By Tarik Hadj-Hamou and Bertrand Breul
Final closures of old abandoned landfills often pose technical and logistical challenges. A small (5 hectares/12 acres) landfill created by pushing waste into a tidal wetland on the edge of Suisun Bay in Northern California was no exception.
The initially proposed final cover design called for 60cm (23in.) of foundation soil, a 1.5-mm (0.06-in.) HDPE geomembrane, a geocomposite, and 60cm of cover soil. This design resulted in the import and placement of 90,000m3 of fill and a 1.5-hectare (3.7-acre) encroachment in the wetland and raised concern about settlement and damages to the wetland habitat.
A revised design, presented in this article, is based on a 40-mm (160 mil) bituminous geomembrane exposed in the tidal area and covered (geocomposite and cover soil) over the rest of the landfill. The revised design, approved by U.S. Army Corps of Engineers, offered financial and environmental advantages.
Site conditions and history
The landfill, located adjacent to Suisun Bay north of Concord, Calif., covers 5 hectares (12 acres) and forms an asymmetric mound that reaches a maximum elevation of slightly more than 3m above mean sea level (MSL) near its eastern edge along an access road. The western half of the landfill is at an elevation of 1-1.5m above MSL.
A tidal wetland surrounds the landfill to the north, west, and south. The landfill contains an estimated 95,000–105,000m3 of municipal waste and cover soil. In addition to a sharp break in slope, the boundary is also clearly defined by the edge of water near the toe of the slope. Terrestrial plant life extends only to the edge of the water.
Historical aerial photographs indicated that the landfill was created by the progressive disposal of waste placed directly on native soil outward from the road on the eastern boundary. Apparently, this area was not excavated before waste was discarded here. Based on the topographic evaluation, the waste was estimated at up to 3m thick; however, the waste may be unevenly distributed and the ratio of waste-to-soil in the fill may vary. Figure 1 shows an aerial photograph of the landfill.
Surface water in the wetland to the west of the landfill is influenced by the ebb and flooding of tidal water in Suisun Bay, which is part of the San Francisco Bay system. Water elevations change daily because of tidal fluctuation and are typically on the order of 0.5m (20in.).
Surface water elevations were measured between the approximate extremes of 0.5 and 0.9 meters (National Geodetic Vertical Datum, 1929) during the wet and dry season tidal influence surveys in 2005 and 2006. Although lower water levels may not readily occur at the site because the area does not drain well, higher water levels are likely during extreme tides and storm events.
The landfill is underlain by a soft clay deposit known locally as San Francisco Bay mud (bay mud). Silty clay is the predominant lithology of the bay mud, although peat lenses are present beneath the landfill.
Three levels of bay mud were identified at the site with the upper level being the newer softer clay, underlain by “recent bay mud” and “older bay mud.” The distinction between the three levels is important when analyzing settlement potential, which is discussed later in this article.
Based on the history of loading and the geologic conditions, a schematic profile of the conditions at the landfill can be established as shown on Figure 2. Because the bay mud is not consolidated, the weight of the refuse in the landfill has likely compressed the underlying bay mud to some extent.
Three zones can be identified in the profile shown in Figure 2:
- Zone 1: the untouched wetland area where no waste or fill has ever been deposited.
- Zone 2: the tidal fluctuation zone and the area where the waste mass ranges from zero to the average of 2.3m, but where no foundation fill was placed.
- Zone 3: the central portion of the landfill where there is 2.3m of waste and up to 3m of fill.
A landfill cover was selected to satisfy these remedial action objectives:
- protect human health and environmental receptors from contact with landfill contents.
- protect human health and the environment from exposure to leachate.
- protect human health and the environment from subsurface landfill gas migration.
The landfill cover selected consisted, from bottom to top: 1) a 0.3m-thick layer of topsoil, 2) a biotic barrier layer, 3) a 0.3m-thick low-permeability clay layer, and 4) a 0.6m-thick foundation layer.
The selected remedy included waste consolidation. The intent of the waste consolidation was to minimize the volume of imported soil while creating grades that promote surface drainage.
In addition, the waste consolidation effort minimized the surface area and successfully held the new footprint of the landfill to the same dimensions as the existing landfill. The landfill consolidation thus prevented any enlargement of the landfill footprint.
The proposed waste consolidation included dewatering the excavation area, excavating waste on the perimeter of the landfill, relocating the waste to the central portions of the landfill, and constructing a perimeter containment dike for the landfill waste, the foundation layer of soil, and the landfill cap itself. A linear low-density polyethylene (LLDPE) synthetic geomembrane was eventually selected to replace the 0.3m-thick low-permeability clay layer.
When construction began for waste consolidation, the surface of the landfill was stripped of vegetation prior to preliminary grading. On June 15, 2006, and again on July 6, 2006, munitions and explosives of concern (MEC) were found in soil excavated as part of construction activities for the landfill cap. The discovery of these objects led to a stopwork order and the request for the design of a new cover system and construction approach.
A new design was proposed, predicated on keeping the same design as the 2004 cover but placing sufficient fill to create a hill with uniform slopes and eliminate the need for excavating and reconsolidating the waste.
These slopes were designed at an interior 3% grade with perimeter slopes at 10%. In addition to requiring substantially more fill, the redesign required a cap with a larger footprint to cover waste in the low areas under water. The increased footprint extended beyond the existing waste into wetland areas that previously were not filled. New fill would have been placed to the north, west, and south of the existing landfill footprint.
