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Dewatering Spillway Gates with a Reverse Needle Cofferdam

After operating since 1944 with minor maintenance, the eight spillway gates at Brilliant Dam in British Columbia, Canada, needed repair. However, the structure lacks a spillway gate isolation system for the vertical lift roller gates. Operator FortisBC Inc. put together a design-build team that investigated alternatives to resolve the situation and proposed to install a reverse needle cofferdam using "floating needles." This system was installed at Brilliant Dam in late March 2008, and FortisBC is now able to rehabilitate the gates.

Understanding the situation

The 145-MW Brilliant Dam and Power Plant on the Kootenay River is owned by Columbia Power Corporation and Columbia Basin Trust and operated by FortisBC. The 140-foot-high concrete gravity dam and associated four-unit powerhouse were completed to provide power for a smelter located about 20 miles south.

The spillway consists of eight bays, each containing a 34-foot-wide by 36-foot-high steel roller gate. A steel superstructure supports two travelling hoists above the spillway deck that lift the gates by means of dual threaded stems fixed to the upper corners of each roller gate leaf.

By January 2006, the spillway roller gates were in need of repair, maintenance and painting. Exposed steel portions had deteriorated paint coating. Because the dam design did not incorporate a gate isolation system, the gates had never been dewatered and submerged portions of the gate, seals and embedded guides had never been thoroughly inspected. FortisBC desired a spillway gate isolation system to be able to repair and paint these gates in the "dry" to extend their useful service life.

Approaches to gate isolation

FortisBC investigated two options for isolating the gates at Brilliant Dam: horizontally and vertically spanning gate isolation systems.

Horizontally spanning systems

This conventional approach for gate isolation involves providing a bearing surface at the pier noses or vertical slots in the piers upstream of the gate. Thus a vertical U-shaped sealing plane is required to provide an effective bearing and seal surface to install the horizontally spanning gate isolation systems. Unfortunately at Brilliant Dam, the pier noses extend 10 feet upstream of the crest. Although two locations were possible for the sealing plane - at the pier noses and between piers on the overflow spillway crest upstream of the gate - neither was attractive because of the cost and amount of work required.

Vertically spanning systems (needle cofferdam)

Another approach is to provide a needle cofferdam: a sill, piers, horizontal support girder between piers, and series of beams placed vertically between the sill and horizontal girder. The vertical beams, called needles, can consist of timbers, sheet-piles or a panelized system. These are placed adjacent to each other to provide the damming surface. Typically, the support beam is a fabricated plate girder, truss or hollow structural section (HSS) tube.

Needle cofferdams allow two-thirds of the water load to be carried by the sill and one-third by the support beam. The primary limiting factor is the height of the needles; the needle structural capacity is exceeded quickly as water depth increases. Needle cofferdams generally have been limited to about 27 feet - the maximum depth at which steel sheet piles can be used without bracing.

For a "reverse" needle cofferdam, the vertical needles pull against rather than bear against the support beam. A reverse needle cofferdam is used where there is insufficient space to locate the support beam between the gate and the spillway crest face. While this arrangement complicates the needle connection to the support beam, it minimizes pier modifications for installation. This type of cofferdam was selected for use at Brilliant Dam because of its lower fabrication cost; minimal pier modifications; no underwater structure modifications required; and ease of installation, removal and translation from bay to bay. However, this situation required design of needles that could be used to dewater to a depth of 42 feet.

Designing the reverse needle cofferdam

The cofferdam used the vertical up-stream face of the spillway crest, the pier walls and a support beam located just above maximum pool, Elevation 1480 feet. The reservoir varies from 1473 feet to 1477 feet. The transition from the vertical crest face to an ogee shape occurs at 1439.07 feet. Thus, total head is 42 feet when 1 foot of freeboard is included. The distance between piers is 48 feet, and the clear span of the spillway bay at the vertical crest face location is 36 feet.

