Deep stormwater infrastructure doesn’t always happen in perfect ground conditions—especially beneath an active airport. At O’Hare International Airport, a major stormwater improvement program required multiple vertical shafts to be connected into the airport’s combined stormwater system. From the outset, the success of those connections depended on one thing: controlling soil behavior at depth before excavation began.
That requirement led the project team to polyurethane soil stabilization early in the planning process.
Project Background
The project involved 20’ – 30’ diameter poured in place concrete vertical shafts extending approximately 30 to 50 feet below grade. Each shaft was constructed with temporary wood lagging intentionally left in place inside the poured walls, allowing future lateral excavation to form stormwater connections.
While the shaft construction itself progressed as planned, subsurface conditions surrounding the connection zones told a different story. The soils consisted primarily of very loose, weak, silty clay—material that could not reliably maintain shape or strength once disturbed. Without treatment, lateral excavation would carry significant unnecessary risk to both safety and constructability.
To address these conditions proactively, the project team engaged Lincoln Company, LLC of Chicago to stabilize the soils surrounding the planned stormwater connections.
The Challenge
The challenge was not a failing structure or an unstable shaft bottom. It was how to excavate laterally with control and predictability in weak silty clays at depth.
Excavation would require cutting through the shaft wall and wood lagging, then advancing outward into soils with low stand-up capability. In an active airport environment, even minor ground loss or instability could have outsized consequences. The soils needed to be improved before excavation could safely proceed.
The Solution
Lincoln implemented a targeted soil stabilization grouting program using NCFI-4RH (formerly Terrathane 24-003) structural polyurethane to modify in situ soil behavior around the shaft connection zones.
Injection was performed through the existing wood lagging, allowing treatment of the surrounding soils without removing shaft components or introducing additional excavation risk. This approach stabilized the ground in place and preserved the integrity of the shaft structure.
As the NCFI-4RH polyurethane was injected, it interacted directly with the silty clay soils. Rather than forming discrete void fills, the material densified the soil, increasing soil cohesion while increasing its ability to stand during excavation. The polyurethane displaced pore water, permeated weaker zones, and expanded to create a densified, interconnected soil mass capable of supporting controlled lateral excavation.
With soil behavior fundamentally changed, excavation could proceed as planned.
Quality Control and Verification
Because excavation would occur at depth, verification of soil improvement was critical.
Before injection, soil density probes were performed at every planned injection location to establish baseline conditions. These measurements informed injection planning, including material volumes, treatment focus, and anticipated response.
Following stabilization, post-injection density probes were taken at the same locations, providing direct, measurable confirmation of soil densification. Comparing pre- and post-treatment results allowed the team to validate performance, adjust injection quantities as needed, and ensure consistency across all connection zones.
This data-driven approach reduced uncertainty and supported safe, controlled excavation.
The Outcome
With the surrounding soils stabilized and verified, crews were able to excavate laterally from the shafts, form the stormwater connections, and advance construction without loss of ground or disruption.
The stabilization effort helped maintain safety at depth, improved constructability, and allowed critical stormwater infrastructure work to proceed efficiently within a high-consequence airport environment.
Why Experience Matters
Projects involving deep shafts, weak soils, and operationally sensitive facilities demand more than materials alone. They require experience, judgment, and disciplined quality control.
Lincoln’s long history working in deep excavation and soil stabilization environments positioned the team to manage risk proactively—using polyurethane not as a reactionary fix, but as a planned tool to support excavation success.
At O’Hare International Airport, stabilizing the soils before excavation made the difference between uncertainty and control.
