6.3         Static Stress Reduction

Geofoam backfilling has a number of configurations that have been used in real applications. A compressible strip or inclusion can help reduce lateral earth stresses on structures without requiring movement of the structure. Whatever the thickness of a compressible inclusion, even as wide as shown in photo 6-1, the lateral pressure will not be reduced less than the active state of stresses. As shown in figure 6-7 the 1.8m height reinforced concrete wall was solved using a finite difference analysis technique for both the cases of sand backfill and foam backfill. The wall was fixed at the base while the upper end is left free, a cantilever condition. The concrete wall is 0.3m thick and can experience very low deformations under the lateral pressure effect. The problem was solved again with a 0.3m thick compressible inclusion. The results show that the total pressure in the case of the compressible inclusion is less than that in the case of sand although it never reached the active state of stresses and both cases, the sand and the compressible inclusion are less than the case of the at rest earth pressure.

Another configuration, which is widely used, is shown in figures 6-8 and 6-9. Foam blocks are arranged in a way to have a front slope that meets the sand repose angle. The vertical stresses shown in the figures is small in the foam sand contact zones hence the lateral pressure is small in the same zone despite the case of stress; active, passive or at rest. Hence lateral stresses applied on the wall are considered negligible (figure 6-7). Real applications have the wall totally attached to the foam. In such case any external lateral force will be transferred to the wall. In figure 6-10 the same configuration of the foam blocks just mentioned was used next to a 2.7m height wall in Syracuse Mall (Sun, 1997). Three cells were placed at two different heights of the concrete wall. Cell 1 is placed near the top of the wall while cells 2 and 3 were placed near the bottom of the wall. The wall is free of supports at the top. A reinforced concrete slab was flushed to the top of the wall. The pressure on the wall was negligible at the beginning (figure 6-11). The increase at the top, which could be the results of placing the concrete slab or any other reason, increased the stresses applied on the wall. The stresses tend to reduce with time. As was mentioned in chapter three that the time dependent effect is minimal on geofoam when subjected to working stresses. That reduction in stresses could be explained as the effect of the soft soil settlement beneath the sand next to the foam blocks that tends to tilt the foam blocks.

From the previous discussion it can be shown that in any geofoam configuration used high stresses can be experienced by the adjacent structures although one configuration tends to induce stresses less than the others. Since geofoam is a self-standing material as shown in the photos in chapter 5, a gap can separate the geofoam fill from the wall. The gap can be calculated such that zero stresses are induced all the time on the wall.

Figure 6‑1 Stress Distribution Using Different Mathematical Modeling

Figure 6‑2 Vertical Stresses Behind a Sand Backfill Wall with Foam Blocks

Figure 6‑3 Horizontal Stresses Behind a Sand Backfill Wall with Foam Blocks

Figure 6‑4 Syracuse Mall Geofoam Configuration (after Sun, 1997)

Figure 6‑5 Syracuse Mall Field Readings