The Smithsonian’s National Museum of Natural History’s west wing basement was previously used to store vertebrate zoology, invertebrate zoology, botany, and entomology collections; supplies; and miscellaneous exhibition artifacts, as well as to provide building support spaces. In 2003, the Smithsonian Institution (SI) commissioned AECOM to develop a plan to relocate the Office of Facilities Engineering and Operations functions to the vacant basement.
Prior to proceeding with the design, the downtown Washington, D.C., area experienced severe flooding, and several buildings on and around Constitution Avenue lost power and experienced significant water damage. The east and west wing basements received 1 inch and 4 inches of water, respectively, during this event.
Several factors contributed to the water infiltration in the west wing basement, including:
- the size and intensity of the rainfall;
- fluctuating groundwater elevations with ineffective building waterproofing;
- existing arched vault openings in the adjacent main building;
- inadequate surface runoff collection and conveyance systems;
- reliance on municipal infrastructure for discharge; and
- inadequate safeguards and backup systems.
At the SI’s request, AECOM carefully investigated, researched, evaluated, and analyzed contributing factors to the basement flooding problems. The firm then recommended strategies to mitigate future water infiltration and eventually designed the solutions to safeguard the building’s basement.
Investigation, research, and evaluation
AECOM’s research was divided into several phases and began with detailed interviews of the museum’s personnel and maintenance staff to uncover any water infiltration patterns throughout the building’s history. This phase also included investigating record information and verifying site topographic and utility survey information.
The next phase included investigations to document conditions occurring at the site. The AECOM team mapped and inspected existing rain leaders from the roof and explored the building basements and main building-wing interconnects (void and tunnel spaces). This phase also included water testing of several building joints and installing four monitoring wells at locations around the building.
The team found that water was flowing into the basement from several locations and for several reasons. Subsurface flows were flowing through existing arched vaults in the main building north foundation wall into void spaces between the main (original) building and the building wing (above the wing basement elevations). Once in these void spaces, groundwater seeped through the concrete masonry unit infill walls into the basement spaces. Similarly, the team found deficient waterstops within several building joints at the interface between the west wing and main building and around the basement exterior walls.
In addition, the team identified undersized drains and undersized storm pipes on the property, which caused water to build up and enter directly into the building through the loading ramps at both the east and west wings. Exacerbating this problem, the adjacent public infrastructure that the onsite drainage systems tie into includes a severely taxed combined sewer system and a drainage system, which are subject to local flooding and Potomac River tidal influences, making the systems more prone to failure.
Recommendations and design solutions
Based on the field investigations, interviews, and analyses, the AECOM team proposed two classifications of design solutions to mitigate flooding in the basements. Priority 1 addressed solutions to provide protection from rainwater and flooding, resulting from a 15-year storm. Priority 2 addressed solutions to provide protection from rainwater and flooding, resulting from much larger storms, when the adjacent facilities are anticipated to be non-functioning. Together, these solutions provide multiple layers of protection from water flooding into the west wing basement, including the following protections:
• Fill and waterproof arched vault openings — The vaulted wall openings along the main building foundation wall near the west wing are the primary source for the water infiltration. The team grout-injected the vaulted openings, sealing the openings to mitigate any further water infiltration from these access points.
• Seal expansion joint between the main building and west wing basement addition — The north expansion joints between the west wing basement walls and the main building foundation are no longer effective in preventing water infiltration. The team replaced the expansion joint material by injecting the cracks with a polyurethane resin from the exterior. Sealing the expansion joints from the exterior was the preferred method of repair because the exterior was exposed during the drain replacement and the wall waterproofing.
• Re-route lower trench drain to discharge to sump pump system connected to exterior site storm drainage — The depressed basement loading ramps posed a significant flooding threat because of the steep slope toward the building and because the ramps are flush with the basement floor. The trench drains at the building face were previously connected to the interior (buried) plumbing system with small-diameter piping. To remedy these conditions, AECOM recommended two corrective actions. The first includes disconnecting the lower trench drain pipes on the east and west wings from the floor drain system inside the basements and installing new storm piping to pump stormwater flow into the storm sewer running in the parking lots. AECOM also recommended the trench drains to be deeper and wider to avoid ponding and overflow. Second, the team recommended grades at the top of the truck wells be modified slightly to minimize the overflow of site ponding during extreme water events, which cause system backups.
