The following information supplements PDDM Section 9.5.1.

While roadway excavation is not classified for measurement or payment purposes, it may be categorized for computation of earthwork design and mass purposes as the following types:

1. Common Material. Common material is largely earth or earth with detached boulders less than 0.5 cuyd [0.5 m3].
2. Rippable Rock. Rippable rock refers to material ready for excavating after being loosened by a ripper.
3. Solid Rock. Solid rock includes hard rock in place, ledge rock and boulders requiring drilling and blasting equipment for removal. Any blasting work will be performed according to the rock blasting section specifications.

Determination of Excavation and Embankment Volumes

Use the average end-area method of determining volumes. The total volume of earthwork is the sum of the volumes of the prismoids formed by adjacent cross sections.

When using the average end-area method, the prismoid is treated as a prism whose cross section is the mean of the two end areas of the prismoid. Equation 9.5.1A(1) presents the formula for use in the average end-area method.

Equation 9.5.1A(1)

Where:

V = Volume, cubic yard [m³]
A₁ and A₂ = Cross sectional end areas, square feet [m²]
L = Distance between the cross sections, ft [m]

This formula is approximately correct. Due to its simplicity and substantial accuracy in the majority of cases, it has become the formula in common use. It gives results, in general, larger than the true volume.

When the earthwork center of mass (centroid of the area of cut or fill) is not centered about the roadway, and the alignment is in curvature, the actual volume calculated is not correct since the true distance between the end area centroids will differ from the distance along the centerline. In this case it may be necessary to adjust excavation volumes for curvature in order to properly account for earthwork. In many cases this is not necessary since the eccentricities about centerline of the earthwork mass tend to equalize themselves over the route.

Shrink and Swell Factors

Using data furnished by the Geotechnical Unit, the designer must check the characteristics of the material to be excavated or placed in embankments. The excavation used for embankments will range from rock to earth and have shrink/swell factors assigned for design purposes.

The values shown in Exhibit 5.1 A may be used for estimating purposes prior to obtaining the project specific information from the Geotechnical Unit.

1. Subject to average ±5% variation.
2. Mass densities are subject to adjustments in accordance with modified swell and shrinkage factors.
3. Based on average in-situ densities. A negative number represents a shrinkage. Factors subject to ±33% variation.

Roadway excavation is typically measured in the original, undisturbed position. The specifications must clearly state the place and method of measurement because almost all materials change volume in their movement from cut to fill.

Excavated common material will expand beyond its original volume in the transporting vehicle but will typically shrink below the excavated volume when compacted into the fill. To illustrate, 1 cuyd [1 m3] of earth in the cut may use 1.25 cuyd [1.25 m3] of space in the transporting vehicle, and finally occupy only 0.65 to 0.85 cuyd [0.65 to 0.85 m3] in the embankment. This, of course, depends on its original density and the compactive effort applied. This difference between the original volume in a cut and the final volume in a fill is the shrink.

Excavated solid rock placed in a fill typically occupies a larger volume. This change in volume is the swell. When the voids in the rock embankment become filled with earth or other fine material, the volume in the fill will just about equal the combined volumes in the two source locations.

For light soil excavation and for fills constructed on swampy ground subject to settlement, the shrink may range from 20 to 40 percent or even greater. For moderate soil excavation, the shrink ranges from ten to 25 percent. For heavy soil excavation with deep cuts and fills, expect a range of approximately 15 percent shrink to five percent swell. Shrink generally includes the slight waste in transporting material from cut to fill and the loss for material that escapes beyond the toe of slopes. Embankments that are slightly overbuilt also contribute to an apparent shrink.

The underlying soil below embankments may also settle and compact, or be displaced, due to placement and compaction of the embankment, contributing to an apparent shrink. Settlement results in apparent shrinkage, but one is not directly proportional to the other. Do not confuse shrink with subsidence. Subsidence is settlement of the entire embankment due to weak foundation conditions (e.g., placing heavy fill on swampy soil).

A swell of 5 to 25 percent is often anticipated in rock excavation depending upon the proportion of solid rock and upon the size of the rock placed in the fill. Rock blasting, especially cushion blasting, is inexact and may result in slightly over-excavated slopes, contributing to an apparent swell.

When available, the design should consider actual field shrink and swell factors for like material used on adjoining projects.

Balancing Earthwork

It frequently happens that the material from the adjacent cuts is not sufficient to make the intervening fill. In this case, material is borrowed from outside the construction limits.

When there is an excess of excavated material, it may be necessary to dispose of the material. Instead of long hauls, it may be more economical to dispose of the material by widening shoulders or placing the material in disposal areas than to pay hauling costs.

If the earthwork is not in balance, the designer should try to adjust grade line or centerline so it is in balance. When a balanced project is not practical or desirable, the designer either disposes of excess material or borrows material to obtain a balance. Designated disposal or borrow areas require clearance for proper ownership, rights-of-use, environmental concerns and applicable permits.

Waste areas for the disposal of excess material and/or borrow areas should be shown on the plans.

Haul

Haul consists of transporting material from its original position to its final location. The cost to haul material is required to estimate the unit price of various items of work.

Haul costs are based on hauling one cubic yard [cubic meter] of material a distance of 1 mile [1 km] or 1 ton [1 metric ton] of material a distance of 1 mile [1 km] using the shortest practical route. Haul costs are generally based on a rate per unit of time for the hauling equipment multiplied by the actual time needed to move the material. This is quite simple when calculating costs to haul from crusher sites to the middle of a project. It is more complicated to estimate the costs to haul material between balance points on a grading project. The use of a mass diagram as described below will provide the quantity of haul within balance points as well as other helpful information. The haul cost will be much greater for haul in the uphill direction than in the downhill direction.