Figure 2. Highway, railway, and seaport locations on Boubyan Island. Figure 5. Adopted load distribution. The combined degree of consolidation of the clay layer due to horizontal and vertical drainage is calculated as shown in Equation (2) based on Carrillo’s (1942) theoretical solution (Bo, Arulrajah, and Nikraz 2007), where Uj), is the combined degree of consolidation at time (t) due to horizontal and vertical drainage, U;, is the degree of consolidation at time (f) with respect to horizontal drainage only, and U, is the degree of consolidation at time (f) with respect to vertical drainage only. The settlement time rate, S,, is calculated using Equation (3). The primary consolidation settlement for normally conso- lidated clay soil was calculated by Equation (1), where S is total primary settlement of the soil layer, H is the thickness of the soil layer, e9 is the initial void ratio, ov,’ is the initia effective stress at the middle of the soil layer, and Ove is the final effective stress at the middle of the soil layer. To more accurately estimate the settlement, the clay layer is subdivided into 10 sub-layers, where the primary settlement of the entire clay layer is the summation of the primary settlement of al sub-layers. The final effective stress ov, includes the initia effective stress ovy’ along with the additional stress due to loading Ao,, which is calculated assuming that the loading is spread within the soft layer at a 1:1 vertical-to-horizonta scale. Figure 5 illustrates the parameters used in Equation (1). Table 2. Design parameter predictions. The predictions listed in Table 2 indicate that variations in the coefficient of consolidation would have a significant impact on the time to 90% consolidation (ie. from 89 days to 372 days) and thus a significant impact on the construction schedule. Therefore, to determine the true behaviour of the ground under surcharge loading, the values of the coefficients of consolidation, C; and C,, were verified by back-calculation from the settlement results measured in the field and by those resulting from the numerical modelling. Figure 8. Plaxis model geometry of the road embankment model with the generated mesh. A finite element mesh was then generated to establish the initial conditions, which includes the effective stresses and pore pressure. Figure 8 illustrates the Plaxis geometry of the road embankment model with the generated mesh. Consolidation is considered in the calculation type in solving the model, as doing so allows the excess pore pressure changes in the soft soil to be monitored. Table 3 shows the construction stages adopted by the Plaxis model to simulate the actual procedure for the full-scale field model. Figure 10. Settlement data of SP-35, SP-36, and SP-37 and the Plaxis model. Figure 12. Water level measured at standpipe SW-8. Figure 11. Active pore water pressure data of KX-19, KX-20, and the Plaxi: model as well as excess pore water pressure from the Plaxis model. Table 5. Settlement measured by the Asaoka method and the Plaxis model. Table 6. Back-calculated consolidation coefficients. Figure 14. Plot of S,.;vs. S, for SP-36 showing S- = 544.8 mm. Figure 15. Plot of S,.,vs. 5, for SP-37 showing S; = 421.4 mm. Figure 16. Plaxis results showing unloading (heave)-reloading-secondary set tlement and changes in the excess pore water pressure dissipation durinc reloading. This study presented an overview of the ground improvement design of soft clay at Boubyan Island using surcharge with PVD. The performance of the design predictions was verified by field monitoring of an instrumented full-scale section under the same surcharge loading and with the same PVD design.
