Page 245 - Civil Engineering Project Management, Fourth Edition

18
Earthworks and

pipelines

18.1 Excavating and earth-placing machinery

Bulldozers (‘dozers’) are used for cutting and grading work, for pushing scrap-
ers to assist in their loading, stripping borrowpits, and for spreading and com-
pacting 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 com-
paction, so that a Caterpillar ‘D8’ 115h.p. weighing about 15t, or its equivalent,
is the machine required; not a ‘D6’ weighing 7.5t 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, turn-
ing 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 down-
grade, 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