Rapid impact compaction (ric) - HENGDA

RIC typically employs a 7 tonne weight dropped repeatedly through 1.2 m onto a 1.5 m diameter steel articulated compaction foot. Whilst the energy per blow is not large (typically 8.4t.m), the equipment permits a large number of impacts to be applied at a rate of about 40 blows per minute. The operator monitors and can record the number of impacts, the total energy input applied, the foot penetration per blow and the cumulative penetration. When a specified parameter is reached, for example, foot penetration or set per blow, the equipment is moved to the next treatment/tamping point. 

As the foot remains in contact with the ground, the energy is applied more efficiently in compacting the ground than in conventional drop weight dynamic compaction where the weight may fall on an irregular surface in such a way that much of the energy is dissipated in deforming the irregularities of the ground. Both field trials and laboratory simulations of RIC have shown that the manner in which the ground responds to treatment is a “top-down” process, compared to DC which is a “bottom-up” process. The first few blows in rapid impact compaction create a dense plug of soil immediately beneath the compaction foot. Further blows advance this plug deeper, which compacts soil in a deeper layer. This process progresses until little further penetration of the compaction foot can be achieved with increasing blows. The effect of the compaction process is confined largely to the ground vertically below the compaction point and treatment is therefore carried out on a closely spaced square or triangular pattern or sequenced on an arc about the centre of rotation of the base machine for the RIC equipment. Additional passes are typically offset from the primary pass to ensure effective treatment coverage. The carrier vehicle is typically a hydraulic excavator. 

The selection of the compaction method (DC or RIC) and plant type for a particular project, will depend on ground and groundwater conditions, and requirements for design and execution. Each system has merits and limitations. It is important that these are understood and considered in the design and application of DC/RIC on a particular site and in the context of the prevailing ground conditions. Indeed, it may be necessary for more than one technique to be employed at a particular site to gain maximum benefit.

Typically, the RIC method is used for the treatment of essentially granular fills in order to improve their geotechnical properties (stiffness and bearing capacity) and to reduce settlement. RIC design firstly involves geotechnical characterisation of the soils to be treated, with emphasis placed on quantifying in-situ relative density and grading characteristics. Groundwater level is an important factor for consideration of suitability of the RIC method as shallow groundwater level can act as a hydraulic barrier reducing effective energy transfer to the fill materials. However, it is the “compaction trial” (discussed under testing and quality control), which provides the designer with the necessary information to permit refinement of the design. With ground improvement techniques involving surface impact such as RIC there cannot be direct control of treatment depth, as would be the case with vibro stone columns. A critical element of RIC design therefore is the depth to which a particular treatment is effective.

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.

Rapid Impact Compaction with the Rapid Impact Compactor (RIC) is an innovative method in the field of near surface and deep compaction techniques. The RIC is a dynamic compaction device based on piling hammer technology. Dynamic energy is imparted by a falling weight dropping from a controlled height onto a patented foot. The foot of the device remains in contact with the ground; thus, the energy is transferred to the ground safely and efficiently. 

The RIC, imparts energy by dropping a 5 to 9 tonne weight from a relatively small height of 1.2 m at a blow rate of 30 to 80 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.