Page 246 - Civil Engineering Project Management, Fourth Edition

Civil Engineering Project Management
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the wear on its large balloon tyres which are expensive. The motorized scraper
gives the lowest cost of excavation per cubic metre of any machine, but it needs
a wide area to excavate or fill and only gentle gradients on its haul road. It can-
not 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
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bucket of 2–5m 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 7m 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 fit-
ted 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 excava-
tion 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 oper-
ation 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 20–25m 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. Atrained operator
can be skilled at placing the bucket accurately to a desired position. The drag-
line 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
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lorries. It may have a bucket size of up to 5m ; it is very mobile and, being soft
tyred, can traverse public roads.