As was mentioned in chapter two a number of creep research tests were done on EPS geofoam. This section highlights the characteristic of the typical EPS geofoam creep curve in both short term and long-term monitoring period.

Figure 3-60 presents the strain of a 0.1x0.1x0.12 prismatic type IX specimen over a short period of time. The specimen is subjected to stresses equal 100% of the strength at 5% strain. The total load is applied in less than 2 seconds. In three days most of the strain is developed and the strain line began to level out. This can be seen also in the logarithmic scale in figure 3-61 where the end portion of the S-shape line began to level out.

The strain rate of the same test is drawn in figure 3-62. The creep is in its primary stage and the secondary stage appears that it will not be reached in this case, as the strain rate will continue to decrease with time. The evidence of such behavior is shown from the rate of the strain rate curve in figure 3-63. In this figure the rate of the strain rate is always negative and it tends to asymptote to a non-zero value. The strain rate will decrease and likely not to reach the secondary stage under a stress equals to a 100% of the strength at 5% strain or less.

Figure 3‑1 Time vs. Strain for Type XI EPS

Figure 3‑2 Log Time vs. Strain for Type XI EPS

Figure 3‑3 Strain Rate for Type XI EPS

Figure 3‑4 Rate of Strain Rate for Type XI EPS

As was mentioned earlier the design stress is a factor of the strength at 5% strain. Results of two specimens tested at 30% of the strength at 5% strain are shown in figure 3-64. The specimens are of two different sizes and shapes. The large specimen is a 0.6m cube and the small one is a 0.2m height prism of a 0.1m square cross section. Both specimens are of the same EPS type. Edge effects appear in the results of the small specimen as the creep is higher and the initial settlement is also higher than that of the large specimen. Extrapolating the strain of the large specimen using a power law to 20 years using the 7 hours data will give less than 0.1 % creep strain as shown in figure 3-65. It’s too bold to do so, but the idea is to show that creep of a material that has behavior as shown in figures 2-60 through 3-64 is likely not to be significant under low stresses.

Figure 3‑5 Creep of Two Specimen Sizes Type XI EPS

Figure 3‑6 Creep Equation for Type XI EPS

Figures 3-66 and 3-67 shows that the strain tends not to increase any more after 8 hours for the 0.6m cube specimen subjected to stress equal to 30% of the strength at 5% strain. The small size specimen will continue to creep as shown in figure 3-67. This is because of the edge effect.

Long-term creep tests are performed on 0.05m cube specimens. The data were collected manually by reading the displacement from a dial gauge. The tests were performed on type VIII EPS geofoam. Results were taken for a period of 517 days. The stresses are 30%, 50% and 70% respectively of the strength at 5% strain. The strain rate is shown in figure 3-68. The non-uniform shape is because of the manual reading of the data. The trend for all three samples shows that creep is still in the primary stage.

Figure 3-69 shows the strain rate for the first 7 days. Error due to manual data acquisition appears as early as the first few hours in the 30% stress loaded specimen. As was mentioned earlier under low stresses creep tends to vanish with in few hours as long as the specimen is uniformly loaded and has parallel horizontal loading surfaces.

Figure 3‑7 Strain Rate for 0.6m Cube Type XI EPS

Figure 3‑8 Strain Rate for Two Type XI Sample Sizes

Figure 3‑9 Long Term Strain Rate for Type I EPS

Figure 3‑10 First Week Strain Rate for Type I EPS