Increasing ground condition certainty by using combined geophysical methods for geotechnical baseline reports

Abstract

Yarra Valley Water’s (YVW) Community Sewerage Program delivers new sewerage infrastructure to properties in Melbourne’s existing communities. Areas like the Dandenong Ranges pose construction challenges due to steep terrain and the presence of rhyodacite boulders, or ‘floaters’. Trenchless technology is an effective solution for sewer installation in such conditions, but contractors often face variable and unpredictable ground conditions, leading to project delays, latent condition cost increases, and extended community disruption. To manage these risks, YVW developed and implemented the use of Geotechnical Baseline Reports (GBRs) for inclusion into its construction contract. These reports attempt to establish ground condition expectations, reducing uncertainty. YVW’s approach was previously presented at NDDU2023 in a paper titled “How geotechnical baseline reports and geophysical methods resulted in greater ground condition certainty...”(Anderson et al., 2023) This paper builds on the previous methodology by integrating different geophysical techniques. YVW, in collaboration with its engineering designer Stantec (under the Ripple joint venture) and their partners Veris and Tetra Tech Coffey, used Ground Penetrating Radar (GPR) as the primary technique. GPR results were calibrated with Electrical Resistivity Imaging (ERI) and traditional borehole sampling, producing classified heat maps that highlighted subsurface conditions. Melbourne Pipelines, YVW’s delivery partner contractor, utilised the advanced GBR in preparation for trenchless construction and recorded site conditions to verify the accuracy of the new GBR approach. Results verify the predicted number of rhyodacite boulders per 100m of sewer generally matched actual site conditions. This improved certainty around ground conditions compared to the previous GBRs and enabled the contractor to deliver the project with negligible delays, minimal cost impacts, and with no lengthy community disruptions. This validated the effectiveness of enhanced GBR methodology in the challenging geological environments. 1 No-Dig Down Under 2025 Increasing ground condition certainty using by combined geophysical methods for geotechnical baseline reports Dillon Van Heer, Xavier Wren and Leigh Sinclair