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Ground Stability, Foundations and Substructures 133
Photograph 3.13 Dynamic compaction (http://www.roger-bullivant.co.uk).
Ground conditions suitable for dynamic compaction include natural granular soils,
made-up ground and land-filled refuse sites. The technique can also be used as part of a
more significant earthworks operation, where the ground is built up in layers, compacted
and consolidated. Where fill is built up in layers, the fill may take the form of unmodified
material (as previously excavated) or soils which are modified or stabilised using additives,
such as quicklime and pulverised fuel ash (PFA), cement.
To achieve the desired effect, several passes may be required. Careful monitoring and
testing is required; grid levels may need to be taken before and after each pass. Trial drops
should be taken to determine the optimum treatment regime, monitor the imprint and
depths, and measure pore water pressures, as necessary. To determine the allowable bearing
capacity, accurate measurements are taken of the penetration achieved by application of
particular energy (known load from a known height). Analysis of the levels can be used to
calculate the amount of void closure and the degree of densification. Using dynamic compaction,
bearing capacities of 50–150 kN/m 2 can be achieved. Greater bearing capacities
may be achievable, depending on the ground condition.
Different shaped tamper heads are available with a variety of weights, depending on the
degree of consolidation and compaction required (Figure 3.55). Figure 3.56 shows a typical
pattern of work. Three passes are used in this example to achieve the required compaction
and consolidation. Initial tamping is undertaken using a single pointed tamper; the tamper
is up to 2.5 m long with a mass of 10 tonnes. The ground is tamped on a grid with the