Dewatering for Clean Closure

At a power station located in the mid-Atlantic region, fly ash, bottom ash and boiler slag from discontinued coalfired operations was stored in five surface impoundments, some of which date back to the mid-1950s. Acting in compliance with current environmental regulations, the owner consolidated the four smaller impoundments, which are unlined, into the fifth and largest of the ponds, which will ultimately be capped. In the first phase of the consolidation work, Moretrench was contracted by the general contractor to dewater a 37-acre pond, the largest of the four to be remediated, to facilitate complete ash removal. When removal was complete, the pond area became a wetland.

BACKGROUND
The CCR was deposited as a slurry by means of a discharge pipe located at the eastern bank of the pond. The coarser bottom ash settled closer to the pipe, while finer ash spread and settled on the western, northern and southern banks. No formal geotechnical exploration had been performed in recent history; however, historic data from the 1960s suggested ash deposits were as deep as 44 ft. Prior to the start of dewatering operations, exploratory boreholes at select locations confirmed the historical data.

Hydraulic dredging was originally considered to excavate the pond, but this method would likely extend the project schedule well past mandatory milestones. The general contractor therefore recommended pre-drainage dewatering in conjunction with conventional excavation, an approach that would cut eight months off the time needed for hydraulic dredging, reduce overall cost, and also guarantee 100% ash removal. Moretrench was contacted by the general contractor early in the planning stage and worked closely with the project team to develop and implement the dewatering program.

PILOT DEWATERING TESTS
Comprehensive pilot tests were conducted for both wellpoints and deep wells to enable development of the clean closure plan. Given the very limited amount of geotechnical information available, five deep wells were drilled along an approximately 500-ft length of the perimeter access road and into the underlying aquifer to evaluate the connection between the surrounding aquifer and the pond ash. A series of wellpoints and piezometers was jetted at 10-ft centers within the ash itself to obtain reliable data on, amongst other hydrogeologic properties, hydraulic conductivity and radius of influence. Tensiometers were installed, hand augering done, and test pits dug to correlate ash stability to water levels.

Results of the pumping tests indicated that there was a connection between the ash and the surrounding aquifer, although only impacting the deepest portion of the pond. The extremely shallow drawdown gradient evident from the wellpoint pumping test also indicated that the radius of influence of a row of wellpoints was relatively wide, allowing production wellpoints to be spaced further apart. Overall, the stable, near vertical cuts achieved in the test pits confirmed that a wellpoint system would be effective in stabilizing the ash for removal.

For comparative purposes, a rim ditch was also excavated to determine flow rates. Given that the pumping volume was significantly less than that of the wellpoint system, it was confirmed that a wellpoint system was required to stabilize the ash for removal within the available timeframe.

PRODUCTION DEWATERING
The dewatering program was structured to allow pre-drainage and excavation to be accomplished in stages, beginning at the eastern bank closest to the slurry discharge pipe. This area was considered to require the greatest pre-drainage effort. Dewatering these deposits first would effectively reduce the amount of drainage required in the less permeable fly ash.

A series of wellpoint lines was laid out in parallel rows of variable spacing and connected to a pump station on the southwestern bank. Within each line, wellpoints were jetted at 10-ft on center. Dewatering began two weeks after wellpoint installation was initiated, with pumped water discharged to a nearby lined pond. Once drawdown had been accomplished, as indicated by piezometric levels, an initial ash pre-cut was undertaken to remove the top 10-15 ft of drained material. Moretrench then worked within the pond to install further wellpoints from the newly created partial subgrade elevation to target the bottom portion of the pond. The original deep wells installed for the pilot program were pumped throughout the production wellpoint dewatering to ensure that the deepest areas of the pond ash would be stable during excavation.

At the completion of the dewatering operation, the system has pumped more than 9,000,000 gallons of water. The
ash was able to be removed safely in 10-ft lifts at a rate of up to 10,000 yd3 per day using conventional construction
equipment, and trucked and placed in a dry condition.