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Compaction of materials (NG10.2)

Unbound granular and cohesive materials

For granular or cohesive materials, a vibrating roller may be unsuitable in small openings because of the restricted manoeuvrability of the large heavy rollers required to achieve adequate levels of compaction with an acceptable number of passes.

Compaction of backfill is a requirement to mitigate the risk of long term settlement. However, definitive measurements for density are often relatively complicated and not necessarily suitable for regular monitoring of works in reinstatements. These test methods include nuclear density testing (BS 1924-2).

There are standard tests available that may be used to measure performance (which can be correlated with adequate compaction/density). The main test methods are:

  1. Lightweight deflectometer to BS 1924-2 for soils, unbound mixtures and HBMs.
  2. California Bearing Ratio to BS 1377-4 for soils.

There are other tests methods available not covered by British Standards such as:

  1. Ground penetrating radar
  2. Surface wave propagation
  3. Penetrometer
  4. Clegg impact soil tester

In case of dispute about compacting backfill, the order of preference is definitive measurement and then standard tests (British Standards). The other test methods are limited in suitability dependent on their established correlation for the materials being tested.

Bituminous mixtures

  1. With some combinations of compaction plant and certain types of bituminous mixtures, the following may result if compaction is continued as the material approaches its maximum density:
    1. Migration of fines or binder to the surface.
    2. Development of shear surfaces and/or crushing of aggregates.
  2. Provided that the material has been laid and compacted within the appropriate temperature range, fewer passes will be required when any signs of distress become apparent.
  3. Asphalt maximum density values, used in the calculation of air voids content, are specific to particular asphalt mixtures incorporating constituents from specific sources. Any variation in mix proportions or constituents requires the maximum density to be re-established.
  4. Although consistent asphalt supply may allow an established maximum density for a particular mixture and source to be used for some time in routine situations, the definitive method to be used in the event of dispute will require the maximum density to be determined for the mixture actually used. The maximum density may be determined from bulk samples, if available, or from material obtained from additional core samples.
  5. When taking cores near surface apparatus, S10.2.10 requires a minimum clearance of 100 mm to avoid damaging the apparatus or structure it is bedded on. However, it is possible that some surface apparatus may have wider than normal flanges and there may be instances where a greater clearance is required to avoid damage. If doubt exists, liaison with the owner of the apparatus should be undertaken in advance.
  6. Where more than two cores are taken, one can be used for determination of maximum density value (BS EN 12697-5) and the remaining cores must be used for bulk density determination using the wax sealing method. This is the preferred method. Provision for the alternative sealing methods is included for when only a single core is available. This is done on the basis that removal of wax post-determination of air voids, and pre-determination of bulk density can be problematic.

    As general guidance on air voids testing, it is important that care is taken to prevent damage to the specimen whilst sealing it. The seal must cover the specimen entirely, including voids that technically form part of its volume. It is equally important to prevent the sealing material from penetrating any internal voids or creating voids between the seal and the specimen or in seal folds. Trimming the bottom of a core sample may be done to avoid testing voided material compacted against unbound material (see BS EN 12697-5 paragraph 7.3). However, this can only be done if the thickness of the layer (after trimming) is still within the layer thickness specification.

    Testing asphalt surfacing which has been opened to traffic will not always give results representative of the as-built air voids condition. In particular, the process of coring may damage the sample. This requires specific consideration in terms of test procedure and results interpretation.

Modular surfacing materials

Depending on the size and type of paving module to be laid, and the extent of the area to be surfaced etc., the use of additional mechanical compaction may become necessary.