Coal Tar Pughole Settlement Assessment

The Project

Trilogy was commissioned to undertake a geotechnical settlement assessment at a brownfield redevelopment site in inner-suburban Adelaide. The specific scope was narrow but technically demanding: assess the settlement risk to proposed road pavements and a gravity-fed stormwater system, both of which crossed directly over mapped pughole extents containing coal tar-saturated fill up to approximately 10 m deep.

Challenge

Gravity stormwater infrastructure cannot tolerate the same settlement as a road. Both alignments crossed a 10-metre deep former industrial-era pughole whose fill was partially saturated with free-phase coal tar — a material that does not consolidate in the conventional sense, ruling out the most cost-effective mitigation option before analysis had even begun.

Investigation

Three boreholes were drilled to 25 m BGL using rotary, rock roller, and diamond coring — penetrating the full pughole sequence and into the underlying Hindmarsh Clay. Four CPT soundings across the pughole extents provided continuous high-resolution stiffness profiling. Environmental borehole records from prior Trilogy site investigations were integrated into the ground model, extending coverage of pughole depth and lateral extent without additional fieldwork.

The subsurface model was constructed in Leapfrog, then imported into PLAXIS 3D via the Central hosting platform — representing road pavement, stormwater trench geometry, and backfill sequence in three dimensions so that trench-to-pughole interaction effects could be captured explicitly.

PLAXIS 3D FEM settlement model showing stress distribution around tank and wet well

PLAXIS 3D finite element model output showing the settlement trough distribution around the stormwater infrastructure and adjacent structures. Red zones indicate peak deformation concentrations — consistently located at the confluence of trench alignments and road intersections within the pughole extents.

PLAXIS 3D finite element model showing layered soil stratigraphy and applied loads for pughole settlement analysis

PLAXIS 3D model geometry showing the layered stratigraphy — natural alluvial soils, pughole fill, capping layers, and pavement — with uniform distributed loads applied to the road surface. The model was constructed from the Leapfrog 3D geological sections imported via the Central platform, allowing interaction between the trench excavation and surrounding compressible fill to be captured in three dimensions.

Five FEM Sensitivity Cases

Case 1 (base): representative parameters for granular pughole fill (E₅₀,ref = 5 MPa) — max settlement 32 mm, differential 31 mm
Case 2 (0.50 reduction factor): max 45 mm, differential 44 mm
Case 3 (0.33 reduction factor): max 60 mm, differential 59 mm
Case 4 (0.33 reduction, 40-year consolidation): long-term additional settlement 0.05 mm — confirming coal tar fill does not consolidate
Case 5 (0.33 reduction, 40 kPa construction load, 10-year duration): max 131 mm, differential 121 mm — the governing case

Dark coal tar-saturated core recovered from pughole fill — push tube drilling rig

Core recovered from within the pughole fill showing the characteristic dark, oily coal tar-saturated material. This material — behaving elastically under load with minimal permanent consolidation — was the defining analytical challenge of the settlement assessment.

Why Surcharging Was Ruled Out

Evidence from prior investigations at the site confirmed that coal tar-impacted soils had not consolidated under pseudo-preloading conditions — rebounding elastically rather than undergoing permanent volumetric change. Sensitivity Case 4 confirmed this: long-term consolidation over 40 years added just 0.05 mm. Surcharging — typically the most cost-effective mitigation for soft or compressible fill — was definitively closed off as an option.

Five-Option MCA

With settlement quantified and surcharging eliminated, Trilogy assessed five mitigation options through a Multi-Criteria Assessment covering cost, effectiveness, timeframe, installation complexity, maintenance burden, and residual risk:

Pile and beam system (~$0.5–1.0M): highest cost, highest certainty — bypasses pughole entirely
Controlled modulus columns (CMCs): preferred option — cost-effective rigid inclusion ground improvement, compatible with the constrained site conditions
Deep dynamic compaction: limited effectiveness in coal tar-saturated materials
Surcharge preloading: ruled out on technical grounds (coal tar behaviour)
Accept and monitor: not appropriate for gravity stormwater infrastructure

Geotechnical drill rig on site — investigation

Geotechnical drilling underway at the investigation site. The drill rig is positioned on the cleared brownfield site, with the consultant's field vehicle and equipment set-up visible behind — typical of the urban brownfield investigation program.

Outcomes

Settlement quantified to ±30 mm precision across five scenarios — converting qualitative risk into actionable design numbers
Coal tar behaviour analytically confirmed — surcharging definitively ruled out, saving the client pursuing an ineffective mitigation path
CMCs identified as the preferred solution through a structured MCA — a cost-effective rigid inclusion method compatible with the site constraints
Governing case (131 mm under construction loading) identified — providing the structural engineer with the design envelope for trench and junction details
Concentrated settlement troughs at trench intersections identified as the governing serviceability mechanism — not generalised pavement settlement

Coal Tar Pughole Settlement & 3D FEM Analysis