First large-scale landfill closure in California to use artificial turf closure system
By Christopher M. Richgels
The Crazy Horse Landfill (CHLF) is located on a 160-acre parcel west of, and adjacent to, Crazy Horse Canyon Road about nine miles north of the city of Salinas in northern Monterey County, Calif. The landfill, owned and maintained by the Salinas Valley Solid Waste Authority (SVSWA), had been in service for 75 years, with final in-place waste volume estimated at 4.3 million cubic yards.
Final cover design for the closure of the CHLF went through three iterations before the SVSWA settled on an artificial turf final cover system. The project marks the first large-scale implementation of this type of cover system in California.
2.0 Project background
2.1 Site history
The CHLF began operation in 1934 as a burn dump in an area referred to as Module 1 and continued in that operation method until 1966.
In 1966, Module 1 disposal operations were converted to a sanitary landfill operation, which continued until about 1972. The landfill was closed in 1988 when an 80-mil, high-density polyethylene (HDPE) geomembrane final cover and overlying vegetative soil layer was put in place. This membrane and soil layer served as a corrective action measure.
The U.S. Environmental Protection Agency (EPA) placed Module 1 of the landfill on the National Priorities List (NPL), and the landfill became a Superfund site in 1990. The primary landfill continued to receive non-hazardous residential, commercial, and industrial solid waste until December 2009, when the SVSWA ceased all fill operations.
The primary landfill is a canyon fill of approximately 66 acres. Three phases (15 acres total) of the main landfill on the western side were lined in accordance with Subtitle D—solid (“household”) waste—regulations after Oct. 9, 1993. The lined modules were constructed with a composite liner system. The landfill base consisted of six unlined acres beneath Module 1, 51 unlined acres beneath the primary landfill, and 15 acres of lined module.
2.2 Site conditions pertaining to design and construction
The SVSWA’s final cover system would have to meet five criteria:
- The system had to withstand the environmental conditions present, including wind erosion and uplift, rainfall erosion, concentrated flow erosion, ultraviolet light degradation, and traffic.
- The system had to provide a static factor of safety (FS) of at least 1.5 and resist seismic deformation due to the San Andreas Fault zone located about five miles northeast of the site.
- The final cover system had to provide excellent groundwater protection to halt continued impact to the local groundwater supply.
- Due to an old cross-traffic style highway interchange, risk to motorists had to be avoided during construction.
- The new system could not create additional nuisance conditions for nearby residents, many of whom had been concerned about the landfill site operations and impacts for years.
In addition, surface water that could be affected by the landfill would provide groundwater recharge, contact (i.e., swimming) recreation, and wildlife habitat.
Access to the site is approximately 1.5 miles from U.S. Highway 101. The interchange consisted of a left-turn lane from the southbound lanes of 101 (crossing northbound traffic) at a blind corner to oncoming northbound traffic. This stretch of U.S. 101 and the interchange had been the site of many catastrophic, sometimes fatal, accidents.
Adjacent land uses near the site include cattle grazing and residences located 560ft from the landfill on the northwest and approximately 360ft on the southwest. Homeowners often registered complaints with the SVSWA regarding dust generation, windblown litter, odors, and general site appearance.
3.0 Final cover systems considered and rejected
3.1 Evapotranspirative (ET) cover system
The ET cover system is an aesthetically pleasing approach to capping landfills. However, ET covers may require substantial amounts of soil—and unless a soil supply is on-site—imported soil can become cosly. The CHLF had no available on-site soil supply.
In addition, hydrological investigations at the site revealed it was too wet for an ET cover. The initial site design had used rainfall data from the nearby Salinas Municipal Airport. The CHLF, situated on the windward face of the north Gabilan Range, receives significantly more rainfall than Salinas because the landfill location is the first elevation change encountered by Pacific storms as they make landfall from Monterey Bay. The ET cover needed to be thicker than initially envisioned to provide storage capacity for the anticipated percolation.
Based on the site’s climatological conditions and soil import cost, an ET cover design for the CHLF project was ultimately rejected.
