Remoulded Strength of High Plasticity Marine Silt Retrieved from Maintenance Dredging

摘要

Maintenance dredging of marine waterways is necessary to ensure the efficiency and safety of the vessels navigating into the shallow waters. The dislodged sediments, commonly known as dredged soils are generally disposed of as a geo-waste. From the geotechnical point of view, the soil, mainly fine-grained particles like silt and clay, is too poorly for engineering applications. However, with suitable handling and treatment, it can be potentially reused as any sound geo-materials. Two feasible yet economical options for improving the remoulded strength of the soft material are as follows: (1) to partially pre-drain the soil in a backfill to improve the plasticity, and (2) to lightly solidify the soil with small dosages of binder, like cement. The present study examines the remoulded strength of high plasticity silt retrieved from a dredged site. The fine-grained soil was originally found to be in liquefied form (water content is 1.75 times the soil’s liquid limit) with negligible shear strength or resistance. The material was semi-dried to form 60 mm thick soil beds at 0.90, 1.25 and 1.76 times the liquid limit. A separate set of soil beds (water content = 1.25 times the liquid limit) were also prepared to determine the efficacy of light solidification in the remoulded strength, i.e. with 1, 3 and 5 % cement addition in the soil. The fall cone and vane shear tests were next carried out for each soil bed at predetermined intervals up to a week. The strength gain over time was recorded as reduction in the cone penetration and corresponding increase in the shear strength measurements. Overall the remoulded soil was found to gain strength with time without further drainage, due to the thixotropic mechanism of particles rearrangement. Besides, light solidification resulted in markedly improved strength of the remoulded material with less time required. In short, strength lost due to remoulding of the soil could be recovered with either methods, though time would be the determining factor in the choice made. It is concluded that dredged marine silt is reusable with carefully designed backfill procedure to allow for strength recovery, i.e. adequate rest period after pre-drainage, as well as the accelerated approach via light solidification.