Chapter 6: Dump Trucks
The most common hauling equipment used for mining and construction
work are the 20 and 40 ton dump trucks
USE
The 20 ton dump truck is primarily used for over the road hauling,
while the 40 ton is used for off-road (in-pit) use.
The 20 ton can be increased by adding a separate trailer for
an additional 20 ton capacity.

Figure 6-1. 20 Ton Dump
Truck
CAPACITY
The capacity of hauling equipment is expressed in one of
three ways:
-
gravimetrically by the weight of the load it will carry (in
tons),
-
by its struck rear-dump body volume (in cubic yards),
-
or by its heaped rear-dump body capacity (in cubic yards).
The hauling capacity of dump trucks is normally expressed
gravimetrically: 20-ton or 40-ton. Conversely, the capacity of
loading equipment is normally expressed in cubic yards. The unit
weight of the various materials to be transported may vary from as
little as 1,700 pounds per LCY for dry clay, to 3,500 pounds per LCY
for trap rock (see Table 1-2, in Introduction to Earthmoving, for
weights of common materials). Always make sure that the volumetric
load does not exceed the gravimetric capacity of the truck.
OPERATION
Loading
For maximum efficiency, fill trucks as close to their rated
hauling capacity as practical. Adjust the load size if haul roads
are in poor condition or if the trucks must traverse steep grades.
Overloading will cause higher fuel consumption, reduced tire life,
and increased mechanical failures.
Use spotting markers when trucks are hauling from a hopper, a
grizzly ramp, or a stockpile. Spotting markers are also beneficial
when excavators (such as a dragline, a clamshell, a loader, a
backhoe, or a hoe) are used to load hauling equipment. They
facilitate prompt and accurate vehicle spotting which improves
loading efficiency.
Spot trucks as close to the bank as possible when loading
with an excavator. Ensure that the trucks are within the working
radius of the dragline, the clamshell, or the hoe bucket. When using
a loader, position the truck and loader so that the two machines
form a V. This arrangement will reduce the loader cycle time (Figure
6-2).

Figure 6-2 Truck and Loader V-Positioning for Loading
Maintaining Proper Speed
Haul at the highest safe speed and in the proper gear,
without speeding. Speeding is unsafe and hard on equipment. When
several trucks are hauling, it is essential to maintain the proper
speed to prevent hauling delays or bottlenecks at the loading and
dumping sites. Use separate haul roads to and from the dump site, if
possible. Keep haul roads well maintained, with a minimum grade. Use
one-way traffic patterns to increase efficiency.
Dumping (Unloading)
Always use spotters to control dumping operations. When
dumping material that requires spreading, move the truck forward
slowly while dumping the load. This makes spreading easier.
Establish alternative dumping locations to maintain truck spacing
when poor footing or difficult spotting slow the dumping operation.
Preventive Maintenance
Keep
truck bodies clean and in good condition. Accumulations of rust,
dirt, dried concrete, or bituminous materials hamper production.
Consider the time spent cleaning and oiling dump bodies,
particularly for asphalt or concrete hauling, when computing
transportation requirements.
-
Clean
truck bodies thoroughly at the end of the day. When used to haul
wet concrete mix, spray the dump beds with water before loading
and clean them thoroughly as soon as practical after dumping.
-
Coat the walls and sides of truck bodies with diesel fuel or oil
to prevent bituminous materials (plant-mix asphalt) from
sticking
PRODUCTION ESTIMATES
The production capacity of the loading equipment is normally
the hauling operation’s controlling factor. Never keep loading
equipment waiting. If there are not enough trucks, there will be a
loss in production.
Number of Trucks Required
Use the following formula to estimate the number of trucks
required to keep loading equipment operating at maximum capacity:

•
The numeral 1 in the formula is a safety factor against the
necessity for closing down loading equipment due to lack of hauling
equipment. If all operations are on schedule, one truck will always
be standing byat the loader, ready for spotting.
•
The truck cycle time is the time required for a truck to complete
one cycle of operation. One complete cycle is the time a loaded
truck takes to travel to the dump site, unload, return to the
loading unit, and be reloaded.
•
The loader cycle time is the time it takes the loading equipment to
load the truck, plus any time lost by the loading equipment while
waiting for the truck to be spotted.
NOTE: After the job has started, the number of trucks
required may vary because of changes in haul road conditions,
reductions or increases in haul length, or changes in conditions at
either the loading or unloading areas.
Number of Standby Trucks Required
Identify, based on the normal cycle time, the number of
standby trucks that should be available to replace trucks that
develop mechanical trouble. The number of standby trucks needed
depends largely on the mechanical condition of the active trucks as
well as the size and importance of the job. In the case of a small
fleet and a single loading unit, the ratio of standby trucks to
active trucks may be as high as 1:4. On larger jobs, the ratio is
smaller. Standby trucks need not be idle; use them on lower priority
tasks from which they can easily be diverted.
EXAMPLE
How
many 20-ton trucks (hauling 18 LCY per load) will it take to support
a wheel loader having a 10-cubic-yard heaped-bucket capacity? The
haul-unit cycle time is 20 minutes excluding loading time. The
loader cycle time per bucket load is 0.5 minute. Consider a
60-minute working hour.
Step 1.
Determine the number of bucket loads required to fill a truck.

Using only one bucket load would mean that the truck would only haul
10 LCY per trip. Using two bucket loads would mean that the truck
would haul 20 LCY per trip and the extra material would spill out
during the loading process.
Step 2.
Determine the loading time per haul unit.

Considering one bucket load per truck

Considering
two bucket loads per truck

Step 3.
Determine the number of hauling units needed to support the loading
unit.
Considering one bucket load per truck


Considering two bucket loads per truck


Step 4.
Determine the production based on the number of hauling units used.
The loader will control the production because of the one extra
truck added to the formula. Therefore, there is always a truck
waiting at the loader.

Using one bucket load per truck will require 42, 20-ton dump trucks.

Using two bucket loads per truck will require 22, 20-ton dump
trucks.

With an understanding of the effect of the different choices,
determine the number of trucks to use on the haul and how many
bucket loads to place on each truck. This illustrates that the
capacity of both the loader and the trucks are set numbers.
Therefore, there is a relationship between bucket loads and
haul-unit capacity, which in practice must be an integer number.