4.4 Excavation Supported by Diaphragm Wall with Two Struts

4.4.1 Model preparation and test procedure

In test Ts3, two struts were used to provide support to the retaining wall. The first strut was installed at the retained ground level before the test. During the centrifuge test, the second strut was installed in-flight at 75 mm level when the Zncl was released to this elevation. The technique of installing the lower strut has been described in detail in Chapter 3.

The c_u of the upper soft clay was 17.3 kPa to 19.5 kPa and that of lower layer was 187 kPa. The thicknesses of the two layers were the same as those of test To1 and Ts1. The excavation sequence and instrumentation are identical to those of test Ts1 shown in Figure 4.14.

4.4.2 Test observations

The variation of surface settlements with time are shown in Figure 4.22. The settlement profiles are displayed in Figure 4.23. Both the magnitudes of surface settlements and the shape of surface settlement profile of this test are clearly different and much smaller than those in tests To1 and Ts1. The maximum settlement occurs close to the wall and the magnitude of settlement decreases sharply with distance away from the wall. A detailed comparison of the observed settlement profiles and pore pressures will be provided later in this chapter.

The variations of bending moments of the wall against time are shown in Figure 4.24. The action of the lower strut changed the bending moment of the wall significantly. Figure 4.25 gives the profiles of bending moment along the wall at three different times.

The photographs of the experiment at different excavation stages are shown in Figure 4.26. Before the lower strut was installed, the mid-wall was observed to move towards the cut. After the lower strut was installed at stage 4 and upon further excavation, the lower part of the wall was observed to rotate around the support point of lower strut as the toe restraint was lost.

The failure surface was limited to only the lower part of the retained soil and not fully developed to the ground surface as in all the previous cases. The local shear failure surface also exhibited a circular slip starting from the toe. This illustrates the importance of embedment of the toe to prevent failure. The stress concentration around the toe means that the failure will propagate from that point.

The failure pattern after the test is shown in Figure 4.28. Due to the support provided by the two struts, the excavation system was much more stable in general. The ground settlement was also limited effectively by such a retaining structure. The lower strut was functional to maintain the stability of the wall, as compared to Ts1 ( Figure 4.27). However, as the lower soil was weak and the embedment of wall might not be deep enough, local shear failure near the wall toe still occurred, as shown after stage 5 in Figure 4.26.