Page 252 - Civil Engineering Project Management, Fourth Edition

Civil Engineering Project Management
232
in the road or break a half shaft. For heavy construction traffic, a road must be
thick enough; have deep drainage ditches either side; be made from good inter-
locking angular large material at the base and similar smaller material above;
and be formed to a camber or crossfall which sheds rainwater. Poor construc-
tion is more liable to occur on flat ground where the temptation is strong not to
dig out more than seems necessary, and not to dig deep enough side ditches to
keep the road construction dry. But once the proper precautions are taken the
road will stand up and need little more than re-grading and rolling from time
to time to keep the surface in good condition and able to shed rainwater.
18.7 Trenching for pipelines

The hydraulic hoe or backacter is the machine most widely used for trench
excavation for pipelines. In hard ground, rock or roads, the trenching machine
might be used which has been described in Section 18.1. Depths for water and
gas pipelines are usually the pipe diameter plus 1m. For sewers, greater depths
are often required to maintain falls. When flexible plastic pipes are used, espe-
cially in the smaller diameters, pipe joints can be made above ground, the pipe
being snaked in. Bottoming of the trench can be achieved by using a straight-
edged bucket without teeth, and the backhoe can also place soft material or
concrete into a trench on which to bed pipes or fully surround them. Provided
no men are allowed in the trench, timbering can thus be avoided. When large
diameter steel pipes with welded joints have to be laid, a string of several pipes
may be welded up alongside the trench, and dozers equipped with side lifting
booms can lower the string of pipes into the prepared trench. This reduces the
amount of timbering and excavation of joint holes necessary which need only
be arranged where successive strings have to be jointed together.
The principal defects occurring on pipelines come from defective joints and
pipe fracture due to settlement of a pipe on a hard band, large stone or lump of
rock in the base of the trench. The use of the hydraulic hoe makes the prepar-
ation of an even bed for the pipe easier to achieve, especially on suitable selected
soft granular fill. However, the base of the trench and the bedding along each
length of pipe must be carefully boned in before the pipe is lowered to ensure
each pipe is fully supported along its body.
For non-flexible pipes of ductile iron, asbestos cement, steel or concrete it will
be necessary to joint them after laying. Sufficient access is then required for the
jointer to make the joint properly, and support to the trench sides will be essen-
tial in every case where there is not absolute certainty there can be no slip of
material into the trench. Falls of material into trenches are a major hazard in civil
engineering, and adoption of a consistent, rigorously applied safety approach
is the only way to prevent accidents. The damaging weight of even a small fall
of earth must be borne in mind.
While it will be obvious that gravity sewers must be laid to a fall, it is some-
times not appreciated that pressurized trunk water mains should be laid to a