Hobart's development along the Derwent estuary has pushed construction onto reclaimed land and deep alluvial deposits left by ancient glacial outwash. These areas, particularly around Sullivans Cove and the industrial zones at Macquarie Point, sit on loose sands and silty fills that lack natural density. We design vibrocompaction programs specifically for these conditions, combining deep probe data with ensayo CPT readings to map the loose zones before specifying probe spacing and energy levels. Our approach ensures the ground reaches the relative density required for raft foundations or shallow footings without over-treating areas that already meet target values.
A properly designed vibrocompaction grid in Hobart's sandy fills can double the soil's relative density within a single working week.
Methodology and scope
- Probe spacing (typically 2.5 to 4.0 metres in triangular grid)
- Retraction rate and vibrator frequency for the sand gradation
- Backfill material selection to match the native soil's friction angle
Local considerations
The vibratory probe itself can reach 30 metres in depth, but in Hobart the limiting factor is often the presence of old timber piles or buried debris from the waterfront's industrial past. Striking an obstruction at depth may require repositioning the probe point and re-evaluating the grid coverage. We also watch for lateral displacement of adjacent fill during compaction — if the ground heaves more than 50 mm near existing services, we adjust the probe sequence or reduce the vibrator's amplitude. Our team logs every probe's penetration curve in real time to catch anomalies early.
Applicable standards
AS 1726 – Geotechnical Site Investigations, AS 4678 – Earth Retaining Structures (lateral effects of compaction), FHWA HI-97-013 – Vibratory Probe Compaction Guidelines
Associated technical services
Deep Loose Sand Treatment
For sites where the loose sand layer exceeds 4 metres, typically in reclaimed estuarine zones. We design a deep probe pattern with backfill to achieve relative densities above 80 % at depths between 3 and 8 metres. The service includes pre- and post-treatment CPT verification.
Shallow Fill Compaction (0–4 m)
Aimed at urban infill sites where the loose layer is thinner but still requires improvement for shallow footings. Spacing is tighter (2.5 m triangular) and we often combine the compaction with controlled fill placement to minimise settlement under service loads.
Typical parameters
Frequently asked questions
What is the typical cost range for a vibrocompaction design in Hobart?
The cost for a vibrocompaction design package in Hobart generally falls between AU$2,510 and AU$7,590, depending on the site area, required treatment depth, and the number of verification tests. The final quote is adjusted after the initial CPT survey confirms the loose layer profile.
How long does a vibrocompaction program take on a Hobart site?
A typical treatment program for a 500 m² site in Hobart takes 3 to 5 working days, including probe installation, compaction, and post-treatment testing. Larger sites or sites with buried obstructions may require an additional 1 to 2 days for re-probing and grid adjustments.
Can vibrocompaction treat soils below the water table in Hobart?
Yes, vibrocompaction is effective below the water table because the vibratory probe displaces pore water and densifies the sand matrix directly. In Hobart's waterfront sites where the water table sits at 1.5 to 2.0 metres depth, we design the probe sequence to avoid liquefying the surrounding soil during treatment.
What verification tests do you recommend after vibrocompaction?
We recommend CPT soundings at a rate of one test per 200 m² of treated area, plus at least two plate load tests on representative zones. The CPT results confirm the cone resistance profile, while the plate tests verify the modulus of subgrade reaction for foundation design.