Rehabilitation to Reduce Piping Potential in the Skelton Dam Artesian Pressure Drainage System

FPL Energy-Maine Hydro LLC, a wholly owned division of Florida Power and Light, owns and operates the Skelton Hydroelectric project. The project includes a 1,200-foot-long, 80-foot-high earth embankment section constructed in the 1940s on a foundation of soft to medium marine clay which is underlain by a sand stratum just above bedrock. During design of the dam, it was recognized that there was a potential for development of artesian pressure in the confined aquifer formed by the sand stratum via a hydraulic connection to the reservoir which represented a potential for heave at the toe of the dam and the potential for compromised slope stability. To relieve this pressure, several 8-inch diameter relief wells with casings that extend to the ground surface were constructed. The relief wells drain by gravity through 4-inch laterals into east and west collection pipes consisting of 36-inch diameter reinforced concrete, buried about 40 feet below the downstream slope of the dam. A 36-inch diameter, 90-foot-long side collection pipe (referred to as the spur) is attached to the west collection pipe, which picks up the discharge from one of the 8-inch relief wells, as well as six additional two-inch diameter relief wells. The 2-inch diameter relief wells are different than the 8-inch wells in that they were installed shortly after construction of the dam to relieve unexpectedly high artesian pressure and discharge directly up into the spur (rather than into the spur via lateral pipes). Therefore, if something went wrong with the 2-inch wells, they could not be easily controlled, and can not be directly monitored from the ground surface as the 8-inch wells can be. The screens for the 2-inch wells were apparently not adequately sized for the sand deposit, since there has always been a problem with sand migrating into the spur. These relief wells were apparently discharging sand into the spur for many years, and the situation was worsened when one of the wells was damaged in 2000 during an attempt to assess and remediate its condition. Since 2000, several attempts have been made to mitigate the sand discharge from the wells, and in 2002, two electric pump wells were drilled adjacent to the spur which mostly stemmed the flow of water and sand into the spur. However, during high tailwater levels, the 2-inch wells apparently would discharge despite the pumping, continuing to carry additional sand into the spur. This was evidenced by a continued buildup of sand sediment in the bottom of the spur observed during the annual confined space inspections of the spur.

Making matters more difficult, access to the spur requires confined space precautions, and is only done by experienced suited divers with air/communication/rescue umbilical lines connected to a surface dive station. Access requires that the diver (attached to the umbilical line) climb down the 40 foot ladder in a manhole, crawl through the 150+ foot long west collection pipe to the spur, and then turn 90 degrees to enter the approximately 90 foot long spur.

The continued sand discharge, difficult access and limited surveillance ability resulted in a serious concern with regard to the potential for piping into the drainage system. As a result, the decision was made to construct a concrete plug, consisting of about 21.5 yards of concrete in the area of the spur where the 2-inch wells were located. Because of the difficult access, constructing this plug presented a number of interesting challenges in design and construction including: assuring potential voids outside the pipe were filled; allow continued discharge of an 8-inch well beyond the plug after the plug was placed; pipe preparation and remove of a significant amount of sand and debris; construction of special bulkheads to form the placement of 5 cells that made up the plug, and testing to assure that adequate depressurization of the artesian layer would occur after the installation of the plug. The proposed paper will discuss the history and events leading to the plugging/grouting of the spur as well as the installation of the plug and piezometric performance following the plugging. This should make an interesting and useful case history for other owners with relief well/toe drainage systems. 16 pp., 10 figures, 6 references.