In March 2001, routine monitoring of perimeter probes around the Davis County (Utah) Solid Waste Management and Energy Recovery Special Service District landfill revealed that high concentrations of landfill gas—greater than 50% in some cases—were present at the property line.

By Dick Sprague and David Lutz

March 2001 testing around the Davis County (Utah) Solid Waste Management and Energy Recovery Special Services District landfill area showed that the landfill gas had migrated beyond the property line, and, since methane is commonly explosive in concentrations between 5 and 10%, Wasatch Energy Systems (WES), the district's operating name, recognized that the migration of landfill gas could result in dangerous conditions. To meet its commitments to the Utah Department of Environmental Quality and the adjacent landowners, WES opted to use the design-build process to quickly implement the needed solution to control the migration of landfill gas.

Gas Migration at Davis County

The integrated solid waste management utility in Layton, UT, consists of a waste-to-energy (WTE) plant, which burns approximately half of the collected solid waste; a landfill, which accepts ash from the WTE plant as well as difficult-to-burn wastes; and various recycling facilities. The 25 ac. Davis County Landfill, part of the WES system, includes both a lined Subtitle D - compliant landfill cell and a historic, unlined landfill cell.

The landfill began operating in the early 1950s, and the WTE plant went on-line in 1987 to complement and extend the life of the landfill. The historic cell started as an end-dump canyon fill approximately 50 years ago and was closed during the fall of 2000. As part of the closure, WES installed 11 horizontal wells to collect landfill gas accumulating directly below the closure cap. These horizontal wells augmented three vertical wells previously constructed in various parts of the landfill.

The discovery that landfill gas, which results from anaerobic digestion of solid wastes, was migrating away from the westernmost property of the landfill propelled WES to commit to implementing a landfill-gas migration control system by mid-December 2001.

The chosen landfill gas migration control system consists of four main elements: a flare station to burn landfill gas; a gas extraction system, consisting of wells within the landfill to reduce pressure, driving gas outward; a soil-gas extraction system, consisting of wells at the property line to draw gases back to the site; and monitoring wells on the adjacent property.

WES Implements Design-Build Solution

In May 2001, WES released a request for proposals (RFP) for engineering services to design the landfill-gas migration control system. As part of the responses to the RFP, HDR Inc. and another consultant suggested that the needs of WES might be better met through the design-build method, particularly since WES had committed to a tight schedule. In July, WES accepted the design-build proposal and jointly entered into a two-part design-build contract with HDR.

Several agencies have developed two-part model contracts for design-build projects. The first part is intended to provide the owner with a conceptual design and a firm price for the second part, which includes the final design and construction of the project.

As part of the initial contract, WES and HDR entered into an agreement to provide conceptual (30%) design and a proposal for the final design and construction, including a guaranteed-maximum price for the construction. As part of the conceptual design, which was completed on August 31, the contractor also obtained bids from qualified drillers to construct landfill-gas extraction wells; bids from qualified suppliers of landfill-gas flaring equipment; and a bid from a local contractor to perform the balance of construction.

These bids became the basis for the proposal of the final design-build contract, which WES accepted on September 27, approximately two weeks behind the schedule contained in the initial proposal and 2% below the bid. Work began in early October, with a planned completion date of December 14.

Drilling Problems Lead to Delays

From the beginning, the drilling subcontractor encountered difficulties. Despite having drilled wells into this landfill previously, the drilling proved more difficult and time-consuming than anticipated. In addition, the subcontractor subcontracted the drilling for the soil-gas extraction wells and the monitoring wells; his first two lower-tier subcontractors essentially failed to perform. Because of these repeated failures, HDR took responsibility for locating a qualified third lower-tier subcontractor to complete the work in parallel with the second subcontractor.

The final wells were not completed until January 18, approximately five weeks behind the original schedule. In general, a delay in schedule almost always results in a problem project, particularly a cost overrun. Through active management of the overall project, the contractor was able to avoid most if not all of these problems.

Furthermore, the flare station, provided by a subcontractor, completed its final testing on December 14, as had been scheduled, and WES signed an acceptance of the flare station on December 18. By this date, approximately 60% of the landfill gas extraction wells were available and on-line, providing the owner with continuous vacuum on the landfill itself and essentially meeting the owner's commitment to have a system running by mid-December.

Design-Build Process Results in Cost-Savings

During early February 2002, WES received a financial reconciliation for the project involving a deductive change order of almost $30,000. WES and HDR then agreed to conduct a "lessons-learned" session following several months of operation of the system.

Dick Sprague is with HDR Inc. in Denver, CO. David Lutz is with HDR Inc. in Kansas City, MO.

MSW - May/June 2004