Rapid Impact Compaction Technology FAQs

1. What is Rapid Impact Compaction (RIC)?

RIC is a process where loose subsurface soils are improved using an excavator-mounted 7.5 ton hydraulic hammer.  The hammer is rapidly raised and lowered onto a 5 ft diameter plate, which densifies the soil, in-place, without the need for undercut and replace.  The average compaction point is hit 40 per minute.

2. Can RIC be used for a site where the loose granular soil is underlain by clay?

One advantage of RIC is that the drop height and number of blows can be varied based on the soil conditions.  Through a test program, we will work with the Geotechnical Engineer of Record (GER) to determine the appropriate improvement criteria and RIC set-up for various areas of the site.  For a site with a mixed soil profile and varying thicknesses of sand and clay, the ability to accurately control the amount of energy delivered to the ground is critical as it allows one to improve the loose overlying loose soil without liquefying the fine grained soils below – providing more uniform compaction.

3. Which soils can be compacted with RIC?

Sands, gravels, silts, sandy clays, and debris fills have been successfully compacted with the RIC.

4. Will a shallow depth to the ground water table preclude the use of RIC?

Usually not.  A water table depth of 4 to 5 ft below the ground working surface is ideal when compacting clean sands and gravels.

5. What is a typical depth of improvement?

Depending on the existing soil type and condition – improvement to depths of 20 ft can be achieved.

6. What level of vibration magnitude should be expected when using RIC?

Vibration, measured in terms of peak particle velocity (PPV), has been observed to attenuate to below 2 inches per second (ips) at a distance of 30 ft from the RIC impact point.  A PPV of 2 ips or less should not be a cause for concern for most modern structures.

7. What is “rapid” about RIC?

The RIC compactor is mounted on a CAT345 excavator so that moving around the site is easy.  The compactor consists of a 7.5-ton weight falling approximately 36 inches onto an anvil in contact with the ground at a rate of approximately 45 blows per minute thereby compacting approximately 800 sf of area per hour.  Onboard diagnostics equipment allows the compaction effort to be stopped when optimum compaction has been achieved.

8. If I use RIC, what bearing pressure can I recommend?

Use of RIC will result in an increase in soil density, stiffness, and angle of internal friction as measured by an increase in SPT N-value, CPT tip resistance or other means of insitu test.  The recommended approach is to determine what level of improvement is desired and discuss that required improvement with your technical representative for feasibility.  For example, a 2-story commercial light industrial structure is to be constructed on a site underlain by up to 10 ft of existing sandy fill soils.  SPT N-values range between 4 and 8 blows per foot (bpf) in the fill.  The geotechnical engineer’s correlation between SPT N-Value and soil stiffness for footing settlement analyses indicates that an average N-value in the fill needs be 10 bpf.  The geotechnical engineer would perform settlement analyses using the foundation sizes and loading provided by the structural engineer to confirm that the footings will perform acceptably if the fills are improved to 10 bpf.  A review of the borings logs indicates that this level of improvement is achievable with RIC.  The geotechnical engineer would then complete his or her report with a recommendation that RIC be used to compact the fills in place and that an N-value of 10 bpf will be required.

9. What cost per square foot should I use to estimate the cost of RIC?

Providing the building area, project location, and geotechnical engineering report to your RIC technical representative will allow him or her to assess your project for feasibility and develop a budget cost for RIC.

10. Why should I use RIC versus deep dynamic compaction or other forms of ground improvement?

RIC is the right answer when:

  • Over-excavation and replacement is not feasible due to environmental or practical reasons
  • Safety is an issue (no weight falling from great heights)
  • Vibrations need to be managed (< 2 ips at 30 feet)
  • Specific levels of improvement are required
  • Compaction energy needs to be carefully controlled