Project managment

Placing and compacting fill

When the contractor assesses the amount of filling he will need to transport to achieve a given earthwork construction, he has to allow for:
• fill after compaction occupying more, or less, volume than it does in the borrowpit;
• settlement of the formation under the weight of new fill as placed;
• further compression of the fill after placement under the weight of the fill above.

Standards will be set in the specification for the permitted moisture content of the fill before it is compacted, and its density after compaction. For example, the specification may stipulate that fill type A must be compacted at a moisture content between ‘optimum 1 per cent’ and ‘optimum 2 per cent’; while fill type B must be compacted at a moisture content between ‘optimum 1 per cent’ and ‘optimum 3 per cent’. The optimum value is that determined by the standard compaction test, whether it is the 2.5 kg hammer method (the original ‘Proctor’ test) for embankments, or the 4.5 kg hammer method as used for roads. The density to be achieved will be specified as some percentage (e.g. 90 or 95 per cent) of the optimum under standard testing. Samples will have to be taken from the fill to find the optimum moisture content and density under the standard compaction test specified, and in situ density tests (see Section 12.11) must then be undertaken by the resident engineer to ensure the right density is achieved. Normally tests on fill materials will have taken place prior to the design of the earthwork and the results of these tests and the method of testing, etc. must form part of the data accompanying the specification. Care has to be taken to ensure that the method of specifying the required end result covers the range of materials likely to be encountered. It is then up to the contractor, from his experience, to know what type of plant he must use to compact the fill to the required standard.
Achieving the required moisture content may present difficulty. In wet weather the borrowpit material may be too wet to use and the formation may be too wet to work on. The resident engineer may have to instruct the contractor to cease working when such conditions occur. There is little that can be done to protect borrowpit material against excess rainfall. The formation can be partly protected against rainfall by rolling it to a fall with a smooth wheeled roller at the end of each day’s placing. Sometimes an attempt to protect the formation by laying sheeting over it is adopted, but this is seldom practicable if the site is windy. If the material is too dry for placing it must be watered. Although watering at the borrowpit can be helpful it is usual to water-spray the spread material from water bowsers. Some mixing of the material by dozer may be necessary after watering to avoid only the surface material being wetted. In hot dry climates more than the theoretical amount of water may need to be added because of the high evaporation rate applying. Aconsiderable amount of water may be needed, involving the use of more than one water bowser.


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