The reuse of existing foundations can offer significant schedule, cost, and sustainability benefits on large renovation projects, but it also introduces geotechnical challenges that are not always straightforward to address. One such challenge is how to increase bearing capacity and control settlement beneath existing foundations when demolition and replacement are not feasible.
This article summarizes a polyurethane grouting case history from the renovation and expansion of the Kentucky International Convention Center (KICC) in Louisville, Kentucky. The project required reuse of spread footing foundations that were originally designed for a lower structural load. Polyurethane grouting was ultimately selected as the ground improvement method. While the project achieved good performance, it also highlights the current limitations in design methodologies for polyurethane grouting and the need for additional case histories to inform future practice.
Project Background
In 2016, the Commonwealth of Kentucky issued construction documents for a major renovation and vertical expansion of the Kentucky International Convention Center. The existing structure included a basement exhibit hall and a single story above grade supported on spread footings. The renovation plans called for demolition of the above grade structure while retaining the basement and the existing foundations.
The new structure included two stories above grade, including a large second-story exhibition space. This increased the applied bearing pressure on the existing foundations from approximately 5 ksf to 7.5 ksf. The project performance criteria required that any additional settlement following construction be limited to less than one quarter inch.
The original specifications anticipated permeation grouting beneath select foundations. During procurement, polyurethane grouting was proposed as an alternative and was accepted by the owner. WSP provided the geotechnical design for the grouting program, and the work was performed by URETEK USA Inc.
Subsurface Conditions and Foundation System
The site is located several blocks south of the Ohio River in downtown Louisville. Subsurface information was available from both the original 1970s investigation and a supplemental geotechnical exploration performed in 2015. The foundations were supported on native sandy soils, generally consisting of an upper poorly graded sand and a lower, denser well-graded sand interpreted as glacial outwash.
The upper sand had average corrected SPT blow counts on the order of N60 equals 10, while the lower sand averaged approximately N60 equals 35. Groundwater was well below the foundation bearing elevations. Column footings ranged from approximately 7 to 14 feet in width, with embedment depths varying across the site.
These conditions were generally favorable for foundation reuse, provided that both bearing capacity and settlement performance could be improved.
Why Polyurethane Grouting
Polyurethane grouting has been used for slab lifting for decades and has increasingly been applied to ground improvement beneath foundations. The grout used for this project was a two-component polymer that expands rapidly due to gas generation during the chemical reaction. The material gains most of its strength within hours and is hydrophobic, allowing it to be injected below the groundwater table.
One of the challenges with polyurethane grouting from a design standpoint is that its behavior in soil does not neatly match traditional grouting categories. Based on field observations and published research, three general modes of ground improvement can occur: permeation in relatively permeable soils, densification when sufficient confinement is present, and formation of what is often described as a polymer lattice where grout forms veins and webs within the soil mass.
The relative contribution of these mechanisms can vary significantly depending on soil type, confinement, and injection geometry. This uncertainty played a central role in how the design approach was developed.
Design Approach and Assumptions
The primary design objectives were to achieve sufficient bearing capacity for the increased loads and to limit incremental settlement to less than one quarter inch. Bearing capacity was not expected to govern once some level of cementation or apparent cohesion was introduced. Settlement control was the critical design issue.
An area and volume replacement ratio approach, commonly used for stone column design, was adapted for this project. The basic idea was to estimate an effective composite soil mass consisting of untreated soil and injected polymer, then assign equivalent material properties for analysis.
For shear strength, the friction angle of the sand was assumed to remain unchanged, while an apparent cohesion proportional to the volume replacement ratio was added. For settlement, a direct proration of modulus values was not practical because the modulus of the polymer itself is relatively low compared to dense sand.
Instead, the design relied on estimating the densification effect of the grouting on the surrounding soil. This required several assumptions, including the size of the zone of influence around each injection, the amount of injected polymer contributing to densification, and the percentage of polymer that might migrate outside the assumed influence zone.
Standard SPT-based correlations were used to relate relative density, unit weight, and settlement modulus. While relying on multiple empirical correlations is not ideal, conservative assumptions were intentionally built into the process. The resulting post-grouting settlement moduli were used in Schmertmann-type settlement analyses to estimate incremental settlement under the increased loading.
Multiple design cases were evaluated to account for variations in footing size, embedment depth, and subsurface conditions. In some locations, treatment depths were extended beyond what the calculations strictly required based on engineering judgment.
Construction and Quality Control
The grouting program was performed from late 2016 into early 2017. A typical sequence involved exposing the tops of footings, installing grout tubes on a four-foot grid, and injecting from the bottom upward. Each injection was terminated once a target grout quantity was reached or when upward movement of the footing was detected using laser level monitoring.
In total, approximately 55 foundations were treated, with roughly 1,800 individual injections and over 90,000 pounds of polyurethane grout placed.
Quality control for polyurethane grouting beneath existing foundations presents challenges. The original project specifications were written for permeation grouting and did not translate directly. Intrusive testing beneath the footings was not practical, and no load testing was performed.
Instead, quality control relied primarily on monitoring grout takes and on frequent survey monitoring of columns above treated foundations throughout construction.
Observed Performance
Survey monitoring was conducted several times per week during much of the construction period. The data set contained some inconsistencies due to construction activity, point resets, and varying measurement frequency. Despite these limitations, the available survey data indicated that the grouted foundations generally performed within the project settlement criteria.
Based on the survey information available during construction, the additional settlement of grouted foundations remained within tolerable limits. While monitoring did not extend through full building completion, the results were consistent with the design intent and compared favorably to nearby foundations that were supported using conventional underpinning methods.
The project was completed in 2018, and reuse of the existing foundation system contributed to reduced construction duration and helped the project achieve LEED Silver certification.
Lessons Learned and Need for Further Study
While the project achieved good performance, it should not be viewed as a validation of any single design method for polyurethane grouting. Key uncertainties remain, including the true extent of densification, the size of the effective influence zone, and the relative contribution of permeation versus lattice formation.
This case history highlights both the potential and the limitations of current practice. Polyurethane grouting can be a powerful tool for foundation reuse, but its design still relies heavily on engineering judgment and conservative assumptions.
Additional case studies, instrumented test programs, and research are needed to develop more reliable design models. As with many ground improvement techniques, broader adoption will depend on improved understanding of the underlying mechanisms and performance predictability.
Personal Anectdotes
This article is based on a paper written by me and Mr. Aaron Rogers of URETEK USA for the Geotechnical Frontiers 2025 conference, held at the very same convention center that is the subject of the paper! It was fun to be presenting mere feet from the columns that were being supported by the grouted footings, and it was my first chance to see the completed project! You can find the full paper here.
Mr. Post and Mr. Rogers at the Geotechnical Frontiers 2025 conference
Disclaimer
The author is a full-time employee of WSP USA Inc. The views expressed on this website are his own and not those of WSP.
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