Dynamic Compaction for Soft Ground Improvement: Applicability & Key Application Points

Dynamic Compaction, also known as Dynamic Consolidation, is a highly efficient and cost-effective ground improvement technique. It densifies deep soil layers using the enormous impact energy generated by dropping a heavy weight from a significant height, thereby significantly increasing bearing capacity, reducing settlement, and mitigating collapsibility and liquefaction risks.

1.Core Principle

A crane lifts an 8–30 tonne tamper to a height of 6–30 meters before allowing it to free-fall, generating an impact energy of 1000–8000 kN・m. The resulting stress waves propagate through the soil and bring about the following changes:

• Dynamic Compaction: Soil particles rearrange, reducing voids and increasing density.
• Pore Pressure Dissipation: Excess pore water and air are expelled to lower soil compressibility.
• Soil Structure Remodeling: A uniform and densified soil matrix is formed with greatly improved bearing capacity.

2.Primary Applicability (Soft Ground Suitable for Direct Dynamic Compaction)

（1）Plain fill, miscellaneous fill and construction waste fill foundations
For cut-and-fill soils, site spoil and construction debris fills which are loose and underconsolidated, dynamic compaction delivers outstanding densification results.

（2）Sandy soft ground
It works well on silty sand, fine sand, medium sand and loose sand layers to eliminate liquefaction potential, improve density and enhance bearing capacity.

（3）Gravel and cobble soft ground
These soils have large inter-particle voids. Dynamic compaction can effectively compact the soil with a large reinforcement depth and remarkable treatment effect.

（4）Collapsible loess and recently deposited loess
As a conventional treatment for typical collapsible soft foundations, dynamic compaction can eliminate loess collapsibility and reduce compressional settlement.

（5）Low-saturation silt and silty clay
Direct compaction is applicable to soft soil layers with low moisture content and stiff-plastic to plastic consistency.

（6）Hydraulic reclamation soft ground (sandy hydraulic fill)
High-energy dynamic compaction is a standard construction method for sandy hydraulic fills in port and tidal flat reclamation projects.

3.High-Saturation Soft Soils: Supplementary Treatment Required

Direct compaction on these typical soft soils will easily form “rubber soil”, resulting in soil disturbance rather than effective densification.
（1）Silt and silty mud
（2）High-saturation soft clay and flow-plastic silty clay
（3）Saturated tidal flat mud and swampy soil

Treatment Solutions (Combined Use with Dynamic Compaction)
（1）Pre-install Prefabricated Vertical Drains (PVDs) for vertical drainage
（2）Combine with dewatering and preloading
（3）Adopt dynamic replacement method: backfill rock or crushed stone into compaction pits to form replacement columns. This method is ideal for soft ground treatment of plant roads, storage yards and isolated foundations of general factory buildings.

4.Ground Unsuitable for Dynamic Compaction

Extra-thick silt layers and fluid mud
Organic soils and peat soils
Sites with extremely high groundwater levels, fully saturated soils and no drainage access
Areas adjacent to residential zones or precision workshops (Vibration caused by dynamic compaction may disturb residents and damage buildings)
Recommended alternatives: Vacuum preloading with PVDs, cement deep mixing piles, CFG piles and cast-in-place piles.

5.Key Technical Parameters

Tamper: 10–40 tonnes, diameter: 2–3 m
Drop height: 10–30 m. Higher impact energy leads to deeper reinforcement.
Compaction passes: Divided into primary compaction, secondary compaction and full compaction; 3–10 blows per compaction point.
Reinforcement depth: Generally 6–10 m; over 15 m for high-energy dynamic compaction.

6.Main Advantages

Remarkable treatment effect: Bearing capacity increases by 2–5 times, and compressibility decreases by 2–10 times.
Short construction period: Around 50% faster than pile foundation and soil replacement methods.
Cost-effective: Cuts investment by 50%–70% compared with pile foundations.
Eco-friendly: No additional building materials required and no construction waste generated.

7.Engineering Applications

Widely applied in subgrades of expressways and railways, airport runways, port yards, factory buildings, large residential communities, land reclamation, nuclear power plants and other infrastructure projects.

Summary

In conclusion, the application of dynamic compaction shall be determined according to actual site conditions. It can be directly adopted for loose fills and sandy soils. Supplementary measures are required for high-saturation soft ground. This method is not recommended for extra-thick fluid mud and peat soils, where alternative technologies shall be applied to guarantee foundation quality.

If you need dynamic compaction plans or on-site technical consultation, please feel free to contact us. We will provide customized solutions based on project conditions to ensure safe and efficient construction.