LANDFILL LINER DESIGN AND CONSTRUCTION

Abstract
Two landfill liner design and construction projects varied dramatically because of the assumptions and protocol employed early in the design/construction process. Despite the best intentions, these two similar landfills experienced very different construction challenges. One project was constructed on time with few challenges; the other was fraught with numerous challenges and a completion delay of nearly one year. Based on this experience, we offer the following recommendations for landfill owners planning liner projects:

  • Confirm material property assumptions early in the design/construction process. Ideally, laboratory testing of potential materials should be performed to confirm design assumptions.
  • Incorporate appropriate laboratory and field testing protocols to ensure the materials meet the requirements prior to shipping and confirm their performance prior to placement.
  • Early in the design process, confirm waste fill placement protocol assumptions to be employed by the landfill operator.
  • Review all environmental requirements early in the design phase of a project. Perform all necessary evaluations and obtain appropriate regulatory approvals prior to procuring a contractor for a project.
  • Cooperate with the contractor and seek to resolve issues amicably. Extensive delays do not have to result in expensive contractor claims.

Introduction | Comparing the Landfills | Challenges and Lessons Learned | Conclusion

Introduction
Brown, Vence & Associates provided construction management services to the Salinas Valley Solid Waste Authority on landfill liner construction projects at Johnson Canyon Road and Crazy Horse landfills. Our role included administration and coordination of the contractor, engineer, and construction quality assurance consultant. The landfill owner had procured a design consultant and prepared a design prior to our involvement in the projects. Also, the owner had primary responsibility for environmental clearances for state and local regulatory agencies. The designers of each project were retained to review shop drawings and submittals, attend periodic meetings with the contractor, and advise the owner accordingly.


Comparing the Landfills

Similarities
Both landfills are located in a seismically active region of California near the San Andreas Fault system and could experience ground motions of up to 0.4 g from earthquakes with a magnitude of 7.2 on the Richter scale. Both liner systems were required to provide containment during potentially deformation-causing earthquakes. Both landfills were required to provide a minimum factor of safety of 1.5 under both static and dynamic conditions as required by state regulations. Both landfills were constructed by the same contractor, who won each project separately under standard procurement procedures for public works construction. The contractor used the same project manager and generally the same site staff by sequencing the construction of one site to occur immediately following the other.

The liner design requirements were also similar. The "as bid" Johnson Canyon composite liner configuration included the application of a double-sided, heat-burnished, non-woven GCL placed on prepared subgrade (both side slopes and floor), a 60-mil single-textured HDPE geomembrane placed on the slope (textured side down), a 60-mil double-textured HDPE geomembrane placed on the module floor, a geocomposite drain net placed on the side slopes, and a sand drainage layer placed on the module floor. Finally, a 1-foot-thick operations layer was placed on the module floor, and an 18-inch-thick operations layer was placed on the slopes to a height of 15 feet above the floor subgrade. The Crazy Horse slope liner included a double non-woven GCL, a 80-mil, single-sided textured HDPE geomembrane with the textured side down in immediate and continuous contact with a GCL. The slope area was covered by a 16-oz non-woven geotextile overlain by a 24-inch-thick operations layer composed of select on-site materials. The bottom liner consisted of a 80-mil, double-sided textured, HDPE geomembrane overlying a 24-inch-thick low-permeability soil layer consisting of native materials blended with bentonite to achieve a permeability of less than 1 x 10-7 cm/sec. The geomembrane was covered with a 1-foot-thick layer of pea gravel and a 2-foot-thick operations layer composed of select on-site materials.

In summary, both landfills were designed under the same general seismic requirements. Both site designs included similar composite liner designs consisting of GCL placed on native soil overlain with a HDPE liner. Both sites were to be constructed by the same contractor using the same staff in generally the same time frame.

Differences
The physical layouts of the sites are quite different. Johnson Canyon, despite its name, is located on an alluvial fan. The expansion area included a horizontal expansion contiguous with, and west of, the existing landfill. The expansion area included approximately 8 acres of rectangular excavation with 3:1 side slopes and a depth from existing grade of 30 feet. The refuse-fill height for the final design was anticipated to be approximately 80 feet. One distinguishing characteristic of this new cell was that only two of the four slopes were lined. The slope west of the new module was unlined to allow future horizontal expansion in the permitted area of the landfill. The slope south of the module also was unlined to allow for possible future expansion. Additionally, in anticipation of future horizontal expansions and waste placement west and south of the project expansion area, the cell's unlined slopes were not buttressed.

