AS 1726 sets the standard for geotechnical site investigation in Australia, and in Bunbury it is particularly relevant because of the city's extensive sandy Quaternary deposits and shallow water table found along the Leschenault Estuary and the coast. These loose granular soils are prone to settlement under dynamic loads and can liquefy during a seismic event. Vibrocompaction design is the most efficient ground improvement technique to densify such sands in situ, achieving relative densities above 70% and reducing void ratios without excavating or replacing the material. Before we design the compaction pattern, we run a site investigation that often includes an ensayo CPT to profile soil layering continuously, and a set of granulometria tests to confirm the fines content is below the 15-20% threshold required for effective vibrocompaction.
In Bunbury's sandy coastal soils, vibrocompaction can reduce liquefaction potential by converting loose fills into dense, uniform deposits with SPT blow counts above 20.
Methodology and scope
Local considerations
Our team deploys a Keller-type vibroflot unit, a cylindrical probe 300 mm in diameter and 2.5 m long, which penetrates the ground by its own weight and water jetting. In Bunbury, where the water table is high, the jetting water returns to the surface carrying fine particles; if not properly managed this can cause surface settlement of adjacent structures. We control this by monitoring flow rates and installing sump pumps that recirculate water within the site boundary. Another risk is the presence of the Tamala Limestone caprock, which can deflect the probe and leave untreated zones. We anticipate this by reviewing borehole logs and adjusting the compaction grid locally.
Applicable standards
AS 1726:2017 Geotechnical site investigations, AS 4678:2002 Earth-retaining structures, Australian Standard for Foundation Design (AS 2159:2009, relevant for ground improvement acceptance), FHWA Ground Improvement Manual (for vibrocompaction design guidance)
Associated technical services
Deep Vibrocompaction Design for Liquefaction Mitigation
Full design package including CPT-based liquefaction triggering analysis (Idriss & Boulanger 2008), probe pattern layout, and post-treatment verification testing. Suitable for residential subdivisions and light industrial developments in the South Bunbury and Withers areas.
Shallow Vibrocompaction for Settlement Control
Design for depths up to 6 m, focusing on reducing differential settlement under strip footings and slab-on-grade. Includes pre- and post-treatment density testing using the sand cone method and nuclear gauge. Common for commercial sites around the Bunbury CBD and along the Bussell Highway corridor.
Typical parameters
Frequently asked questions
How much does vibrocompaction design cost in Bunbury?
The total cost for vibrocompaction design, including site investigation, laboratory testing, and the compaction design report, typically ranges between AU$2,550 and AU$8,400 depending on the site area, depth of treatment, and number of verification tests required. A fixed price quote is provided after the initial site walkover.
What is the difference between vibrocompaction and dynamic compaction?
Vibrocompaction uses a vibrating probe inserted into the ground to rearrange sand particles into a denser state, working best in saturated clean sands with low fines content. Dynamic compaction uses a heavy weight dropped from a crane to densify the soil through impact energy. Vibrocompaction is preferred in Bunbury where the water table is high and adjacent structures limit the use of high-energy impact methods.
How deep can vibrocompaction treat Bunbury soils?
Standard vibroflot units reach depths between 6 and 12 m, depending on the power of the vibrator and the soil resistance. In Bunbury, where the loose sands extend to approximately 8-10 m before encountering the Tamala Limestone, treatment to 8 m is common. For deeper loose layers, a more powerful vibroflot or a combination with stone columns may be necessary.
What laboratory tests are needed before vibrocompaction design?
Key tests include sieve analysis to determine fines content and grain size distribution, Atterberg limits to assess plasticity of any fines, and a Proctor compaction reference for moisture-density relationships. We also run a CPT or SPT sounding to profile soil layers. All results are interpreted per AS 1726 to confirm suitability for vibrocompaction.