Design and Construction of an Embankment on Slurred Fly Ash

Arizona Public Service Company (APS) is a joint owner and the operator of the Four Corners Power Plant near Farmington, New Mexico. The steam generators at the facility are fueled with pulverized coal. Fly ash and bottom ash coal combustion products, generated at the facility by three of the five generating units are disposed on the site. The fly ash for these units is captured using wet venturi scrubbers and is then transported in slurry form to a new active fly ash disposal pond. APS has elected to construct this new Lined Ash Impoundment (LAI) and Lined Decant Water Pond (LDWP) at the Four Corners site. The new LAI and LDWP will be constructed within the footprints of two existing fly ash impoundments. A key component of LAI will be a 40-foot high embankment constructed largely using compacted bottom ash generated at the facility. The embankment will be constructed in two stages, a 20-foot high starter embankment to be raised to 40-feet over the next three years as part of the second stage. The LAI embankment will be constructed using the downstream tailings method of deposition. The embankment will be founded on old, slurried fly ash. This paper describes the design and construction of the LAI and the LDWD. The geotechnical investigation program included hollow stem auger borings and cone penetration testing. The embankment was designed with 3:1 (horizontal to vertical) upstream and downstream slopes. The embankment is founded on a 3-foot thick layer of bottom ash, reinforced with two layers of bi-axial geogrids. Instrumentation at the embankment includes nested vibrating wire piezometers to monitor pore pressure development and dissipation in the underlying fly ash, and settlement plates to monitor vertical displacement. A major obstacle during construction of the LAI was to support larger construction equipment on the soft, uncompacted fly ash. In the embankment areas, a layer of bottom ash was pushed out across the soft material to provide support for construction equipment. Two layers of geogrid were incorporated in the embankment foundations over the soft fly ash. Normal bottom ash placement operations were allowed after the placement of the foundation. In the pond interior the softer material was stabilized using a progression of heavier equipment with occasional placement of bottom ash in areas of extremely soft fly ash. At the time of preparing this paper, construction of the 20-foot embankment has been completed and construction has commenced on raising the embankment to 40 feet. Pore pressure dissipation within the existing fly ash has been relatively rapid, generally confirming the results of field pore pressure dissipation tests performed during the geotechnical investigation. 13 pp., 9 references.