Structural design group in order to develop an action plan for either repairing the foundations or installing new foundations and structures. The conclusion of the field inspection was that stability of the tower was in jeopardy due to the severity of the corrosion. Severe corrosion was observed in all three foundations with the middle tower being by far the worst. The combination of severe corrosion and possibly tower loading had caused several anchors to sever and displace downward. The grillage top of the middle tower foundation had the most vertical displacement with a total of about 8 ½” (21.59 centimeters) difference between the elevations of the wide flanges composing the grillage. The top surface of the grillage was sloping at about 12 degrees from horizontal. This extreme uneven vertical displacement of the grillage top created the possibility that the tower could potentially slide out of the base cup and off of the foundation, especially in the event of an extreme wind loading event such as a hurricane. See Photograph 5

Photograph 5 – Middle Phase Tower Foundation


The corrosion of the foundations was so severe that any type of repair would not be possible without an extended line outage. Vibrations or disturbances near the foundations could cause the middle tower to slide off the foundation cup and likely result in an extended line outage. It was also concluded that any work necessary to repair the foundations could not be performed safely without a line outage. This 230kV transmission line is a critical line that must be available during the peak summer months. Another very important consideration in developing options for either repairing the foundations or installing new foundations and structures was the 2010 BP Gulf oil spill. The Southern Company transmission system operating contingencies related to the BP oil spill required that this line be available at all times during the period that the oil spill could possibly impact Gulf coastal areas of the Southern Company system. Therefore, an extended outage of this line would not be allowed until after the summer peak and after all chances of oil reaching Gulf coastal areas of the Southern Company system had passed. This also meant that the structure would be extremely vulnerable to possible hurricane wind loads that could cause the middle tower to slide out of its base cup. Because of the risk of foundation failure that could result in an extended line outage on this critical line, the Alabama Power Company transmission line maintenance organization concluded that the foundations would either have to be repaired or a new tower and foundations would have to be installed as soon as possible to ensure the availability of this critical line. The scope of work necessary to install new foundations would require an extended line outage. In order to keep the line energized while a new structure and foundations could be installed, a very expensive temporary by-pass line constructed in wetlands would have to be installed in order to isolate the three tower angle structure. The costs associated with building a temporary by-pass line around the existing structure, installing new foundations and a new three pole angle structure and then removing the temporary line and structures was estimated as follows:

1. Install two temporary by-pass structures with deep foundations and string the line around the existing structure - $514,000.

2. Install three new foundations to a minimum depth of 40 feet required for geotechnical conditions - $70,000 each or $210,000 total.

3. Remove existing aluminum lattice structure and install a new steel three pole angle structure on new foundations - $100,000.

4. Restring the line to the new three pole structure, cut in the new line section and remove the temporary line - $70,000.

Therefore, the total estimated costs that would have been associated with replacing the corroded structures during peak summer months or during the BP Gulf oil spill crisis were $514,000 + $210,000 + $100,000 + $70,000 = $894,000.


Costs Saving Alternative Solution The Southern Company Transmission Civil/Structural group proposed a two step alternative to the very expensive plan to install a new structure and foundations. The proposed alternative was as follows:

1. Phase 1 - Install temporary precast concrete foundations and structural steel frame supports immediately to stabilize the existing structure and ensure adequate support during the critical summer months, hurricane season and during the oil spill crisis.

2. Phase 2 - Install new redundant foundations around the existing grillage foundations after the summer months, hurricane season and BP oil spill crisis had passed. A line outage could also be scheduled if required.

Phase 1 - Temporary Foundations and Structural Steel Framing For the temporary foundations a design was developed to install concrete mat foundations and steel framing that would provide backup support of the tower through the summer peak and hurricane season until an outage could be scheduled and a permanent solution installed. Geotechnical testing indicated that there were fairly dense sands just under the surface muck soils and down to about 5 feet (1.52 meters) below the surface. Soils from 5 feet (1.52 meters) deep to about 30 feet (9.14 meters) deep were determine to be very soft silty clay with essentially no strength. Medium to dense sands were encountered at about 30 feet (9.14 meters) below grade, which matched the installation depth of the existing helical anchors. Surface soil conditions were determined to be unsuitable for long term support of the structure. However, the surface soils were considered adequate for short term support of the tower on floating mat foundations. Temporary foundations would also provide a means of supporting the tower while construction work required to install permanent foundations could be performed safely and possibly without a line outage. The tapered profile of the lattice towers provided a means of clamping a structural steel frame to the lattice structure that would provide additional lateral and vertical support of the towers. This assumed that the guying would prevent any significant moment transfer to the temporary frame during the short timeframe that the temporary frame was to be in place. The temporary structural steel support could be clamped around the base of the towers with the primary intent of preventing the towers from shifting laterally and sliding off of the grillage tops. It was recognized that there were some risks involved in supporting the lattice tower in such a manner. The steel framing and clamps would not allow for total articulation and rotation of the tower bases and could possibly result in localized overstresses and buckling of the main aluminum angle members in the event of an extreme loading event such as extreme winds associated with a hurricane. These risks were


considered acceptable for the intended short duration that the temporary foundations would be in service. On August 6, 2010 temporary precast concrete foundations and steel frames were installed for each of the three towers of the structure. See Photographs 6 - 9

