The Basics of Earthwork Takeoff

A site design that incorporates the best use and minimum movement of soil is a winner for everyone involved in construction. Unfortunately, the design process does not always meet these standards, and this may lead to expensive change orders and trucking costs. 

Software that can incorporate data from a variety of sources (PDF files, CAD files, paper plans and topographical data) not only decreases the time to prepare an estimate and validate the site analysis results but also minimizes a contractor’s financial risks.

Listed below are some of the major factors that must be addressed by site-work estimators and considerations that should be made when evaluating takeoff software.

Acquire Accurate Topographical Data

Every inch of error in elevation on a 25-acre site can affect the job by more than 3,000 yards. Sites developed with the existing grades determined by uncorrected aerial surveys or sparse spot elevation data are at risk. If there is any doubt about accuracy, the supplied existing topography should be independently verified.  A GPS rover or robotic total station makes the process of collecting elevation data easy.

Use Soil Borings as Insurance

Sites should be designed to minimize soil import or export by assessing the suitability of stripped and excavated material for use elsewhere on the site. In a perfect world, this “balanced site” would require no import or export. An expensive mistake could be made without considering the on-site material’s suitability.

Material generated from the cut operation, for example, may not be suitable as fill, and the fill under structural areas (buildings, paving, etc.) might require an import of more expensive quarry fill. The site might also yield more low quality material than required in non-structural (landscaped) areas. An expensive haul-off and/or an expensive import would be the result.

A generous number of soil borings or test pits are required for an accurate prediction of the cut material’s quality. While a balanced site cannot always be achieved within the site’s design goals (drainage, aesthetics, etc.), knowledge of material quality from a sufficient number of soil borings helps prevent unanticipated project cost overruns.

Make Adjustments for Topsoil Removal and/or Demolition

If the site requires topsoil stripping or demolition (for example, removing old walks and paved areas, building slabs, etc.), these values must be subtracted before the cut and fill quantities are calculated. If the stripping is not deeper than the cut, then the total amount of excavation is not changed, but these two must be kept separate for pricing purposes. In fill areas, stripping will increase fill requirements by as much as the stripping volume in some cases.

Allow for Material Placement

For proper site takeoff, an adjustment for building pads (the finished floor minus the slab and base material), paving thickness and topsoil respread must be made. Consider a topsoil replacement requirement of 4 inches. In a fill area, the amount of regular fill needed will be reduced by the 4 inches of topsoil respread. In a cut area, the site must be over-excavated by 4 inches to leave room for the topsoil placed later. Cut and fill calculations must be made to the bottom of the paving footprint and building slab.

Compensate for Material Shrinkage and Expansion

Undisturbed soils will normally expand when excavated and shrink when used as fill. Net shrinkage (bank volume to compacted volume) of 5 to 15 percent is not unusual. The loss of material due to shrinkage can affect a site balance tremendously. For example, not considering a 10 percent shrinkage on a 100,000 cubic yards or m3 “balanced site” would result in a shortage of 10,000 yards of material when excavated and re-compacted.

Work with Insufficient Design Details

Takeoff software must also provide additional tools for plans with insufficient design detail (offsets, slope to daylight, etc.). These tools allow estimators to use their excavating expertise to fill in the blanks when designs are lacking.

The right software will dramatically improve productivity, giving lower overhead per bid and more bids in less time. The improved accuracy and supporting documentation minimizes risk. Overall, contractors can win more jobs and attain higher profits per job.

Construction Business Owner, March 2011