Sedimentation and consolidation behaviour of fly ash-based geopolymer stabilised dredged mud

Ports conduct maintenance and capital dredging campaigns to maintain channel depths, improve navigational safety of vessels, and to cater for larger ships with deeper draughts. Most of the soft dredged material derived from these dredging campaigns is beneficially used as a fill material for land reclamation purposes. Port authorities undertake land reclamation works to address land scarcity and environmental constraints that are associated with dredged material placement alternatives. Land reclaimed with soft dredged mud has geotechnical challenges of slow self-weight consolidation, high compressibility and low bearing capacity. To overcome these geotechnical challenges and to alleviate risk of structures settlement, dredged material stabilisation techniques such as chemical admixtures, electrokinetic, stone columns, prefabricated vertical drains (PVDs), and surcharging are implemented. For these existing dredged material stabilisation methods to be applicable, the land reclamation fill material must have settled, consolidated and gained sufficient strength and stiffness to be traversed by ground improvement plant and workforce. By then, the reclaimed (the man-made) ground becomes similar to a naturally formed soft ground that civil and geotechnical engineers have no control on its soft soil formation processes. On dredging and land reclamation with soft dredged mud slurry project site, dredge cutters and drag heads rip and disintegrate marine sediment turning it into high water content dredged mud slurry. Then, the dredged mud slurry is pumped into land reclamation containment ponds to settle and self-weight consolidate to form soft land reclamation fill material. The dredged mud slurry takes several hours from the moment it is placed into containment ponds to the commencement of the land reclamation fill material formation. This time slot is sufficient for an early geotechnical intervention to stabilise the dredged mud while it is still in its slurry stage, prior to the formation of the soft land reclamation fill material. Stabilising the soft dredged mud slurry provides an opportunity to manipulate its sedimentation behaviour that controls microstructure, consolidation and compressibility characteristics of the resulting land reclamation fill material. This research study has investigated feasibility of stabilising 400% water content dredged mud slurry that is derived from Port of Townsville, Queensland Australia, using fly ash-based geopolymer binder at 6%, 12% and 18% by weight. The fly ash-based geopolymer binder is chosen for its tolerance to high water content nature of the dredged mud slurry, binding attributes, cost effectiveness, and environmental benefits. The study examined the influence of the fly ash-based geopolymer stabilisation on the sedimentation and consolidation behaviours, mineralogy and microstructure of the fly ash-based geopolymer stabilised dredged mud. Settling column tests were conducted to investigate the sedimentation behaviour of untreated and fly ash-based geopolymer stabilised dredged mud slurries. Standard one-dimensional consolidation (Oedometer) tests were used to evaluate the compressibility and consolidation characteristics of the untreated and the fly ash-based geopolymer stabilised dredged mud sediments. X-ray diffractometer (XRD) and scanning electron microscopy (SEM) with energy dispersive spectrometer (EDS) techniques were deployed to analyse the mineralogy and microstructure of the untreated and the fly ash-based geopolymer stabilised dredged mud. The study found it is feasible to stabilise high water content dredged mud slurry with fly ash-based geopolymer binder. Fly ash-based geopolymer gel coating dredged mud particles in the slurry was found to be the main stabilisation mechanism. It is noted the geopolymer gel coating dredged mud particles in the slurry has exacerbated flocculation of the stabilised dredged mud slurry, extended its flocculation duration, reduced settling time and shorten overall sedimentation duration. The SEM with EDS analysis showed the fly ash-based geopolymer stabilisation has altered the microstructure of stabilised dredged mud, changed its particles arrangement and reduced its desiccation shrinkage cracks. Subsequently, the fly ash-based geopolymer stabilisation has improved the compressibility and consolidation properties of the stabilised dredged mud by reducing its coefficient of volume compressibility (mᵥ) and increasing its coefficient of consolidation (cᵥ) and permeability coefficient (k). However, the XRD analysis found no correlation between the fly ash-based geopolymer stabilisation and the mineralogy of the fly ash-based geopolymer stabilised dredged mud

Geotechnical site investigations for dredging works - Port of Townsville

In order to accommodate larger vessels with greater draughts, the Port of Townsville has conducted a major capital dredging campaign in 1993 to deepen its access channels and inner harbour from -10.7 m to -12.0 m Lowest Astronomical Tide. To determine the subsurface conditions of the dredging project site, the port has conducted offshore geotechnical site investigations. The factual information obtained on soil stratigraphy, classifications, and geotechnical properties has guided dredgeability assessment, cost estimates and dredging operations planning. Subsequent to provisions of guidance to design and tendering stages, the geotechnical site information reduced dredging project risk levels and contributed to a successful project execution without major delays or cost overruns for unforeseen ground conditions. This paper describes the geotechnical site characterisation for dredging works that were undertaken by the Port of Townsville, emphasising on site geological setting, scope of the investigations, field works, laboratory testing, and the obtained results

