AS 1726:2017
Geotechnical Site Investigations
AS 1726:2017 specifies the requirements for geotechnical site investigations for engineering purposes, covering the planning of investigation programmes, field investigation methods (boreholes, test pits, geophysical methods, in-situ testing), sampling, laboratory testing, and the preparation of geotechnical investigation reports. It defines the levels of investigation appropriate to project complexity and risk (preliminary, design, construction, post-construction), prescribes minimum requirements for soil classification (USCS), descriptions of soil and rock, sampling procedures (disturbed, undisturbed, U50 tube, SPT), and reporting standards for inclusion in design documentation. The 2017 edition replaced AS 1726-1993 and aligns the standard with current geotechnical practice, including updated geophysical method coverage, revised in-situ testing methodology (CPT, vane shear, pressuremeter), and explicit treatment of contamination assessment scope. AS 1726 is the controlling reference for geotechnical investigation in Australia and is referenced from AS 2870 (residential slabs and footings), AS 3798 (earthworks), AS 5100.3 (bridge foundations), and AS 4678 (retaining structures). It is the standard cited in geotechnical investigation reports relied upon by structural engineers for foundation design and existing-foundation assessment.
AS 1726 is the basis on which TRSC structural engineers rely on geotechnical investigation outputs from third-party geotechnical consultants. Three application points matter for existing-asset structural practice. First, the standard's level-of-investigation framework directly determines the engineering reliance that can be placed on the geotechnical input. A preliminary-level investigation (typical desktop assessment with limited borehole data) supports concept-level design but does not support continuing-life Form 15 certification of existing foundations; a design-level investigation (boreholes to refusal, in-situ testing, laboratory characterisation) supports detailed structural assessment. TRSC's existing-asset structural assessments of foundation-sensitive structures (heritage settlements, pile-supported high-rise, transfer-truss buildings, retaining structures) require AS 1726 design-level geotechnical input as a precondition for the structural Form 15 — preliminary-level data is insufficient to certify foundation adequacy. Second, AS 1726 sampling and laboratory testing provisions govern the input to soil-structure interaction analysis. For existing-asset assessment of buildings showing differential settlement (the Q1 Tower foundation review and several Brisbane CBD heritage settlement investigations are examples), the structural engineer's settlement analysis depends entirely on the AS 1726-compliant geotechnical input. The standard requires undisturbed sampling at structural foundation depth, laboratory consolidation and shear testing, and water-table characterisation; without these inputs, settlement-back-analysis is speculative. TRSC's structural settlement investigations are commissioned alongside an AS 1726 design-level geotechnical investigation as a coordinated package, with the geotechnical report explicitly addressing the soil-structure interaction questions the structural assessment requires. Third, AS 1726 reporting standards govern the interface between geotechnical and structural disciplines. The standard requires geotechnical investigation reports to include borehole logs, laboratory test results, geological cross-sections, and recommended characteristic soil parameters with explicit confidence levels — outputs that the structural engineer reads, validates, and relies upon. Where the geotechnical report does not meet AS 1726 reporting standards (which is more common than it should be — preliminary or budget-constrained reports frequently omit confidence-level treatment, characteristic-parameter derivation, or sufficient borehole density), the structural engineer cannot proceed with confidence-level Form 15 certification. TRSC's protocol is to specify AS 1726 design-level investigation explicitly in our geotechnical-coordination scope, and to read the resulting report against the standard's reporting checklist before proceeding with structural assessment. For seismic ground assessment per AS 1170.4, the AS 1726 sub-soil class determination is the bridge between geotechnical and structural — and a wrong sub-soil class (typically Class B assumed when Class C or D is the actual condition) under-estimates the design seismic action by a factor of 1.5 or more.
TRSC Form 15 RPEQ certifications for foundation-sensitive existing-asset structural adequacy reference AS 1726:2017 as the geotechnical-investigation basis. The Form 15 is conditional on the foundation system being supported by an AS 1726-compliant geotechnical investigation at the appropriate level (typically design-level for continuing-life or change-of-use certification, or preliminary-level only where the foundation is explicitly excluded from the structural certification scope). The Form 15 file retains the geotechnical report, the AS 1726 level-of-investigation classification, the characteristic soil parameters used in structural calculation, and the AS 1170.4 sub-soil class determination where seismic action is decision-controlling. Where the geotechnical investigation is inadequate against AS 1726 reporting standards, the Form 15 cannot be issued — TRSC's protocol is to commission supplementary investigation rather than proceed on inadequate data.
Engineering questions about AS 1726:2017
What level of AS 1726 investigation is required for an existing-asset structural assessment?
How does AS 1726 affect AS 1170.4 seismic assessment?
Does TRSC provide geotechnical investigation services?
How is heritage building settlement assessed?
- GovernmentAS 1726:2017 — Standards Australia