2 3 4A new northern access road located to the north of the existing Airport Drive on a currently undeveloped area was proposed by Brisbane Airport Corporation to link the new duplicated Gateway Motorway to the Brisbane international and domestic terminals. One of many design and construction challenges faced in this project was to build five drainage structures on deep soft ground at specified locations to enable excavation of a drainage channel underneath the pavement in future, to divert flood water across the alignment without affecting traffic or causing adverse impacts on the structure itself and adjacent embankments. Considering the presence of deep compressible clays, it was critical to employ appropriate ground improvement techniques to reduce in-service settlements along the embankment and in particular adjacent to rigid piled structures. In addition to the appropriate ground treatment, design of cost-effective retaining structures to confine earth behind the abutment piles, with a view to controlling ground displacements and improving long-term stability, also became a critical item to be resolved during the design phase. This paper discusses the design of the structure approaches, the retaining structures and the pile foundations associated with the construction of drainage structure A within the west area of the project.
In the paper by Yang et al. (2015), the focus is on drainage structure A, but readers should be aware that there are other more significant structures of interest on the Northern Access Road Project (NARP), presumably also delivered under the overall design and construct (D&C) contract. As a frequent user of the Northern Access Road, the discusser has observed at least two rounds of resurfacing at two other major bridge approaches due to significant settlements affecting riding quality and, presumably, user safety. In the discusser's experience, excessive post-construction settlement (PCS) can also put piled bridge foundations at potential risk due to the inevitable lateral foundation movements at abutments.The discusser has over 30 years of experience in the design of road embankments in the Brisbane River floodplain that encompasses the project site, including monitoring the longterm settlement performance of several embankments. It was therefore a genuine surprise that the paper by Yang et al. (2015) promoted the innovation/success achieved when, close by within the same project and compressible regional marine clay formation, multiple pavement surface corrections have been deployed. A serious drawback with the paper is the absence of long-term monitoring, which restricts the topic to design/predictive aspects only. This discussion therefore explores both the interpretation of settlement data and comparison with actual performance.The settlement performance at the key monitoring point S24 (control line MC10 CH11050) is worthy of closer scrutiny, noting that S23 and S25 share a similar form. Figure 9 from the original paper has been digitised and reproduced for S24 as Figure 10 and an Asaoka prediction (Asaoka, 1978) is shown in Figure 11 for the post-filling portion of the settlement record. The discusser has reviewed various soft soil projects for a variety of clients over a number of years and, invariably, the Asaoka method of settlement prediction has been a tool of choice for consultants (including some of the authors) in supporting their assessment of settlement progress.
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