In preparing the bills, the quantities should be accurately taken off drawings in accordance with the method of measurement. The quantities billed should not contain hidden reserves by over-measuring them when preparing a bill.
There may be a temptation to do this when, for instance, billing the trench excavation for a pipeline. But if the engineer increases the length at greater depth and decreases that at shallow depth to compensate, he may give the contractor a false impression of the nature of the work. It needs to be borne in mind that sometimes it is the practice to agree bill quantity for an item for payment if there is no obviously large variation from what the drawings show.
Hence quantities should represent a best estimate of what will occur, in order to be fair to both contractor and employer.
The problem of rock. A problem occurs when billing rock which may be suspected but whose incidence is not known as in the case of a long pipeline where it is impracticable to sink enough borings in advance to discover the depth and extent of rock everywhere. Sometimes a provisional quantity is put in for rock, but if the extent of rock is not known, the problem is to decide what provisional quantity is to be put in the bill? Also how can the tenderer price such an item when the actual quantity to be encountered is only provisional.
Instead, it is suggested, a provisional sum should be included in the bill for rock excavation, and a price for excavating rock should be agreed with the contractor, if rock is encountered.
However, a consequent problem is that, if rock is encountered, it will almost certainly delay the work, so the contractor will put in a delay claim. Despite this, there is much to be said for negotiating a rate when rock is encountered, because widely different methods and therefore costs per unit excavated will apply according to the nature and direction of bedding of the rock encountered (see Section 15.7).
Provisional quantities for other matters should likewise only be used with care. They should relate to something known to be required, the quantity being a reasonable judgement as to what might be required. This could apply to such matters as bedding pipes on soft material, or bedding and haunching pipes in concrete, or fully surrounding pipes in concrete where the actual extent of such work depends on the site conditions encountered.