Consolidation behavior of fly ash-based geopolymer-stabilized dredged mud

Ports use soft dredged material for land reclamation to address land scarcity and environmental constraints. Land reclaimed with soft dredged material has the properties of high compressibility and slow consolidation that need to be improved. This laboratory study investigated the effect of a fly ash-based geopolymer binder on the consolidation behavior of soft dredged mud derived from the Port of Townsville, Queensland, Australia. For geopolymer synthesization, an alkali activator solution to aluminosilicate source material mix designs that is used in the geopolymer concrete industry was adopted. The dredged mud sediments were reconstituted at 400% water content dredged mud slurries of nontreated and geopolymer stabilized at 6, 12, and 18% by weight. A series of standard one-dimensional consolidation tests was conducted to study the consolidation and compressibility characteristics of the dredged mud sediments. Taylor’s square root of time curve-fitting method was used to estimate the coefficient of consolidation. The study found that the fly ash-based geopolymer stabilization improved the compressibility, permeability, and consolidation characteristics of the dredged mud sediments. The findings indicate the feasibility of stabilizing dredged mud while still in its slurry form

Influence of fly ash-based geopolymer binder on the sedimentation behaviour of dredged mud

This laboratory study investigated the sedimentation behavior of a fly ash–based geopolymer-stabilized dredged-mud slurry extracted from the Port of Townsville, Queensland, Australia. Settling-column tests were conducted to study the sedimentation behavior of 400%-water-content dredged mud of untreated and geopolymer-stabilized slurries at 6, 12, and 18% content by weight. Dredged-mud slurry interface height movements with elapsed time were recorded, and their settling patterns were observed. Mineralogical and microstructural characteristics of dried, untreated, and geopolymer-stabilized dredged-mud sediments were determined using X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) analysis. The study found that a fly ash–based geopolymer gel coating the dredged-mud particles in the slurry led to a flocculated settling behavior, and the geopolymer stabilization reduced the overall dredged-mud slurry sedimentation duration. XRD and SEM/EDS analysis showed that the geopolymer stabilization altered the microstructure of stabilized dredged-mud sediment and reduced its desiccation shrinkage cracks

Settlement analysis of eastern reclamation area: Port of Townsville

The Port of Townsville's successive maintenance and capital dredging campaigns for the last two decades have resulted in the deposition of over two million cubic metres of soft dredged material into the port's eastern reclamation area. The dredged mud slurry is hydraulically placed into containment ponds and left to settle and self-weight consolidate, thereby forming 50 hectares of reclaimed land. In addition to challenges of low bearing capacity and high compressibility, land reclaimed with compressible dredged material would take decades to complete the consolidation process by natural means. Ground improvement techniques are usually applied to enhance strength and compressibility characteristics, to expedite the lengthy consolidation duration, and allow early land utilization. To ascertain geotechnical properties of the dredged fill material and subsurface soil of its eastern reclamation area, the Port of Townsville in collaboration with its geotechnical consultants, has conducted a series of field investigations and laboratory testing. Using soil parameters obtained from geotechnical investigations, the Port of Townsville has carried out ground settlement analysis on its eastern reclamation area under a range of applied loadings to evaluate the anticipated ground settlement. The estimated settlement values were utilized to refine the established consolidation lead times guidelines, ground improvement methods selection, and to appreciate likely performance of the treated ground. This paper discusses the Port of Townsville’s eastern reclamation area site description, geology, geotechnical investigations, site categorization, assumptions made to allow a rational ground settlement analysis, settlement assessment method used, obtained results and observations

Maintenance dredging at Port of Townsville

The Port of Townsville conducts regular annual maintenance dredging to maintain depths of its harbor basin and approach channels for the navigational safety of the vessels against the natural accumulation of marine sediments. In addition to the regular maintenance dredging, the port undertakes emergency dredging in cases where large quantities of sediments are mobilized and deposited in port waters by cyclone or major flood events. The maintenance dredging material derived from the port may be disposed at sea or on land in accordance with relevant state and commonwealth regulations. For the land disposal, the dredged mud slurry is hydraulically placed into containment ponds and left to undergo sedimentation and self-weight consolidation to form fill material for land reclamation. This paper provides an overview of the maintenance dredging at the Port of Townsville and emphasis on maintenance dredging requirements, sediment quality, bathymetry, dredging methods used, and dredged material disposal options

Capital dredging at Port of Townsville

In order to improve navigational safety and operational efficiency of vessels and to cater for larger ships with deeper draughts, the Port of Townsville conducts capital dredging. The port's capital dredging campaigns over the last 40 years has enabled deepening and widening of the existing inner harbor basin and approach channels, beside development of outer harbor. To determine the technical parameters of quantity, quality and types of materials to be dredged, the Port of Townsville undertakes hydrographic survey; offshore geophysical and geotechnical investigations that define dredging projects. While necessary to support the economic growth of the region, major port development works such as capital dredging have the potential significantly impact on the natural environment. To ensure the safety of the surrounding environment, the Port of Townsville has implemented integrated proactive approaches that consider environmental matters from capital dredging project planning and design stages. In accordance with relevant state and commonwealth regulations, the capital dredging material derived from the Port of Townsville may be placed at a designated ocean site or brought ashore for land reclamation. This paper provides an overview of the capital dredging at the Port of Townsville, with emphasis on geological setting, geotechnical investigations, capital dredging requirements, bathymetry, dredging methods/plant and dredged material placement options