Stabilizing Roadway Subgrades and Bridge Approaches

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Stabilizing Roadway Subgrades and Bridge Approaches Using Deep Foamjection®: A Modern Approach to Infrastructure Resilience

Will Sublette, P.E.

Geotechnical Engineer

Across the U.S., aging transportation infrastructure faces increasing pressure not only from traffic loading and climatic events but also from the subsurface environment. Differential settlement, voids, and unstable soils threaten the integrity of pavements and bridge abutments, often long before surface deterioration is visible. This article presents a geotechnically sound approach to remediating these challenges using Deep Foamjection®, a polyurethane-based ground improvement method developed by HMI. The method is evaluated in the context of subsurface failure mechanisms and compared with traditional remediation techniques.

Subsurface Failures in Transportation Infrastructure

Roadways and bridge abutments are inherently sensitive to the behavior of subsurface soils. Common failure indicators—surface cracking, rutting, and slab displacement—often trace back to issues beneath the surface such as:

Water infiltration and erosion, often through poorly sealed joints or utility corridors
Poorly compacted structural fill, a legacy of outdated or under-controlled construction practices
Organic or low-strength native soils, incapable of supporting modern traffic loads
Voids from leaky culverts or abandoned utility trenches
Differential settlement at bridge approaches, where transition zones create stress concentration
Slope instability, especially in soft cohesive soils or under saturated conditions

In many DOT-maintained systems, maintenance crews resort to surface grinding, asphalt patching, or sub-base replacement—repairs that are often short-lived due to their failure to address the underlying geotechnical problems.

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Deep Foamjection®: Principles of Operation

HMI’s Deep Foamjection® method involves the controlled injection of high-density polyurethane foam into subsurface soils. The process leverages the expansive properties of structural polyurethane to compact, stabilize, and lift deficient soils. The operation includes:

Targeted depth control, achieved through soil profiling and tools such as the Dynamic Cone Penetrometer (DCP)
Void filling and hydro-isolation, as the closed-cell foam blocks further water ingress
In-situ densification, through radial and vertical expansion that increases soil modulus
Real-time observation, allowing for lift and pressure adjustments to avoid overburden heave or undermining of adjacent structures

Notably, the polyurethane foam used by HMI cures rapidly (often within 15 minutes), is hydrophobic, and has compressive strengths ranging from 50 psi to over 100 psi depending on formulation and confinement conditions.

Engineering Benefits and Verification

The Deep Foamjection® method addresses critical failure mechanisms through soil-structure interaction improvements, rather than surface-level treatments. Key geotechnical benefits include:

Improved load transfer efficiency by reducing differential movement
Restoration of bearing capacity in loose or disturbed soils
Sealing of seepage pathways, reducing hydraulic conductivity around structures

Verification is conducted using tools such as DCP, plate load testing, and ground-penetrating radar. These tests can establish pre- and post-treatment stiffness profiles and verify that the foam reached design depths and achieved sufficient soil densification.

From a construction logistics perspective, Deep Foamjection® can be performed with minimal surface disturbance. Traffic lanes often remain open during application—a major benefit for urban arterials and bridge decks.

Application Case Types

Common applications of Deep Foamjection® in transportation infrastructure include:

Bridge approach slabs subject to differential settlement
Highway void fill behind MSE walls or retaining structures
Roadway slope stabilization in cut-and-fill transitions
Leaking culverts and underdrain systems, where foam acts as a water barrier
Rail crossings, where slab stability is critical under dynamic loads
Highway Joint Stabilization

Conclusion

Deep Foamjection® offers a technically robust, minimally invasive method for addressing subsurface geotechnical deficiencies in transportation infrastructure. By combining engineered polyurethane chemistry with modern installation and verification methods, this technique offers DOTs and contractors a reliable alternative to legacy methods. It provides both immediate structural benefits and long-term durability, especially in environments where soil conditions are unpredictable or access is constrained.

For engineers and transportation agencies seeking to mitigate subsurface risks while maintaining operational continuity, Deep Foamjection® presents a practical, tested solution that deserves strong consideration.

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