Pearl District High Rise
A creative dewatering strategy delivered savings for this modern mixed-use high rise development in Portland’s trendy Pearl District.
Security Properties wanted to redevelop a site in downtown Portland’s popular Pearl District as a modern mixed-use complex of apartments and ground floor businesses. The design included an underground parking level, retail space and two residential towers of 11 and 15 floors. Security Properties contacted J. Gordon with GeoEngineers to perform a suite of multidisciplinary services, including site-specific hazard analysis, seismic modeling, and hydrogeologic, geotechnical, environmental and dewatering recommendations. Redeveloping a full city block in a dense urban environment is never easy, but the Pearl District High Rise brought several specific challenges.
During preliminary subsurface investigations, GeoEngineers confirmed that the proposed parking level would be below the permanent groundwater table at the site. That meant temporary dewatering during construction as well as designing permanent dewatering systems to protect the foundation and other ground improvements at risk of hydrostatic uplift and lateral pressures—especially during a 100-year flood event. GeoEngineers’ geotechnical team recommended a mat foundation with a permanent system of cost-effective pressure relief valves to mitigate uplift risks if the groundwater rises.
Temporary dewatering can be costly, so Security Properties asked GeoEngineers to evaluate potential strategies and estimate the quantity of water that would need to be pumped out. GeoEngineers knew that the volume of discharge could vary in this part of Portland by an order of magnitude, and so could the costs. The site was almost entirely covered by an existing building, so the team drilled a large-diameter sampling well nearby in the public right of way. Michael Kenrick and GeoEngineers’ hydrogeologists used this well to perform an intense 24-hour pumping test to more accurately understand the groundwater conditions and water quality parameters. Data from the test helped the team determine where the water could be discharged and develop a cost-effective construction dewatering plan.
A known plume of underground zinc contamination near the site added a significant environmental wrinkle to consider in GeoEngineers’ temporary dewatering plans for the construction phase. The environmental team knew that any groundwater with zinc concentrations above regulatory limits couldn’t be discharged into the local stormwater system. Although zinc does not threaten human health, at high levels it could negatively affect certain fish species in the nearby Willamette River. All groundwater discharge had to be tested, and when it exceeded permitted zinc limits our client would be required to discharge water pumped from the site as wastewater instead, a much more costly process.
To reduce wastewater discharge costs for our client, Cris Watkins and the environmental team developed a creative method to keep discharged water below the zinc concentration limits for stormwater. The site is on high-energy Pleistocene flood deposits from Lake Missoula, which created a high degree of complexity and variability in groundwater movement and transport. A single well might contain different concentrations of zinc from day to day. During a pilot test, the team tested water samples from each of the eight 60-foot wells around the site. Then, during construction, GeoEngineers carefully adjusted the pumping rate of each to reduce the amount of water pumped out of more heavily contaminated wells. By balancing the pump rates according to current zinc levels, the team was able to discharge primarily to the stormwater system, significantly reducing dewatering costs during construction.
The geotechnical team investigated the site and provided recommendations for the mat foundation, including below slab drainage and pressure relief valves to protect the structure from potential hydrostatic uplift during significant flood events. GeoEngineers also delivered specifications for earthwork, below grade walls, a construction crane support pad and temporary tieback and soldier pile shoring.
Environmental Services and Contaminated Media Management Plan
GeoEngineers provided environmental services for the project, including phase I and II environmental site assessments and a contaminated media management plan to identify potential contaminants of concern and outline a strategy for testing, handling and disposing of any affected material encountered during construction. During construction field staff also monitored the site’s temporary erosion and sediment controls to verify regulatory compliance.
24-Hour Pumping Test
GeoEngineers performed a 24-hour pumping test to evaluate hydrogeologic conditions, dewatering strategies, discharge options and related costs. The team worked hard to account for complex variables like groundwater draw-down from other nearby construction dewatering operations.
GeoEngineers secured an NPDES permit for stormwater discharge and managed a complex construction dewatering process. The team prepared a construction dewatering plan that included dewatering well installation, pumping rates, treatment systems, and compliance sampling and reporting before discharge. The team used an innovative strategy to reduce the levels of zinc in discharged groundwater. Staff tested wells and adjusted the pumping rates regularly to preferentially pull water from less contaminated wells. This approach reduced the client’s dewatering costs significantly.
Site-Specific Hazard Analysis
The large size of the podium and 15-story residential tower required a site-specific seismic hazard analysis under City of Portland structural codes, so GeoEngineers called on its in-house experts on seismic modeling to perform an analysis and deliver a site-specific response spectrum.