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 borrowpit must be examined to point out to the contractor where suitable and unsuitable 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 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 granular 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 material 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 specification;
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 measured 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.