projects

Seminole Road Landfill Closure Construction, Cell Construction, and Environmental Monitoring and Gas Collection & Control System (GCCS) Operations & Maintenance Services

DeKalb County, GA | 2013-2020

Scope of Services

Notable Accomplishments

Oasis’s work on the site is critical not only to the proper operation and NSPS compliance of the GCCS, but also enhances the County’s ability to generate revenue from beneficial re-use projects at the site, which include the following:

• Conversion of landfill gas to electricity under a long-term third-party Power Purchase Agreement to Georgia Power
• Conversion of landfill gas to natural gas for Atlanta Gas Light pipeline injection
• Compressed natural gas (CNG) for vehicle fueling (both County sanitation CNG-powered vehicles and public CNG vehicles.)

Throughout the time that Oasis has been in responsible charge with overseeing environmental monitoring at Seminole Road Landfill, DeKalb County has been in full environmental compliance.

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Project summary

The DeKalb County Seminole Road Landfill encompasses over 1,100 acres and
comprises the closed Phase 1/2/2A areas, as well as the active Phase 3 and
future Phase 4 area with over 50 million cubic yards of disposal capacity and
an expected life of 70+ years. DeKalb Sanitation is one of the largest public
solid waste management operations in the southeastern U.S.

Oasis Consulting Services team members worked at the DeKalb County
Seminole Road Landfill on the first lined cell, when Mr. Glenn Wallace served
as Project Manager in 1996. Mr. Wallace’s responsibilities included working with County officials to forecast cost savings methods for the County. During construction of Cell 3, Mr. Wallace and Oasis’s President, Mr. Majid Zibanejadrad, advised DeKalb County to bridge unsuitable subgrade soil and to perform pre-consolidation loading of unsuitable subgrade soils. The task was performed by Valley Crest Contractors. As a result of the combined expertise of Mr. Wallace and Mr. Zibanejadrad, the County saved upwards of $2m in construction costs.

Phase 2A Unit 1 (20 Acres, 2001) and Unit 2 (18 Acres, 2004)

Beginning in 2001 Oasis was involved with the Phase 2A Unit 1 project, a key step in the landfill’s growth with regard to maintaining waste disposal services for the citizens of DeKalb County. After completion of the construction and filling of the Phase 2A Subtitle D lined cell in 2001, the county was ready to embark on the Phase 2A Fill-Between Area, the first “piggyback” liner system approved in the state of Georgia. This system was designed to allow filling on top of the existing intermediate closed slopes of the unlined Phase 1 and 2 areas, where a Subtitle D liner system was to be constructed over the existing slopes and connected to the Phase 2A cell. This Upslope Liner System in the “fill-between area” was initially approved by the Georgia Environmental Protection Division (GAEPD) in the early 1990s, but the approved design was conceptual and had not included detailed considerations for geotechnical/ settlement issues, or for managing landfill gas. The design was then updated to address these issues during the process of preparing bidding and construction documents.

A settlement and stability analysis were performed, which resulted in the decision to place a geosynthetic reinforcement on top of the existing landfill surface. To prevent the buildup of landfill gas under the liner, an innovative gas collection system was incorporated into the design. The gas collection system conveyed gas to the active Phase 1 and 2 gas collection and control system (GCCS) via solid pipes exiting the upslope liner at the perimeter anchor. “Finger valves” were then installed at the GCCS connection points so that the system could be monitored and adjusted both to control flow and to maintain gas quality.

Under subcontract to ARCADIS U.S., Inc. (ARCADIS), Oasis was responsible for Construction Quality Assurance (CQA) on all aspects of construction for both Unit 1 (20 acres) in 2001, and Unit 2 (18 acres) in 2004. Oasis CQA monitors performed the following CQA activities for both Units:

• Vegetative and topsoil stripping of the existing intermediate cover surface
• Repair of leachate outbreaks prior to surface preparation
• Placement of a geogrid reinforcement layer and connections
• Drilling oversight for vertical gas collection wells into the existing waste mass
• Placement of a #57 stone gas transmission layer and perforated horizontal gas collection piping
• Placement of non-woven geotextile over stone as a separation layer
• Borrow site soils segregation and bentonite amendment to clay soils
• Installation of a composite liner system consisting of 24 inches of 1×10-7 cm/sec permeability clay, overlain by 60-mil HDPE textured geomembrane liner, with a geocomposite drainage layer for enhanced leachate collection and 2 feet of manufactured non-carbonate sand protective cover
• Installation of “finger valve” gas collection controls, and connection to the existing GCCS.

