DESIGN/CONSTRUCTION OVERSIGHT UNIT 0 WASTEWATER IMPOUNDMENT
Johnston Power Plant | Glenrock, Wyoming
WET was tasked with providing engineering design and construction oversight services for a new Coal Combustion Residuals (CCR) wastewater holding pond for a coal-fired power plant in Glenrock, Wyoming. WET’s engineering services consisted of the full engineering design of a 25-acre lined surface water impoundment, permitting, and construction support through completion of the project.
The design of the Unit 0 Pond was required to meet both State (Wyoming DEQ) and Federal (EPA) requirements for wastewater and CCR impoundments, respectively. These included requirements for the facility to be double lined with a bottom component of at least 2 feet of compacted clay, or an equivalent product meeting or exceeding the permeability requirements of a compacted clay liner, and a top component of a geomembrane material. The area was determined not to have a suitable source of clay, therefore an alternative material was evaluated and specified for the project. The alternative material selected was a geosynthetic clay liner (GCL). The GCL product was evaluated by a lab using site-specific permeant under design loading to determine the expected permeability of the product under design loading scenarios. The product was shown to exceed the minimum requirements of a compacted clay liner, which saved the client money in potential hauling costs from a suitable clay source.
The pond required an outlet system to be designed to convey process inflows from the Unit 0 Pond to an existing clearwater pond located nearly 0.5 miles away with very minimal elevation change, which did not allow for a conventional gravity discharge piping system. The solution to this problem was to design an outlet system to convey the inflows exiting the Unit 0 Pond through a new HDPE pipeline by passive head-differential. The system saved the client a substantial amount on both construction and maintenance costs versus a conventional discharge system that would have required pumps, controls, and backup generators.
Once the engineering design tasks were completed, WET prepared construction plans, design specifications, and other statement of work elements for the contractor bid documents. WET also aided in the contractor selection process by providing reviews of the bids for adherence to the statement of work and design specifications. During the bid process, WET provided clarification documents consisting of plan and specification amendments, along with technical answers to prospective contractor questions during the bid phase.
WET provided technical support to the power plant’s construction oversite team during the construction phase of the project by reviewing and evaluating proposed design deviations during construction. Additionally, WET reviewed the company quality assurance testing to ensure the testing met the requirements of the project specifications.
Once the project was substantially complete, WET prepared and submitted certification documents required by the EPA. Completion of the certification documents allowed the facility to be placed into operation in November of 2019. The project was completed on time and within budget.