4  Slope Stabilization With EPS Geofoam

4.5         Route 23A

4.5.1        Background

Geofoam was used for stabilizing a nearby 100m long section of Route 23A east of the Village of Jewett Center in Greene County, NY (Jutkofsky, 1998). In 1966 New York State Department of Public Works reconstructed the portion of Route 23A between the Village of Hunter and Jewett Center. Shortly after the reconstruction, a 100-m-long section of the roadway embankment began to move laterally towards Schoharie Creek, as shown in figure 4-12. A scarp developed on the pavement as shown in photo 4-1. Frequent patching was required to maintain a normal grade through the following years.

In 1978, a subsurface exploration program began. The general subsurface profile up to a maximum explored depth of 21 m, consist of compact gravelly silt (clayey); over layered clayey silt (silty clay); underlain by clayey silt gravelly. Ground water was observed to vary from at 1.5 to 5m below surface. In 1979, a cluster of 22 horizontal drains was installed in a fan-shaped pattern along the toe of the slope in an attempt to reduce the movement by lowering the water table. At the same time, a monitoring program began. Inclinometer A, shown in figure 4-13, was installed near the center of the failure area to determine the zone and the rate of movement. Inclinometer reading was taken for the following 14 years. 

Photo 4‑1 Main Crack before 1996

Figure 4‑1 Cross Section before and after 1996 (after Sheeley 2000)

Figure 4‑2 Positions of the Slope Indicators

As the results of the inclinometer showed progressive movement, the horizontal drain treatment (figure 4-12) was considered unsuccessful. That was partly due to the low hydraulic conductivity nature of clayey soils and also their low strength.  In 1994, a permanent treatment was pursued. The solution was utilizing EPS geofoam on a horizontal sand filter as shown in figure 4-14. Utilizing EPS geofoam reduced the driving force while using a sand filter provided a positive drainage that lowered the water table to strengthen the soil. This results in fast reconstruction, without property- taking from the adjoining homeowners and maintaining the same geometry of the slope.

A 15m long sheeting was required to handle two construction problems; retaining surrounding soils during excavation and functioning as a safety barrier for the detour traffic during construction. The excavation level was chosen to be the 100-year flood level at 5m below surface and geofoam was placed above elevation 426.72m. A typical cross section in figure 4-14 shows 0.6m crushed stone - filter below 5 layers of foam. Besides functioning as a drainage blanket the horizontal sand layer established a clean stabilized working platform over the soft saturated soils. A 0.6m wide crushed stone column extends between the foam blocks and the sheeting as shown. The main purpose of this arrangement was to lower the water table to the bottom drainage layer. The vertical layer functioned as a protection to the foam from damage as the sheeting was extracted. That filled the voids when the sheeting was vibrated out.

Figure 4‑3 Typical Cross Section in Route 23A

Photo 4‑2 The Crushed Stone Filter

Photo 4‑3 Pouring the Concrete Slab

Photo 4‑4Backfilling over the Foam Side

The foam used was type VIII Expanded Polystyrene with nominal density of 20 kg/m³. The blocks were 0.6m X 1.2m X 2.4m. The minimum shear strength of such type was between 159kpa (ASTM D732). The minimum compressive strength at 10% deformation according to ASTM D1621 was 90 kg/m³. A 0.1m reinforced concrete slab was placed on the top of the 3m height foam blocks to serve as a protection from petroleum spilling and to distribute the traffic load on the foam.

In August 1995 inclinometer B, shown in figure 4-14, was installed. In October sheet pile driving began. Excavation started after about 70 percent of the sheeting was installed. In November the foam blocks were placed. Backfilling began after several geofoam courses were placed. Earth fill was placed over the stepped face of the block mass. This operation continued as geofoam courses were added up to the finish grade. Only the earth fill was compacted.

By December 1995, the first half of the blocks was in place and ready to receive the concrete slab as shown in photo 4-3. The graded sub base crushed stone was placed after concrete curing and backfilling as shown in photo 4-4. The sheeting was removed in January 1996.  Placing of 0.23 m asphalt pavement was completed in April 1996. The total thickness of the base and the subbase ranged from 0.6m to 1.2m because of the road banking and inclined geometry. Figure 4-15 shows the construction sequence.

Figure 4‑4 Construction Time Line