For large open excavations, such as when road cuttings have to be made and the material tipped to form embankments, or for building an earth dam from open borrow pit areas, the motorscraper is the most economical machine for excavating, transporting and placing clays and clay-sand mixes. But the gradients traversed need to be gentle and the motorscraper cannot pick up hard bands of material or rock, unless ripping beforehand can break up the material sufficiently.
If hard or rocky material has to be excavated, the face shovel loading to dump trucks has to be used, the trucks commonly having a capacity of 50–60 t, sometimes larger. However, neither scrapers nor dump trucks can traverse public roads.
If the excavated material has to be routed off site via public roads to some dumping area, the excavated material has to be carted away by tipping lorries licensed for use on the public highway. Tipping lorries have a lesser capacity than dump trucks, usually in the range 10–30 t. Afactor often having considerable influence when needing to transport material along public roads, is the reaction of the local road and public authorities who may object to the extra construction traffic and mud on the roads. If the local authority has also to give planning permission for dumping spoil on some given land, such permission may only be granted subject to restriction on the size of lorries used and their frequency of passage. This situation cannot be left for tenderers to find out; the employer has to obtain the necessary permissions and the contract must reproduce exactly the conditions laid down by the planning or other authority concerned and require the contractor to conform to them. If the restrictions limit the size and frequency of tipping lorries, the contractor may be forced to temporarily stockpile excavated material on site and double handle it in order to conform to his intended programme for construction and the haulage conditions laid down. This will raise his costs for excavation.
Assuming there are no planning restrictions, the contractor needs to choose that combination of excavating plant and haulage vehicles which achieves the match the timing of empty vehicles back from the dumping ground and their loading capacity. This means that the excavator bucket size and loading cycle time must be such that one haulage truck is loaded and moving away by the time the next vehicle arrives. Hence, the cycle loading time for the excavator must be known. Thus if 10m3 haulage vehicles return at 5 min intervals, and the cycle loading time is 1.5 min, only three cycles of loading are possible so an excavator bucket size of 3.3m3 is required. Alternatively if the cycle loading time could be 1.25 min a 2.5m3 bucket would suffice. Allowance has to be made for the bulking factor and unit weight of the material to be excavated. The bulking of granular or soft material may range 1.1–1.3, through 1.4 for hard clays, to 1.6–1.7 for broken rock. Clays, clay–sand mixtures, gravels and sands may weigh 1.6–1.9 t/m3 in situ while rock and hard materials may vary 1.9–2.6 t/m3 in situ. The excavator bucket size has to allow for the bulking factor: for example, a 2m3 bucket may only lift and load 1.4m3 loose material at 1.4 bulking factor, so it will need seven loading cycles to fill a 10-m3 tipper wagon. If this is too long a loading time for the required rate of output, an excavator with a larger capacity bucket is required.
Correct assessment of the bulking factor is financially important to the contractor, particularly in relation to the use of tipping lorries for offsite deposition of material. Whereas dump trucks used on site can be heaped, tipping lorries have a limited cubic capacity and payload, neither of which can be exceeded.
Thus if a bulking factor of 1.2 applies, a 10m3 lorry will take away the equivalent of 8.3m3 net excavation; but if the bulking factor is 1.35 the 10m3 lorry will take away only 7.4m3 net excavation. If the contractor has based his price on the former but experiences the latter, he would find his price for disposal of material off site 12 per cent too low. This could mean no profit on the operation or a large financial loss, since there may be many thousands of cubic metres of material involved. In practice a contractor’s past experience will guide him as to what plant to use, taking into account many other practical matters which apply, such as reliability of different types of plant, need for standby, margins for hold-ups, length and nature of haulage road, cost of transporting plant to and from the job, and hire rates for different sizes of excavator and haulage vehicles.