Dealing with Unacceptable Soil Conditions

Construction, by nature, has its share of unforeseen conditions. One of the most common is the soil at a construction site. The bid/contract documents provided for a project may include a subsurface exploration and foundation evaluation that would include soil boring data provided by an engineering service. A soil bore can give you a glance at the subsurface, type of material, density, and moisture content, as well as possible water table at any given depth.

Typically, soil boring depth for a one-story commercial building ranges from 10 to 22 feet below the surface. This information helps contractors evaluate the foundation conditions on site and informs the excavator in determining if the subsurface material that he excavates, can be reused as compact fill or trucked off site. If the excavator does not have to truck material off site or truck fill material on site, this is known as the site being balanced.

When excavating a site for footings and column pads, the sizes of the footings and column pads are based on the minimum soil bearing capacity (e.g., 3,000 pounds per square foot.) A recalculation of the footing design may be required if the existing soil does not meet the bearing capacity. Having an engineer testing in the trench at the time the footings are being dug can help uncover potential problems quickly or clear the way to form the footings.

When the excavator digs to the planned subgrade (where the footing will be placed), the engineer will examine and test the soil’s density. If the soil structure meets specifications, the engineer may request the bottom of the trench to be compacted due to the teeth of the bucket breaking the surface.

If the soil does not meet specifications, the engineer will discuss the conditions with the contractor. Costs and availability of materials can limit the options to present to the owner.

From my own experience, here are a few possibilities the engineer may consider.  

1. As stated above, they may consider redesigning the width of the footing to spread the weight of the building across more of the surface area in the trench. This is why footings are known as spread footings.
2. If you have to undercut a few inches, you may want to add compacted fill back up to the planned subgrade, extend the height of the footings, or extend the height of the wall to the proper top of wall height.
3. Concrete slurry can be an option when undercutting. Concrete slurry is a controlled density fill that is self-compacting cementitious material used primarily as a backfill in lieu of compacted fill. Trenches can be filled back to subgrade with slurry and then cured before forming of footings. Slurry can have a compressive straight of 1200 psi (pounds per square inch).
4. Larger undercuts can be filled with approved engineer fill and compacted in lifts. The size of the compact machine will determine compaction specifications. Widening the trench may be part of this solution.
5. Pile driving is when poles are driven down into the bedrock in order to give support to the footings. This option is used when unacceptable soil cannot be removed, when too much must be removed, or for much heavier buildings.

As land becomes more expensive, property that was once low grade may have been filled to give the lot better curb appeal for sale. If the lot has not been filled properly, this can become very costly during construction. If you are looking at property and planning to build, you may want to ask if the property has been filled. If it has, asking for the proper documentation may save you money and headaches down the line.