Civil Engineering Project Management (4th Edition)
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are then more clearly revealed. There is not time to conduct in situ density tests:<br />
the contractor has to know immediately whether he can continue placing the<br />
material. Familiarity with the behaviour of suitable material as it is compacted<br />
and trafficked will soon indicate its characteristic behaviour. Excess of granular<br />
material, for example, sand or gravel in clays is easily observed, while too<br />
much clay or silt in a clay-sand mix is evidenced by the behaviour of the material<br />
under traffic. ‘Cushioning’ or ‘bounce’ under the wheels of lorries passing<br />
across the formation are signs of inadequate compaction which may be due to<br />
the material being too wet or containing too much clay or silt. Severe rutting by<br />
lorries can indicate material too wet or too clayey. Change of colour of a clay, on<br />
the other hand, may not indicate any change of suitability. The contractor must<br />
be warned immediately when material being placed appears unsuitable. If the<br />
placing is stopped after a few loads of unsuitable material have been delivered,<br />
these can usually be ‘lost’ by dozing the material out to mix it with previously<br />
placed suitable material.<br />
Purpose mixing of two different kinds of fill is seldom practicable. It may be<br />
difficult to ensure that loads of the two materials are delivered in the right ratio<br />
and, if they are not clearly distinguishable apart by appearance, the mixing may<br />
be haphazard and incomplete. If two dissimilar materials must be used, the<br />
designer should preferably devise some means of zoning each separately. When<br />
zoning is adopted, the resident engineer should check from time to time that a<br />
supposed difference between materials is occurring because material from a<br />
borrowpit can change its composition gradually.<br />
In situ density tests need to be taken to prove compliance with the specification;<br />
but the sand replacement method as described in Section 12.11 takes some<br />
hours to complete – so it is a record of past achievement and cannot be used as<br />
an instant control measure. The moisture content can possibly be quickly measured<br />
by using an appropriate moisture meter, but judging by eye can be equally<br />
effective and has the advantage that the whole area of placing can be kept under<br />
survey. The compaction equipment used by the contractor will vary according<br />
to the nature of the fill. Apart from the use of a large dozer to spread, compact<br />
and vibrate fill in place; the passage of laden dump trucks across a formation<br />
achieves a substantial degree of compaction. Hence, the contractor will usually<br />
arrange a method of placing material that makes effective use of the compactive<br />
effort of the delivery vehicles.<br />
18.6 Site roads<br />
Earthworks and pipelines 231<br />
A contractor who pays insufficient attention to the right construction of site<br />
haulage roads runs the risk the road will begin to break up and cause delay just<br />
at some crucial time of construction, such as when autumn rains begin and the<br />
contractor is hoping to get filling finished before the heavier rainfall of winter<br />
occurs and delays construction. Pushing hardcore into the worst patches is no<br />
real solution, and more troubles come when haulage lorries get bogged down