E 242 Earthwork Computations (Superseded)

Note: The information in this page is superseded by E 242 Geotechnical Report.

The Geology and Soils Engineering Section of the Street Opening and Widening Division should be consulted when any appreciable amount of earthwork is involved. This Section can determine the earthwork volume factor (swell or shrinkage), the type of soil, the amount of compaction required, the maximum slope of cut and fill permitted, and other factors that may be needed to determine earth volumes  and   stability.  Reference is also made to Standard Specifications – 1970 Edition and the Hillside Grading Ordinance, No. 123, 970, which cover earthwork and grading construction practices and the discussions of related practices in this Part of the Manual. See Subsection E 622.11 Flexible Pavement Design, and Section E 680 Side Slopes, and E 020B.

E 242.1 Volume Calculations

Three methods are used in determining earthwork quantities:

1. Prismoidal formula —This is the most accurate method since it takes into account the area in the middle of the prismoidal section.
2. Average end area — This is not as accurate as the prismoidal formula, but it is acceptable for most purposes. The City generally uses this method, not making any corrections for alignment curvature.
3. Contour interval — This is generally acceptable for quick preliminary studies.  It may also be used for odd-shaped construction, such as grade separations or complex intersections. It is much more rapid than the other two methods. The earth- work volume is computed by getting the product of the area bounded by a closed contour and the contour interval.

E 242.2 Limit of Economical Haul

The limiting distance beyond which it is not economical to overhaul yardage is a distance usually designated as the limit of economical haul. For hauls beyond this limit, it is cheaper to waste the excess excavation and haul from a borrow pit for supply of fill material, provided a borrow pit with suitable material is available within a reasonable distance. The limit of economical haul may be determined by the formula: 

Where L = Limit of economical haul in stations 

B=Cost of borrow per cubic yard

O= Cost of overhaul per station in yards 

F= Free-haul distance in stations

The City presently uses 1000 feet (10 stations) as the free-haul distance.

E 242.3 Volume Computation Factor

Allowance must be made for the change in volume of excavated materials from original volume in cut to the final volume in after shrinkage and settlement takes place. Since most soils shrink as a result of this process, a shrinkage correction factor must be applied for earth volume computations.  The percentage allowed for this shrinkage varies with different soils; as an average it is 10%. Occasionally, rock locations are encountered which expand rather than shrink and an average figure that may be used is 10%. However, since certain soils or rock may shrink or expand as much as 50%, it is advisable to contact the Geology and Soils Section of the Street Opening and Widening Division and request the shrinkage or expansion factor to be used for a specific project.