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The repeated dropping of a heavy weight onto the ground surface is one of the simplest and most basic methods of ground improvement. The major use of the method in the UK has been to compact loose, partially saturated soil or fill; hence ground treatment by high-energy surface impacts is commonly referred to as dynamic compaction. The ground is compacted in a predetermined pattern by repeated impacts of a large mass (hereafter referred to as a weight). The technique is generally applied to natural granular soils and partially saturated fills, where the objective is to reduce the volume of voids between constituent particles, producing an increase in density and an overall improvement in engineering properties. The weight may be dropped in 'free fall' from a large crane or within guides. A variation of this technique is the rapid impact compactor, where a hydraulic piling hammer impacts an articulated foot that remains in contact with the ground surface. This report contains a technical specification for ground treatment using dynamic compaction by either the falling-weight or the rapid impact method. Notes for guidance and information explain and clarify the particular clauses of the specification. Appendix 1 outlines commonly adopted contractual arrangements under which dynamic compaction is carried out and suggests acceptance of levels of risk by the various parties appropriate to different types of contract. 59 pages.