OWNER:
Engelhard Corporation

ENGINEER:
Tensar Environmental Systems, Inc.

CONTRACTOR:
Earl Boyett Contracting Company, Inc.

The Engelhard Corporation needed to close three existing nitrate sludge ponds at their facility in Attapulgus, Georgia with an interim soil cap. The ponds have surface areas ranging from 1.6 acres to 2.1 acres and sludge depths of approximately 20 feet. All three ponds were covered with an existing HDPE liner that remained in place during construction. A construction plan and sequence was developed in order to satisfy certain specified criteria. The soil cap was required to 1) be constructed with a maximum thickness of 2 feet with minimal disturbance of the soft nitrate sludge; 2) be capable of supporting construction equipment and intermittent light vehicular traffic; and 3) provide a final cap geometry that would promote gravity drainage of rainfall. The undisturbed shear strength of the nitrate sludge in all three ponds was highly variable and ranged from a high of approximately 200 psf to a low of less than 10 psf. In addition, the sludge was very sensitive and tended to liquefy under vibration and loading. In order to satisfy the design criteria, biaxial geogrids were used to reinforce the soil cap. A layer of non-woven geotextile was also placed over the existing liner to provide a positive separation in the event the existing liner ruptured. Construction of the soil caps over all three nitrate ponds was successfully completed in 1996.