THE CONSOLIDATION BEHAVIOUR OF ALLUVIAL SOFT CLAY IN GLADSTONE, CENTRAL QUEENSLAND

Infrastructure, port facilities and road embankments built on existing soft clay sediments impose various geotechnical challenges. A trial embankment has been constructed at the Wiggins Island Coal Export Terminal (WICET), Gladstone, Queensland to investigate the consolidation behaviour of soft, quaternary alluvial sediments. Large settlement associated with prolonged consolidation time is of particular interest. To predict the magnitude and rate of settlement to allow the design to meet often tight construction schedules, consolidation parameters such as compression and recompression indices, pre-consolidation pressure, coefficients of primary and secondary consolidation need to be reliably estimated. In this paper the consolidation properties of soft clay at WICET were evaluated using three approaches, which are: i) laboratory testing (oedometer); ii) field testing (CPTU with pore pressure dissipation); and iii) back analysis of field monitoring results from an instrumented trial embankment. Derivation of the parameters from the above approaches are discussed, with the results compared and assessed. The benefits of back-analysing instrumentation data to improve the construction programme are also discussed. 1 PROJECT INTRODUCTION 1.1 PROJECT OVERVIEW The Wiggins Island Coal Export Terminal (WICET) project involves the construction of a new coal export terminal at Golding Point, to the west of the existing RG Tanna Coal Terminal in Gladstone Harbour. This important coal transport infrastructure will form part of the existing Port of Gladstone. Construction for Stage One of the terminal will include: • A rail receival facility designed to handle 7600 tonnes per hour • A 5.5 km long overland conveyor • Stockyard area for 1.9 million tonnes of coal • Materials handling and sampling systems feeding the 2km long jetty conveyor • Single berth with a travelling ship loader to fill ships at 8250 tonnes per hour • Channels and wharf to accept a range of vessels from 40,000 dwt to 220,000 dwt • 132kV substation, access roads, workshops, administration offices and amenities. The proximity of WICET to the mouth of the Calliope River and Port Curtis means that there is a need to build over recently deposited sediments which are often still very soft. Construction in and around these marine environments can raise significant challenges for engineers and contractors alike. As part of the project development over 2.3 million cubic metres of dredge material is required to be stored in two onshore containment facilities, termed Reclamation Area B and Reclamation C (shown in Figure 1). Both reclamation areas are formed using compacted earth filled bunds. These bunds vary in height from 4 m to 6 m, with a total length of external and internal bunds measuring 14 kilometres. Over 90% of these bunds are located on the soft, recently deposited sediments. The focus of this paper is on the trial bund constructed in Reclamation Area B. For the reclamation areas the behaviour of the soft underlying soils under embankment loading is an important consideration for many reasons. Issues associated with soft soils often arise during both the construction and service phases of the project and can have cost, schedule and safety implications if not addressed properly. Essential to the management of these issues is an adequate understanding of the behaviour of soft soil foundation under loading. To characterise the soft soil to allow the design of a competitive earthworks scheme, the in-situ shear strength and consolidation parameters, such as compression and recompression indices, drainage path length, thickness of compressible layer, pre-consolidation pressure and coefficient of consolidation need to be reliably assessed. However, competing against this aim is the significant natural variability of these properties, which ultimately lead to the geotechnical model erring on the side of caution or conservatism at preliminary stage, rather than best-estimate. THE CONSOLIDATION BEHAVIOUR OF ALLUVIAL SOFT CLAY IN GLADSTONE, CENTRAL QUEENSLAND KEMP et al. Australian Geomechanics Vol 48 No 1 March 2013 134 Figure 1: Project Overview The aim of this paper is to examine the natural variability of these soft soils and understand how well we are able to model the magnitude and rate of consolidation settlement through comparison with field monitoring data collected at an instrumented trial bund. The paper examines the soil properties calculated from field and laboratory testing and compares them against those determined from back-analysis methods. The back-analysis has been undertaken using two methods; Asaoka (1979) and Hyperbolic (Tan, 1995), in conjunction with a simple curve fitting approach based on 1-D consolidation theory. The results determined by each of these methods is compared and appraised. Finally, the project benefits associated with the implementation of the trial bund, refinement of the geotechnical model and subsequent field monitoring programme, are discussed. 2 GEOTECHNICAL MODEL 2.1 SITE GEOLOGY The coastal area to the north-west of Gladstone is dominated by soft, fine grained deposits associated with Quaternary tidal fluctuations. These Quaternary age deposits consist predominately of estuarine mud made up of clay, silt and some fine sands. The estuarine mud is typically underlain by older undifferentiated alluvial flood plain deposits and residual soils of the Wandilla formation. These recent Quaternary age deposits are of particular interest due to both their geographical dominance across the Project Site and also their engineering properties. A simplified geological map, showing the extent of the Quaternary Alluvium within the project site, is shown in Figure 2. Figure 2: Geological Map showing location of Quaternary Alluvium within Project Site Reclamation Area B