CHAPTER
6
Fundamentals of Earthmoving
1. A contractor is planning to use a Caterpillar D6R tracked dozer with angle blade to clear and
grub a construction site. The ground conditions are very muddy with an estimated rolling
resistance of 250 pounds per ton and an estimated coefficient of traction of 0.6. The
construction site is located at an elevation of 4,000 feet, and the average temperature conditions
will be about 60o F. A performance chart for the dozer is shown in Figure 6.3. The dozer
weighs 40,000 pounds and is equipped with a 4-cycle, naturally aspirated diesel engine.
a. What is the maximum usable tractive force that can be generated by the dozer under the
site conditions described?
The first step is to determine the maximum usable drawbar pull that can be developed
by the dozer before the tracks start to spin. Using Equation 6.12,
b. What is the maximum speed the dozer can clear and grub when operating up a 3% slope?
Next, we need to determine the total resisting force, which in the case of a tracked dozer is
only grade resistance. Using Equations 6.3 and 6.4,
Since the drawbar pull required to overcome the total resisting force is less than the maximum
usable drawbar pull determined from the coefficient of traction, the dozer can perform the job.
The dozer is powered with a 4-cycle, naturally aspirated diesel engine, so we use equation
6.9 to compute the derating factor:
6 – 2
Since the dozer engine loses 9% of its performance, we need to determine an adjusted drawbar
pull using Equation 6.11 to enable reading a maximum speed from the manufacturer’s
performance chart, because the data on the chart is only for standard operating conditions.
Now we can read the maximum speed in first gear from Figure 6.3 to be 2.3 mph.
6 – 3
2. A wheeled scraper is being used to excavate the side of a hill to construct a level site for the
construction of an office building. The scraper weighs 150,000 pounds loaded and 90,000
pounds empty and is equipped with radial tires. The weight distribution is shown in Table
6.4. Only the front tires are powered, and the coefficient of traction for the haul road is 0.3.
Table 6.4 Weight Distribution for Problem 6.2
Loaded
Empty
60% rear tires
40% rear tires
40% front tires
60% front tires
a. What is the total resisting force when the scraper is operated empty up a 3% slope over a
road surface with a tire penetration estimated to be 2 inches?
b. What is the total resisting force when the scraper is operated fully loaded up a 5% slope
over a road with a tire penetration estimated to be 0.5 inch?
c. What is the maximum usable tractive force that can be generated by the scraper when
operated empty?
3. A tracked dozer (operating weight = 39,000 lb.) is being used to excavate for the foundation
of a new four-story office building. The dozer is powered with a 4-cycle, naturally aspirated
165-horesepower engine. The construction site is very muddy (the coefficient of traction is
estimated to be 0.7). Manufacturer’s data are shown in Table 6.5. The site is located at an
elevation of about 4,000 feet. A ramp at a 6% slope will be used to remove spoil from the
excavation.
Table 6.5 Manufacturer’s Data for Problem 6.3
Gear
Drawbar Pull
1
27,530 lb.
2
20,969 lb.
3
15,740 lb.
a. What is the total resisting force?
b. What is the maximum tractive force that can be applied without the tracks slipping?
c. What is the maximum drawbar pull available to move a load when pushing spoil up the
6% ramp in first gear?
6 – 5
4. A contractor owns a Caterpillar D6R tracked dozer and plans to use it to clear and grub a
project site and then to excavate for the basements of condominiums to be built on the site.
The dozer is powered with a 6-cylinder turbocharged diesel engine and weighs 40,000
pounds. A manufacturer’s performance chart for the D6R dozer is shown in Figure 6.3. The
project site is at an elevation of 4,000 feet, and the coefficient of traction for the soil
conditions is estimated to be 0.7.
a. What is the maximum usable drawbar pull that the dozer can develop before the tracks
begin to slip?
b. What is the maximum speed the dozer can clear and grub up a 10% slope while operating
in second gear?
First, we need to determine the total resisting force, which in the case of a tracked dozer is only
grade resistance. Using Equations 6.3 and 6.4,
Since the drawbar pull required to overcome the total resisting force is less than the maximum
usable drawbar pull determined from the coefficient of traction, the dozer can perform the job. The
dozer is powered with a turbocharged engine, so no derating factor is required below 10,000 feet
of elevation. Therefore, the maximum speed can be read directly from Figure 6.3 by finding the
minimum drawbar pull required (4,000 lb.) on the left axis, reading across to intersect the second
gear performance line, and reading down to determine the maximum speed, which in this case is
3.8 mph in 2nd gear.
c. What is the maximum speed the dozer can push the excavated soil up a 15% ramp out of
the basement area while operating in first gear?
