Lime-clay modification and its application in the construction of man-made islands

Due to the scarcity of land following rapid development in many countries, the dredged mud may be used as fill materials to develop new land in coastal zones. Several major infrastructure facilities like ports or airports are strategically situated on these artificial lands. However, soft dredged deposits are intrinsically low in strength, and very high in compressibility. As a result of these inherently undesirable engineering characteristics, an appropriate ground improvement technique has to be employed so as to meet the engineering requirements necessary for the design and construction of overhead infrastructure facilities. The placement of a surcharge preload on a soft foundation facilitated with vertical drains is the most commonplace method used worldwide. Surcharge preloading with vertical drains is used to accelerate the rate of primary consolidation settlement and reduce the magnitude of the post construction secondary compression settlement. In this study, an attempt was made to provide insight into some factors, aimed at improving the efficiency of the preloading used in a man-made island. A series of laboratory tests was performed to evaluate the effect of clay fabric, altered due to the addition of hydrated lime into slurry, on the primary and secondary consolidation behaviour of the dredged mud samples. As a prime objective, a unique testing method was proposed to determine the threshold value of lime below which the maximum alteration in clay fabric takes place. This threshold is known as Lime Modification Optimum and presents the boundary between modification (i.e. ion exchange and flocculation) and solidification (i.e. formation of the pozzolanic products) phases in limeclay reaction. It was found that, for the type of the dredged mud used in this study, 4 percent of lime (dry weight basis) should be considered as lime modification optimum. A set of oedometer tests was conducted in both normally consolidated (NC) and artificially overconsolidated (OC) stages on both natural and lime treated samples. The laboratory findings revealed that with an increase of lime up to 4 percent, the increase of coefficient of consolidation, cᵥ , was observed up to ten fold,. Observation of the test results suggested that in the compression range, the values of the compression index, (Cc), increase when increasing the lime content whereas in the recompression range, the value of the recompression index, (Cr), gradually decreases with increasing % lime. This finding indicated that with increasing % lime, the magnitude of primary consolidation in the NC state increases while in the OC state it decreases. The secondary compression index, (Cα), in both the compression and recompression range decreases with increasing % lime. In the compression range the decreasing of (Cα)together with an increase of (Cc), results in a gradual reduction of (Cα)/Cc) as the amount of lime increases. It is worthwhile stating that the surcharging effort was found to be more effective in a further reduction of the secondary compression index of the natural dredged mud in comparison with lime treated dredged mud. In this study, empirical correlations between coefficient of consolidation, void ratio and corresponding effective stress was proposed and evaluated. It was found that for the type of the dredged mud used in this study, irrespective of the percentages of added lime, the coefficient of consolidation can be obtained by knowing the state of void ratio and effective stress at any instance during consolidation. Further analyses of the laboratory results by means of Plaxis (finite element model software) and analytical approach suggested that the pretreatment of the dredged slurry with hydrated lime prior to discharging into a containment pond during the construction of man-made islands, may contribute significantly towards the project cost saving strategy