How Undercut Decisions Quietly Drive Up Construction Costs
A Practical Look at Where Projects Lose Money Before Work Begins
By Nathan McNallie | MCS Geotechnical Engineering
Introduction
On many projects, undercut recommendations are treated as straightforward instructions. A geotechnical report calls for removal and replacement of a certain depth of material, and the assumption is that the requirement applies uniformly across the site.
In practice, that approach can introduce significant and often unnecessary cost.
Undercut decisions are rarely wrong from an engineering standpoint. However, they are frequently applied more broadly than necessary, especially when subsurface conditions vary across a site.
Where the Cost Starts
A typical recommendation might read:
“Undercut unsuitable soils to a depth of 2–3 feet below subgrade.”
On paper, that seems reasonable. In the field, it can translate to:
Large volumes of excavation
Increased hauling and disposal
Imported material costs
Additional compaction and labor
Across a building footprint or site, this can quickly become one of the more expensive line items—often without much discussion.
Why These Recommendations Get Applied Broadly
There are a few reasons this happens.
Subsurface exploration is limited by nature. Borings provide a snapshot of conditions at specific locations, not a continuous profile of the entire site. To account for this uncertainty, recommendations are often written conservatively.
There is also a tendency during construction to apply recommendations uniformly. It simplifies execution and avoids the risk of missing a problematic area.
While both of these approaches are understandable, they can lead to a “one-size-fits-all” application of what was intended to be a conditional recommendation.
What Often Gets Missed
Soil conditions are rarely uniform across a site. Even within short distances, material can change in strength, moisture content, and suitability for support.
When undercut is applied uniformly:
Areas with adequate support may be unnecessarily removed
Excavation quantities increase beyond what is required
Costs rise without a corresponding benefit in performance
In many cases, portions of a site meet the required criteria without full removal. The challenge is identifying those areas early enough to adjust the approach.
The Impact on Cost and Schedule
The financial impact is not limited to excavation alone.
Additional undercut can affect:
Haul-off and disposal costs
Import of structural fill
Labor and equipment time
Construction sequencing
It can also introduce schedule delays, particularly if unexpected conditions are encountered after work begins.
These impacts tend to accumulate quietly. By the time they are recognized, the work is already underway.
A More Targeted Approach
Undercut does not need to be treated as an all-or-nothing decision.
A more effective approach is to:
Review recommendations in the context of site variability
Identify areas where conditions differ from borings
Adjust excavation limits based on actual field conditions
This allows for targeted removal where necessary, while avoiding unnecessary work in areas that already meet performance requirements.
It also requires coordination between the geotechnical engineer, contractor, and project team before and during construction.
Where Value Is Found
The opportunity is not in eliminating undercut entirely, but in applying it more precisely.
Small adjustments in how these recommendations are interpreted can result in:
Reduced excavation quantities
Lower material costs
More efficient construction
These are not theoretical savings—they are tied directly to how the work is executed on site.
Conclusion
Undercut recommendations are a necessary part of many projects, but how they are applied makes a significant difference.
Treating them as fixed requirements across an entire site can lead to unnecessary cost. Interpreting them in the context of actual conditions allows for a more efficient and controlled approach.
In most cases, the goal is not to challenge the recommendation itself, but to ensure it is applied in a way that reflects the reality of the site.
About the Author
Nathan McNallie is a senior geotechnical consultant with experience in construction materials testing, report review, and construction advisory services. He focuses on practical interpretation of geotechnical data to improve constructability and reduce project cost and risk.