First, we need to determine the total resisting force, which in the case of a tracked dozer is only
grade resistance. Using Equations 6.3 and 6.4,
Since the drawbar pull required to overcome the total resisting force is less than the maximum
usable drawbar pull determined from the coefficient of traction, the dozer can perform the job. The
dozer is powered with a turbocharged engine, so no derating factor is required below 10,000 feet
of elevation. Therefore, the maximum speed can be read directly from Figure 6.3 by finding the
minimum drawbar pull required (6,000 lb.) on the left axis, reading across to intersect the second
gear performance line, and reading down to determine the maximum speed, which in this case is
2.3 mph in 1st gear.
6 – 6
5. A fully loaded wheeled scraper (empty weight = 97,000 lb.; loaded weight = 172,000 lb.)
must travel up a 3% slope on an unmaintained haul road (rolling resistance is estimated to be
200 pounds per ton and the coefficient of traction is estimated to be 0.45). The scraper is
powered with a 4-cycle, naturally-aspirated 450-horsepower engine. The construction site is
located at an elevation of 5,000 feet. Manufacturer’s data are shown in Table 6.5.
Table 6.6 Manufacturer’s Data for Problem 6.7
Gear
Speed
1
3.4 mph
2
4.8 mph
3
6.0 mph
4
9.0 mph
Only the front tires are powered, and the weight distribution is shown in Table 6.7.
Table 6.7 Weight Distribution for Problem 6.8
Loaded
Empty
50% rear tires
37% rear tires
50% front tires
63% front tires
a. What is the total resisting force?
Since the rimpull required to overcome the total resisting force is less than the maximum
usable rimpull determined with the coefficient of traction, the scrapers can perform the hauling
task.
6 – 7
b. What is the maximum available rimpull in each gear, if the gear train efficiency is 80%?
Now we need to determine the available rimpull developed by the scraper in each of the
gears. Since we have no manufacturer’s performance chart, we must use Equation
6.7 to estimate the available rimpull.
Let’s start with 4th gear. Using Equation 6.7,
Equation 6.7 estimates available rimpull under standard conditions; and we must determine a
derating factor, since the project site is at 5,000 feet.
Using Equation 6.9,
Since the scraper engines lose 3 percent of their sea level performance at 5,000 feet, the available
rimpull in 4th gear at 5,000 feet is
Since the available rimpull in 4th gear (13,200 lb.) is less than that required to overcome the total
resisting force (22,360 lb.), the scraper cannot perform the hauling task in 4th gear.
Let’s look at 3rd gear. Using Equation 6.7,
Adjusting for the 5,000 feet elevation.
Since the available rimpull in 3rd gear (19,800 lb.) is less than that required to overcome the total
resisting force (22,360 lb.), the scraper cannot perform the hauling task in 3rd gear.
Let’s look at 2nd gear. Using Equation 6.7,
Adjusting for the 5,000 feet elevation,
6 – 8
Since the available rimpull in 2nd gear (24,750 lb.) is greater than the total resisting force
c. What is the maximum speed the scraper can operate at on the haul road when loaded?
Since the available rimpull in 2nd gear (24,750 lb.) is greater than the total resisting force
d. What would be the maximum speed, if the haul road were properly maintained with a
rolling resistance of 50 pounds per ton?
Since the rimpull required to overcome the total resisting force is less than the maximum
usable rimpull determined with the coefficient of traction, the scrapers can perform the hauling
task.
We need to determine the available rimpull developed by the scraper in each of the gears. Since
we have no manufacturer’s performance chart, we must use Equation 6.7 to estimate the
available rimpull.
Let’s start with 4th gear. Using Equation 6.7,
Equation 6.7 estimates available rimpull under standard conditions; and we must determine a
derating factor, since the project site is at 5,000 feet.
Using Equation 6.9,
6 – 9
Since the scraper engines lose 3 percent of their sea level performance at 5,000 feet, the available
rimpull in 4th gear at 5,000 feet is