Page 250 - Civil Engineering Project Management, Fourth Edition

Civil Engineering Project Management
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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 stand-
ard compaction test, whether it is the 2.5kg hammer method (the original
‘Proctor’ test) for embankments, or the 4.5kg 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 con-
tractor 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 for-
mation 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. A considerable amount of water
may be needed, involving the use of more than one water bowser.

18.5 Watching fill quality

When fill from a borrowpit is of variable quality the resident engineer needs
an inspector to watch the fill quality as placed, with power to reject unsuitable
material or call in the resident engineer in cases of doubt. Although the borrow-
pit must be examined to point out to the contractor where suitable and unsuit-
able materials appear to exist, the actual watch on material quality must take
place as it is dumped, spread and compacted. The characteristics of a material