Site work is one of the trickiest parts of construction estimating, because it involves several variables that are often unknown when a project starts.
Getting the estimate right at the beginning of the project matters, and a little vigilance can produce significant savings. Earthwork is usually the item that makes or breaks the number.
The hard part for an estimator is assessing hidden factors: the type of soil, the required slope of the banks, groundwater conditions, whether dewatering is needed, and whether bracing or piling will be required for stability.
Technology gives us many convenient approaches to site work estimating — 3D cut-and-fill models, for example, reveal a lot of detail and sharpen your judgment. Even so, estimators need to look at every factor that is critical to an accurate estimate.
Here are the key aspects estimators and contractors should consider when working out a site work estimate.
Answer these critical questions first
- What do the project specifications require?
- How much excavation is required, and where will it be done?
- How will the fill be done? How will excess earth be handled, and where is the dump site? These answers are essential to your total cost.
Site management matters too:
- How many other contractors are involved?
- Who is responsible for clearing and grubbing? How will trees in the earthwork area be handled, and by whom?
- What are the trenching requirements for electrical and mechanical work?
Determine the soil type
A soil boring report is usually included in the project documents, and it's the starting point for assessing soil type. However, the owner or engineer/architect may not take responsibility for the accuracy of those reports.
In that case, be extra careful to check the drawing notes and any other information that helps identify the actual soil type.
It's also prudent to check the soil yourself. Visit the site with a long-handled shovel or post-hole digger, check the soil type, and record it in your notes.
Estimators and contractors should state their assumptions up front when quoting. If ground conditions turn out different from the documents, you'll have grounds for a justified claim. And if soil investigation is required, build the testing costs into your bid.
Calculate swell and compaction
A quantity takeoff for excavation requires an understanding of how different earth materials are measured and how they expand and compact. Excavation is measured in cubic yards (27 cubic feet = 1 cubic yard).
Bank material is earth in its natural, undisturbed state, measured in bank cubic yards (bcy). Excavation disturbs the earth and makes it swell into loose material, which is measured in loose cubic yards (lcy).
Typical swell percentages are 10–18% for sand and gravel, 15–25% for loam, 20–35% for clay, and 40–70% for solid rock.
The opposite happens when earth is compacted during filling and backfilling: the material shrinks. Sand, gravel, loam, and clay typically compact to 85–100% of their bank volume, while solid rock can end up at around 130%. Compacted material is measured in compacted cubic yards (ccy) using these shrinkage factors.
These percentages are field averages. To determine accurate swell and shrinkage for a specific soil, use lab testing where possible.
Determine the equipment requirements
Equipment is one of the biggest drivers of site work cost. Projects with heavy earthwork may need heavy machinery.
Most mid-size to large projects use earthmoving equipment such as power shovels, front-end loaders, dump trucks, backhoes, and trenching machines.
The final equipment choice depends on the project's requirements, the equipment the contractor owns, and each machine's capacity. Relying on rental equipment can make a bid less competitive, so price rentals carefully.
Understand the grading requirements
Every site needs some earthwork adjustment, so cut and fill quantities have to be worked out meticulously. Material that is cut can often be reused in fill areas.
If fill exceeds cut, you'll also need to determine how much material has to be imported.
Hauling — distance, quantities, and equipment — is critical when estimating site grading.
The site plan is the estimator's starting point for cut and fill. Its elevations and contour lines show how the existing ground varies against a set benchmark.
Several methods can be used to determine grading quantities, but 3D models for site visualization and cut-and-fill have made the intricacies of this process much easier to estimate.
Conclusion
These are the key areas to focus on when estimating site work. Depending on the project, there may be additional requirements — topsoil stripping, backfill for deep foundations, excavation that needs special equipment, or engineered fill. Each of these needs to be determined accurately to produce a reliable site work estimate and quantity takeoff.