Page 248 - Civil Engineering Project Management, Fourth Edition

Civil Engineering Project Management
228
the last 150–225mm of excavation, any loose material being either removed by
hand labour or rolled back with a light roller before placing of the base course for
a road or blinding concrete.
The presence of springs in a soft formation material exacerbates formation
finishing problems. Usually the specification will require spring water to be
led away by grips or drains to a pump sump which is continuously dewatered
to prevent softening of the formation. If springs are encountered and have not
been anticipated, or the method of dealing with them is not specified or shown
on the drawings, the resident engineer should report the situation to the engin-
eer. Special measures are often required to deal with springs to ensure safety
of the structure to be built on a formation containing them.
18.3 Haulage of excavated material

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 excav-
ating, 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 suffi-
ciently. 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 50–60t,
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 10–30t. A factor often having consider-
able 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 con-
struction 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 repro-
duce exactly the conditions laid down by the planning or other authority con-
cerned 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 temporar-
ily 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
required excavation rate at lowest cost. The face shovel or backhoe output must