This master’s thesis investigated the effects of biopolymers on the geomechanical properties of dredged sediment generated during the dredging and maintenance of navigation channels at the Port of Koper. The sediment has a high natural water content and contains dissolved salts, which limit its potential applications. The effects of adding 1 wt.% guar gum, locust bean gum, xanthan gum, chitosan and calcium alginate were investigated. The biopolymers were mixed into the sediment at its natural water content in powder form. Undrained shear strength was monitored using a laboratory vane apparatus during the first 72 h after mixture preparation. During air-drying of the specimens, crack development and surface shrinkage were monitored by sequential photography and binary image analysis and quantified using the crack area and surface shrinkage ratio (DPR). The water-retention properties of the sediment were determined by measuring matric suction with a HYPROP device and total suction with a WP4-T device. The shrinkage limits of the untreated sediment and the sediment treated with calcium alginate were also determined. The addition of all the investigated biopolymers increased the initial undrained shear strength of the sediment. The highest value, 6.23 kPa, was measured for the sediment treated with guar gum; however, its effect was short-lived. By contrast, the undrained shear strength of the chitosan-treated sediment remained approximately constant throughout the observation period. Calcium alginate did not affect the shrinkage limit of the sediment and was the only investigated additive for which cracks developed on the specimen surface. The other four biopolymers prevented crack formation; however, the surface area of the specimens decreased because they separated from the mould, creating a gap between the mould and the specimen edge. In the untreated sediment and the mixtures containing xanthan gum or calcium alginate, shrinkage began at a water content approximately corresponding to field capacity, whereas the transition to slower shrinkage occurred at a water content approximately corresponding to the wilting point. The addition of biopolymers had no clearly discernible effect on the water-retention curves of the sediment. Owing to the presence of salts, the estimated contribution of osmotic suction was considered when interpreting the total suction measurements. The effects of the mass of dissolved salts and drying temperature on the determined water content were also examined. Despite the observed improvement, the undrained shear strengths achieved were too low for the sediment to be used in earthworks without further treatment. The results indicate that the effects of the biopolymers were more pronounced in modifying shrinkage behaviour and limiting crack formation than in improving the strength and water-retention properties of the sediment.
|
