Abstract:
This paper presents a case study on the design of a replacement rail bridge along the Southwest Mainline at Dodson Road crossing, Bunbury, Western Australia, a single narrow-gauge track operated by TransWA. The site posed geotechnical challenges, including underlying acid sulphate soils and a ~3 m thick, very dense sand layer at the east abutment, requiring a tailored foundation solution. A comprehensive geotechnical investigation, incorporating borehole drilling and CPTu testing, guided the evaluation of suitable piling options under the site's geotechnical and environmental constraints. Initially, steel tubular piles were proposed to mitigate acid sulphate soil handling issues associated with bored piling. Considering the commercial and technical aspects, a comparative analysis was undertaken between 762 mm steel tubular piles and 400 mm square concrete displacement piles, assessing axial and lateral load capacities as well as pile drivability. The dense sand layer presented significant risks for concrete pile refusal or damage, prompting a trial piling program to evaluate performance in situ. The trial pile results confirmed that concrete displacement piles could effectively penetrate the dense sand without compromising structural integrity, offering a viable and constructible solution. The project ultimately adopted concrete displacement piles, supported by performance monitoring and load testing to ensure compliance with design criteria. A total of fourteen piles has been installed at each abutment and satisfying the design load and set criteria. This case study highlights the importance of integrating detailed geotechnical analysis with field verification in environmentally sensitive and technically complex conditions. It demonstrates how early-stage investigation, comparative design evaluation, and trial implementation can inform optimal pile selection, mitigate drivability risks, and support sustainable infrastructure outcomes. The findings contribute to best practices in geotechnical engineering for transport projects, particularly where environmental considerations and challenging soil conditions exist.