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6
FUNDAMENTALS OF EARTHMOVING
Introduction
Earthmoving is the process of moving soil or rock from one location to another, and in some cases,
processing it to meet contract specification requirements. Earthmoving construction tasks include exca-
vating, loading, hauling, spreading, compacting, and grading. To determine equipment productivity for
a specific task, you must understand both the physical characteristics of the material to be moved and the
performance characteristics of the equipment to be used. Some material characteristics were discussed
in Chapter 5, and others are discussed in this chapter. Performance characteristics of equipment used in
earthmoving will be discussed in Chapters 7 through 15.
The sequence of earthmoving tasks involved in site work include:
•Clearing and grubbing, which means the removal of vegetation
•Removal of topsoil, which contains organic material and is unsuitable for construction use
•Excavation for utilities and building foundation
•Removal of excess excavated material not needed for backfill
•Backfill of utility trenches and around foundation, and compaction of fill material
•Grading of site to meet contract requirements
•Spreading and compacting soil in areas to be paved
•Spreading topsoil and landscaping
As discussed in Chapters 1 and 4, equipment selection and productivity estimation are essential to
our ability to estimate the cost of site work activities. Before studying specific types of equipment,
we will first examine the earthmoving principles that are used to determine equipment productivity.
In this chapter, we will discuss some of the effiects of soil type and job site conditions on equipment
performance.
Rolling Resistance
Rolling resistance is the resistance of the equipment operating surface to the forward or reverse move-
ment of a piece of wheeled equipment as illustrated in Figure 6.1. It does not apply to tracked equipment,
because the idler, rollers, and sprocket always run on steel track rails. It results from internal friction of
the wheel bearings, tire exing, and penetration of the operating surface due to the pressure of the tires.
For example, loose sand or mud provide more rolling resistance than does a compacted clay surface,
because of tire penetration into the sand or mud. Penetration is not necessary if the operating surface
deects under load. Rolling resistance is expressed in pounds of resistance per ton of gross vehicle
FIGURE 6.1 Rolling Resistance
Fundamentals of Earthmoving 71
weight. Gross vehicle weight is the weight of the piece of equipment without load plus the weight of
any load that it is carrying. It varies with the size, air pressure, and tread of the tires and the condition
of the operating surface.
Experience has shown that the minimum rolling resistance (no penetration of the operating surface)
is about 40 pounds per ton of gross weight on the tires (30 pounds per ton for radial or dual tires).
Additional rolling resistance due to tire penetration is about 30 pounds per ton of gross weight on the
tires per inch of penetration. Thus, the rolling resistance for a given operating surface may be estimated
with Equation 6.1:
Rolling Resistance lb tonR lb
tonper in in of(./) [.
.)( .=+30 ttire penetration)] (6.1)
where R is 40 lb./ton for non-radial single tires or 30 lb./ton for radial or dual tires
Resistance due to internal friction has been considered by manufacturers in the development of
tracked equipment performance charts. Rolling resistance must be considered, however, if a tracked
piece of equipment is used to tow a wheeled vehicle. The resisting force due to rolling resistance will
be the rolling resistance times the weight of the trailer.
It is not possible to predetermine the rolling resistance of a given operating surface without measur-
ing tire penetration. Table 6.1 provides representative values that can be used for estimating purposes,
if tire penetration is not known.
The resisting force caused by the rolling resistance (FRR) that acts against the forward or reverse
movement of the equipment can be estimated with Equation 6.2:
FlbRolling Resistance lb
tonGrossVehicle
RR (.)[ .][=
()
]Weight tons (6.2)
Grade Resistance
Grade resistance is the force due to gravity that a piece of equipment must overcome when moving
up a grade as illustrated in Figure 6.2. When the equipment moves down a grade, the force of gravity
that assists movement is called grade assistance. Grades are usually described in terms of percent slope
which is equal to the change in elevation divided by the horizontal distance times 100. For example, a
slope that rises 5 feet per 100 feet would be a 5% slope.
TABLE 6.1 Representative Rolling Resistances for Various Types of Operating Surfaces
Type Operating Surface Rolling Resistance
Asphalt or concrete 40 lb./ton
Dirt surface: Smooth, hard, dry; well-maintained; free of loose material 50 lb./ton
Dirt surface: Dry, but not firmly packed; some loose material 70 lb./ton
Dirt surface: Soft; poorly maintained 120 lb./ton
Dirt surface: Deeply rutted 300 lb./ton
Sand or gravel: Packed 60 lb./ton
Sand or gravel: Loose 200 lb./ton
Adapted from Production and Cost Estimating of Material Movement with Earthmoving Equipment
72 Fundamentals of Earthmoving
Grade resistance (or grade assistance) is equal to 20 pounds per ton of gross weight on the tires or
tracks per percent slope. Note that grade resistance afiects both wheeled and tracked equipment. Grade resistance
can be calculated using Equation 6.3:
Grad
eResistancelbton lb
tonper slopeslope(./)(.
%)(%=20 )) (6.3)
The resisting force caused by the grade resistance (FGR) that acts against the forward or reverse move-
ment of the item of equipment can be found using Equation 6.4:
FlbGrade Resistance lb
tonGrossVehicleW
GR (.)[ .][=
eeighttons
()
] (6.4)
Total Resistance
The total resisting force (FR) acting against the forward or reverse movement of an item of equipment
is the sum of the force due to rolling resistance and the force due to grade resistance:
FlbFlb
Fl
b
RR
RG
R
(.)(.)
(.
)
=+
(6.5)
Sometimes the total resisting force is expressed in percentage terms and called the efiective grade.
Some manufacturers provide equipment performance charts that allow using the effiective grade to
determine equipment speed. The effiective grade can be determined using Equation 6.6:
FIGURE 6.2 Grade Resistance and Rimpull
Fundamentals of Earthmoving 73
EffectiveGrade ActualGrade
Rolling Resistance lb
%%
()
=
()
+
..
.
%
ton
lb
tonper slope
20
(6.6)
Now let’s look at an example problem involving both rolling resistance and grade resistance. A piece
of construction equipment must generate enough force to overcome the total resisting force, if it is to move and perform
the desired task.
Example 6.1: Determine Total Resisting Forces for Wheeled Equipment
A contractor plans to use elevating scrapers to construct a 3-foot compacted fill that that will support
the concrete slab foundations for a warehouse complex. The empty weight of the scrapers is 38.5 tons,
and the contractor has not equipped the four-wheeled scrapers with radial tires. The 2-mile haul road
between the borrow site and the construction site has an average uphill grade of 4% when traveling
from the borrow site to the construction site. When fully loaded, the scrapers will carry 21.5 tons (loose
volume) of fill material. The contractor inspected the haul road, watching a loaded scraper pass and
estimated that the average tire penetration was about 1.5 inches.
