Retaining Wall Stabilization Using Dual Polyurethane Foam Grouts

Home » Learning & Resources » Case Studies » Tunnel Abandonment Beneath Church Breezeway Addition

PROJECT OVERVIEW

A church located in Manitowoc, Wisconsin was constructing a new addition and needed to convert an existing breezeway into a sloped, ADA-compliant ramp. When the original slab was removed, the construction team discovered a large, abandoned tunnel directly beneath the ramp footprint. An HMI® contractor provided an engineered void-fill solution using HMI’s 201 Lightning high density polyurethane foam and FillFoam™ lightweight geotechnical foam.

THE CHALLENGE

Large unknown tunnel void beneath a structural transition zone
Need for stable, uniform load-bearing conditions
Restricted access preventing traditional backfill methods
Geotechnical uncertainty regarding settlement and structural integrity
Tight construction schedule requiring phased work

ALTERNATIVE SOLUTIONS CONSIDERED

Flowable fill and traditional fill were evaluated but rejected due to weight concerns and limited access. In addition, traditional fill must be placed and compacted in lifts to minimize future settlement, which was not feasible here due to site constraints and restricted access. Structural reconstruction was considered but deemed too costly and invasive. Lightweight geotechnical foam provided the ideal solution with superior flowability, stability, and installation speed.

THE HMI SOLUTION

The contractor used a combination of HMI 201 Lightning polyurethane foam and FillFoam™ to fully seal, fill, and stabilize the void. FillFoam™ offered long-distance flow, extremely lightweight placement, and fast curing—ideal for tunnel abandonment and void remediation.

PROJECT EXECUTION - Phased Approach

BULKHEAD CONSTRUCTION: A bulk wall was constructed to contain material during filling.
SEALING: 100 lbs of HMI 201 Lightning polyurethane foam was applied to seal the bulkhead wall, ceiling areas, and all joints.
VOID FILLING: Within one hour 4,500 lbs of FillFoam™ was installed into the approx. 6′ x 6′ x 14′ tunnel, producing the volume of 20 cubic yards, starting from the rear and filling forward to achieve complete cavity encapsulation.
CAPPING: 200 lbs of HMI 201 Lightning polyurethane foam was applied to cap the FillFoam™ mass and create a rigid sealed surface suitable for the new slab.

After capping was complete, the job site was ready for the concrete ramp installation. It was recommended that the new concrete slab be doweled into the existing foundation to improve structural support above the former tunnel.

ENVIRONMENTAL & SAFETY ADVANTAGES

Using engineered foam materials allowed the team to:

Avoid trucking in several cubic yards of traditional fill
Reduce fuel consumption and overall environmental load
Minimize job-site disruption and equipment footprint
Lower worker exposure to confined or potentially unstable underground spaces
Eliminate dust and debris associated with traditional soil movement

This made the approach both safer and cleaner than conventional remediation methods.

QUANTIFIABLE BENEFITS

The foam-based approach delivered clear measurable advantages:

Over 90% reduction in material weight compared to conventional fill
Tunnel abandonment achieved with 20 cubic yards of fill material
Eliminated the need for importing, placing, or compacting several tons of soil
Achieved complete cavity encapsulation with minimal disruption

🟥 CASE STUDY – Void Remediation – Tunnel Abandonment Beneath Church Breezeway Addition

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