revised final closure plan - Salinas Valley Solid Waste Authority

Revised Final Closure Plan July 2010
Crazy Horse Sanitary Landfill -11- 103-97133
The final cover above is an engineered alternative final cover consistent with 27 CCR 20080. In May
2010, the SVSWA constructed a test pad of the proposed cover system. The Construction Report
documenting the results of the test pad is provided in Appendix N.
The final cover for the current recycling and maintenance area will be asphaltic concrete (AC)
paving. The AC paving will provide a functional final cover while providing usable space for
SVSWA and county use.
Foundation Layer - Title 27 CCR §21090 stipulates that not less than two feet of a soil foundation
layer shall be constructed for the final cover, unless it is shown that differential settlement and
ultimate land use do not adversely affect the integrity of the final cover. Postclosure use for the site
will be as a solar power generation facility on the topdeck (Figure 4A). The remaining areas will be
non-irrigated open-space. The ultimate land use does not involve structures or other uses of the
covered areas that could be affected by differential settlement.
To support vehicle travel on the closed facility during construction activities and the postclosure
period, the foundation layer beneath traveled roadways will be a minimum of two feet thick. The
SVSWA recently conducted a survey of existing foundation soil thickness. It was determined that
onsite soils are available for foundation layer construction, however, it is anticipated that some soil
will require import. Other surfaces of the landfill will be constructed with a minimum 1-foot thick
foundation layer in the final cover system as discussed below.
Low-Permeability Layer - The proposed low-permeability hydraulic barrier layer is provided by
geosynthetic materials (structured LLDPE geomembrane). These geosynthetic materials can tolerate
substantially higher strains up to 10 to 20 percent or greater before yielding, and thus are a superior
cover construction material. Because these materials can tolerate strains 10 times larger than soil
components without adverse impacts, the two-foot foundation layer thickness supportive for
construction of a clay liner is not required for geosynthetic materials. Furthermore, modern
landfilling techniques focus on achieving a high degree of compaction to optimize airspace, and large
containers are typically diverted from the landfill that could otherwise collapse and cause large
differential settlements. As a result, differential settlements at the top of the refuse are expected to be
relatively small at approximately 2 percent or less (Appendix B). Therefore, the foundation layer for
the proposed EAD cover design need only be thick enough to provide a clean, firm surface for the
geosynthetic materials.
Due to the SVSWA’s efforts and practices to divert large appliances/containers from the landfill, the
development of large differential settlements resulting from large voids is considered unlikely. In
addition, the SVSWA is considering utilizing dynamic compaction techniques on the finished topdeck
surface to reduce settlement impacts on the constructed final cover. However, a very conservative
analysis was completed that considered the potential development of a void in the underlying wastes
(Appendix B). This analysis indicates that the resulting strain (2%) are well within the limits that can
be tolerated by the proposed cover materials in the unlikely event that significant voids develop
beneath the cover system. For the CHLF, a foundation layer of one foot is more than sufficient to
achieve this objective and will also reduce the traffic and environmental impact of transporting
additional foundation layer material to the site.
For unlined and lined areas, the low-permeability layer will be a geomembrane placed on the
foundation layer. The geomembrane exceeds the permeability requirement in 27 CCR as discussed in
Appendix D.
Golder Associates