By Russ Miller-Johnson, PE, SE
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The State Office Complex historic buildings in Waterbury, Vermont, were devastated by Tropical Storm Irene’s 2011 flooding. The fully inundated partial basement levels, used for office, storage, and mechanical system functions, were a complete property loss with extensive clean-up and remediation to remove alluvial debris and incipient mold. The mandated renovation project selected 13 of the original 20 buildings for preservation and resilient improvements requiring the interconnected basements to be “flood-proofed” for 500-year event flood criteria. A range of both dry and wet floodproofing options were studied.
The design team developed a stressed-skin type “sandwich” mat foundation system, comprised of steel fiber-reinforced concrete outer layers with a lightweight flowable cellular concrete fill (FCCF) center, for eliminating basement level flooding. The design balanced the settlement resulting from the added infill weight with the competing hydrostatic uplift effects due to the basement level being below the design flood elevation. The system significantly reduced the difficulties of construction in existing basements and met the owner and construction manager’s budget and schedule constraints. Since 2014, the installation has not seen settlement distress; and in July 2023 it withstood flooded conditions approaching the design flood elevation.
The 2011 flooding first reached the partial basement levels by surface flow into window well and utility passages at the outlying buildings. A utility and passageway tunnel system, originally built for the complex’s prior use as a State Hospital, allowed the water to flow freely into all the basements. With depths up to 7 feet, operational relocation costs and service time losses, as well as the clean-up costs were extensive, and the complex was completely and indefinitely shut down. Because basements have limited access and confined space work issues, drying and removing waste, is routinely a long-term problem.
The renovation project was mandated to preserve the selected buildings with flood resilient measures to the 500-year elevation based on Appendix G of the IBC and the FEMA-based Executive Order for Critical facility use function. This elevation was above Irene’s level by about 3 feet as well as above the local Waterbury municipal Design Flood Elevation requirement at 2 feet above the FIRM-mapped Zone AE 100-year Base Flood Elevation. The existing wood-framed ground floors were about 6 inches above the design level. The State of Vermont Building and General Services specified that no open wet floodproofed spaces below that ground level were permitted to remain, so as to eliminate future cleanup and potential for utility use. Preliminary work by the design and construction team determined that raising the buildings above a service crawl space, itself above the design elevation, or dry floodproofing the basements as reinforced concrete tank-like structures was not economically viable. The project team elected to fill the basement in alignment with the State’s focus on resiliency, prioritizing lower future monetary outlays, reduced environmental impact, and the desire to maintain the location as a locus for state operations.
The circa 1890 low-rise buildings consist of interior and exterior multi-wythe load bearing brick masonry walls supporting wood-framed floors and roofs. Limited existing drawings showed stone and brick foundation walls on rough strips of concrete, confirmed by test pits and probing. GEODesign, Inc., the Geotechnical Engineer of Record (GER), found that the surcharge weight of conventional granular earthen fills or naturally balanced fills such as sand, would likely lead to widespread long-term settlement of the underlying silts, clays, and sandy soils. The findings did consider that initial settlements had likely occurred from flooding “pre-load”, which was reported to have been at least 10 feet deep around the buildings in the 1920s before upstream river flood-control measures were constructed.
The GER analyzed a range of potential surcharges in the basements for settlement effects that could limit settlement potential distress to an acceptable design level. At a 500 pounds per square foot (psf) design load for over 9 feet of fill, calculations indicated overall movements up to 1-inch and settlement distortions in the range of L/250, well in excess of a L/2500 reference criteria for unreinforced brick masonry buildings. To achieve even lighter loadings to limit settlements to acceptable levels with just unstructured fills, the use of lightweight polyfoam or foam-enhanced cellular flowable fills would be needed. And to employ those, with the basements almost entirely below the design water elevations, buoyancy-resisting structural measures were required. However, global uplift forces of a very light fill weight that was acceptable for settlement would exceed the permissible buoyancy factor of safety even if all the available building structure was engaged as ballast. Without an apparent convenient, conventional remedy, Engineering Ventures teamed with the GER in an iterative solution study for a mat foundation system to distribute gravity loads and stiffen against settlements movements, in balance with resisting buoyancy loads.
Options that were designed and studied included a “dry” floodproofing tank with a structured reinforced concrete mat base and side walls encapsulating a light fill; a complete fill of a reinforced, sand-lightweight concrete mat; and a coffered, ribbed “waffle” mat with poly-foam infill. While these mat designs limited differential settlements, the long-term sinking effect from total weight and costs were excessive. The team also studied a two-way structured slab at grade with a hold-down pile grid system over foam-based fill, but this solution was also not viable due to cost and schedule constraints.
