Tensar Environmental Systems, Inc.
Earl Boyett Contracting Company, Inc.
Kerr McGee desired to construct closure of a waste settling pond located on their property. The pond, used as a final point of deposition for the plant’s waste by-products, has a surface area of approximately 30 acres and is completely surrounded by a 17-foot high containment dike. The pond contains manganese waste that was transported to the pond location hydraulically and was thus deposited in a slurried state. The thickness of the waste material in the pond ranged from 3 to 15 feet. In general, the surface of the sludge was just below the crest elevation of the dikes for approximately 60% of the rectangular impoundment area, and then sloped downward on a 2.5% grade to a low sump area in one corner.
Due to the depositional process, the waste material is characterized as having low shear strength, with values of in-situ undrained shear strength as low as 20 psf. The shear strength of the waste material was so low that the removal of cover water prior to construction resulted in widespread failures of the pond’s sloped surface. While some portions of the pond surface have dried out over time creating a thin crust, other portions of the pond remained uncrusted at the time of closure. These uncrusted portions of the pond were not capable of supporting even foot traffic.
The initial phase of the approved closure plan for the pond consisted of placing an earthen cap over the entire surface. The earthen cap ranged in thickness from a minimum of 2 feet to a maximum of approximately 6 feet, as controlled by equipment support and grade requirements. Later phases of the closure plan included placement of a geomembrane over the earthen cap, followed by placement of a final soil cover and vegetation.
To allow implementation of the initial phase of the approved closure plan, SES developed a construction plan to accommodate the soft nature of the manganese waste to be covered. This plan included the utilization of various geosynthetics to enhance drainage of and provide reinforcement for the earthen cap. The plan also included tailoring the selection of construction equipment to the expected support conditions. Additionally, a construction sequence was developed that insured that there would be no global stability failures of the sludge under the weight of the cap in areas that involved the sloping sludge surface.
The contract covering construction required by the approved closure plan was awarded in August 1997 at a value of approximately $2.5 million dollars. Installation of the geosynthetics and earthen cap required by the initial phase of the closure plan commenced in August 1997 and was completed over a period of 60 days