Impact Compaction (Dynamic Compaction)


Application of method

Reduce foundation settlements

Densify waste deposits

Prevent soil liquefaction during earthquakes

Increase in-situ density of land reclamation fills



Suitability of method




Energy dissipation over depth

The impact energy (weight x drop height) dissipates with depth.

The effective treatment depth can be estimated using the following formula:

Depth [m] =
, with W = tamper weight in tons, H = drop height in meters.

Entering a weight of 20 tons and drop height of 20 m results in a treatment depth of 10 m. This shows that for commonly available cranes the impact compaction method works most economical to a depth of up to 10 m. For any greater depth cranes of over 150 tons operating weight would be needed.


The different compaction effect over depth between Vibro Compaction and Impact Compaction (also called Dynamic Compaction) is shown in the following sketch.




Specific energy requirement:

Clean sand and gravel20 – 25 mt/m3

Silty sand      25 – 50 mt/m3

Landfills      50 – 100 mt/m3


Cost efficiency in clean sands

The following figure shows that Impact Compaction is most cost efficient in the depth range from 3 m to 7 m. At less than 3 m to 4 m depth HEIC (High Energy Impact Compaction, also known as Impact Roller Compaction) is cheaper in achieving the same results, while at depth greater 6 m Vibro Compaction is often cheaper while achieving increasingly better results with depth.




Impact wave dissipation

The weight impact creates a low frequency wave that travels several hundred meters depending on surface soil conditions (a hard crust surface conducts waves further than a loose soil surface). The safe distance to existing structures is best established in trials. As a rule of thumb, a distance of 50 m is the minimum and 150 m should be safe.



Design steps

1.In cooperation with the architect, structural engineer, and the owner, define design objectives, such as max. admissible settlement, max. differential settlement, design earthquake (Mw and amax ), shear strength.

2.Perform a site investigation. Select the field sounding and lab testing program so that it later can answer all design and QA/QC questions.

3.Calculate with unimproved soil parameters (Es , phi’) to see what settlement (or stability against earthquake liquefaction, or slope stability) would result without treatment.

4.From 3. above derive a sensible definition of what soil parameters need improvement, how much, and in which zones of the project such improvement is needed. Keep in mind that small differences in specified improvement level can mean huge differences in cost.

5.For proper QC conditions in the Technical Specifications, translate the target soil mechanical parameters (Es , phi’) into required sounding resistance values (NSPT, qc) that the contractor can work with.

6.Set up the QA/QC plan to supervise the ongoing compaction works.

7.On large projects: Plan a test installation to allow the contractor to calibrate weight size and drop grid pattern for the local soil conditions.



Installation Process

The weight is dropped in a primary, secondary and often also tertiary grid. The primary grid (widest spacing) is used to achieve compaction at depth. It uses the largest weight and highest drop. The compaction effect at depth is larger as long as the soil surface is not yet densified. The uncompacted soil produces less dispersion of the impact wave. The secondary and tertiary grid are used to achieve compaction at medium and shallow depth. Sometimes a so called “ironing path” with a smaller weight and lower drop height concludes the process. This ironing path can be replaced by roller compaction.





Surface Impact Compaction (also called “Dynamic Compaction”)




Impact Compaction Equipment


Crane

Crawler crane (70 tons to 150 tons, depending on drop weight and height). The crane is designed for heavy duty load cycles. The winch must be capable of lifting the drop weight (so called tamper) with a single-line pull.


Weight (also called Tamper, Pounder)

Specially designed impact weight (typical range is 10 to to 30 to).


Auxilliary equipment

Dozer or wheel loader for filling / leveling of impact craters.








Photos courtesy of Ecosoil GmbH



Quality control of installation

Verify compaction point locations

Verify heave and settlement around impact craters

Measure weight penetration for consecutive drops

In cohesive soils, measure pore pressure

Control logs of number of drops and drop height



Acceptance Testing

Control settlements of compacted site area.

Soil sounding (SPT, CPT, PMT, DMT)

Load testing (Plate load, zone load)