3.2 Subtitle D final cover system
The preliminary closure plan was also evaluated using a textured geomembrane as the barrier layer in the final cover design. However, with the CHLF located approximately five miles from the San Andreas Fault, the final cover design had to provide veneer stability.
The CHLF would be subject to a Peak Ground Acceleration (PGA) between 0.5 and 0.6 gravity according to the California Geological Survey. Initial seismic stability analysis of the final cover veneer indicated a residual interface friction angle on the order of 31 degrees was required to prevent seismic induced deformations greater than 1ft. Linear low-density polyethylene (LLDPE) textured geomembrane cannot provide such high residual interface shear strength. Two existing sideslopes, 2H:1V and 2.3H:1V, added to the challenge.
The average residual interface friction angle for textured LLDPE geomembranes is approximately 18 degrees, with the geotextile component of a geocomposite drainage layer. The slope coefficient for a 2H:1V slope is 0.5, which creates a static factor of safety (FS) less than 1.0: FS=tan(18°)/0.5=0.65. The required minimum static FS is 1.5.
This situation rendered the project unbuildable using typical textured geomembranes.
3.3 Structured geomembrane system
A structured geomembrane that provides a high shear strength was then examined by the SVSWA. This geomembrane considered for the CHLF project consisted of a 50-mil LLDPE sheet with 130-mil studs on the drainage side of the sheet and 170-mil spikes on the underside of the sheet.
An interface shear strength testing program was initiated with the geomembrane and the soils specific to the CHLF. The shear interface of the geomembrane vs. the various soil types proposed for use all delivered acceptable static FS. The CHLF final closure plan was approved, incorporating this structured geomembrane.
However, getting vegetative cover soil to the site proved financially prohibitive. The estimated minimum total miles for soil import was 660,000. Adding that level of heavy truck traffic to U.S. 101 and the local roads was not desirable due to the difficult highway access. The average cost to haul in soil was $13/cy for a total cost of approximately $2.6 million. The design, while buildable, was too expensive.
4.0 Final closure design and system approved
4.1 Artificial turf closure system
In 2010, the SVSWA examined an alternative final cover system consisting of five layers (see Figure 2) from bottom to top:
- A minimum 1ft-thick (2ft-thick in traveled areas) soil foundation layer beneath a structured geomembrane.
- A structured geomembrane low-permeability layer.
- A woven polypropylene geotextile layer over the geomembrane.
- A geosynthetic erosion protection layer of engineered synthetic turf with tufted grass blades.
- A sand ballast layer placed in the artificial grass blades to provide anchoring against wind and water erosion.
Initially, the design engineer and the primary regulating agency were skeptical of the product’s ability to serve as an adequate alternative to the Subtitle D prescriptive standard final cover. The SVSWA decided to revise the project description after reviewing initial data and material costs regarding the artificial turf system.
The estimated materials cost of $1.75/sf could be paid with available closure funds. With regard to engineer and agency skepticism, a testing program was already started to prove the system’s ability to withstand extreme environmental conditions: wind erosion and uplift, rainfall erosion, concentrated flow erosion, ultraviolet radiation degradation, and traffic.
4.2 Wind erosion and uplift
An exposed geomembrane can be susceptible to wind damage from uplift forces, particularly at leeward slope hinges. Exposed geomembranes can also require ballasting with sandbags to resist these uplift forces.
Wind tunnel testing revealed unexpected results for the material with regard to wind uplift. At a wind speed of approximately 60ft/sec (40 mph), the uplift force peaked and then began a decline at 80ft/sec (55mph). The uplift force reduced to zero at an approximate wind speed of 100ft/sec (70mph) and became a downward force at higher wind speeds tested (up to 120mph or Category 3 hurricane wind speeds).
The design wind speed for the California Pacific Coast was 85mph in 2010. For the CHLF final cover system, the minimum 8psf of sand ballast specified for the perimeter and sideslope access roads (along the critical slope hinge) was more than sufficient to resist wind uplift.
4.3 Rainfall erosion
Subtitle D landfill covers are allowed erosive soil loss up to 2 tons/acre, a restriction that also applies to the artificial turf sand ballast.
