In the morning of Aug. 29, 2005, hurricane Katrina arrived in full force. One of the most devastating storms in the history of the United States, she pushed across the coastal areas of Louisiana, Mississippi and Alabama with 125-mph (200-kmph)-plus winds gusting up to 180 mph (225 kmph). Combined with storm surges of about 40 ft (12 m), the result was a catastrophic loss of not only personal property, but also critical infrastructure that provided local utilities with the resources they needed to meet the electric service demand of its customers.

Mississippi Power Co. (MPC; Gulfport, Mississippi, U.S.) was one of the utilities impacted by the hurricane. Service to all 195,000 of its retail and territorial wholesale customers was lost in a matter of hours. The infrastructure damage included the loss of 65% of MPC's distribution system, 97% of its generation fleet and 92%, or more than 1900 miles (3000 km), of its high-voltage transmission lines.

More than 300 transmission poles and towers were damaged. Specifically included in this count was the loss of 12 transmission towers that held up more than 6 miles (10 km) of the Ocean Springs—Moss Point East 230-kV transmission line. This line crosses an east-west path through two coastal counties and is a vital link for reliable electricity to area customers.

The loss and damage to the 230-kV transmission line presented a unique opportunity and challenge for engineers and construction personnel to not only restore the line, but to make it a better, stronger asset. A large number of towers and length of conductor had to be replaced, but the uniqueness of the line was attributed to the fact that it was built in the heavy marsh of the Pascagoula River and would have to be rebuilt in the same location. This not only presented design challenges, but also challenges to mitigate the potential for environmental impacts during construction.

The Situation

The redesign and reconstruction of the 230-kV line was initiated in January 2006. The design effort included a change in the towers being used and a new foundation system that would better secure the towers to withstand future natural events, with much consideration being given to the fact that the towers would be erected in a severe marshland environment.

The new towers were designed using guyed galvanized steel, instead of aluminum like the old ones. The foundation system design included the use of concrete pilings as opposed to steel to mitigate corrosion, and an anchor-and-guy design normally associated with the stabilization of higher-voltage towers, such as in 500-kV construction. Subsequent testing proved the towers would be much more secure with this design, which more than justified the additional cost.

Many challenges exist when construction is required in severe wetlands or marsh. All 12 towers being replaced on this critical line had to be re-erected in such conditions. These challenges center not only on the working conditions and the added safety issues working in such conditions pose, but also on the potential environmental impacts of such a project. The success of the project was not only measured by the fact that the line was built back stronger than before in a very timely, efficient and economically justified manner, but also by the fact that the project was completed accident free and with minimal impact to the environment.

During construction, an osprey nest occupied by recently hatched eggs was located on one of the existing dead-end towers that had to be taken down due to its damaged foundation. Work on this structure began only after the offspring had left the nest. The work was accomplished, and the nest was safely relocated intact.

Improvements

The location of this transmission line falls within the 150-mph (240-kmph) extreme wind-loading requirement of the National Electric Safety Code (NESC). The new design allows for an infrastructure that, after construction, should withstand 165-mph (265-kmph) winds, which exceeds the NESC requirement. The stronger asset will provide a more reliable transmission resource in the future. This transmission line is a key element in supplying the Mississippi Gulf Coast's electric needs because it is part of the network connecting two of MPC's large generating plants. This line is critical to providing reliability to MPC's customers and, if left standing after a natural disaster, would be a huge asset in emergency-response restoration efforts.

Foundations

The foundation design for the transmission-line rebuild proved challenging. The existing steel-pile foundations consisted of three H-shape pile sections on a batter with a steel W-shape pile cap. Although they appeared to survive the storm without suffering any significant damage, several of the foundations exhibited significant signs of corrosion within the tidal zone. In some locations, the cross section of the piles had been reduced by as much as 70%. Since the tower-replacement process would require heavy-equipment access at each structure location, it made sense to replace all of the foundations within the marsh.

To begin the foundation design process as quickly as possible, soil tests were necessary to determine the type of foundation needed for each location. Another important consideration for the foundation type was the availability of materials following the devastation of Hurricane Katrina. Many of the surrounding pre-cast concrete manufacturers had suffered extensive damage to their production capacity.

After attaining the necessary soil information and considering the availability of materials, the foundation of choice consisted of three square pre-cast, pre-stressed concrete piles driven on a batter. The concrete piles would eliminate the corrosion problems that had plagued the existing steel foundations. Based on some preliminary discussions with a local pile driving contractor, the cross section of the piles was limited to a 14-inch (356-mm)-square section to reduce the handling weight in the difficult marsh conditions.

