8.2.3 Testing requirements

The designer should provide a table that clearly sets out the testing requirements for earthworks materials. This may be in the form presented in Table NG1/1 of SHW [1]. The designer should include either the frequency or number of tests dependent on the size or duration of the works being undertaken.

NOTE Table 9 is based on Table 3/1 in HA 44/91 [17] as an example.

The types of test should be related to the material properties specified in SHW [1] Appendix 6/1, including a check test to ensure that the required density has been achieved if required by the designer/overseeing organization.

The preferred method of specifying moisture limits on clays in the UK is the MCV, which is quick to measure on site and for which there is a substantial base of experience; typical values commonly used are:

• Class 2A (wet cohesive) 8–12;
• Class 2B (dry cohesive) 12–16;
• Class 2C (stoney cohesive) 8–16.

These limits may be varied depending on site-specific relationship testing to fundamental properties, as described in Table 10 (based on Table 4/2 in HA 44/91 [17]).

The MCV should not be used for stoney clays if there is insufficient matrix (typically less than 50%–55%) for the test and in such cases reliance on moisture content is necessary (Oliphant and Winter [56]).

NOTE An example of classification and acceptability criteria is given in HA 44/91 [17], Annex A.

SHW [1] Appendices and Tables should be developed to reflect local knowledge of materials and experience of particular equipment.

8.3 Specification of earthworks for minor works

The SHW [1] has been developed for large highway schemes, however it may be used effectively on minor projects. As a minimum, this should include: Appendices 0/1, 0/2 and 1/5 together with the appropriate 600 series appendices, e.g., Appendix 6/1. Further guidance may be found in the Notes for Guidance to the Specification for Highway Works [2].

8.4 Additional requirements for deep fill areas/buildings and structures

Collapse compression upon groundwater inundation is a major hazard for buildings and other structures on significant thicknesses of fill; therefore the specification of placement and compaction of the fill should be designed to eliminate collapse potential.

NOTE The risk of collapse upon inundation is particularly high where fill is placed below the potential groundwater level (e.g. infilling of a quarry), but is present at many other sites due to risk of inundation following a water main burst.

It should be noted that, the collapse potential of some fills will not be eliminated despite the achievement of a field dry density equivalent to at least 95% of the maximum dry density achieved using the British Standard compaction tests (see BS 1377-4). Consequently, in such cases, dry density should not be relied upon to provide an adequate measurement for compaction specification. Where there is an unacceptable risk of collapse upon inundation the specification should include a requirement for all fill to be compacted to < 5% air voids.

See Charles et al [39] and BRE Digest 427 [37].

8.5 Alternative earthworks specifications

8.5.1 General

NOTE As described in 8.1, it is assumed that the default situation in the UK is that earthworks are undertaken in accordance with the SHW [1] (subject to any additional requirements to address specific risks described within this standard).

The SHW [1] is commonly adopted for earthworks; however, if an alternative specification is used (e.g. Model specification for fills, Trenter and Charles [40], or a company in-house earthworks specification) then the specification should provide as a minimum the following information:

• a) types of materials permitted for use in the earthworks together with material properties;
• b) performance requirements to be met;
• c) requirements for the disposal of unsuitable material;
• d) requirements for placement, spreading and compaction of the earthworks materials;
• e) requirements for the treatment of exposed surfaces;
• f) requirements for the testing and verification of compliance.

This subclause sets out the recommendations for any alternative to the SHW; if the earthworks specification conforms to these recommendations and addresses the requirements of this standard then those earthworks may be considered as conforming to this standard.

8.5.2 End product

For this form of specification the designer specifies the degree of compaction necessary for the given material by reference to criteria linked to either serviceability or ultimate limit states; the level of compaction required should be expressed in terms of selected geotechnical properties e.g. percentage of maximum dry density and is supported by rigorous on-site testing.

An end product specification may be used to control earthworks provided the approach will adequately control the various issues that effect earthworks. For instance the specified parameters should not concentrate on the issue of stiffness (or shear strength) alone since the control of air voids in compacted material is important in restricting the potential for excessive settlement if an increase occurs in the moisture content of the material. Where an end product specification is adopted, the employer's requirements may set overall targets to be achieved without detailing the methods used to achieve the targets. In this case the following minimum requirements should be addressed (acceptable limits may be set on these criteria or this may be stated as being for the earthworks designer to assess).

• Materials used should be chemically suitable for the environment in which they are used; some material might require treatment (e.g. stabilization or remediation) and consent prior to use.
• Materials used should be durable (not prone to deterioration) and non-biodegradable.
• The earthworks should provide a stable finished surface that will not suffer unacceptable post construction settlement or movement.
• The earthworks should provide a surface of sufficient stiffness (and or shear strength) for the intended end use (if a stiffness value to be achieved is specified then the value will need to consider both the imposed load and settlement induced by the loaded area).

NOTE That the works can be constructed, maintained and demolished safely are requirements of the CDM 2007 Regulations [10].

The above criteria are associated with showing that the works should be constructed so that they are suitable for the proposed end use. To achieve these objectives the earthworks contractor and the designer should consider a range of practical issues, and to deliver this should effectively require that a method of working/specification is put in place (whoever writes this document will effectively become an earthworks designer).

8.5.3 Performance specification

A performance specification should be designed in terms of the required serviceability limit state: e.g. "the maximum differential settlement should not exceed 1:200 over a defined length, five years after construction and the maximum settlement in any one area should not exceed 25 mm".

This may be considered an onerous form of specification from a contractual viewpoint as it seeks to place the risk for all future events on the Contractor and might be very difficult to monitor in practice (for example, see Virginia TRC [57]).

COMMENTARY ON 8.5.3

The way that control of earthworks is being developed in the USA and Europe is of direct relevance to future earthworks construction in the UK. Intelligent compaction or continuous compaction control has the potential to improve infrastructure performance, reduce costs, reduce construction programmes and improve site safety.

A compaction control approach using modulus and moisture content (plus air voids) has the potential to fit well with performance specifications and could be monitored in real time using roller-mounted devices (see Mooney and White [58].