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.
Table 8 Classification of earthworks materials in the UK by the Highways Agency
Type Class Description Typical use
General granular fill 1A Well graded granular material General fill
1B Uniformly graded granular material
1C Coarse granular material
General cohesive fill 2A Wet cohesive material General fill
2B Dry cohesive material
2C Stony cohesive material
2D Silty cohesive material
2E Reclaimed pfa cohesive material
General chalk fill 3 Chalk General fill
Landscape fill 4 Various Fill for landscape areas
Topsoil fill 5A Topsoil or turf existing on site Topsoiling
5B Imported topsoil
Selected granular fill 6A Selected well graded granular material Below water
6B Selected coarse granular material Starter layer
6C Selected uniformly graded granular material Starter layer
6D Selected uniformly graded granular material Starter layer below pfa
6E Selected granular material For cement stabilization to form capping – class 9A
6F1 Selected granular material (fine grading) Capping
6F2 Selected granular material (coarse grading) Capping
6F3 Selected granular material recycled bituminous/asphaltic materials, etc.) Capping
6F4 Selected/imported (unbound) granular material that conforms to BS EN 13285 (fine grading) Capping
6F5 Selected/imported (unbound) granular material that conforms to BS EN 13285 (coarse grading) Capping
6G Selected granular material Gabion filling
6H Selected granular material Drainage layer For reinforced soil and anchored earth structures
6I Selected well graded granular material Fill
6J Selected uniformly graded granular material Fill
6K Selected granular material Lower bedding for: Corrugated steel buried structures
6L Selected uniformly graded granular material Upper bedding for:
6M Selected granular material Surround to:
6N Selected well graded granular material Fill to structures
6P Selected granular material Fill to structures
6Q Well graded, uniformly graded or coarse granular material Overlying fill for corrugated steel buried structures
6R Selected granular material For stabilization with lime and cement to form capping – class 9F
6S Selected well graded granular material Filter layer below subbase
Selected cohesive fill 7A Selected cohesive material Fill to structures
7B Selected conditioned pfa cohesive material Fill to structures and reinforced soil
7C Selected wet cohesive material Fill to reinforce soil
7D Selected stony cohesive material Fill to reinforce soil
7E Selected cohesive material For stabilization to form capping to: Lime – class 9D
7F Selected silty cohesive material Cement – class 9B
Cement – class 9C
7G Selected conditioned pfa cohesive material Overlying fill for corrugated steel buried structures
7H Wet, dry, stony or silty cohesive material and chalk For stabilization with lime and cement to form capping – class 9E
7I Selected cohesive material
Miscellaneous fill 8 Class 1, class 2 or class 3 material Lower trench fill
Stabilized materials 9A Cement stabilized well graded granular material Capping
9B Cement stabilized silty cohesive material
9C Cement stabilized conditioned pfa cohesive
9D Lime stabilized cohesive material
9E Lime and cement stabilized cohesive material
  9F Lime and cement stabilized well graded material  
NOTE Clays and cohesive soils are shown in shaded rows.