Dewatering Fly Ash

Dewatering Fly Ash for Remediation

Fly ash has properties and behaviors that present unique challenges during excavation,  handling and disposal. Fly ash material has historically been stored in ponds and is  known to be unstable and sensitive to vibration when saturated. When saturated fly ash  is subjected to shear strain, it densifies and expels water, resulting in a near total loss of  shear strength. In this state, the ash becomes a viscous fluid and may begin to slide or  flow. This process may result in overtopping or breaching of impoundments and makes  excavation and handling difficult to impossible.  Using a coal ash dredge makes this easier.

Changing the water content in the ash by only a few percentage points has a dramatic  effect on its behavior, allowing stable, near vertical cuts suitable for conventional mass  excavation. The increase in strength happens when a reduction in water content  changes the pore pressure from slightly positive to slightly negative, imparting apparent  cohesion and shear strength to the ash. 

The purpose of this paper is to discuss two different methods for lowering the water content of fly ash: one where the ash was impounded in low permeability soil and one where the ash was stored directly in contact with high permeability soil. In the first instance, dewatering was conducted with an interior system of closely spaced wellpoints. In the second case, dewatering was performed using widely spaced, high capacity deep wells. In both cases, dewatering allowed for the safe and efficient handling of the ash. These projects demonstrate both the feasibility and desirability of dewatering for these types of operations. 

  • Considerations
  • Landfills
  • Wet-to-dry ash handling system conversions
  • Handling and disposal systems
  • Closure of wet and dry ash disposal facilities
  • Environmental Studies and Permitting:
  • Permitting
  • Preliminary and detailed site investigations
  • Environmental assessments
  • Environmental investigations and remediation
  • Are There any Beneficial Uses?
  • Underground and surface mine reclamation
  • Coal yard base and pond liner material (roller compacted concrete)
  • Leachate collection system media
  • Protective soil cover systems
  • Ash Handling & Disposal:
  • Bottom Ash Handling
  • Fly Ash Handling
  • Pond Retrofit/Closure
  • DEWATERING IN A LINED POND 

The first project was a pilot dewatering test at the Pennsylvania Electric Company’s  Seward Generating Station in Johnstown, Pennsylvania. 

The project was extensively documented in a 1985 report published by the Electric  Power Research Institute.1 The station had been in operation since the 1920s. Until  1980, both bottom ash and fly ash were randomly deposited via slurry in storage ponds.  Two ponds were used at this plant; when the first pond was full, ash deposition would  switch to the second pond and the first pond would be mucked out. The ash would then  be transported and deposited at a final disposal site. 

After the closure of the storage ponds in 1980, the State of Pennsylvania required that  the ponds be emptied and returned to original grade. This presented two problems to  the plant owner: 1) because of the low angle of repose of the near-liquid wet ash, the  final disposal site did not have the capacity to store all of the ash in the ponds unless it 

was pre-drained, and 2) the state required the project to be completed in under two  years, requiring a relatively rapid method of pre-draining. 

At the time of the project, Ash Pond 1 only contained approximately 1.2 m of ash.  Therefore, it was decided to remove that ash without dewatering as it was thought that  the benefits of pre-drainage would not be worth the cost for such a thin layer. Ash Pond  2, however, contained between 2.1 and 3.7 m of ash and the plant owner decided to  use dewatering techniques for pre-drainage. 

Over the course of a nearly 11-day pump test with all 23 wellpoints pumping, the array’s  flow rate dropped from an initial value of 57 L/min to a final value of 13 L/min  Drawdowns of over 0.60 m were observed at the end of the test in three of the four  piezometers. 

Hand auguring was conducted up to 30 m away from the wellpoint line in order to  observe actual conditions in the pond just before turning off the wellpoints. Similar to  the pre-pumping condition, a 1 to 1.2 m thick stable crust was encountered. This time,  however, there was an approximate 0.9 m thick zone of thixotropic material beneath the  stable layer. This material would tolerate the digging action but flow when subject to  vibration. Below this zone the ash remained in a flowing condition. 

The hand auguring revealed that longer-term pumping could increase the thickness of  the excavatable zone (as compared to pre-pumping conditions) and that the thickness  increased closer to the wellpoint line. This improvement in ash characteristics extended  for a distance of approximately 12 m from the wellpoint line. Test pits dug near the  perimeter dike also revealed poor conditions immediately adjacent to the dike even  when pumping nearby. However, it was theorized that poorly controlled runoff from the  dike could have been a contributing factor to this phenomenon. 

Values of bulk hydraulic conductivity calculated from the pump test for the ash were on  the order of 10-2 cm/s. This value is much greater than the values typically assumed for  fly ash and is likely due to the co-mingling and layering of coarser bottom ash with the  relatively fine fly ash. 

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Christophe Rude

Christophe Rude

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