Stabilization-solidification of high water content dredged sediment : Strength, compressibility and durability evaluations

Abstract: Dredging activities at ports and harbors are inevitable for safe navigation of ships and vessels. The outcomes of dredging are huge volumes of dredged materials, which can range from very fine and contaminated sediments to sands and gravels. The coarse sands and gravels can be directly used in civil engineering applications. The fine dredged sediments (DS) are usually associated with high water content, low shear strength, high compressibility and presence of contaminants. However, these unfavorable properties do not exclude the suitability of fined dredged sediments for use in geotechnical applications. Stabilization-solidification technology provides a comprehensive treatment method for improving strength, reducing the compressibility and mobilizing the contaminants to be less mobile.These properties make the stabilized fine dredged material (SDM) suitable for use in civil engineering applications (e.g. road embankment or structural backfill in land reclamation).However, stabilization-solidification is not a magic wand by which every geotechnical property is improved for better. In cold region climates, repetitive freeze–thaw cycles have detrimental effects to the strength and hydraulic properties of the SDM. Consequently, the applications and long term performance of the SDM under repetitive freeze-thaw cycles are still uncertain.Successful stabilization–solidification of the DS and the performance of the SDM depend on stabilization methods and materials. Process stabilization-solidification (PSS) is convenient technology for amending high water content DS with binders. The use of composite binders for stabilization–solidification of the DS is increasing due to increased artificial pozzolanas that can be used as supplementary cementitious materials (SCM). Primary binders such as cement can be supplemented with SCM (e.g. fly ash and ground granulated blast furnace slag). Cement hydration is a complex process with a complex series of unknown chemical reactions. The hydration of cement incorporating SCM is more complicated due to the co–existence of cement hydration and the pozzolanic reactions of the SCM. The fabric of dredged sediments formed under different physicochemical environments affects the reactivity of binders. The physic ochemical interactions between binders and the DS that influences the strength, compressibility and durability of high water content stabilized dredged sediment are examined and presented in this thesis. The findings of this study show that the use of fly ash (FA) and ground granulated blast furnace slag (GGBS) delays strength development of composite binder (CB)-treated DS. Irrespective of the amounts of CB, the improved strength depends on the amount of cement in the blend. The unconfined compressive strength increases with increasing the cement quantity Three phases of hydration mechanisms determine the compressibility behaviour of the SDM during curing. These are induction phase (IP), nucleation and crystallization phase (NCP), andhardening phase (HP). The IP occurs immediately after mixing. A protective layer is formed on the particle surface of binders, which prevent further penetration of water and then increases resistance to deformations. The evaluated tangent modulus increases to maximum value followed by abrupt drop to lower values at effective vertical stress, which is equals topreconsolidation stress. NCP follows when the protective layer changes to a more permeable membrane, which permits inward flow of water molecules, and outward migration of calcium ion and silicate ions. The tangent modulus of the SDM in NCP is small and increases linearly with effective vertical stress. The SDM in NCP is characterized by loss in apparent preconsolidation stress and tangent modulus. HP occurs as a result of increased thickness and stiffness of the protective layer. The compressibility of the SDM in HP is reduced significantly due to increased apparent preconsolidation pressure and tangent modulus. It is concluded that the maximum tangent modulus of untreated DS determines the maximum deformation of the SDM in all phases of hydration processes.Healing of the damaged SDM due to repetitive freeze-thaw action depend on the type of binder. The inclusion of SCM on one hand increases the healing of the SDM with reduced strength. This occurs during thaw consolidation. On the other hand, inclusion of SCM causes increased HC of the SDM. Considering healing potential on the damaged SDM with reduced strength, increased hydraulic conductivity causes increased rate of dissipation of excess pore pressure, reducedundrained conditions, and improved strength (enhanced outcome). In order to maintain its strength and hydraulic conductivity, the SDM requires protection from severe damage of repetitive freeze–thaw cycles. It can be beneficial to place the SDM below frost depth or use protective cover of geosythetic clay liners (GCL)