AS 3798:2007

AS 3798 is decision-controlling on TRSC engagements involving foundation-on-fill — typically heritage adaptive-reuse projects on filled urban sites (Spring Hill, Fortitude Valley, parts of West End and Newstead in Brisbane, where 19th-century filling on tidal swamp and gully sites is widespread), and post-construction settlement assessment of buildings constructed on fill that did not meet AS 3798 acceptance criteria at placement. Three application points matter for existing-asset practice. First, the standard's fill-classification framework determines the engineering basis for relying on as-installed fill as foundation support. Controlled fill (Level 1 or 2) placed under documented Class A or B supervision can be relied upon at AS 3798 design parameters; uncontrolled fill cannot be relied upon without site-specific investigation and load-testing. Heritage urban sites in Brisbane have extensive uncontrolled fill from 19th and early 20th century landfilling, frequently with no records of placement, compaction or material composition. For continuing-life or change-of-use Form 15 certification of buildings on uncontrolled fill, the assessment requires AS 1726 design-level investigation through the fill profile, in-situ load testing where decision-controlling, and explicit AS 3798 acknowledgement that the as-installed fill cannot be assumed to meet controlled-fill acceptance criteria. Second, AS 3798 compaction acceptance criteria differ for different uses — 95 percent standard Proctor for general structural fill, 98 percent modified Proctor for heavy-duty pavement and high-rise footing platforms, with corresponding minimum density-test frequencies. For existing structures on fill, the assessment must determine whether the as-installed fill meets the relevant acceptance criteria, typically through reconstructive review of placement records (where they exist) supplemented by current density testing through targeted boreholes. Third, AS 3798 inspection-level requirements (Class A, B, C) determine the basis on which the structural engineer can rely on the geotechnical fill report. For existing fill platforms supporting heavy structural loading (transfer-truss buildings, high-rise pile caps on improved fill), Class A inspection is typically the only basis that supports continuing-life Form 15 certification — and pre-2007 fill platforms placed under earlier inspection frameworks may not have the documentation required. TRSC's protocol on heritage urban adaptive-reuse projects in known fill zones is to commission AS 1726 design-level investigation through the fill, AS 3798 retrospective fill-classification assessment, and where decision-controlling, in-situ plate load testing or pile load testing to establish the as-installed bearing capacity. The Q1 Tower foundation review and several Brisbane CBD heritage settlement investigations have used this combined framework. Stormwater and subsoil-drainage provisions in AS 3798 are also commonly under-applied — fill platforms with inadequate drainage routinely show progressive settlement decades after placement as fill saturates and consolidates.