5.3 Wall with Shallow Embedment and with an Upper Strut - Test Ts4

5.3.1 Changes in pore water pressure

The excavation was simulated by releasing the zinc chloride solution from the rubber bag during centrifuge flight. The excavation process was divided into 5 stages and the corresponding model time and excavation depth are shown in the table of Figure 5.1. The test was monitored by a pore pressure transducer placed inside the rubber bag, as shown by PPTzncl curve in Figure 5.6. For the first 300 seconds, the excavation depth increased from zero to about 41 mm. Subsequently, the excavation progressed at a much slower rate and only reached the maximum excavation depth of 50 mm at 3000 sec. It is suspected that this slow rate of excavation was caused by partial blockage in the drain pipe.

A close-up view of the pore pressure changes during the excavation is shown in Figure 5.7. It indicates that pore pressure dropped about 6.5 kPa as measured by PPT1 and PPT5. A rebound of about 2 kPa was observed after the excavation. PPT2 and PPT4 cannot give good data due to the problem of signal transmission. Although the contours of pore water pressure cannot be plotted with so few transducers, rough trend can still be found as PPT1 is nearest to the wall and PPT5 located at a lower elevation (see Figure 5.1 for location) show a significant drop in pore pressure. PPT6 shows the smallest change in pore pressure and PPT3 shows an intermediate change as it is located between PPT1 and PPT6. The rebound of pore pressure after the excavation exhibits a similar trend as observed in other tests.

5.3.2 Ground surface settlements

The surface settlements versus time data measured by LVDT1 to LVDT4 are plotted in Figure 5.8. The settlement profiles along the ground surface are shown in Figure 5.9. The magnitude of settlement in this test was significantly smaller than those in test Ts1 (Figures 4.19 and 4.32) at the same excavation depth. The retaining system in this test did not fail when the excavation reached 50 mm (Figure 5.10). Figure 5.9 shows that before the appearance of a slip surface, the surface settlement decreased with increase in distance away from the wall. This observation will be further discussed in the following section.