8 Specification of earthworks fill materials

8.1 General

A specification should adequately describe the design requirements, be easily understood by the parties to the contract, be practicable and capable of both enforcement and measurement, and not be unnecessarily costly or time consuming in its application. It should be capable of being monitored by an effective form of quality assurance procedure.

NOTE 1 There are three main types of specification for earthworks used in the UK:

• method;
• end-product;
• performance.

Method specification defines how compaction should be conducted in terms of the types of compaction plant, method of operation, number of passes of the plant and the final thickness of the compacted layer. In the UK the SHW [1] has been developed from research using full scale testing of plant (Parsons [38]) and should be used as the preferred approach.

However alternative specifications are not precluded and some relevant information is provided in 8.3 and 8.4 to illustrate the issues that designers should consider when assessing the suitability of an alternative form of earthworks specification.

When the SHW [1] forms the basis of the earthworks specification the document should preferably refer to the SHW rather than repeat the clauses from the SHW (e.g. "The Specification for earthworks shall be Series 600 of the Specification for Highway Works dated 20xx").

NOTE 2 The content of the SHW [1] is updated on a regular basis by the Highways Agency. Within this standard references to the SHW are undated throughout, and consideration was given to the content of the SHW at the time of preparation of this standard.

Any reference made to the SHW [1] within an earthworks specification should state clearly the version of the SHW upon which the specification is based. When invoked as part of a contract document then the edition of the SHW should be stated at the start of the document.

The SHW [1] 600 series may be used as the specification for earthworks within an overall contract specification that is not based on the SHW.

The SHW [1] should be used to satisfy the compaction requirements of BS EN 1997-1:2004, 5.3.3 and the testing requirements of BS EN 1997-1:2004, 5.3.4.

Engineered fills which are used to produce suitably shaped landforms for structures should be constructed to high standards to minimize the risk of ground movements causing damage to property built on shallow foundations. Specifications based on those developed for highway embankments are not necessarily appropriate for fills on which buildings will be founded since acceptable settlement is likely to be significantly smaller for a building than for a road; hence a more stringent specification might be necessary than for highway purposes (see 7.2).

Highway schemes are often major civil engineering projects, whereas schemes involving low-rise buildings founded upon engineered fill are often relatively small in scale. Control procedures should be appropriate to the scale of project and criticality of settlement tolerance (see 7.4.2).

Control procedures for large highway projects may not always be the most suitable for fill being placed as, for example, part of a small housing development.

NOTE 3 However, the SHW [1] has reached an extensive level of use across UK industry, and it is set out in such a way that allows the designer to tailor the requirements to suit the scheme; consequently it provides the most suitable document for incorporation into BS 6031 by the approach set out in 8.2.

8.2 Specification of earthworks by SHW approach

8.2.1 Required documentation

The appropriate appendices should be provided to enact several of the SHW [1] clauses. As a minimum Appendix 6/1 (including Table 6/1), Appendix 6/2 and Appendix 6/3 should be provided. Appendices 6/8, 6/12, 6/14 and 6/15 should usually be provided. Other appendices should only be supplied when the specific works covered are proposed as part of the scheme. The designer should avoid excessive paperwork for relatively simple schemes as this can hide the important details of which the parties involved need to be aware.

8.2.2 Compaction requirements

It is important that the designer decides whether a material is to be controlled by "method" or "end product" compaction; attempting to combine both approaches for the same material is not appropriate.

However, it should be realized that a limited amount of testing should be undertaken during method compaction to verify that the method proposed or adopted is appropriate. Similarly, end product controlled compaction should be monitored to ensure that the criteria and consistency will be achieved.

NOTE 1 The Highways Agency in the UK has adopted a standard classification of earthworks materials. A summary of the classification, first published in Reeves et al [41], is presented in Table 8.

NOTE 2 Method compaction is designed to deliver 90% compaction by BS 1377 compaction test for general fill and 95% for class 6 or end-product fill .The compactive effort stipulated in SHW [1] Table 6/4 is designed to produce an adequate state of compaction (usually 10% air voids or less) at a conservative (low) moisture content for the particular class of soil (see SHW [1] NG 612 for more details). See 7.6.4 for further explanation.

NOTE 3 There may be variations in testing procedure and/or interpretation of test results to take account of local variations in soil characteristics, e.g.:

• testing moisture content at various gradings can be beneficial, e.g. the moisture-susceptible (< 425 μm) fraction of glacial till, or < 20 mm fraction of Class 2C fill in order to relate to other earthwork relationship test data; and
• many of the standard tests included in BS 1377 cannot be usefully employed on some of the UK soils primarily due to their coarse nature (> 10% of material is retained on the 37,5 mm sieve). In order to overcome such problems and still enable classification testing to be performed on earthworks materials local variations to these methods have been adopted together with suitable acceptance criteria.