Photographs 6 and 7 – Temporary Steel Framing and Foundations

Photographs 8 and 9 – Temporary Steel Framing and Foundations The total cost for material and installation of the temporary foundations was approximately: 6 precast pads 4’-0 (1.22 meters) x 8’-0 (2.44 meters) x 0’-9 (0.23 meters) @ $720 each, delivered = $4,320 Structural steel, ungalvanized = $3,593 Labor and equipment to install = $16,000 Total = $23,913 Phase 2 - Permanent Foundations One objective of the design of the temporary support was to provide support for the towers while independent and redundant foundations could be installed around the existing steel grillage foundations and incorporating the existing wide flange grillage tops as part of new foundations. As noted, the wide flange tops were in excellent condition with a good coating of galvanizing remaining. A design was developed that would involve installing micropiles around the existing grillage foundations for deep


support, pouring concrete mat foundations up to the top flange of the existing grillages and then removing the temporary foundations and support frame. The towers would remain in the cups of the original grillage foundations throughout the construction of the new and redundant foundations. For the deep foundation components, both micropiles and helical anchors were considered. Micropiles were chosen even though helical anchor piles would have been better suited for the soil conditions. It was concluded that micropiles would be less likely to impact the existing helical anchors during installation and possibly cause movement of the original grillage foundations and a forced line outage. Drawings for the installation of the original helical anchors where available, but based on field observations the batter of the helical anchors did not exactly match the installation drawings. Helical anchors can also be deflected during installation by underground obstructions. So, the exact batter and location of the existing helical anchors was not known. Therefore, the conclusion was that for this situation grouted micropiles would be the best choice for the new deep foundations. Another consideration in the selection of grouted micropiles encased in concrete mats was the highly corrosive organic soils near the ground line. The significant corrosion in the existing helical anchors was isolated to the layer of corrosive organic soils and the oxygenized zone or within the top 6 inches (15.24 centimeters) to 12 inches (30.48 centimeters) of soil. Even though the micropiles would be grouted and protected from the corrosive soils, additional protection was provided by designing the mat foundation thickness so that the bottom of concrete would be below the corrosive soils and oxygen line. To protect the concrete from the corrosive soils, the mix design would include silica fume. A detailed description of the new and redundant foundations is as follows:

• Four grouted micropiles per tower would be installed to a depth of approximately 35 feet (10.67 meters).

• An epoxy paint would be applied to the wide flange members of the grillage and the base cup of the tower.

• A 4’-0” (1.22 meters) x 8’-0” (2.44 meters) x 2’-6” ( 0.76 meters) thick reinforced concrete mat foundation was designed for each tower to be cast over the new micropiles and around the existing helical anchor grillage foundations.

• An anchored bracket and retainer was designed for the middle tower for restraining the tower so that it could not slide out of the base cup.

• The temporary structural steel framing and precast concrete pads would be removed once the new redundant foundations were constructed.

The above described foundations were installed in May of 2011. The work was accomplished safely and without any interruption to the energized 230kV line. The following photographs document each major task involved in the construction of the permanent foundations. See Photographs 10 - 24


Photographs 10 and 11 – Micropile Installation

Photographs 12 and 13 – Concrete Mat Foundation Forming

Photograph 14 – Middle Tower Exposed Grillage Foundation with Severely Corroded and Displaced Anchors


Photographs 15 and 16 – Concrete Mat Foundation Micropiles and Reinforcing

Photographs 17 and 18 – Concrete Mat Construction

Photographs 19 and 20 – Concrete Mat Construction Complete with Temporary Supports Still in Place


Photographs 21 and 22 – Completed Micropile Mat Foundations with Temporary Supports Removed

Photographs 23 and 24 – Middle Tower Completed Foundations with Cup Retainer Bracket


Total Estimated Costs Savings The total installed cost of the three new foundations was $125,500. Therefore, the estimated total cost to construct the new redundant foundations was $23,913 (costs of temporary supports) + $125,500 = $149,413. The installation of temporary foundations provided a means for safely installing new tower foundations without the need for a line outage and with considerable cost savings compared to the estimated costs of constructing new foundations and structures. The total estimated costs savings associated with the chosen plan was approximately $894,000 - $149,413 = $744,587. Photograph 25 shows the completed project with three new micropile supported mat foundations and the temporary support frames and foundations removed.

Photograph 25 – Completed Project


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