6          Reducing Lateral Pressure on Structures

6.2         EPS Geofoam Subjected to Large Strains

In chapter two a literature survey was shown on the mechanical properties of geofoam. Chapter three also presented some of the mechanical properties of geofoam. Most of those properties are for small strain applications. Using its properties under large strains, geofoam can be utilized in reducing lateral pressure on structures experiencing large impact forces, blasts, large dynamic excitations or soil expansion. In this sections two tests are presented. These tests are separated from chapter three as they are for large strains, which can be experienced on few applications mainly in reducing lateral pressure on structures.

Photo 6-3 shows the uniaxial compression test of EPS geofoam. The test was performed in a strain-controlled mode with a strain rate of 4% per min. A stack of two blocks was compressed to more than 75% of its initial height. Photo 6-4 shows the stacks after loading compared to a similar original size. Figure 6-1 shows the stress strain curve for a type I geofoam. The stress is increased by increasing the strain with an initial modulus of 4.7MPa up to approximately 1% strain. A plateau showed with a slope of 0.2MPa until the stains reached more than 50%, this could be explained to be buckling (Weaire and Hutzler, 1999). The slope began to increase although it did not reach the initial modulus when reaching a 73% strain. At this strain level the stresses reached a value of approximately 400kPa. Upon unloading the blocks experienced rebound as shown in the figure. The same results are shown in figure 6-2 for the higher density type IX geofoam.  Upon unloading a rebound of 15% was obtained.

The two figures, figure 6-3 and figure 6-4, show that energy can be absorbed by geofoam while experiencing limiting stress levels. This can be useful for structures experiencing large forces while its strength is limited to a certain value. The appropriate geofoam type can be chosen for such structures to absorb the required amount of energy.

The previous test was performed for one cycle with 0.1m cube specimens under load control and were tested for a number of cycles following the pattern shown in figure 6-5. Such a pattern was chosen to study the effect of repeated loading on the stress-strain curve of EPS geofoam when subjected to different stress levels. Strains reached 10 % after the end of the first cycle and exceeded 8% at the end of the last three cycles. The stress strain curve shown in figure 6-6 shows that the stiffness of the material decreased sharply after reaching a value of 1% strain in one of the loading in the first cycle. In the last three cycles the stiffness of the material was maintained at a certain level. The area under the stress strain curve was decreased in the last three cycles but is almost constant for the same stress level. From the previous discussion it can be seen that even under repeating loading foam can absorb energy and the larger the amount of strain it the more the energy dissipated using foam.

Photo 6‑1 1.2m Height of Foam Subjected to Uniaxial Compression

Photo 6‑2 Compressed and Uncompressed Stacks

Figure 6‑1 Type I EPS Geofoam Subjected to Uniaxial Compression

Figure 6‑2 Type IX EPS Geofoam Subjected to Uniaxial Compression

Figure 6‑3 Strain Energy Density for Different Stress Levels

Figure 6‑4 Strain Energy Density for Different Strain Levels

Figure 6‑5 Loading Pattern and Corresponding Displacement Response

Figure 6‑6 Type XI Stress Strain Curves Under Cyclic Loading