Aubian Engineering conceived of a needle design using steel HSS tubes based on its experience with segmental floating bulkheads. The needle uses Aubian's patented system of sandwiching steel HSS tubes between steel plates to provide the structural capacity required while creating buoyancy and ballasting chambers.1 Schnabel Engineering provided design of the pier modifications, support beam and, through Aubian Engineering's license, the floating needles.

Floating needles

The needle design is an iterative process between fulfilling structural requirements, buoyant height needed above the reservoir, buoyant stability in all three axes when the bulkhead is floating vertically, and horizontal stability when all ballast chambers are empty. The HSS tubes were sized to meet CAN/CSA S16-01 standards, especially the width-to-thickness ratios of the steel elements. The cover plate thickness and their extent on the HSS tubes were governed by structural and buoyant stability requirements.

One limitation of a needle is the buoyant height needed above the reservoir water surface. For the floating needle to be stable vertically, its center of gravity needs to be below the buoyant centroid. As more of the needle is exposed above the reservoir by removing ballast, the buoyant centroid is lowered until it reaches the center of gravity. At this point, the needle is unstable and will begin to rotate horizontally. The needles were designed for an exposed height of 9 feet, the height needed for the needle to reach the support beam when the reservoir is at 1473 feet, minimum pool.

Each floating needle consists of two or three HSS 24 by 19 by 3/8 inch tubes welded to a downstream flat sealing plate and upstream cover and intermediate plates to create ballasting and buoyant chambers. Three needle configurations were used. The two end needles have a downstream sealing plate width of 76.5 inches and use two HSS tubes. The two intermediate needles have a downstream sealing plate width of 78.5 inches and use two HSS tubes. The center needle has a downstream sealing width of 118 inches and uses three HSS tubes. The HSS tube and intermediate chambers are sealed at each end. A set of valves are used to add or purge water from the ballast chambers for each needle. The needle floats in a horizontal position when empty and rotates to a vertical position when ballasted.

At Brilliant Dam, on the Kootenay River in British Columbia, Canada, the eight spillway bays were designed without a means to isolate the gates for repair and maintenance. Project operator FortisBC had a reverse needle cofferdam installed for dewatering.

Each needle is connected at its top to the downstream support beam using bolts. At the bottom, each needle bears against the upstream vertical face of the spillway crest. An elastomeric bearing pad runs along the bottom of each needle to make a seal between the needle bottom and crest face. A flap seal is used between each needle to prevent water from passing through the joints. An adjustable flap seal is used at the joint between the end needles and piers.

Support beam assembly

The support beam assembly consists of an upstream beam to transfer the needle loads to the piers, a downstream beam to support the individual needle top end reactions and 12 tension rods to connect the two beams together.

The upstream beam, with a length of 47 feet to span between piers, consists of a HSS 42 by 24 by 3/4 inch tube with upstream and downstream ¾-inch-thick cover plates welded to the 24-inch-wide sides. This beam rests on sills cut in the nose of each concrete pier. Elastomeric bearing pads mounted to the upstream beam transfer the beam end reactions to the pier concrete. The upstream beam was placed upstream of the needles because there was insufficient space on the piers between the sealing plane and gates to transfer support beam loads. Also, pier modifications would have interfered with post-tensioned steel anchors previously installed in each pier.

The downstream beam supports the top reaction of the vertical needles. This horizontal load is transferred to the upstream beam using 12 equally spaced high-strength (150 kips per square inch) threaded rods. The needles are connected to the downstream beam using two bolts through each needle. At each pier, the downstream beam rests on a flat ledge containing a sliding seat. This beam moves downstream about 1 inch due to deflection of the upstream beam when the intervening space is dewatered.

Pier modifications

To minimize the exposed height of the floating needles, the bottom of the support beam assembly was set as low as possible, 1480 feet, maximum pool. This was accommodated by saw cutting and demolishing 6.5 feet of the upper potions of the pier nose above the reservoir to provide a level surface to support the upstream beam and a vertical surface to transfer the upstream beam end reactions to the piers.

