Dynamic Compaction Design for Sydney Sites

The alluvial flats of Homebush and the deep fill at Pyrmont respond very differently to dynamic compaction. At Homebush, high groundwater and soft estuarine clays require a high-energy pattern with a 20-tonne tamper dropped from 20 m to densify the underlying sands. On the Pyrmont foreshore, where former industrial waste and sandstone rubble dominate, a lower energy pass with a 15-tonne weight and a 10 m drop height suffices. In both cases, before we design the compaction grid, we run calicatas exploratorias to log the stratigraphy and locate obstructions. Our approach in Sydney always tailors the impact energy, drop spacing, and number of passes to the actual ground conditions encountered.

Illustrative image of Dynamic compaction design in Sydney

A 20-tonne tamper dropped from 20 m on a 5 m grid typically achieves 80–90% relative density in granular fill to 8 m depth.

Our service areas

Scope of work

Sydney's development over the last three decades has pushed construction onto marginal land — former brick pits at St Marys, dredged sand at Botany Bay, and demolition fill at Barangaroo. These sites all share a common problem: loose, heterogeneous fill that settles unevenly under load. Dynamic compaction design for Sydney sites must account for the variable nature of these fills. The technique works by transmitting high-energy surface impacts into the ground, forcing particles into a denser arrangement. We typically use a 15 to 20-tonne weight dropped from 10 to 25 m on a 5 to 8 m grid, followed by a low-energy ironing pass. Verification involves placa de carga tests and CPT soundings to confirm modulus improvement. Where the fill is more than 8 m deep, we often combine the design with precarga to accelerate consolidation of underlying soft layers.

Technical reference — Sydney

Area-specific notes

The coastal humidity and high water table across much of Sydney create a real risk: pore pressure build-up during compaction can reduce effective stress and limit densification. In the Botany Sands aquifer, for example, we monitor pore pressure with piezometers and pause compaction when it exceeds 50% of the overburden stress. Another risk is vibration damage to adjacent structures. For sites near heritage buildings in The Rocks, we limit peak particle velocity to 15 mm/s and use a single-drop sequence rather than a continuous pattern. Good dynamic compaction design in Sydney always includes a pre-construction vibration assessment and a contingency plan for wet weather delays.

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Standards used

AS 1726:2017 Geotechnical site investigations, AS 4678:2002 Earth retaining structures, AS 1289 Standard test method for soil compaction determination at shallow depths

Technical parameters

Parameter	Typical value
Drop Weight	15–20 t
Drop Height	10–25 m
Grid Spacing	5–8 m (primary), 2–4 m (ironing)
Energy per Drop	150–500 t·m
Number of Passes	2–3 (primary + ironing)
Depth of Improvement	6–10 m (granular fill)

Quick answers

What is the typical cost of dynamic compaction design in Sydney?

The cost usually ranges from AU$1,880 to AU$6,250 depending on site size, fill depth, and the number of verification tests required. A small residential infill lot may be at the lower end, while a large industrial site with deep fill and multiple passes can reach the upper end.

How deep can dynamic compaction improve the ground in Sydney?

In granular fills common around Sydney — such as sandstone rubble or sand — improvement typically reaches 6 to 10 m depth with a 20-tonne weight dropped from 20 m. For deeper fills, we recommend combining the design with preloading or vibro stone columns.

Can dynamic compaction be used near existing buildings in Sydney?

Yes, but with restrictions. Peak particle velocity must be kept below 15 mm/s for heritage structures and below 25 mm/s for modern buildings. We use a single-drop sequence and monitor vibrations continuously. Pre-construction condition surveys of adjacent properties are standard.

What verification tests are done after dynamic compaction in Sydney?

We typically run plate load tests (AS 1289) to measure bearing modulus, CPT soundings for continuous profiling, and SPT borings to check relative density. If the fill contains large cobbles, we may also use seismic cone tests to evaluate shear-wave velocity improvement.

Location and service area

We serve projects across Sydney and its metropolitan area.

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