What “Unsuitable Soils” Actually Means (And Why It Gets Overused)
What “Unsuitable Soils” Actually Means (And Why It Gets Overused)
A Practical Look at One of the Most Misunderstood Terms in Construction
By Nathan McNallie | MCS Geotechnical Engineering
Introduction
“Unsuitable soils” is one of the most common phrases in geotechnical reports and field discussions. It appears in recommendations, specifications, and daily conversations on site.
It also tends to be one of the least clearly defined.
On paper, the term suggests a clear condition: material that cannot support the intended load or performance requirements. In practice, it is often applied more broadly than necessary, leading to additional excavation, imported material, and increased project cost.
What the Term Is Supposed to Mean
From an engineering standpoint, “unsuitable” typically refers to soils that do not meet the requirements for support, stability, or compaction.
This can include:
Low strength or bearing capacity
Excessive moisture content
High compressibility
Organic or deleterious material
In these cases, the recommendation to remove or improve the material is appropriate.
The challenge is that these conditions are not always constant, and they are not always present across an entire site.
Why It Becomes Vague in Practice
Unlike structural materials, soil behavior is highly dependent on conditions.
A cohesive soil that performs adequately in one state may become unstable when moisture increases. A material that appears unsuitable during excavation may meet requirements after drying or stabilization.
Because of this variability, the definition of “unsuitable” often shifts from a specific condition to a general label.
At that point, the term becomes less about measured performance and more about avoiding risk.
How It Gets Over-Applied
In the field, decisions are often made quickly and conservatively. When conditions are uncertain, the default approach is to remove and replace material rather than evaluate alternatives.
This leads to:
Blanket undercut across large areas
Removal of soils that could be improved in place
Limited reassessment once excavation begins
It is an understandable approach, particularly under schedule pressure. However, it can also result in unnecessary work.
What Often Gets Missed
Soil conditions rarely change uniformly across a site. Even within a small area, strength, moisture, and composition can vary.
When “unsuitable soils” is applied broadly:
Areas that meet performance requirements may still be removed
Opportunities for stabilization or reuse are overlooked
Costs increase without a corresponding improvement in performance
In many cases, the issue is not the presence of unsuitable material, but the assumption that it exists everywhere.
The Cost Impact
The financial impact of over-applying this term is often underestimated.
Additional removal leads to:
Increased excavation quantities
Haul-off and disposal costs
Imported structural fill
Additional labor and equipment time
It can also affect sequencing and introduce delays, particularly if conditions are reassessed late in the process.
These costs tend to accumulate gradually, rather than appearing as a single obvious issue.
A More Practical Approach
“Unsuitable soils” should be treated as a condition to evaluate, not a blanket designation.
A more effective approach includes:
Verifying conditions during excavation
Distinguishing between localized and widespread issues
Considering stabilization or moisture conditioning where appropriate
Adjusting recommendations based on actual field performance
This requires coordination between the project team, but it allows for more precise and efficient decisions.
Conclusion
The term “unsuitable soils” is necessary, but its application often extends beyond its intended meaning.
When used as a general label rather than a specific condition, it can drive unnecessary excavation and increased project cost.
A more targeted interpretation—based on actual site conditions—can reduce cost while maintaining performance and stability.
About the Author
Nathan McNallie is a 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.