In contrast, Crazy Horse, a canyon landfill expansion, included a horizontal expansion area adjacent and contiguous to the existing landfill footprint. The expansion area included steep side-slope excavations into the native sandy soils. The excavation slopes were designed to be 1.5:1 (horizontal to vertical) with a vertical height of approximately 60 feet. A mid- slope drainage bench was designed to reduce the length of the liner deployment and to provide a location for a mid-slope anchor embedment. The refuse depth for the final design was anticipated to be approximately 100 feet. Unlike Johnson Canyon, all of the slopes were lined. Also, the daylight portion of the canyon on the western edge included a small soil buttress.

In summary, the Johnson Canyon excavation area and installation requirements, such as liner deployment, maneuvering areas, and slope inclinations, were much easier than Crazy Horse, which posed seemingly more design and construction challenges.


Challenges and Lessons Learned

Challenge 1: Protected Species Found at Site
During review of the owner's board action checklist prior to commencement of the Johnson Canyon construction, an environmental mitigation measure was discovered that required evaluation of potentially affected protected species at the site: the California Tiger Salamander and the Western Spade-Footed Toad. An initial field investigation by a wildlife biologist indicated the possible presence of the species, and further action was required. A meeting with the State Department of Fish and Game revealed that an extensive wildlife evaluation would be required before excavating for the liner. The construction was delayed until spring 1998 to allow biologists to survey the population of the two species at the site. After well over $100,000 was spent on biological surveys and laboratory genetic testing of approximately 110 salamanders, experts concluded the salamander species was no longer present at the site. Apparently, a more robust species of salamander from Oklahoma had been introduced in the early 1900's, and cross breeding had essentially eliminated the California Tiger Salamander from the genetic pool. The Western Spade-Footed Toad was found at the site but could be accommodated using proper relocation techniques. The State Department of Fish and Game approved several mitigation measures that would not affect the liner construction project, and construction began in early summer following this approval.

Lessons Learned
1. Evaluate environmental issues, associated mitigation measures, and their potential effect on the project early in the design phase. If complications are possible, meet with appropriate regulatory agencies to evaluate potential solutions before engaging a contractor.
2. If the effects of environmental issues on the schedule are unknown, specify that such issues may cause delay but that such delay is noncompensatory for the contractor.

Challenge 2: Field Testing Discovers Material Variability
The Johnson Canyon specifications included provisions for sampling and testing the GCL delivered to the site to demonstrate internal shear strength requirements. The specifications also contained a provision for the GCL to be tested in the laboratory prior to shipping.

The specifications delineated the internal strength requirement of the GCL material in addition to requiring it to be heat burnished. The purpose of the internal strength requirement was to ensure the material would not delaminate under lateral static or seismic forces. The purpose of the heat burnishing was to increase internal shear strengths.

Heat burnishing is a method of melting the surface fibers of a needle-punched, woven geotextile. The needle-punched fibers extend from one woven side of the GCL, through bentonite clay and the other woven side of the GCL. The fibers typically extend perpendicular to the woven surface and are shorn approximately 1/8 inch long. When done properly, burnishing the fibers melts the shorn fiber, forming a tiny ball at the surface of the woven fabric. This tiny ball acts as a lock, restraining the needle-punched fiber from being pulled through the GCL when experiencing horizontal shear.

However, not all GCLs experience increased internal strength when they are burnished. The primary reason for reduced strength is that if the burnishing surface is too hot, rather than melting the surface fiber into a tiny ball, the ball melts away, leaving the fiber shorn at the woven surface. This renders the needle-punched fibers dramatically diminished under horizontal shear conditions.

The contractor performed laboratory tests and submitted results indicating the burnished GCL met the strength requirements. However, after the material was delivered to the site, both testing and visual observation of field samples revealed variability in the burnishing. Samples collected from heavily burnished areas lacking the tiny balls were tested. The internal shear strength of these areas was well below the minimum specified level. Samples taken from the entire lot revealed similar results. The manufacturer performed its own independent tests and reached the same conclusions. It offered to replace the GCL with new unburnished material, and the designer agreed to remove the burnishing requirement. Laboratory and field tests on the unburnished GCL met the specified internal strength requirements.

In contrast, the laboratory and field testing procedure for the Crazy Horse project did not evaluate the GCL material to the same extent. The laboratory test procedures prior to shipping were similar to Johnson Canyon and required an evaluation of the internal strength of the material. The field testing procedures required one sample to be tested to confirm internal strength. It should be noted, however, that the material specifications did not require heat burnishing.

Lessons Learned
1. Specify laboratory testing procedures exhaustive enough to identify fatal flaws in the material performance before shipping.
2. Specify field testing procedures exhaustive enough to identify variability in material quality.
3. Specify the performance of the material, not the manufacturing processes of the material (unless those manufacturing processes are an indicator or otherwise reveal properties of the material performance). Different manufacturers make similar products in different ways. By specifying one manufacturer's processing procedures, it may negatively affect other manufacturers' products.
4. To avoid extended overhead delay claims, specify any delay related to the performance testing to be a contractor responsibility.

