Dynamic Compaction (DC) for ground improvement
Dynamic Compaction (DC) densifies loose granular soils and fills by repeatedly dropping a heavy weight from a crane on a planned grid. It can be considered where large, open sites need improved bearing performance and reduced settlement before construction.
Suitable ground conditions and applications
Can be considered where…
- Loose sands and gravels need densifying to depth.
- Granular or mixed fills, including old, uncontrolled fills, must be improved before building.
- Collapsible soils need treatment before loading or wetting.
- Large, open areas are to be treated: industrial plots, storage yards, roads and platforms.
Limitations
- Saturated fine-grained soils (clays and silts) respond poorly.
- A shallow groundwater table may require lowering or a raised working platform.
- Vibration and flying debris need clear space from structures, services and people.
- The site must be accessible to a heavy crawler crane on a stable working surface.
Suitability is confirmed from the site investigation and the performance requirement.
How it works
A crawler crane lifts a steel or concrete weight (the pounder) and drops it in free fall onto the ground at planned grid points. Each impact sends stress waves into the soil, pushing particles into a denser arrangement and closing voids. The surface settles into craters, which are backfilled and levelled between phases.
Energy is usually applied in two or more high-energy phases on offset grids, so the deeper layers are treated first and the ground between points is reached later. A final low-energy ironing pass compacts the loosened surface layer.
Design inputs
- Site investigation: soil profile, in-situ test results (CPT/SPT), fines content, groundwater level.
- Performance criteria: required bearing pressure, allowable total and differential settlement, target test values.
- Required depth of improvement.
- Energy parameters: pounder weight, drop height, drops per point, number of phases.
- Grid layout: point spacing and phase offsets.
- Constraints: distance to structures, services and boundaries; vibration limits.
Depth of improvement is commonly first estimated with the empirical relationship D ≈ n·√(W·H), where D is the depth of improvement (m), W the pounder weight (t), H the drop height (m), and n an empirical coefficient that depends on the soil. This is a planning estimate only. The actual depth is confirmed by trial and testing.
Trial and calibration
- Baseline in-situ tests (e.g. CPT) before treatment.
- Treatment with trial parameters, recording crater depth after each drop to build the drop-penetration response.
- Surface level survey to measure induced settlement and any heave.
- Post-treatment tests compared with the baseline and the acceptance criteria.
- Parameters adjusted and agreed with the consultant before production.
Execution controls
- Setting out of every print position against the approved grid.
- Number of drops per point and drop height per phase, recorded for each print.
- Crater depth measurement against the agreed stopping criteria.
- Waiting periods between phases where needed for pore-water pressure to dissipate.
- Backfill volume and level survey after each phase to track induced settlement.
- Daily production records available to the consultant.
Verification and acceptance
Design intent, production control and acceptance verification are kept separate, so it is always clear what was required, what was done and what was proven.
What is required
Performance criteria and target test values agreed with the consultant.
What was done
Point-by-point records of energy, drops, crater depth and levels.
What was proven
Post-treatment CPT/SPT, plate load tests or settlement monitoring against the criteria.
Deliverables
- Method statement and risk assessment
- Treatment grid and phasing drawings
- Trial area report with agreed production parameters
- Daily production records
- Level surveys and induced-settlement records
- Verification test reports
- As-built treatment records
Common questions
How deep can Dynamic Compaction improve the ground?
It depends on the weight, the drop height and the soil. The depth is estimated at design stage and then confirmed by a trial area and testing.
Which soils are suitable for Dynamic Compaction?
Loose sands and gravels, granular and mixed fills, and collapsible soils. Saturated clays and silts respond poorly.
How is the result verified?
By comparing in-situ tests such as CPT before and after treatment against the agreed criteria, with plate load tests or settlement monitoring where specified.
Discuss whether Dynamic Compaction (DC) fits your ground conditions.
Share the site location, the investigation report and the performance requirement.