Earthworks are increasingly required on development sites across the UK for a variety of reasons. From creating level plateaus on sloping sites through to raising a site above flood levels, it is extremely important that earthworks are correctly specified and controlled to ensure the re-engineered ground is fit for purpose.
The primary guidance used in the specification of earthworks is ‘Series 600 Earthworks’ of Volume 1 Specification for Highway Works by the Department for Transport. Whilst this document was produced to be used in relation to highways works, it is widely used in the industry in relation to all earthworks.
Series 600 sets out two methods of specifying and controlling earthworks contracts; these are end product and method specification. Which method is adopted really depends on what purpose the engineered fill is to serve. In general terms, if the earthworks fill is just supporting roads, infrastructure or open space then a method specification can be used, whereas if it is supporting a structure then it is best to adopt an end product method.
The method specification outlined in Series 600 is based on extensive research by TRL and sets out the layer thickness a fill material should be placed, and the number of passes required by various different types of compaction plant to provide an acceptable level of compaction.
Method specification type earthworks require extensive up-front testing so that material to be used in the earthworks can be correctly classified prior to selection of the compaction method.
In contrast, end product specifications do not set out how the earthworks should be performed but does set out the properties which the compacted fill must achieve, such as a minimum degree of compaction, a maximum amount of air voids, a minimum shear strength or a minimum CBR value.
Once these are set, the earthworks contractor has to undertake site trials to prove that the method of compaction they are proposing will achieve the required criteria and validation testing is required during the works to prove they are consistently being met.
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