structural RESILIENCE historic STRUCTURES New Solution Needed Following Flooding in Historic Office Complex Buildings
A renovation project found success with a newly developed mat foundation system. By Russ Miller-Johnson, PE, SE
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he 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
Flooding at the State Office Complex in Waterbury, Vermont, in July 2023 reached the approximate 50-year level. This was 3 feet above the previous basement floor level of the preserved, historic buildings. Photo courtesy of fluidstateconsulting.com.
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 M AY 2024
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