The greatest danger for sand erosion would occur along the toe of the landfill sideslopes. Slope erosion testing using ASTM test method D6459 confirmed that no sand loss occurred at 2.63in. and 4.65in. of rainfall per hour. At 6.6in./hr (above design rainfall intensity), 184.45 grams of the finer grained component in the sand were collected at the 8ft × 40ft test plot outlet. This was equivalent to an erosive soil loss of 0.03 tons/acre.
4.4 Concentrated flow erosion
Based on concentrated flow erosion testing (ASTM test method D6460), the decision was made to replace typical final closure drainage infrastructure—concrete lined ditches and channels, overside corrugated metal pipe drains, and riprap energy dispersion aprons—with a system that uses a sand-cement ballast infill, rather than just sand, to increase hydraulic shear resistance up to a measured 15psf (equivalent to a 12-in. D50 riprap).
During the first winter after installation, an intense series of storms occurred. Inspections during and after the storms revealed no observable damage to the surfaces.
4.5 Ultraviolet radiation degradation
The SVSWA wanted to know how long the exposed artificial grass tips would last under UV radiation exposure.
Tensile tests were conducted on artificial grass samples from a field test location near Phoenix, Ariz., at five- and seven-year exposure conditions. The retained strength in the HDPE grass was measured at 89.7% and 83.8%, respectively, from an original tensile strength of 20.2lbs/in. Projecting out to 30 years of field exposure from these two data points, the turf-retained tensile strength was estimated at approximately 60%.
This longevity property was reassessed in 2012. As shown in Figure 3, the 30-year exposure tensile strength projection increased to 70% based on four data points shown. Projecting out another log cycle from the 10-year data set, the service life estimate of the system could be more than 100 years using a 15% retained strength parameter (grass blade strength required to retain the sand ballast).
Because periodic final cover inspection and maintenance is required post-closure, the cover system had to withstand traffic from pickups and other heavier vehicles.
A traffic analysis was conducted that considered burst resistance, tensile strength, puncture resistance, and vehicle slide while braking. After extensive testing, the final cover system proved acceptable in all aspects.
4.7 Integration of solar power array station
The final cover system is supportive of a solar power array installation with zero impact to the system’s synthetic materials.
One of the highest solar panel conversion efficiency loss factors is dirt and dust accumulation on the panel surface. The lack of soil beneath the solar arrays means a significant reduction in the amount of maintenance needed to keep the arrays clean. And since completion of the Crazy Horse project, that vision has become a reality on a recent installation at the Hartford Landfill owned by the Connecticut Resources Recovery Authority. Figure 4 (an aerial shot of the Hartford Landfill) and 5 show solar panels deployed in a five-acre area.
5.1 Project costs
The initial project budget was $10.4 million for materials, installation, construction management, and quality assurance costs.
The completed project, from finish grading to geomembrane installation, including construction management and quality assurance cost, was approximately $10.5 million.
The annual postclosure maintenance cost is estimated at $46,600 per year. The CHLF closure design eliminated the soil component, all vegetation maintenance costs, and reduced drainage maintenance costs by approximately 70%. The total savings over a nominal 30-year postclosure cost for the reduced final cover maintenance is projected at $1.4 million. To date, the site owner has had to perform no significant final cover maintenance activities.
5.2 Reduced carbon footprint
The final cover system eliminated about 11,000 truck trips to import soil to the site. The project’s carbon footprint was reduced by 70% due to the combined project size reductions of soil import elimination and reduced heavy equipment needs.
5.3 Benefits to local community
Typical landfill closures require specialized equipment and labor due to their complicated combinations of earthworks and specialty geosynthetic materials.
With a CHLF artificial turf system that is relatively easy to construct, the SVSWA could hire local labor to perform the work since Monterey County was experiencing high unemployment rates at the time.
The closed landfill, with its artificial green grass top layer mottled with gray sand infill, is barely discernible from the natural hillside behind it. Aesthetic and odor complaints from neighbors abutting the landfill have diminished, with many of the residents contacting the SVSWA to compliment the appearance of the site. Intrusive and noisy postclosure care efforts, including slope reconstruction, revegetation, fertilization, and mowing, are now reduced to essentially zero.