5.4 Wall with Deeper Embedment and Upper Strut - Ts5

The only difference between this test and test Ts4 is that the wall embedment depth in the stiff soil layer is 50 mm, which is deeper than 25 mm depth for test Ts4. This is to investigate the effect of wall embedment depth.

5.4.1 Changes in pore water pressures

The excavation sequence is indicated by PPTzncl curve in Figure 5.11. In contrary to test Ts4, the excavation progressed smoothly and was completed in 300 seconds.

The pore pressure changes in the soil as excavation proceeds are shown in Figure 5.11. The details of pore pressure changes during excavation are shown in a larger scale in Figure 5.12. It shows a 8.5 kPa drop of pore pressure for PPT1. A rebound of 2 kPa occurred after the excavation was completed. The readings by PPT4 and PPT6 were also illustrated in the same figure which shows the pore pressure changes at the same elevation but at different locations away from the wall.

5.4.2 Ground surface settlements

The surface settlements with time measured by LVDT1 to LVDT4 are plotted in Figure 5.13. The settlement profiles along the surface are shown in Figure 5.14. The magnitudes of settlement for this test and test Ts4 (Figure 5.9) were nearly the same up to an excavation depth of 30 mm. A significant difference in settlement appeared after this. The total settlement at the end of excavation was much larger in test Ts4 than test Ts5. This suggested that after 30 mm of excavation, the effect of embedment becomes important.

In order to investigate the long term behaviour of an excavation, the post-excavation observation continued for another 6 hours. The measured pore pressure changes and surface settlements are presented in Figures 5.15 and 5.16, respectively. It was found that the dissipation of pore pressures and the surface settlement continued throughout the 6 hours, which is equivalent to a prototype time of about 7 years. The rate of change of the above measurements are small but still could be observed. Such phenomenon was also reported by Bolton and Powrie (1987). The reasons of such long term changes will be discussed in a later section.