A track-mounted drilling rig set up near the Murray River banks in Albury-Wodonga is a familiar sight for our crews. We deploy continuous flight auger rigs with SPT hammer assemblies to profile the deep alluvial sequences that dominate the subsurface here. These deposits, often exceeding 30 metres in thickness, demand careful interpretation. The rig delivers undisturbed samples through a split-spoon sampler at 1.5-metre intervals, following AS 1726 procedures. Data from this fieldwork feeds directly into the tunnel face stability model, allowing us to calibrate support pressures before a single ring is erected. For low-cohesion sands near the river channel we complement the investigation with ensayo CPT to obtain continuous cone resistance profiles, while stiff clay horizons are cross-checked using calicatas exploratorias to verify stratigraphy boundaries visually.
Interbedded alluvial deposits in Albury-Wodonga create anisotropic stiffness conditions that control tunnel deformation response more than any single soil parameter.
Methodology and scope
Local considerations
A common oversight among contractors in Albury-Wodonga is assuming uniform alluvial behaviour across the entire tunnel alignment. The Murray River meanders have left behind abandoned channels and point-bar deposits that create abrupt lateral changes in soil stiffness and permeability. If the geotechnical analysis for soft soil tunnels does not capture these transitions, the tunnel boring machine can encounter a sudden loss of face support when crossing from a stiff clay into a loose sand pocket. The consequence is often a face collapse that propagates to the surface, damaging adjacent utilities and delaying the project by weeks. We mitigate this by running a continuous MASW survey along the alignment to map velocity contrasts, then targeting boreholes at the anomalies revealed by the geophysical profile.
Applicable standards
AS 1726:2017 Geotechnical Site Investigations, AS 4678:2002 Earth Retaining Structures, AS/NZS 1170.0:2002 Structural Design Actions – General Principles, FHWA-NHI-10-034 Technical Manual for Soft Ground Tunnel Design
Associated technical services
In-Situ Pore Pressure Monitoring
Installation of vibrating wire piezometers at tunnel horizon depth to track seasonal pore pressure cycles and calibrate consolidation parameters.
Advanced Triaxial Testing
Consolidated undrained and drained triaxial tests with local strain measurement to derive effective stress strength and stiffness parameters for numerical modelling.
Face Stability Analysis
Limit equilibrium and finite element analysis using the derived geotechnical parameters to determine the required support pressure for the tunnel face.
Settlement Trough Prediction
Empirical and numerical methods (Peck formula and 2D/3D FEM) to estimate surface and subsurface settlements caused by tunnel excavation in soft soils.
Typical parameters
Frequently asked questions
Why is geotechnical analysis for soft soil tunnels in Albury-Wodonga different from other regions?
The alluvial deposits here are highly heterogeneous due to the Murray River's meandering history. Loose sand pockets and soft clay layers alternate abruptly, creating anisotropic stiffness conditions that standard isotropic models fail to capture. Our analysis specifically targets these lateral variations using continuous geophysical profiling and targeted boreholes.
What is the typical cost range for a tunnel geotechnical investigation in Albury-Wodonga?
For a typical road or utility tunnel up to 500 metres in length, the cost ranges between AU$7,440 and AU$28,320 depending on borehole density, laboratory testing scope, and the complexity of the numerical modelling required.
How is the high groundwater table handled during the investigation?
We install standpipe piezometers and vibrating wire instruments at multiple depths to establish the hydrostatic profile. During drilling, we use polymer-based drilling fluids to prevent formation disturbance and maintain borehole stability below the water table.
What happens if the analysis reveals a liquefaction risk in the sand layers?
If the sand lenses fall below the cyclic resistance ratio threshold (based on NCEER 2001 procedures), we recommend ground improvement such as jet grouting or deep soil mixing prior to tunnelling, or alternatively a closed-face TBM with active face pressure control to manage the risk.