Guilford Reservoir

Located near Loyola University in northern Baltimore City, the reconstruction of Guilford Reservoir involved a complete drainage of the existing open reservoir and subsequent installation of two 240-ft diameter 20-million-gallon tanks. Guilford is one of several reservoirs around the city transitioning to an enclosed underground structure to eliminate exposure of the city’s drinking water to environmental contaminants, as mandated by the Environmental Protection Agency in 2006. 

In the original project foundation design, each tank was to be supported by 176 drilled shafts with a diameter of 42 inches and length of 45-68 feet bearing a minimum of two feet into competent 65 TSF rock. Working design loads on each tank included 625 tons in compression, 50 tons in uplift, and 31 tons in lateral capacities for each drilled shaft. Due to site geology and historic use, negative skin friction of residual soil was included in the foundation design. 

In the fall of 2016, FTG performed a geotechnical site investigation that included rock coring in 38 locations per tank. The results of the core samples revealed that the top of rock was up to 96 feet below grade with top of competent rock up to 110 feet, much deeper than originally thought. FTG developed a load testing program consisting of an axial compressive Statnamic test to 1250 tons, static tension test to 100 tons, and lateral test to 62 tons on various test shafts within the site.  

The results of the load testing program resulted in a re-design of the drilled shafts to have a maximum shaft length of 60 feet with load carrying capacities developed in side shear of disintegrated rock or prior rock refusal.

Installation of production shafts were visually assessed with a downhole video inspection device (mini-SID) to verify shaft bearing material and cleanliness. The overall drilled shaft redesign saved the project from multi-million dollar change orders that would have been necessary to chase the depth of rock required by the original project specification.