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 but made easier with a coal ash dredge.

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 article 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. 

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. 

Ash Pond 2 had approximate dimensions of 122 m by 183 m and was constructed on  an impermeable native clay layer with a perimeter clay fill dike

Test pits dug in the ash prior to installation of any drainage devices showed a relatively stable crust approximately 1 to 1.2 m thick underlain by a zone of flowing ash that would fill a test pit as quickly as the pit could be excavated. Phreatic levels in existing observation wells in the ash were 0.30 to 0.60 m below the top of the flowing ash zone.

The first test performed involved pumping on wellpoint #3 for 35 minutes using a  centrifugal pump. The flow rate for the test was 1 L/min and the maximum drawdowns  observed at a distance of 1.5 and 3 m from the test wellpoint were 0.35 and 0.20 m respectively. 

The second test performed using this array involved pumping on wellpoint #2 for 310 minutes using an eductor. The flow rate for the test was 1 L/min and the maximum drawdowns observed at a distance of 1.5 and 2.1 m from the test wellpoint were 0.40 and 0.36 m respectively. 

Both wellpoint and eductor systems are capable of pumping the well yields observed during the test. However, eductor systems require installation of two pipes (supply and return) while wellpoint systems require only one pipe (vacuum header). Given the similar results from the two tests and the relative simplicity of implementing a wellpoint system versus an eductor system, it was decided to move ahead with a more comprehensive wellpoint test. 

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. 

A comprehensive human health and ecological risk assessment were conducted on ash not removed during the initial time-critical dredging work. The ash present was found to be commingled with contamination from the Department of Energy (DOE) Oak Ridge Reservation site. Oak Ridge Associated Universities conducted independent medical screening and concluded that there were no adverse health impacts caused by the coal ash spill. The analysis also included extensive geochemistry studies, sediment and pore water bioassays, benthic macroinvertebrate assessments, two-dimensional sediment ash fate, and transport modeling, and groundwater modeling.