The enlarged landfill footprint would have encroached on surrounding surface water areas to the north, west, and south. The footprint of the landfill covered 5 hectares (12 acres), whereas the footprint of the 2008 cover would have covered 6 hectares (15 acres). Approximately 1.2 hectares (3 acres) of new fill were to be placed in the wetland. A schematic cross section of the 2008 final cover is shown in Figure 3.
The main issues associated with the 2008 cover design were:
- encroachment on the wetland.
- consolidation settlement of the soft bay mud not previously loaded by waste or fill.
- import of more than 90,000m3 of fill.
This design was abandoned because of overrun in fill quantities and the costs associated with encroaching on the wetland.
The cover proposed and approved by the regulatory agencies and presented in this paper was designed in accordance with the following set of guidelines:
- cover over the full footprint of waste.
- no encroachment on the Site 2 wetland.
- no additional fill import.
- revegetation and post-closure usage.
To satisfy these guidelines in addition to regulatory requirements, the remedial action objectives (RAOs) and all applicable or relevant and appropriate requirements (ARARs), a hybrid multilayer cover was designed. The hybrid multilayer cover consists of an exposed geomembrane in the tidal fluctuation zone defined in Section 4.0 and a multilayer cover over the remainder of the landfill.
The exposed geomembrane is a 40mm-thick bituminous geomembrane. This geomembrane is four times the thickness of the initially proposed LLDPE geomembrane and five times heavier. It is inert and not sensitive to UV rays and, therefore, can remain exposed.
The multilayer cover includes (bottom to top): 1) a 0.45m-thick foundation layer, 2) a 40mm bituminous geomembrane, 3) a biotic (drainage) layer, and 4) a 0.3m-thick vegetative soil cover.
The tidal fluctuation zone is defined as the zone that extends from the limit of waste to the highest tide elevation. This highest tide elevation was calculated to be 2.1m (7ft) above MSL. A schematic of the proposed final cover is shown in Figure 4.
It is anticipated that settlement of the landfill will be mostly due to consolidation of the bay mud underlying the site and it is assumed that settlement of the waste has occurred.
Three levels of bay mud were identified at the site, with the upper level being the newer softer clay with the highest potential for consolidation. Consolidation is a time-dependent process and occurs during a number of years; therefore, rate of settlement, time to maximum settlement, and maximum settlement are calculated. The rate of consolidation is an inherent property of the clay and varies with the age of the bay mud.
Maximum settlements were calculated at the center of the landfill where the loading (height of fill) will be the largest and at the toe, near the landfill boundary, where the loading from the cover will be the least. The time to reach these maximum values was estimated to be 27 years from the end of construction. The calculated long-term settlement at the center of the landfill is on the order of 0.15m and at the edge on the order of 0.30m.
The relatively low value calculated for the center of the landfill is due to the fact that the grading plan developed for the landfill is based on redistributing fill already in place. By removing the fill, the loading is reduced and the final settlement is less.
The calculated values of settlement at the center and landfill boundary lead to a theoretical maximum differential settlement of 0.15m between center and landfill boundary. This differential settlement does not affect the overall drainage of the cover. Assuming a 0.3m settlement at the entry point of the longest drainage swale and 0.15m of settlement at the exit reduces the average slope of the swale from 2.1% to 1.9%.
Under a load of finite extent, such as an embankment, soft and compressible soil tends to experience vertical movement (settle) as well as some horizontal movement.
As the load (i.e., fill) is applied, the soil compresses but also “squeezes” laterally. The loading of the middle part of the landfill will induce some lateral movement of the area not loaded (i.e., the area of exposed 40-mm bituminous geomembrane).
The lateral movement at the landfill boundary was estimated to be on the order of 5cm (2in.). Assuming the geomembrane is firmly anchored at the landfill boundary and at the collector trench, the 5cm extension has to be accommodated by the geomembrane. The minimum length of exposed 40-mm bituminous geomembrane is 12m leading to a maximum strain of 0.43%, an insignificant level of strain for this type of geomembrane.
The static and pseudostatic stability analyses performed indicate that the slopes are stable under static conditions with factors of safety in excess of 1.5 and that the calculated permanent seismic displacements are on the order of a couple of inches and therefore do not exceed the accepted displacement criterion of 0.3m.
The hybrid system for the final cover at this small landfill in the bay area near San Francisco relies on an exposed 40mm-thick (160mil-thick) bituminous geomembrane on the portion of the landfill subject to tidal influence and on a standard multilayer cover in the area of the landfill that is not subject to tidal influence. The hybrid final cover system was installed during spring and summer 2012 as seen in Figure 6.
Selecting this cover system for this site underlain by soft compressible clay (San Francisco Bay mud) offered the following:
- Full footprint of the waste mass covered.
- No wetland encroachment.
- Use of the available on-site soil.
- No additional soil import.
- Minimized settlement
and lateral spreading.
- Reduced risk of cracks in cover.
- Minimized tensile stresses
- Minimized traffic disruptions in neighborhoods by eliminating 1,350 truck round-trips.
- Reduced emission of greenhouse gas (GHG) by reducing truck traffic.
- Protected wetland from sediment contamination (cover soil).
- Accommodated tidal fluctuation and the effect of rising sea level with no potential damage to cover.
Tarik Hadj-Hamou, SLR International Corp. (formerly SES Inc.), Irvine, Calif., USA
Bertrand Breul, Axter Coletanche, Irvine, Calif., USA