Crust Formation in Disposed Dredgings

Abstract

In this paper, the findings of an experimental investigation aimed at determination of parameters required to model the behavior of sea bottom sediments dredged from the sea and pumped via a pipeline to a confined land disposal site, as part of the Golden Horn Rehabilitation Project (Istanbul, Turkey) are presented. The dredged slurry samples from Golden Horn are allowed to dry under variable conditions in the laboratory, and their shrinkage and dessication behaviors are experimentally investigated. The undrained shear strengths of the dried samples are also investigated with different experimental techniques. Slurry samples are also allowed to dry under the influence of an artificially-generated wind action to study the effects of accelareted drying. In addition, the surface of some samples were covered with phosphogibs and tuff powder prior to drying under the laboratory conditions. Laboratory vane, falling cone, and loading tests were used to determine the shear strength of dried samples. Experimental results have shown that a rather homogeneous but not very stiff crust is formed in samples dried under laboratory conditions, whereas the relatively thinner crust formed under wind action in the laboratory is observed to be much stiffer but more heteregeneous with deep tension cracks. The phosphogibs and tuff powder covers inhibited the evaporation and surface crust formation, and led to very low values of undrained shear strength. Among the three different types of tests used to determine the shear strength, vane and falling cone tests are observed to give comparable results, whereas the values obtained indirectly from the loading tests turned out to be much lower and variable over a wide range. From the free shrinkage tests performed on dredged samples from Golden Horn, the shrinkage void ratio has an average value of 1.33, and it is not greatly affected by preloading. It is observed that both slurry and preloaded samples have undergone more or less isotropic shrinkage. The α-function which is used to characterize the ratio of free vertical and lateral strains and its variation with the void ratio is experimentally determined. The experimental values for the α-function are compared with the values calculated from a relationship based on the assumption that free shrinkage strains are isotropic and small strain theory is valid. For slurry samples a good agreement is observed between experimental and calculated values, but for overconsolidated samples the results are far from being in agreement with each other.