E 242 Geotechnical Report

The BOE Geotechnical Engineering Division (GED) should be consulted as early in the design process as possible when any appreciable amount of disturbance of the land surface is involved. Disturbances of the land surface may include moving, removing, placing or replacing soil or earth, by excavation, cutting or filling. The Project Manager or Project Engineer should request a geotechnical report from GED to obtain the parameters needed at the beginning of design, such as site earthwork and grading, foundation type, allowable load capacities, earth pressures, settlement estimates, shoring requirements, R-value/overlay thickness, etc. 

The Project Engineer is responsible for incorporating the Geotechnical Report into the project design, design documents, and cost estimate. Refer to Procedures 7.3 Identifying Geotechnical Restraints and 8.4 - Geotechnical Report of the BOE Project Delivery Manual for guidance on identifying a project's geotechnical restraints, initiating subsurface investigations, and developing and completing geotechnical reports and reporting standards.

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 Geotechnical Engineering Division (GED) and request the shrinkage or expansion factor to be used for a specific project.