With this unique design, the five 44-foot-long needles can hold back 41 feet of water when the reservoir is at maximum pool. The 36-foot-wide by 44-feet-high needle cofferdam holds back a reservoir load of about 2 million pounds at maximum pool. The support beam ends transfer about 330,000 pounds to each pier during isolation of a spillway gate.

Once the reverse needle cofferdam was installed in Spillway Bay 2 at Brilliant Dam, the bay was dewatered so owner FortisBC could begin work on necessary gate repair, maintenance and painting.

Construction

Dix hired Associated Underwater Services and NUS Group to perform an underwater survey of each spillway bay to provide data needed for the design and sealing arrangements for the cofferdam. Also, NUS Group performed a visual survey of the concrete surfaces where the needles were to seal and patched those areas as required. And Kodiak Measurement Services performed a level survey of the pier surfaces for the divers' use and an alignment survey to locate the bearing pads to be installed on the pier surfaces. The cofferdam was designed based on these measurements and the surface variations found.

In July and August 2007, Dix demolished the pier noses. Bluegrass Concrete Cutters used a diamond wire saw to cut concrete blocks from the pier noses, while Dix mobilized a crane on flexi-float barges. The crane lifted the blocks from the pier onto the barge and placed them on shore. Dix then located and installed the pier nose bearing seats for the upstream beam on the horizontal and vertical cut concrete pier surfaces.

The original schedule was to have all commissioning completed in March 2008. By accelerating the schedule, Dix took advantage of the summer 2007 window to install the support beam assembly in Spillway Bay 2, as a barge-mounted crane was needed. Because the needles float, no barges were needed for commissioning in March 2008. The needles were fabricated by Northwest Steel-Fab.

Fabrication of the needles consists of: welding the HSS tubes to the downstream plate, welding end and cover plates, painting the interior of the space between HSS tubes, welding a cover plate to seal the space between the HSS tubes, and welding lifting lugs and other appurtenances. The only difficulty encountered was welding of the HSS tubes to the downstream plate. The ASTM A500 tolerances for these 44-foot-long tubes, particularly for twist, were large. Consequently, the HSS tubes would not lay flat against the plate. This twist had to be taken out of the tubes before they could be welded.

Commissioning

The needles were delivered to Brilliant Dam in late March 2008. Personnel completed assembly of the needles, such as trimming seals and installing valves, then stationed a crane adjacent to the shoreline, closed all valves and placed the needle in the reservoir. The needle was towed by a boat to the spillway bay.

A diver then opened the ballast and air release valves to allow the needle to move to a vertical position. Once vertical, the air release valve was closed and the needle was moved into position between the upstream and downstream support beams. Personnel then connected an air compressor to either add air to raise the needle or release air to lower it for securing to the downstream beam. All needles were installed in this manner. Once complete, a diver inspected the needles' position against the piers and crest and adjusted the seal clamping plates on the end needles that bear against the pier face. Finally, the gate was opened slowly to dewater the intervening space, and any leaks were sealed with cinders.

The only unexpected item was that the end needles would not come in to bear on the vertical crest face. The flap seals were too stiff and would not allow the needle to seat. Divers were able to rig a come-along on the crest and pull the end needles into their correct positions.

With this dewatering system in place, FortisBC could proceed with repairing gate members, replacing seals and repainting the gates for the first time in more than 60 years. One or two spill gates are scheduled for refurbishment per year.

- By Frederick Lux III, P.E., president of Aubian Engineering; Michael Dix, project engineer with Dix Corporation; Dustin Hale, P.Eng., mechanical engineer with FortisBC; and Robert T. Indri, P.E., project engineer with Schnabel Engineering

Note

1Canadian Application No. 2,591,457; U.S. Patent Nos. 7,214,003 and 8,066,449

Reference

Lux III, Frederick, Michael Dix, Dustin Hale and Robert Indri, "Simply Brilliant! New Approach for Isolating Spillway Gates," Proceedings of Dam Safety 2010, Association of State Dam Safety Officials, Lexington, Ky., 2010.

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