Needing to make sewer improvements in downtown Norfolk, the city requested the design of a new 1,700 lf gravity sewer main on reclaimed land in a historic waterfront area. Teamed with Brown and Caldwell, Schnabel’s role was to evaluate the subsurface conditions and provide geotechnical engineering recommendations regarding the design and trenchless installation of the proposed sewer.

At the site, located along Waterside Drive, Atlantic Street, and Main Street N/S intersection, we performed subsurface investigations consisting of soil borings, cone penetration tests, and laboratory testing. We found the area consisted of variable depths of fill, with alluvial sands and clays underneath, and high groundwater. In general, the fill soils were comprised of miscellaneous urban fills consistent with various debris, while the alluvial soils consisted of loose density sands and soft to stiff clays.

Based on these investigations, we assessed the feasibility of various trenchless methods, including auger boring, microtunneling, directional drilling, and pilot tube methods. We evaluated the constructability of potential launch and reception pits; performed feasibility calculations to evaluate jacking forces, settlement, and thrust block capacity; and prepared a geotechnical engineering report presenting the results of the investigations and engineering assessments. We also prepared a geotechnical baseline report and reviewed project specifications for the pits and trenchless installations.

In light of this analysis, the new 16-inch diameter gravity sewer was planned at depths of up to 23 feet below existing grades in highly congested urban streets. Areas of open-cut excavation are severely limited due to potential traffic impacts and a network of congested utilities.