If I use rapid impact compaction what bearing pressure can I recommend?

Use of rapid impact compaction 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.

The RIC, imparts energy by dropping a 5 to 10 tonne weight from a relatively small height of 1.2 m at a blow rate of 40 to 90 times a minute. Depending on the ram weight, the maximum energy delivered per blow is 59 to 106 kNm. Although the energy per blow is small compared to the conventional DC, the rapid blow frequency amply compensates, resulting in a greater power that varies between 2.4 to 6.4 MNm/min. Thus, a much greater total energy input per unit area of a site can be achieved with RIC. Moreover, the energy transfer of the RIC is far more effective due to its foot which stays in contact with the ground during the impact sequence. 

Typical areas of application could include projects such as low-rise structures like housing and schools, embankments, roads and pavement areas. Having the Rapid Impact Compactor mounted on a tracked machine gives it the versatility to move about in narrow and limited height spaces, such as within existing warehouses. With regard to its mobility, the RIC is able to be transported as a single unit, with the impact foot removed and the front end lowered horizontally on a flat-bed trailer. The machine can be ready to work just a few minutes after off-loading. If road restrictions apply, the unit can be easily split into two loads with the excavator travelling separately from the hammer. Re-assembly is achieved in less than two hours. 

RIC utilises a 7 tonne hydraulic piling hammer repeatedly hitting a heavy 1-1.5m diameter steel foot. Recurring blows at each treatment position create imprints, which are subsequently filled with granular material. The pattern is then repeated at offset locations in order to provide treatment depths of around 3m. 

Preliminary trials are an important pre-requisite to any extensive RIC works. Furthermore, as the main RIC works are proceeding, ongoing monitoring and testing is necessary to ensure that the appropriate amount of energy is being applied to the soil profile and that performance requirements are being met. The compaction trial, in particular, is important for the evaluation of ground response. The optimal number of blows per pass is typically taken as the value beyond which continued blows produce negligible further penetration of the compaction foot.