Daniel Electric Generating Plant, owned by Southern Company Services Inc., is located in Jackson County, Mississippi, near Eascatawpa. Ash Pond B was a previously unlined pond, triangular in shape with side dimensions of approximately 1600 ft, 18000 ft and 1000 ft. The ash was a highly stratified mix of predominantly fly ash, approximately 25 to 30 ft in total thickness. The owner planned to completely remove the ash, temporarily store it on site, and install a liner before replacing the ash.
Geotechnical exploration at the site identied two underlying clean, sandy, water-bearing soil units: an upper sand formation and a lower sand formation. The natural groundwater table was well above the bottom of the pond and dewatering of the upper sand was required in order to facilitate ash removal and liner placement.
Pilot Dewatering Tests
Moretrench designed and conducted several full-scale pumping tests to evaluate aquifer parameters for the upper and lower sand strata, ash drainability, communication with nearby open water, and communication between the upper sand aquifer and the ash.
A series of step and constant rate pumping tests was conducted on wells screened in the upper sand only and in both upper and lower strata, with transmissivities and individual well yields indicating that the soils were amenable to dewatering. Separate in situ testing within the ash itself confirmed favorable drainage properties.
From the results of the tests, the project team concluded that while dewatering of the upper sand only would adequately drain the ash, extending the wells into the underlying lower sand would prevent it from acting as a recharge source for the upper sand.
System Design & Operation
The primary dewatering system was designed as a system of 15, large-diameter, high-capacity wells installed to a depth of 115 feet around the perimeter of the pond to lower the groundwater in the upper sand suf ciently below the bottom of the ash. A series shallow wells was jetted through the ash and 10 ft. into the upper sand to provide drainage of the ash as the pond was pumped down.
The pond was pumped down at a controlled rate following activation of the deep well dewatering system. Water levels in the underlying aquifer were continuously maintained below the water level in the overlying pond. The pond water was pumped with a system of oating sumps. Residual and surface water was readily handled from within the ash pond by the excavation contractor with trenches and sumps.
The deep well dewatering system pumped 4,500 gpm for more than a year, maintaining the groundwater below target levels throughout ash removal, liner placement, and replacement of the ash.
Early in the course of the excavation, as the pond was newly unwatered, the conditions were sloppy and unstable. As excavation proceeded, and the ash had more time to drain to the underlying upper sand formation, the conditions within the pond improved significantly. The photographs show the ash, at several stages in the project, cut almost vertically within the excavation. The conditions were excellent for the removal of the ash due to the effectiveness of the perimeter dewatering system.