Apartments, Brompton — Multi-Phase Brownfield Investigation

The Project

Trilogy was commissioned to deliver a geotechnical interpretive investigation at a brownfield redevelopment site in inner-suburban Adelaide, to facilitate detailed design of a 7-storey concrete and masonry apartment building. The 5.8 ha former industrial parcel presented a combination of geotechnical challenges uncommon in a single project — complex fill, soft alluvial soils, extreme durability conditions, and the precision demands of a multi-storey structure.

Core tray from the apartment development geotechnical investigation showing the red-brown oxidised clay and silt profile encountered at depth. The blocky, desiccated texture reflects the drying of recovered core under summer field conditions.

The Comacchio GEO305 compact drill rig operating in a constrained suburban environment. The compact track-mounted rig was selected specifically for access in the urban setting.

Trilogy engineer overseeing drilling operations. Nine boreholes were completed in Phase 1 and Phase 3 (addendum), with CPT soundings supplementing the borehole program in Phase 2.

A Three-Phase Investigation

Phase 1 — Boreholes & SPT. Seven boreholes to 15 m BGL using a Geoprobe 7822DT, with SPT at 1.5 m intervals, plus comprehensive NATA laboratory testing including Atterberg limits, PSD, aggressivity, and UU triaxial testing.

Phase 2 — CPT. When SPT revealed a weak, laterally variable alluvial profile, Trilogy commissioned seven CPT soundings — providing the continuous, high-resolution strength and stiffness data that SPT alone could not deliver in soft layered soils. CPT identified a localised weaker zone that materially influenced both the ground model and the FEM analysis.

Phase 3 — Addendum Deep Drilling. Two additional boreholes using rotary and diamond coring to 24.95 m BGL, confirming basal gravel thickness and characterising the underlying Hindmarsh Clay sequence — findings that materially refined pile capacity parameters.

Core tray from deep addendum drilling showing the variable alluvial sequence — fill and reworked material in the upper columns transitioning to stiffer alluvial clays and gravels at depth. The heterogeneity visible here was a primary driver for the multi-phase investigation approach.

Split spoon SPT sample from borehole TRI-NTG-BH001 at 3 m BGL, showing the brown silty clay of the upper alluvial sequence. SPT N-values in this unit were low to moderate, consistent with the soft to medium stiff consistency encountered.

What We Found

• Variable uncontrolled fill (0.60–3.50 m), including a capping layer and geosynthetic separator above mixed granular/anthropogenic fill
• Natural alluvial silty sands dominant to 7–9 m BGL — loose to medium dense in the upper profile, stiffening at depth
• Dense to very dense basal gravel — generating CPT refusal and SPT practical refusal — the primary founding stratum
• Hindmarsh Clay from ~16–17 m BGL: stiff to very stiff (Cu 270–410 kPa from UU triaxial)
• Very Severe aggressivity in upper fill and silty sand — driving concrete and steel durability design requirements
• Class P (Problem Site) under AS 2870:2011 due to variable uncontrolled fill

Overview of the investigation setup on the brownfield site, with the urban context of the inner-suburban redevelopment precinct visible in the background.

3D geotechnical layer model showing the surcharge loading configuration from the proposed building footprint. The model captures the cap/surcharge geometry used as input to the settlement analysis.

PLAXIS Finite Element Modelling (FEM) total displacement output (scaled 200×). Peak vertical displacement of ~0.19mm at the centre of the loaded area, confirming acceptable settlement within structural tolerances for the proposed development.

Outcomes

• Nine-unit subsurface model constructed from integrated borehole, CPT and laboratory data
• CPT elevated as primary dataset — providing the continuous resolution needed for FEM settlement modelling in soft layered alluvium
• Preliminary pile parameters derived using CPT-inferred stress ratios for each granular unit — a project-specific approach rather than generic correlations
• Settlement screening using CPT-based methods identified differential settlement risk in the northern building area, informing FEM model geometry
• Addendum drilling confirmed basal gravel continuity and Hindmarsh Clay undrained shear strength — materially refining pile end bearing values
• Durability design requirements explicitly characterised by unit — Very Severe conditions in upper profile requiring specific concrete specification and steel protection