5.2 Model Preparation

The soil preparation procedures for tests Ts4, Ts5 and Ts6 have been described in section 3.6. After the soil has been consolidated under conventional laboratory condition, it was then subjected to 100g in the centrifuge for a further 24-hour consolidation under a sand surcharge of 55 kPa. The high-g consolidation processes were monitored by measuring the surface settlements and the dissipation of excess pore water pressure of the soil. The layout of LVDTs and PPTs is illustrated in Figure 5.1. The observed pore pressures and surface settlements are presented in Figures 5.2 and 5.3, respectively. Undrained shear strengths of the soils are partially shown in Figure 3.7.

The excavation process, the installation of model wall, upper strut and latex bag for the three tests essentially followed the same procedure as for test Ts1. The wall was embedded 25 mm into the stiff soil for test Ts4 and 50 mm deep for test Ts5. After the model setup is completed, it was further subjected to another 8 hours of in-flight re-consolidation with a sand surcharge of 20 kPa. The observed pore pressure and surface settlement during the re-consolidation process are shown in Figures 5.4 and 5.5, respectively.

At the end of 24 hours of high-g consolidation and 8 hours high-g reconsolidation, the observed pore water pressures were still higher than those of insitu static state. Due to the very low permeability of the marine clay, an average consolidation ratio of about 75 % to 85 % throughout the depth was achieved.

After the re-consolidation process, the excavation test was performed by draining the zinc chloride solution out of the rubber bag, as in the previous tests. The excavation rate was controlled by the rate of releasing the zinc chloride solution. The excavation rate was about 9 mm/min which is 0.13 m/day in prototype scale.

For tests Ts4 and Ts5, the wall was propped with an upper strut at the retained ground level. The soil conditions for these two tests were very similar, but the penetration of wall into the lower stiff soil layer was different, that is, 25 mm for test Ts4 and 50 mm for test Ts5. Test Ts6 had the same condition as test Ts5 except that the wall was unpropped. This is to compare the behaviour of an excavation system with and without the upper strut under working condition.