The geology shifts dramatically between South Dunedin and the hill suburbs. On the flat, reclaimed harbour sediments and compressible alluvium extend to depth, while the flanks of Signal Hill expose weathered schist and colluvium. For a tunnel alignment passing through either terrain, or transitioning between both, the ground behaviour is rarely uniform. We run targeted in-situ permeability tests early in the programme to define steady-state seepage into the excavation, then pair that with triaxial stress-path testing to assess how the soft matrix will deform under unloading. Understanding the contrast between the stiff residual soils of the peninsula and the estuarine clays of the Taieri Plain is what drives the investigation design.
In Dunedin’s harbour silts, strength loss during excavation can be rapid once the confining pressure is removed. Early pore-pressure measurement is non-negotiable.
Methodology applied in Dunedin
Local geotechnical conditions in Dunedin
Much of South Dunedin’s development sits on reclaimed land that was marsh and tidal inlet before the late 19th century. Early drainage schemes and subsequent fill placement created a layered profile of uncompacted material over natural soft sediments, and that legacy still dictates underground construction risk today. Tunnelling through these deposits without a solid ground model invites face collapse, uncontrolled settlement at surface, and long-term buoyancy issues in the lining. The most common trigger we see is water: even minor leaks can soften the matrix, reduce arching capacity, and accelerate consolidation settlements under adjacent structures. A phased investigation with instrumented boreholes and settlement arrays, tied to staged excavation monitoring, gives the early warning needed to adjust support pressure and grouting before problems become visible at street level.
Our services
The investigation programme is structured to move from preliminary screening to detailed parameter derivation, matching the project phase and the ground complexity.
Soft Ground Investigation & Laboratory Testing
Rotary and wireline coring through the Caversham Formation and harbour sediments, combined with index testing, oedometer consolidation, and triaxial strength tests to build a stress-dependent material model for the tunnel alignment.
Tunnel Face Stability & Deformation Analysis
2D and 3D finite element modelling of sequential excavation, assessing face extrusion, surface settlement troughs, and the influence distance on nearby piled foundations and buried utilities.
Pre-Support & Ground Improvement Design
Evaluation of pipe umbrella systems, face bolting, and compensation grouting to control relaxation in the running ground typical of the South Dunedin reclaimed zone.
Common questions
What site investigation density is recommended for a soft ground tunnel in Dunedin?
NZGS guidelines recommend investigation points every 30 to 50 metres along the alignment in variable ground, with closer spacing across known paleochannels or the harbour margin. We typically supplement boreholes with CPT soundings to get a continuous strength profile through sensitive silts that can be disturbed by sampling.
How do you determine whether a tunnel can be advanced without a TBM?
The decision rests on face stability calculations using the undrained strength and the stability number N. For the soft alluvium common in the flat, N values above 5 without support indicate high risk. We run drained and undrained parametric analyses to define the maximum unsupported span and the required pre-support intensity.
What is the typical cost range for a geotechnical investigation for a tunnel in soft soil?
Depending on the alignment length and the number of boreholes required, a compliant investigation with laboratory testing and reporting generally falls between NZ$8,050 and NZ$24,670. Scope is driven by access constraints and the depth of the tunnel crown.