Challenge 3: Considering Changing Materials
The Johnson Canyon liner designer prepared the liner design report in late 1996. The bid documents, consisting of plans and specifications, were prepared in early 1997. Between this time and spring 1998 when construction actually began, literature was published that revealed new ways to evaluate the seismic design of landfills. The designer began to rethink the design and concluded that if the landfill cell were filled to the permitted height without the benefit of a buttressing effect from the adjacent future cell, the seismic stability safety factor could be too low. The designer suggested a change in the liner materials to account for this situation. The revised design would include a geotextile fabric to be placed on top of the HDPE (which would be revised to be only single-sided, textured side down) as a weakened plane above the composite liner system. The contractor was asked to provide a quote for installing the geotextile material. However, the price quoted was too high to accommodate in the project budget without returning to the owner's board of directors for approval. The designer than reevaluated the design and concluded the sand could meet the weakened plane requirements if the interface shear value of the sand and the HDPE were low enough (see Issue 4 for further discussion about the sand). The problem was resolved by lowering the height of waste anticipated to be placed on the liner. The designer concluded the higher value of interface shear strength provided by the locally available sand could meet this requirement. The limitations included the necessity for the owner to construct the adjacent cell and fill it with wastes to a certain height before filling the current cell to its maximum design height. The future cell would act as a buttress to the existing cell, thereby increasing the resistive forces. With the fill buttress in place, the need for the redesigned weakened plane was reduced. The decisions by the owner to accept the temporary reduced fill capacity and by the designer to return to the original design concept occurred at the same time the new GCL was being delivered to the site and mobilization of the liner installation crew was to occur. Consequently, no project delay costs by the contractor were incurred by the owner.

Lessons Learned
1. It is unlikely that large quantities of new materials can be added or the definition of the material be revised after the contractor has been awarded the project.
2. Knowledge and technology typically change over time. Incorporation of new technology into an ongoing project may require adjusting owner expectations about the use of the landfill. In this case the adjustment included a temporary limitation on the full use of the liner area.
3. When landfill operational issues affect the cost of design, discuss the key issues with the landfill owner and operations group to explore ways of simplifying the design.

Challenge 4: Specifying Naturally Occurring Materials
As described previously, during the Johnson Canyon construction, the designer's concerns about protecting the liner system from deformation during a seismic event using newly acquired information potentially affected the construction schedule and costs. The designer performed a reevaluation and concluded the original design could be used if the specified "rounded sand" demonstrated a certain interface shear value lower than the other components of the composite liner system. However, the construction contract only required "rounded sand" and lacked specific testing requirements regarding interface strength with other liner system components. Laboratory tests were performed on numerous sand sources, and no sand could be found locally that met the revised design interface shear test values. The resolution of this problem, as described above, included the use of locally available sand and filling height and sequence restrictions on the landfill operator.

In contrast, during the design phase for Crazy Horse, samples of the native sand were tested for their performance and potential use in the construction project. The specifications incorporated the test results on native sand from the excavation area. During construction, this sand was used as the protective and drainage layer above the liner system.

Lessons Learned
1. Specify natural products such as sand or soil that occur locally.
2. During design, confirm the locally available natural product has properties that interact appropriately with other products of the liner system.

Challenge 5: Avoiding Costly Delay of Project Claims
Given the circumstances described above (construction delay of nearly one year, removal and replacement of the GCL material, and other related challenges experienced during construction), one would anticipate extended overhead costs. BVA expended considerable effort in communicating with the contractor throughout each phase of the project. For example, prior to delaying the Johnson Canyon project for the environmental issue, a meeting with the contractor was held to explore the owner's options for terminating the project (and reissuing the project for solicitation of competitive bids the following year) or issuing a noncompensatory time extension. The contractor chose the latter. BVA employed this process of intensive communication throughout both projects, with the result that both were completed within the owner's budget. The sum of all change orders from the Johnson Canyon project totaled 3.5 percent of the initial construction bid. Similarly, the sum of change orders for the Crazy Horse project totaled only 1 percent of the initial construction budget.

Lesson Learned
Maintain cooperation with the contractor during a project. Costly delays may be averted through communication and cooperation.

Conclusion
With the advent of Subtitle D, composite landfill liners are here to stay. These liners represent huge capital expenditures for landfill owners. Landfill owners and designers need to use practical approaches to the design and construction of these facilities. When challenges are encountered, we need to evaluate the cause of the challenge to avoid it in the future. The challenges may not be related to the complexity of the site but rather to other circumstances such as the way we prepare and implement our designs, the way products are manufactured, and the limitations of naturally occurring products.

—Timothy J. Raibley

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