Oasis was instrumental in providing the general contractor with direction on the proper segregation of borrow soils from multiple borrow sources to meet the specific requirements of each soil type needed for the various components of the project (i.e., subgrade, general fill, and clay liner). The clay liner required a bentonite amendment to the existing on-site soils to obtain the required permeability, and Oasis provided close oversight to ensure proper metering and blending of the bentonite.

Soils placement required extra effort to attain the proper compaction and the permeability required of the clay liner; however, working on the 3:1 slopes lessened the effectiveness of the compaction equipment. Oasis worked closely with the contractor to adjust the moisture content of the soil and to compact the material near the slope.
Geosynthetics deployment was also a great challenge, and the heavy equipment normally used to deploy the materials had to be used from the top of the slope. Oasis guided the contractor through the unusual procedure for a successfully maneuvered deployment.

Phase 3 Unit 1 – 2005

In 2005 a Major Modification Permit was issued to DeKalb for the Phase 3 and Phase 4 Upgrade, which allowed construction to begin on Phase 3 Unit 1, a 39-acre Subtitle D double-composite-lined disposal cell for Municipal Solid Waste (MSW) generated within the county, and one of the largest cells ever constructed in the State of Georgia. Under subcontract to ARCADIS, Oasis was responsible for CQA on all aspects of construction.

The following CQA activities were monitored by Oasis:

• Underdrain system for foundation stability
• Groundwater gradient control system to control fluctuations in groundwater levels, thereby maintaining minimum liner separation to groundwater
• Approximately 148,000 cubic yards of rock blasting and excavation
• Over 1 million cubic yards of structural fill
• Composite liner system consisting of six inches of 1×10-5 cm/sec permeability clay, overlain by 60-mil HDPE secondary geomembrane liner with geocomposite drainage layer for leak detection, geosynthetic clay liner (GCL), 60-mil HDPE primary geomembrane liner, geocomposite drainage layer for enhanced leachate collection, and 2 feet of manufactured non-carbonate sand protective cover.
• Approximately ½-mile-long mechanically stabilized earthen (MSE) wall with vegetated wire-basket facing, 78 feet maximum height, with high-strength woven geotextile primary reinforcement and select backfill
• Three large stormwater control ponds
• 250,000-gallon welded-steel leachate storage tank

Oasis advised the contractor to use blast rock, which was crushed as deep fill material, to strategically avoid stockpiling rock on the site. This solution also reduced the necessary amount of borrow soil.

Once the secondary geomembrane liner was in place, the heavy equipment normally used for geosynthetics deployment could not be used on top of the cell. The general contractor and geosynthetics installer used rubber-track-mounted equipment, and with guidance from Oasis avoided damaging the materials already placed; successfully deploying the multiple layers of geosynthetic materials.

Phase 2A Closure – 2009

In late 2007, after operations began in the Phase 3 Unit 1 MSW disposal area, the county was allowed to operate two working faces in order to move toward final filling and closure of the 50-acre Phase 2A area, which stopped receiving waste in November 2008.

Of primary importance was the GCCS, which started operating in April 2009 and which supports the Green Energy Facility, where electricity sold to Georgia Power is generated. Continuous operation during the closure process was extremely important to ensure contractual obligations were met.

