Dynamic compaction

How bridges get built · Ground improvement · Dynamic compaction

Also called: heavy tamping, dynamic consolidation

Dynamic compaction — concept diagram — Concept diagram (TheBridgeEng)

How it is done

Confirm the material (granular fill, rubble, collapsible soil) and the groundwater depth, test the ground before treatment, and run a trial area to fix energy, grid, drops per point and number of passes.
Place a granular working platform where the surface is weak, and keep the water table at least about 2 m below the working level, dewatering if needed.
Make the first, high-energy pass on a wide grid of about 5–10 m with 5–15 drops per point, measuring crater depth after each drop and stopping when further drops add little.
Backfill the craters with granular material and allow excess pore pressure to dissipate, which takes days to weeks, before the next pass.
Make the second and any later passes on intermediate grid points to treat the zone between the first prints.
Finish with an ironing pass of low drops on overlapping prints to compact the top 1–2 m.
Verify with CPT, SPT or pressuremeter tests and plate load tests against the targets; monitor vibration at the boundary and the induced settlement.

Key numbers

Depth of improvement D ≈ n √(W·H), with W in tonnes and H in metres; n is about 0.3–0.8, often 0.5.
Tamper typically 10–25 t dropped 10–25 m; heavier tampers and higher drops need special rigs.
Effective depth typically 6–12 m.
Grid 5–10 m on the first pass; 5–15 drops per point; two or three passes plus ironing.
Induced settlementtypically about 3–10 % of the treated thickness.
Keep 30–50 m from vibration-sensitive structures and services unless monitoring shows otherwise.

Where it fits

Loose sand and gravel fill, rubble, reclaimed land, old industrial sites under approaches.
Depths to about 10 m.

Where it does not

Saturated clays and silts (no densification, only pore pressure).
Within 30-50 m of structures or services sensitive to vibration.

Choosing it

Choose dynamic compaction over vibro-compaction for coarse or unsaturated fills, rubble and collapsible soils to about 10 m on open sites: it is cheaper and treats debris a vibroflot cannot penetrate.
Prefer vibro-compaction for deeper, saturated clean sands, and wherever vibration and flying debris near neighbours are unacceptable.
Prefer dynamic replacement or stone columns in silts and soft clays, where tamping alone only raises pore pressure.
Prefer rapid impact compaction for shallow treatment of a few metres on confined sites.

Plant, pace and money

PlantLarge crawler crane (100-200 t) with a free-fall drum and a tamper.

Productivity2,000-5,000 m2 per crane per week.

CostUSD 8-20 per m2.

Risks and controls

What goes wrong

Vibration damage, flying debris, crane fatigue.

Quality assurance

Pre- and post-CPT, plate load tests, vibration monitoring at the site boundary.

Origins

Louis Ménard developed dynamic compaction in France in 1968 and first applied it in 1969 at a residential resort in Mandelieu-la-Napoule; Ménard extended it to clayey soils as dynamic replacement in 1975, and FHWA's guidance (GEC 1) codified US practice.

Examples

Airport and port fills; occasionally bridge approaches on rubble fill.

Case studies

Residential resort, Mandelieu-la-NapouleFrance · 1969First application of dynamic compaction, by Ménard.

Nice Côte d'Azur AirportFrance · 1978Dynamic compaction treated reclaimed land for the airport extension; Ménard reports it as its deepest dynamic compaction application, about 27 m.