The new foundations also needed to provide for the additional strength for the new tower design. The depths of the piles varied from 30 ft to 75 ft (9 m to 23 m) below the mud line, with additional length being added to extend the top of the foundation above the water line and to provide for unexpected soil conditions.

Now that the piles were designed, the next challenge was to develop a pile cap that would allow for the necessary driving tolerances but wouldn't require a significant concrete pour, which would be a problem logistically. Due to the smaller cross section of the piles, the connection to the pile would need to be fixed to adequately transfer the design loads into the supporting piles. Pre-cast concrete caps were considered, but because of weight and connection concerns, the material of choice became galvanized steel. Steel corrosion of the existing foundations was not observed above the tidal-zone area, so the deterioration concerns of the new pile cap were alleviated.

Since the piles were pre-cast concrete, the pile cap connection could not simply be field welded. As a result, a steel-pile driving template was fabricated to control the driving tolerances to accommodate the bolted steel-pile cap connections. This technique proved to be the key to the successful construction of the foundations. The pile cap would not only connect two different materials, but would also support transition from the batter of the piles to a level-bearing surface for the base of the tower.

The primary connection between the steel and concrete was a 16-inch (406-mm)-square steel-tube assembly that slid onto the top of each pile. Once the assemblies were secured at the proper elevation, the voids inside the sleeve assemblies could be filled with a flowable grout to secure the connection. The sleeve assembly was also bolted through the top of the pile to provide additional connection strength. Adapter plates were fabricated to bolt to the top of the tube assembly and transition to the level-bearing surface. A steel wide-flange-beam assembly was then installed to connect all three of the piles to form the tower foundation. The steel assemblies were provided with slotted holes and oriented in such a way as to provide adjustment during the erection process. Finally, the steel caps were coated with a coal tar epoxy to provide additional corrosion protection.

Another technical improvement associated with the structural integrity of the new guyed galvanized steel-lattice tower was the use of 1.75-inch, 10,000-lb (44-mm, 4536-kg) anchors and anchor rods. The original transmission-line construction included the use of 1.5-inch, 5000-lb (33-mm, 2268-kg) anchors and anchor rods. The increased torque capacity of the anchors and rods allowed the helical anchors to achieve a much greater depth during installation.

As the anchor depth increased, the soil bearing increased, which resulted in a greater shear resistance against the helixes, thereby providing increased strength for the new structure. To provide the anchors with protection from corrosion, a 6-inch (152-mm) pipe was installed over the anchors and submerged into the marsh bed. A dense liquid tar was poured into the pipe, creating a moisture-free seal that eliminated the galvanized anchor rods from exposure to the changing marsh conditions.

Completion

The 230-kV line was completed and re-energized in mid-October 2006, just nine months after the project was initiated. Subsequent testing of the new tower and foundation designs proved they would provide a more secure infrastructure in the face of possible future events like Hurricane Katrina. As a result of good communication and planning between a great team, innovative design and construction solutions were developed to meet the challenges of this restoration project in a safe, timely and cost-efficient manner.

Acknowledgments

The authors would like to thank those at MPC and the Southern companies who contributed to the successful completion of this project: Jim Cochran, Steve Craig, Charles Munden, Ronnie Reinike, Rick Allison, K-Rob Thomas, Robert Whatley, Tricia Jermyn, Cyla Clark, Harry Durden, Jason Sapp, Phil Weeks and Ed Price. Thanks also to Vice Construction of Moss Point, Mississippi, for completing this project in a timely, efficient and, most importantly, very safe manner.

This article is based on the 2007 Southeastern Electric Exchange Chairman's Award winning paper titled: “Building Back Better, Stronger and Safely in the Pascagoula Marsh: Rebuild of the Ocean Springs—Moss Point East 230-kV Transmission Line.”

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Jim Myers has more than 31 years of experience in the electric utility business, with an extensive background in generation and transmission planning, and transmission-line design, construction and maintenance. He is currently responsible for managing the planning, design and construction of Mississippi Power Co.'s future transmission system, while maintaining more than 2000 miles (3219 km) of high-voltage transmission line, including oversight of the company's vegetation management program. Prior to his current position, his responsibilities included negotiation and management of bulk power agreements and contracts, and project management for construction of generating facilities across Southern Company's electric system. He holds a bachelor's degree in mathematics from the University of Alabama at Birmingham. jlmyers@southernco.com

Casey Allums is a senior engineer with Southern Company Transmission Design and Construction Civil/Structural support group. He began his career with Alabama Power Co. in 1998 as a student engineer. He earned a bachelor's degree in civil engineering from the University of Alabama at Birmingham in 2001. He is a registered professional engineer in the state of Alabama. clallums@southernco.com