Under subcontract to ARCADIS, Oasis was responsible for overall project management, design support, and CQA on all aspects of construction for the Phase 2A Closure. The design phase began in early 2008; in July 2009 construction started and was completed in January 2011. Oasis provided design leadership for the closure design
and landfill gas expertise for effective and efficient landfill gas control during and after closure construction. The state-of-the-art final closure design incorporated many features over and above the prescriptive Subtitle D requirements to provide long-term stability, effective landfill gas control, and reduced maintenance during
post-closure. Key components of the design included the following:

• Reshaped terraces, with defined ditches and minimum 2 percent slopes for enhanced surface drainage
• Terrace drainage flaps above each terrace to discharge water collected in the geocomposite layer above the 40-mil liner
• Rock toe drains at perimeter liner anchors with solid-pipe discharge points
• Horizontal gas collectors under the 40-mil liner in areas of shallow waste to compensate for the limited radius of influence of vertical wells
• Buried downdrains with domed, grated inlets in terrace ditches
• Bermuda sod on the entire cap, with specifications requiring a temporary irrigation system to be implemented by the contractor
• Perimeter ditches with concrete aprons and gabions at pipe outlets

To ensure that experienced, reputable contractors would bid on the project, Oasis recommended specific qualifications for the bidding process to reduce the county’s risks both during construction and into the post-closure period. After successful award and project commencement, Oasis CQA monitors performed the following CQA activities:

• Monitoring and testing of all facets of the soil layers placed on the cap comprising over 550,000 cubic yards and which included a structural fill/soil bridging layer, a clay infiltration layer, an erosion layer, and a vegetative topsoil layer
• Monitoring of four on-site borrow areas for proper soil segregation
• Monitoring synthetic Subtitle D cap geosynthetics consisting of 50 acres of 40-mil LLDPE liner and a geocomposite drainage layer
• Extension and booting of over 75 GCCS wells, laterals, air lines/force mains, and valves, with minimal downtime of the Green Energy Facility

For geosynthetics deployment the cap was prepared in 3- to 8-acre sections, and liner was deployed in seven to eight mobilizations – under windy conditions on the exposed hill. During the fall and winter of 2009, after the contractor had initially stripped and begun fill placement on a 16-acre area, adverse weather conditions included above-average rainfall and poor drying conditions. Progress was threatened to stall for over 5 months. Oasis devised a plan for modifying the construction sequence so the contractor could proceed in spite of the poor weather conditions. By early January 2011 the closure was successfully completed with the exception of punch list items.

Phase 3 Unit 3 Cell 5 – 2013

In 2011 construction was to begin on the 27-acre Phase 3 Unit 3 Cells 4 and 5, and Oasis was responsible for CQA on all aspects of construction for 13 acres of Cell 5. Oasis CQA monitors performed the following activities:

• Subgrade preparation
• Composite liner system consisting of six inches of 1×10-5 cm/sec permeability clay, overlain by 60-mil HDPE secondary geomembrane liner, with a geocomposite drainage layer for leak detection, geosynthetic clay liner (GCL), 60-mil HDPE primary geomembrane liner, a geocomposite drainage layer for enhanced leachate collection, and 2 feet of manufactured non-carbonate sand protective cover
• Leachate collection and disposal system, including sideslope riser pumping system to convey leachate to an on-site storage tank prior to gravity discharge to the DeKalb County sewer system
• A stormwater diversion system, consisting of a reinforced tarp which was anchored and sandbagged to protect the sand layer and to allow pumping of stormwater from the inactive area prior to initiation of waste disposal operations.

In addition, the paving at the landfill entrance and scalehouse was in poor condition and needed to be both reconstructed and expanded. Heavy-duty reinforced concrete paving was designed and implemented by Oasis engineers.

Currently Oasis is servicing the Environmental Monitoring contract, which started in July 2013. This contract involves performing comprehensive environmental monitoring and reporting, and Gas Collection Control System (GCCS) Operations & Maintenance (O&M) services. Oasis has also been providing NPDES storm water implementation not only for the landfill but also for nine additional sites within DeKalb County. Services include groundwater, surface water, and stormwater monitoring; and include sampling, testing, and analysis, as well as field monitoring of methane concentration levels. The results are reported to the County and to GAEPD. Our contract to provide GCCS O&M services also includes emergency on-call services.