Some of this column’s readers have sent comments and questions, and one of those prompts this month’s column regarding the earthwork on a project. The question posed was how to control the costs of earthwork and this brings to the forefront the importance of an adequate allowance for this division included in an estimate.
Most estimators have dealt with the frustration of coming up with a rational cost factor for dirt work. Finding an earthwork contractor who will actually provide a binding bid is equally aggravating. None of these annoyances justify doing the hard work to attempt to provide a figure to the estimate.
NECA’s Manual of Labor Units is one of the references that can reduce an estimator’s blood pressure. While the manual is not a primer on underground work, it does cover many of the operations required when considering underground installations. Using a reference such as the manual or a manufacturer’s catalogue provides an organized path to coming up with the required costs.
The primary operation of an underground installation is the job walk prior to beginning the estimate. This is the time to examine the site and note existing conditions. Nobody is perfect in their observations, therefore a camera, even one of the cheap throwaway units, is a must. Pictures will often reveal details that may be missed on the walk.
Items to be noted on the walk are the surface cover—whether it’s asphalt paving, concrete or some other method. The depth of such cover is a factor when cutting cost is considered. Just a gentle reminder that any trench requires cuts to be made on two sides; guess who forgot to do that one time? The surface cover, or lack of it, is also a replacement cost to be priced.
As electrical estimators, we are not readily familiar with classifying the underlying “diggability” of the ground. A simple tool to take along is a piece of steel drill rod with a T-handle welded to it and a sharp point at the other end. Prodding the soil with the tool will help to reveal how well the area is compacted, and this will help establish backfill compaction considerations and a time line for opening the ground.
Cost data from previous projects is invaluable to judging the costs for the current estimate. This is one of the many reasons that costs are divided into recognizable categories. Another source for these costs is equipment operators in the geographic area of the project location, as these operators are usually familiar with the peculiarities of your job location.
While many of the operations of the trenching are obvious, other factors may not be as easily recognized. Location of the project is paramount, since pricing concrete for duct banks has a component for deliveries past a given maximum. Concrete deliveries are also time dependent and a limited amount of time is allowed for unloading the product; a factor to consider when laying out the trenching to avoid obstacles for the delivery.
Ever increasing are problems and costs associated with transferring the removed paving surfaces to a legal dump location. The dirt spoils may not be a problem, but should be considered, as few projects will allow the spoils to spread or left at the site.
Duct banks requiring concrete encasement need some special attention. Conduits in trenches can float when poured in concrete and this poses a problem of anchoring the banks firmly. Most duct banks are encased for at least three inches around the conduits and whatever spacing is called for between the conduits. Unless the ducts are to be direct buried in earth, most will be required to encased in concrete.
Specific attention must be directed at the mix called for in the specifications for encasement. Where the banks must also offer some structural value, the mix may require more concrete and heavier gravel. In many instances a slurry mix can be used. Slurry is a mix of one bag of concrete to four, five or even six sacks of sand. Slurry encasement does not offer structural value, but does protect the duct bank.
When calculating excavations, the amount of dirt to be removed is a function of the width, depth and length of the trench. When backfilling, the amount of room of the duct bank must be deducted from the amount of dirt excavated and disposed of. When calculating the space taken up by the duct bank, it is best to use the trade size of the conduit to make the calculation as well as the spacers.
Underground operations need not be a mystery if approached with an organized task analysis; the work is not any different than any other system of the project. Analysis and attention to details pays off with this type of work. EC
DAVID is a professor of electrical technology at Long Beach (Calif.) City College, a consultant and an expert witness. He can be reached at 562.597.1877 or at firstname.lastname@example.org.