Report No FR0432



MARCH 1994



The benefits arising from this work relate to generating information on the performance of compaction plant, the potential effects on buried apparatus and the measurement of the compaction. The actual benefits in the field will depend upon the subsequent use to which the information is put. Potential benefits are listed below.

Excessive number of passes may be avoided hence reducing over compaction of the backfill and improving the standard of reinstatement achieved.

The data generated may be included in a future revision of the HAUC Specification to better reflect the actual performance of compaction equipment.

The work may lead to the development of a certification scheme for assessing the performance of compaction plant.

The work has shown it is possible to relate the Clegg Values to the dry density of a granular material.


  1. To assess the performance of typical small compaction equipment when used to compact granular sub-base Type 1 (GSB 1).
  2. To determine whether this approach to performance assessment could be developed into a formal assessment procedure for compaction equipment.
  3. To provide information on the potential for damage to buried pipes during compaction.
  4. To indicate whether there is a relationship between results from the Clegg Impact Tester and the density of the compacted granular material as measured by a nuclear density gauge.
  5. To provide information on the performance of compaction plant in confined spaces.


  1. There is a trend for minimum width trenches to be cut to reduce both environmental disruption and reinstatement costs. Vibrotampers and plate compactors are both used in these trench environments during reinstatement to compact the backfill material. There is little information regarding their performance.
  2. Plant generally used for compaction has been found to be poor for compaction around boxes and in confined spaces. Specialist equipment has been developed for use in small openings but its performance has not been assessed.
  3. There is concern that the enhanced compaction requirements of the HAUC Specification for reinstatement may result in damage to installed apparatus.
  4. The Clegg Impact Tester is widely used to assess the compaction achieved in the field but there is little information available relating the values generated and the compaction density of the bulk of the material.


  1. This document summarises the performance of the majority of compaction equipment on the market at present. Each piece of equipment has been categorised by assessing the static weight per unit area and the plate design. Their efficiency to compact adeq uately in the correct environment is reported.
  2. During compaction the potential for damage caused to buried apparatus is monitored and reported.
  3. Calibration charts have been developed for both vibrotampers and vibratory plates.


The ability of compaction equipment to adequately compact varies from one piece of equipment to another and details of the variability may be found in Section 3.3.

Vibrotampers compact to a final standard similar to vibratory plates.

The vibrotampers are more proficient at compacting granular material with a reduced number of passes. This causes a gradual breaking up of the granular material on the surface and weakening of the overall structure when compacted to the recommendations within the HAUC Specification.

In general, the stresses transmitted to the pipe by the plate compactors were less than those transmitted by the vibrotampers. Both types of plant can be considered as being adequate to compact the GSB 1 without damaging the pipe provided that the pipes are well supported and covered by finefill material.

Pneumatic pole tampers and small vibrotampers are adequate at compacting a GSB 1 material in a confined space.

The results indicate that in principle it is possible to relate the Clegg Impact Value to the compacted dry density. Further data would be required to confirm the relationship for different materials and various layer thickness.


During compaction close attention should be paid to the variation of the appearance of the surface layer. When the surface of the material begins to appear smooth, this is an indication that the optimum compaction has been achieved. Dusting on the surface is an indication that over compaction of the material is occurring and the material is beginning to breakdown.

The section of the HAUC Specification that relates the number of passes to material and equipment type should be reviewed.

There appears to be wide variation between compaction equipment as to the number of passes required to achieve optimum compaction and the rates at which the optimum compaction can be achieved. This suggests the need for equipment able to compact any given material to its optimum density under site conditions. This approach would move away from the rigid approach laid down by the HAUC Specification(1) and could be developed as an alternative procedure to obtain the correct compaction for a given material type.

Where possible a small vibrotamper should be used to compact around boxes or in other areas of restricted access. Where man entry is not possible, a pneumatic pole tamper should be used.

It must be stressed that engineering judgement should be exercised on site when using the Clegg Impact Tester as surface irregularities greatly affect the readings obtained.

Copies of the report are available from FWR, price 35.00, less 20% to FWR Members.