The Seneca, S.C., Light and Water Department employed the latest technologies in potable water diving earlier this year to repair two damaged clearwells without having to take them out of service. A specialized diving team applied surface epoxies under water to rehabilitate the structural integrity of the tanks.
The Seneca department serves a portion of the South Carolina lakes region and has been stressed by growth from a resort industry that has developed on Lakes Keowee and Hartwell. The department also is serving a new industrial development that includes major industries such as Schlumberger, Cryovac, and Greenfield Industries.
Seneca pulls its water from Lake Keowee, a Duke Energy lake impounded in the early 1970s to serve Duke?s Oconee Nuclear Station. The water plant dates from that time.
Three months prior to the repairs, the department discovered problems with two of its three clearwells when divers from Southeastern Underwater Services Inc. (SUSI) cleaned and inspected the tanks.
SUSI is an 11-year old Greenville, S.C., company formed by Steve Burdsal in 1988 when he took his interest in commercial diving and applied it to the potable water industry. The firm has a history of work in potable water over the last three years in four Southeastern states.
Before entering a potable water tank, the company?s divers don a fully-encapsulating dive suit of vulcanized rubber and undergo decontamination with a 200-ppm chlorine solution. After studying the as-built plans, divers are briefed in a safety meeting to avoid hazards such as the unscreened effluent discharge within the tank. While the discharges are not screened, the potable water is very clear and the diver easily can see the discharge openings. Water levels often fluctuate while the diver is in the tank. Tethered by an umbilical, the diver is protected from straying into any outflowing currents.
The diver?s helmet is equipped with live voice and real time video allowing the tender (diver?s helper) to see what the diver sees. Because of OSHA confined space regulations, a minimum of three crew members are required for each dive. There must be one tender per diver, in addition to a supervisor who can conduct a confined space rescue. The tender?s responsibilities include managing the diver?s umbilical hose, air, voice communications, closed circuit television and lighting. The umbilical supplies filtered surface air because SCUBA (self-contained underwater breathing apparatus) does not provide an adequate safety factor.
During the Seneca project, divers first cleaned two tanks measuring 152 feet in diameter and a third tank measuring 92 feet in diameter. The divers used an Aquavac vacuum system to remove silt and debris.
Areas in two of the clearwells which received sun almost all day showed stress from the constant expansion and contraction caused by temperature cycling. The clearwells originally constructed in 1976 are 25-feet deep, concrete, in-ground tanks. Five feet of the tank wall extends above the ground, exposed to the sunshine. The concrete roof is supported by internal columns with a tongue and groove or keyway pour joining the roof to the tank sidewall.
Most of the problems were between the joint where the roof meets the sidewall. As heat from the sun caused the roof to expand and contract, the sidewall pulled away from the roof. Over the years, the movement back and forth caused the concrete to wear. Large pieces of concrete had fallen from the top of the walls, which support the roof, and numerous smaller cracks and crevices needed attention. In some places exposed rebar held the roof in place. Full color videotapes made by the divers allowed the Seneca water plant personnel to see for themselves the extent of the damage.
Burdsal contacted Stewart Nance, CEO of Raven Lining Systems, to discuss alternatives for repairing the clearwells without taking them out of service. Nance proposed using AquataPoxy A-7, a solventless, 100 percent solids structural epoxy grout. The material is specifically formulated to bond to wet substrates. The epoxy is certified by NSF to ANSI/NSF Standard 61 for drinking water. It can be applied underwater and has been used to patch dam faces, saltwater piers and other underwater infrastructures.
Over the next two weeks SUSI divers used bulk-loaded guns to apply 21 gallons of the epoxy in the two damaged clearwells. The epoxy displaced the water and bonded to the exposed steel rebar and wet concrete, filling the voids and curing into a material that is structurally stronger than the original concrete.
?It was very tedious and painstaking work,? said Burdsal. The two-part epoxy formulation had to be mixed and loaded in the bulk cartridges. Because of the product?s 30-minute pot life, the material is mixed in quantities no larger than the diver can apply in that amount of time.
The sidewall was coded by marking a number on each expansion joint. The numbers helped the diver stay oriented as he worked in the nearly total darkness inside the tank?s watery environment. Except for the damaged areas and the manway hatch, there was no ventilation or light within the tank.
During the application, a live video feed to monitors in the SUSI dive control trailer allowed the Seneca staff to oversee the progress. The application was also captured on videotape for quality assurance and to provide a record for future inspections.
The epoxy is a 100 percent solids epoxy, meaning that it contains no solvent. It cures to a Shore D hardness of 85. Shore D is the ASTM test D2240 that measures hardness. In the test, an item similar to a nail is pressed onto the surface. The amount of indent gives a scale reading from zero to 100, with 100 being the hardest. A common material that has a hardness of 100 would be hardened steel. Silicon caulk sealants have a low Shore D of five to 10.
?What sets this product apart is the proprietary breakthrough specifically formulated around 15 years ago to adhere to underwater or damp or moist surfaces,? said David Stanley, Raven Linings. Designed specifically for potable water, and meeting the NSF61 certification, the epoxy generally is used in concrete drain tanks to smooth walls prior to the final coat.
Burdsal said, ?The product exceeded our expectations for application ease and physical strength. Our client is pleased with the results.? The company expects the rehabilitation will extend the tank life for another ten to 15 years. The repaired damage seals the tanks against further intrusive damage.
?This is an excellent example of how the service industry can make use of technology assistance from supply firms to find solutions to problems for our clients,? said Burdsal. ?We knew we could fill the need if we could find the correct product.? SUSI will monitor the repairs to ensure durability over the next year, but Burdsal said he does not expect any problems.