• Install backflow preventers on storm outfalls — The Constitution Avenue area suffers from inadequate sewer capacity. Facilities tied to the combined sewer system (District of Columbia Water and Sewer Authority) are especially susceptible to backups and interior flooding. Based on the conditions associated with the adjacent infrastructure, a critical component to the project was including backflow prevention at the connection point to the adjacent public facilities. AECOM recommended using elastomeric insert valves within the onsite drainage system because of the limited hydraulic impact, minimizing disruption to the existing conditions.
• Replace undersized parking lot storm drain lines — AECOM’s investigations determined that the onsite storm drainage system was designed for smaller water flow levels and was susceptible to backup during heavy storms. Although adjacent infrastructure capacities were currently unavailable, the team recommended drainage system upgrades to evacuate runoff from the site as expediently as possible. Because the National Museum of Natural History falls within the lower portion of the watershed, quick discharge provides the best opportunity for use of available capacity.
• Place sump pumps on emergency power — The team connected the sump pumps in the basement mechanical rooms on the east and west wing to emergency power. This keeps the floor drain systems functioning properly in case the building loses power.
• Install French drain with sump pumps — The base of the void spaces in the wings was not completely sealed, allowing water to percolate into the voids through the soil. The team installed French drains with sump pumps in the bottom of the voids to effectively prevent them from flooding, especially during events such as flash floods when the groundwater level would be higher and the amount of water entering the voids from the bottom would be increased. The pumping system is now supported by emergency power and will pump water out of the building and into the stormwater collection system. The pump system is also now equipped with an alarm device to notify SI personnel of water activity within the space. During design, AECOM carefully considered the pump location and accessibility within the void to ensure that SI could perform routine maintenance and repairs on the equipment as required.
• Install exterior foundation drains with sump pumps — The team placed exterior foundation drains along the exterior wall and installed a sump pump at the basement entry-level elevation that continuously pumps water away from the building. Water is also pumped into a storm drain onsite. The team replaced all undersized pipe sections for this design solution to be effective.
• Provide retention basin for excess water — Due to the severely taxed nature of the offsite utilities, the team designed a stormwater detention basin to hold and release metered excess runoff during extreme storms. The detention basin will collect water from various locations around the west wing (upper and lower trench drains, foundation drains, and the French drains in the void spaces). The team selected an underground vault for storage due to the historic nature of the property.
Additionally, the team modified the underground storage component of the recommended measures to address flooding on the property to meet sustainability objectives of the SI. The SI is “committed to incorporating principles of sustainable design and energy efficiency into all of its building projects. The result is an optimal balance of cost, environmental, societal, and human benefits while meeting the mission and function of the intended facility.” To facilitate this objective, the storage component was modified to include a filtered discharge component to offset the use of potable water for the irrigation and cooling tower water. The strategy generally requires collection and storage of runoff during rainfall and pumping stored water at controlled rates to meet all or a portion of the irrigation system and cooling tower needs.
Of all of the factors that contributed to the significant flooding of the west wing basement during severe rain storms, groundwater flow through the existing arched openings in the main building wall, increased hydrostatic pressure because of existing soil conditions, and undersized trench drains and storm drains piping were the three most significant. The AECOM team was able to resolve these water infiltration issues with no impact to the building operations or to its occupants. Further, use of the underground storage component can be adapted to meet sustainability objectives of the SI. It is estimated that implementing this strategy will have the potential to reduce water consumption by roughly 2 million gallons annually and is anticipated to produce a 4-percent reduction in potable water use, and meet 70 percent of the irrigation demand for the property.
AECOM completed design documents for the first phase in June 2007 and the final second phase in August 2008. The SI started construction for the mitigation measures in early 2008 for the first phase and completed all remaining design recommendations in the final phase in mid 2009.
David W. Lew, P.E., is a principal structural engineer based in AECOM’s National Capital Office in Arlington, Va. He has more than 32 years of experience with the firm, and more than 20 years of experience working with the Smithsonian Institution, providing analysis and design of structural support systems for specialized exhibit items, such as raised floors, specialized lighting systems, freestanding exhibit structures, and casework.Samuel S. Hemenway, P.E., is a senior civil engineer based in AECOM’s National Capital Office. He has been involved with the site design and design of storm drainage systems for more than 22 years.