Excavating and earth-placing machinery
Bulldozers (‘dozers’) are used for cutting and grading work, for pushing scrapers to assist in their loading, stripping borrowpits, and for spreading and compacting fill. The larger sizes are powerful but are costly to run and maintain, so it is not economic for the contractor to keep one on site for the occasional job. Its principal full-time use is for cutting, or for spreading fill for earthworks in the specified layer thickness and compacting and bonding it to the previously compacted layer. It is the weight and vibration of the dozer that achieves compaction, so that a Caterpillar ‘D8’ 115 h.p. weighing about 15 t, or its equivalent, is the machine required; not a ‘D6’ weighing 7.5 t which is not half as effective in compaction. The dozer cannot shift material very far, it can only spread it locally.
A dozer with gripped tracks can climb a 1 in 2 slope, and may also climb a slope as steep as 1 in 1.5 provided the material of the slope gives adequate grip and is not composed of loose rounded cobbles. On such slopes of 1 in 1.5 or 1 in 2 the dozer must not turn, but must go straight up or down the slope, turning on flatter ground at the top and bottom. It is dangerous to work a dozer (and any kind of tractor) on sidelong ground, particularly if the ground is soft. Dozers cannot traverse metalled roads because of the damage this would cause, and they should not be permitted on finished formation surfaces. Sometimes a flat tracked dozer (i.e. with no grips to the tracks) can be used on a formation if the ground is suitable.
Motorized scrapers are the principal bulk excavation and earth-placing machines, used extensively on road construction or earth dam construction. Their movement needs to be planned so that they pick up material on a downgrade, their weight assisting in loading; if this cannot be managed or the ground is tough, they may need a dozer acting as a pusher when loading. This not only avoids the need for a more expensive higher powered scraper, but reduces the wear on its large balloon tyres which are expensive. The motorized scrape gives the lowest cost of excavation per cubic metre of any machine, but it need a wide area to excavate or fill and only gentle gradients on its haul road. It cannot excavate hard bands or rock, or cut near-vertical sided excavations.
The face shovel, or ‘digger’ can give high outputs in most types of materials, including broken rock. It comes in all sizes from small to ‘giant’; but for typical major excavation jobs (such as quarrying for fill) it would have a relatively large bucket of 2–5m3 capacity. The size adopted depends on what rate of excavation must be achieved, the capacity of dump trucks it feeds to cart away material, and the haul distance to tip or earthworks to be constructed. The face shovel would normally be sized to fill a dump truck in only a few cycles. The machine can only excavate material down to its standing level, and work a limited height of excavation face. Hence, if a deep excavation is required, the face shovel must ‘bench in’ and must leave an access slope for getting out when it has finished excavating. It must stand on firm level ground when working, and is not very mobile. It works in one location for as long as required, moving its position only as excavation proceeds. Its major advantage is its high output and ability to excavate in most materials.
The hydraulic excavator used as a hoe or backacter, cuts towards the machine. It is highly versatile. The larger sizes can cut to a depth of 6 or 7 m and excavate a face of the same height, slewing to load to trucks alongside. It can be used for lifting pipes into trenches, and ‘bumping down’ loose material in the base of a trench with the underside of its bucket. It can usually excavate trenches in all materials except rock; but sometimes has trouble in getting out hard bands of material that are horizontally bedded or which dip away from the machine.
It can have a toothed bucket capable of breaking up a stony formation, or be fitted with a ripper tooth for soft rock or a hydraulic breaker for hard materials, or have a smooth edged bucket for trimming the base of a trench. A wide range of such machines are available, the smallest size often being used on small building sites; the larger sizes being used for large trench excavation and general excavation
of all kinds.
The dragline’s principal use is on river dredging work from the bankside, and for other below water excavation. Although the machine is slow in operation and has a smaller rate of output than an equivalent hydraulic backhoe, it can have a long reach when equipped with a long jib and can excavate below its standing level. With a 15-m jib, it can throw its bucket 20–25 m out from the machine; hence its use for river bed excavation and bankside trimming. The dragline can also be operated to cut and grade an embankment slope below its standing level, or for dumping soil or rock on such a slope. A trained operator can be skilled at placing the bucket accurately to a desired position. The dragline offloads its material to dump trucks, but this tends to be a messy operation because the swing of the bucket on its suspension cable tends to scatter material.
The wheeled loader is widely used for face excavation in soft material, but its predominant use is for shifting heaps of loose spoil and loading them to lorries. It may have a bucket size of up to 5m3; it is very mobile and, being soft tyred, can traverse public roads.
The grab has a low output rate, but is used when sinking shafts in soft material, especially when sinking caissons kentledge fashion. It is also used occasionally for the job of keeping aggregate hoppers filled with concrete aggregates from stocks dumped by delivery lorries at ground level.
The clamshell bucket has a pincer movement, hydraulically operated, and is principally used for the construction of diaphragm walls. The bucket is fixed to a long rod which is lowered and raised down a frame held vertically (or at an angle) so that it can cut trenches up to 30 m deep in soft material, usually up to 0.6 m wide. The machine rotates so the clamshell can be emptied to a waiting dump truck.
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| Trencher in action |
Trenching machines can be used either for excavation of pipe trenches or construction of shallow diaphragm walls. They have a bucket chain cutter delivering material to the side of the trench or by additional conveyor belt can deliver to dump trucks. For hard ground the machine has special cutters cutting a groove at either side of the trench, with a third bucket cutter chain to remove the dumpling of material between.