Page 251 - Civil Engineering Project Management, Fourth Edition

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Earthworks and pipelines
are then more clearly revealed. There is not time to conduct in situ density tests:
the contractor has to know immediately whether he can continue placing the
material. Familiarity with the behaviour of suitable material as it is compacted
and trafficked will soon indicate its characteristic behaviour. Excess of gran-
ular material, for example, sand or gravel in clays is easily observed, while too
much clay or silt in a clay-sand mix is evidenced by the behaviour of the mater-
ial under traffic. ‘Cushioning’ or ‘bounce’ under the wheels of lorries passing
across the formation are signs of inadequate compaction which may be due to
the material being too wet or containing too much clay or silt. Severe rutting by
lorries can indicate material too wet or too clayey. Change of colour of a clay, on
the other hand, may not indicate any change of suitability. The contractor must
be warned immediately when material being placed appears unsuitable. If the
placing is stopped after a few loads of unsuitable material have been delivered,
these can usually be ‘lost’ by dozing the material out to mix it with previously
placed suitable material.
Purpose mixing of two different kinds of fill is seldom practicable. It may be
difficult to ensure that loads of the two materials are delivered in the right ratio
and, if they are not clearly distinguishable apart by appearance, the mixing may
be haphazard and incomplete. If two dissimilar materials must be used, the
designer should preferably devise some means of zoning each separately. When
zoning is adopted, the resident engineer should check from time to time that a
supposed difference between materials is occurring because material from a
borrowpit can change its composition gradually.
In situ density tests need to be taken to prove compliance with the specifica-
tion; but the sand replacement method as described in Section 12.11 takes some
hours to complete – so it is a record of past achievement and cannot be used as
an instant control measure. The moisture content can possibly be quickly meas-
ured by using an appropriate moisture meter, but judging by eye can be equally
effective and has the advantage that the whole area of placing can be kept under
survey. The compaction equipment used by the contractor will vary according
to the nature of the fill. Apart from the use of a large dozer to spread, compact
and vibrate fill in place; the passage of laden dump trucks across a formation
achieves a substantial degree of compaction. Hence, the contractor will usually
arrange a method of placing material that makes effective use of the compactive
effort of the delivery vehicles.

18.6 Site roads

A contractor who pays insufficient attention to the right construction of site
haulage roads runs the risk the road will begin to break up and cause delay just
at some crucial time of construction, such as when autumn rains begin and the
contractor is hoping to get filling finished before the heavier rainfall of winter
occurs and delays construction. Pushing hardcore into the worst patches is no
real solution, and more troubles come when haulage lorries get bogged down