Page 249 - Civil Engineering Project Management, Fourth Edition

Earthworks and pipelines
229
match the timing of empty vehicles back from the dumping ground and their
loading capacity. This means that the excavator bucket size and loading cycle
time must be such that one haulage truck is loaded and moving away by the
time the next vehicle arrives. Hence, the cycle loading time for the excavator
3
must be known. Thus if 10m haulage vehicles return at 5min intervals, and
the cycle loading time is 1.5min, only three cycles of loading are possible so
3
an excavator bucket size of 3.3m is required. Alternatively if the cycle loading
3
time could be 1.25min a 2.5m bucket would suffice. Allowance has to be made
for the bulking factor and unit weight of the material to be excavated. The bulk-
ing of granular or soft material may range 1.1–1.3, through 1.4 for hard clays, to
1.6–1.7 for broken rock. Clays, clay–sand mixtures, gravels and sands may
3
weigh 1.6–1.9t/m in situ while rock and hard materials may vary 1.9–2.6t/m 3
in situ. The excavator bucket size has to allow for the bulking factor: for
3
3
example, a 2m bucket may only lift and load 1.4m loose material at 1.4 bulking
3
factor, so it will need seven loading cycles to fill a 10-m tipper wagon. If this is
too long a loading time for the required rate of output, an excavator with a larger
capacity bucket is required.
Correct assessment of the bulking factor is financially important to the con-
tractor, particularly in relation to the use of tipping lorries for offsite deposition
of material. Whereas dump trucks used on site can be heaped, tipping lorries
have a limited cubic capacity and payload, neither of which can be exceeded.
3
Thus if a bulking factor of 1.2 applies, a 10m lorry will take away the equiva-
3
3
lent of 8.3m net excavation; but if the bulking factor is 1.35 the 10m lorry will
3
take away only 7.4m net excavation. If the contractor has based his price on
the former but experiences the latter, he would find his price for disposal of
material off site 12 per cent too low. This could mean no profit on the operation
or a large financial loss, since there may be many thousands of cubic metres of
material involved. In practice a contractor’s past experience will guide him as
to what plant to use, taking into account many other practical matters which
apply, such as reliability of different types of plant, need for standby, margins
for hold-ups, length and nature of haulage road, cost of transporting plant to
and from the job, and hire rates for different sizes of excavator and haulage
vehicles.
18.4 Placing and compacting fill

When the contractor assesses the amount of filling he will need to transport to
achieve a given earthwork construction, he has to allow for:

• fill after compaction occupying more, or less, volume than it does in the
borrowpit;
• settlement of the formation under the weight of new fill as placed;
• further compression of the fill after placement under the weight of the
fill above.