How Geotechnical Baseline Reports and Geophysical Methods Resulted in Greater Ground Condition Certainty - a Story of Strong Collaboration Between

Abstract

In 2019, Yarra Valley Water’s (YVW) Community Sewerage Program began constructing 49kms of sewer pipes across several communities in the Dandenong Ranges including Kallista, The Patch, Sherbrooke and Sassafras. The sewers are being constructed almost entirely by horizontal directional drilling methods and will see sewerage connection points created for 1,284 properties to replace ageing septic systems. During the first stage of the project in 2019, a higher-than-expected number of rhyodacite boulders, also known as ‘floaters’, were encountered. These floaters delayed project construction, resulted in latent condition claims and exacerbated the disruptions felt by the community. The lessons from this project became the catalyst to improve the way ground conditions would be characterised and communicated for future Community Sewerage projects. This paper explores how YVW and their engineering designer, Jacobs, incorporated the lessons gained from the first stage of the project to begin using targeted geophysical studies to enable the preparation of geotechnical baseline reports (GBRs) that provide greater clarity on expected conditions and support clearer risk allocation for YVW and its construction contractors. The GBRs are the amalgamation of ground penetrating radar surveys, geomorphology assessments and conventional geotechnical investigations and express the number of rhyodacite boulders that can be expected for every 100m of sewer across the project. Melbourne Pipelines is one of the trenchless construction contractors currently constructing the project and this paper discusses the benefit of having greater certainty around challenging ground conditions and how this has enabled more accurate tender pricing, appropriate equipment selection and, ultimately, a smoother construction experience for our delivery partners, residents, and the broader community.