Soil densification by dynamic compaction (DC), also called "heavy tamping" is a well-known compaction method. The method was "rediscovered" by Menard, who transformed the crude tamping method into a rational compaction procedure. Soil is compacted by repeated, systematic application of high energy using a heavy weight (pounder). The imparted energy is transmitted from the ground surface to the deeper soil layers by propagating shear and compression waves types, which force the soil particles into a denser state. In order to assure effective transfer of the applied energy, a 1 to 2 m thick stiff layer usually covers the ground surface. Pounders can be square or circular in shape and made of steel or concrete. Their weights normally range from 5 to 25 tons and drop heights of up to 25  m have been used. Heavier weights and larger drop heights have been used for compaction of deep soil deposits, but are not very common.

Dynamic compaction is carried out in several passes. During each pass, the weight is dropped repeatedly in a predetermined grid pattern. The distance between the compaction points is normally decreased in the subsequent passes and compaction is carried out in-between the previously compacted points. The final pass, also called "ironing pass", usually performed with low compaction energy, is carried out with a reduced drop height. The objective is to densify the superficial soil layers without remoulding the already densified deeper layers. Mayne (1984) presented a detailed description of the dynamic compaction method. Choa (1996) has summarised the experience from dynamic compaction work a major project, the Changi airport in Singapore, fig. 9.

Although the dynamic compaction method appears to be very simple, it requires careful design of the compaction process. The densification effect is strongly influenced by the dynamic response characteristics of the soil to be compacted, but also by the underlying soil layers. Usually, extensive compaction trials are needed to optimise the compaction process with respect to the required energy for achieving specified densification criteria. A major limitation of dynamic compaction is the lack of monitoring and quality control during the production phase. However, for research purposes, the pounder can be equipped with sensors to monitor the applied energy and to record the dynamic response of the soil layer.

The maximum depth which can be achieved by dynamic compaction depends on several factors, such as the geotechnical properties of the soil layer to be compacted, the dynamic soil properties in and below the layer to be compacted (e.g. a soft clay layer below the layer to be densified can significantly reduce the compaction effect), the ground water level, the compaction grid, the number of compaction passes and the time interval between passes. As a general rule, the maximum depth d_max to which a soil deposit can be estimated from the following relationship

dmax = a

(18)

where H is the average drop height and M the mass of the pounder. The factor a should be determined for each site, but varies between 0,3 - 0,5. The typical depth of compaction for drop height of 15 m and a pounder mass of 15 tons is 7 - 8 m.

Figure 9. Dynamic compaction carried out in the trial area of Changi East Reclamation Project, Phase 1B (mass 25 tons, drop height 25 m)