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Duncan Scott: Common pitfalls in earthworks specifications

Earthworks form a key element of many enabling works projects that I have been involved in but there seem to be a number of issues that are not being resolved and are repeated on project after project.

duncan scott talking point

duncan scott talking point

Duncan Scott is technical director at Vertase FLI and FLI QDS

Typical earthworks activities that I’m involved in range from the backfilling of excavations (following the removal of contaminated materials), through to cut and fill exercises to construct development platforms and the construction of road embankments. These tasks are often undertaken using challenging materials that have undergone processing/remediation to render them suitable for re-use, so I have reviewed numerous earthworks specifications prepared by third party consultants to form part of tenders and written a growing number myself.

Through this part of my work I have become aware of a number of issues.

BS6031 provides a useful overview of the fundamental objectives of an earthworks specification, which are to adequately describe the design, be easily understood by all parties; be practicable in both enforcement and measurement and to not be unnecessarily costly or time consuming. BS6031 goes on to describe the three main types of specification for earthworks used in the UK; namely method, end-product and performance. Notwithstanding this source of clarity, it would be my observation that confusion often creeps in to some earthworks specifications which, at best, contradict the fundamental objectives as set out in BS6031 and, at worst, could lead to the under-engineering of fill for its intended purpose.

By far the most common pitfall that is seen in earthworks specifications is the reliance on the Highways Agency Specification for Highway Works (SHW) for fills on which buildings shall be founded. While this may be appropriate for many sites, it is important to remember that SHW was developed as a method specification for highway cuttings and embankments based on work in the 1960s and 1970s by the Transport Research Laboratory.

Strict adherence to SHW with respect to material type and compaction method is intended to produce a compacted material with less than 10% air voids for general earthworks fill materials and greater than 95% maximum dry density (MDD) and generally less than 5% air voids for specialist fill materials. Given that many geotechnical engineers would require that general fill placed beneath buildings has less than 5% air voids to minimise the potential for long-term settlement, strict adherence to SHW Series 600 could be inappropriate if not coupled with end-product verification to confirm the air voids and inform the necessity for further compactive effort.

Another common pitfall seen in earthworks specifications is reliance on the standard 2.5kg Proctor test (light rammer) to characterise the MDD and optimum moisture content (OMC) in the laboratory when using modern heavy compaction plant in the field. Given that the standard Proctor test would exert a compactive effort of 1.2KJ but a typical 13t smooth drum roller could exert a greater compactive effort, it would not be unexpected to hear site technicians declare field dry densities which are well in excess of 100% to the rapture of plant operators and site foremen. To account for the availability of heavy compaction plant, the modified 4.5kg Proctor test (heavy rammer) is made widely commercially available by geotechnical laboratories, exerting a compactive effort of 4.5 times that of the standard test (5.4KJ). However, this modified test is not always requested in earthworks specifications. Given the interrelationship between density, moisture and compactive effort, it would seem disjointed that a greater emphasis is not placed on the matching of density/moisture relationships for materials with compactive effort in the field when specifying earthworks.

As we can all appreciate, failure to undertake earthworks to the required standard can be costly both financially and in terms of reputation, so I would urge the industry to carefully consider specifications in more detail to ensure the right end result.

  • Duncan Scott is technical director with Vertase FLI and FLI QDS


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