Saving a Slope Failure

WHEN A CENTRAL MAINE LANDFILL was recently faced with having to build a new disposal cell to extend site life, the project managers got creative. In 1989,a landslide had nearly doubled the size of the Crossroads Landfil. Following the landslide, the site was purchased by Houston-based Waste Management Inc. (WM),who expanded operations in other areas of the site. By the late 1990s, WM knew that to keep the site open, it had to remedy the area of the landslide while working with the soft foundation conditions. So, the company began a project to relocate the waste from the landslide area, revamp the area and provide new disposal capacity — all while controlling costs.

According to WM, the landslide occurred because the slopes of the landfill cell were too steep and buttressing soils adjacent to the toe of the site had been removed by the prior owner. When the soft foundation clay sheared, the unlined landfill moved 400 feet, doubling its size from 12 to 25 acres in less than 30 seconds. A year after the landslide, WM acquired the facility and covered the failed area with a soil/geosynthetic final cap.

Maine passed a law shortly thereafter restricting landfill expansions to property that was used for active landfilling before 1990. WM had developed landfill cells on other portions of the property during the 1990s but had to construct a new lined disposal cell in the landslide area to keep the site open. It began a project to dig up more than 1 million cubic yards of unlined waste from the landslide area, relocate it to lined cells, then construct a liner system in the area of the landslide. This would allow for disposal in the newly lined area and in adjacent lined cells.

The expansion project has three phases. The biggest challenge to relocating the unlined waste was ensuring the stability of the surrounding waste disposal cells during excavation. Controls were established to ensure that the excavation slopes were not too steep and that excavation was limited to small areas at any given time. Backfilling occurred rapidly. To speed the consolidation of the underlying soft clays and strengthen the landfill foundation as waste is placed in the site, WM installed wick drains in the clay under the expansion area. An underdrain layer beneath the liner collects water from the wicks.

To further increase disposal capacity, WM used mechanically stabilized earth (MSE) berms around the landfill perimeter. Geosynthetic reinforcement created a nearly vertical slope along the exterior portion of the perimeter berms. The MSE berm around the expansion area has a height of about 20 feet and a length of more than 5,500 linear feet. The interior side of the MSE perimeter berm is sloped at a three-to-one horizontal-to-vertical ratio prior to installation of the liner system. Horizontal layers of geogrid reinforcement are installed every 18 inches vertically within the berm soil to allow the exterior slope to be nearly vertical. Each layer of geogrid extends horizontally from the exterior face about 15 to 35 feet into the berm. The exterior facing component consists of geogrid and welded-wire mesh forms filled with vegetated topsoil.

To date, the first two construction phases have been completed, so the unlined waste has been removed from the southern portion of the landfill and the liner has been constructed. Nearly 2,800 liner feet of MSE berms have been placed around the perimeter of the expansion area. Project completion is expected by 2006.

Because of creative design techniques, the Crossroads Landfill will provide close to 4 million cubic yards of new disposal capacity, thereby allowing the facility to continue serving Central Maine for years to come.