At Pennsylvania Electric Companys (Penelec) Seward coal-fired generating station, bottom ash and fly ash from production operations were transferred in slurry form to two temporary holding ponds and allowed to settle out until the surface water could be decanted off. The residue was then mucked out, trucked to a designated permanent disposal site, and dumped. WhenPenelec changed its disposal operations in 1980, the ponds were no longer utilized and since they were designated as temporary holding locations, the Pennsylvania Department of Environmental Protection required their complete removal and restoration of the site within a relatively short timeframe.
However, given that the height of the ash already in the extremities of the permanent disposal pile was near the height of the perimeter dike, together with the tendency of the saturated material to flow, the large quantities of material that would result from closure could not be safely or cost effectively excavated, transported or placed in a saturated condition. Penelec therefore retained Moretrench to evaluate the use of dewatering techniques to stabilize the ash so that it would have less tendency to flow when placed at the disposal site. Ash Pond No. 2 was selected for a 2-phase study consisting of a comprehensive literature review and site visit to determine ash disposal practices followed by a field dewatering test. The successful outcome of the test program resulted in a full-scale dewatering operation 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.
Dewatering Test Program
Ash Pond No. 2 was approximately 400 feet wide by 600 feet long and founded on stiff clay. Ash depths varied from 7 to 12 feet. The overall deposit was non-homogeneous: fly ash and the coarser bottom ash had been introduced into the pond at different locations and at different times, and had also been placed in lifts. The presence of horizontal layers of the coarser material throughout a significant area of the pond meant that drainage effects could be translated for some distance. Gradation analyses indicated that the fly ash, while fine, was no finer than some organic silts that are frequently stabilized by dewatering methods. Test pits dug at several locations prior to wellpoint installation to determine the stability of the ash encountered approximately 3 feet of moist to dry surficial ash over saturated, highly unstable material.
To test installation and pumping techniques, a cluster of five vacuum wellpoints was jetted in place and a washed concrete sand filter introduced in each hole. Pumping tests were conducted, and drawdown in adjacent wells measured. Given the significant drawdown achieved during these tests, a larger test was conducted using a system of 23 wellpoints, with 4 piezometers installed to monitor the effects of the pumping operation. The wellpoint system pumped for approximately 10 days, and water levels in the piezometers dropped between 1 and 3 feet. Based on these results, and post-pumping test pit evaluation, it was decided that pre-drainage dewatering would achieve the desired result of stabilizing the ash for excavation and disposal.
System Design and Operation
Since pumping tests had indicated that the zone of significant improvement extended only 40 feet away from theline of wellpoints, a series of wellpoint lines was installed at 80-foot centers across the pond. Within each line, wellpoints were installed on 10 foot centers. Piezometers were installed between adjacent rows. Wellpoints were also installed around the entire perimeter. Since water quality information indicated a corrosive environment, the system was composed of corrosion-resistant materials.
Removal of the ash began approximately 5 weeks after pumping was initiated and was completed some 4 weeks later. After dewatering, the residual thickness of unstable material was reduced to less than 1 foot. The soil in the transition zone between dry (stable) and saturated (unstable) was thixotropic in nature. It became unstable when vibrated but quickly regained stability when the vibration was removed.
Performing a test section, including laboratory testing of ash properties to verify dewatering system details and the suitability of the technique for the ash encountered, was critical to the success of the dewatering program. Even after such tests, the experience and professional judgment of the Moretrench team was required during system design and operation to ensure a problem-free operation. Good surface water control practiced during pumping effectively expedited lowering of the groundwater levels by preventing excess rainwater infiltration.
As a result of the dewatering program, the stability of the ash was such that excavation could take place without sloughing of the slopes or bogging down of equipment.