While none of the more conventional options studied were viable, the analysis exercise illustrated that a structured fill system that used the full depth of the basement for stiffness could efficiently limit the differential settlements to the comparatively small allowable design limits for the brick masonry walls. The infill solution also armored the basement walls above grade against hydrostatic pressures and potential debris impact. Building off of the understanding that for common concrete structural elements, a lot of the actual cementitious material functions as a placeholder for reinforcing coverage or form-filler and is not used to resist stresses at a given location, the team explored concepts similar to precast concrete sandwich or wood structural insulated panels.
The final solution consisted of a structural mat foundation with 12-inch thick “flange” top and bottom reinforced conventional concrete layers with a lightweight foamed cellular fill (FCCF) shear-based mid-section. The material specifications were developed with the concrete contractor, using a full-range water reducer and a blended aggregate to enhanced flowability of the fiber-rich layers given their inherent stickiness. Establishing a 56-day test period allowed for a relatively higher water cement ratio and the use of fly ash to further allow for uniform placements from a limited amount of access locations. A range of foamed concrete strengths and densities was evaluated for required modulus and strength mechanical properties, permeability, local availability, installation, and cure time logistics, and cost performance implications. The best overall fit for the project was a 32 pounds per cubic foot (pcf) material of 250 pounds per square inch (psi) strength for the middle fill.
The existing posts and complicated cross wall geometries in the basement drove up the cost of conventional steel reinforcing placement in the mat. To alleviate most of the reinforcing placement effort, steel fiber reinforcing in the mix was used for flexural strength and to address minimum shrinkage and temperature criteria. The steel fiber supplier, Fab-Form, provided Helix’s fibers supported by Wicke Herfst Maver’s engineered calculations to confirm dosage rates for strength and stiffness requirements in the mix design. Conventional reinforcing was used to directly engage the bearing walls and to transfer load to the post bases for gravity and uplift load resistance. While the elastic shear design values were used to qualify the middle FCCF material for horizontal shear transfer between the upper and lower concrete layer, to account for a range of anomalies including potential unforeseeable soft soil pockets, placement stoppage joints, catastrophic flood levels, or weak existing brick wall spots, steel reinforcing standees were used as a secondary, ductile and overload horizontal shear transfer mechanism.
Individual building mat foundation analytical models were developed using RAM Elements. The three standard zones of subgrade modulus values were developed and incrementally iterated with the GER using results for displacement and bearing stress review and modification. The design team selected an overall design settlement limit of 1-inch and an L/600 differential settlement limit as target design parameters. While these values exceeded recommendations, the structured mat and overall renovation budget allowed for sufficient mitigation for any minor brick wall repairs that might be required. Analyses were run for each building using a range of mat materials and subgrade stiffnesses. Design properties were varied globally to account for local “soft pocket” possibilities, as well as to address creep effects. The hybrid sandwich mat foundation weighed in at an average density of approximately 60 pounds per cubic foot, with the structural capability to distribute the loads from the bearing walls and the fills to address settlement implications and resist hydrostatic uplift to the walls serving as ballast.
The Architectural team of Goody Clancy and Freeman, French, Freeman concurrently developed related designs and specifications for each option accounting for the moisture effects of below grade walls, building enclosures requirements, and plumbing implications. Additionally staging plans were developed with the construction team, to allow for mat installation to replace existing ground floor framing serving as wall bracing without the need to temporary shoring. As an integral part of the design and pre-construction assessment and estimating work, material testing and survey monitoring plans were developed. These included density, strength and modulus testing for the mix designs during construction for both layers. Surveys taken throughout construction before and after the mat pours found total settlements of up to 5/16-inch, averaging in the range of 3/16-inch, with associated differential settlements not exceeding established L/600 curvature levels. Settlement distress from the mat work has not been observed, and uplift-inducing recent flooding has not resulted in any reported issues.
The preservation of the historic portions of the Waterbury State Office complex for community and environmental well-being required the engagement of the Owner, Construction team and Design consultants in all project phases. Their collaboration, effort and expertise to expand conventional solution boundaries is reflected in the facility’s recent resilient performance against flood conditions similar to those that caused previous damage. The sandwich mat foundation system was found, and now tested, to be an economical solution that met the State’s preservation goals. ■
About the Author
Russ Miller-Johnson, PE, SE, is a Senior Engineer and former Principal with Engineering Ventures in Burlington, Vermont (russmj@engineeringventures.com). Miller-Johnson has served on a broad variety of projects and has designed with many types of materials throughout his sustainability-focused practice.