Transforming What Was Wet and Wasted in Wayzata
Declining Mall Redeveloped to Native Wetland Conditions
By James W. Tiggelaar, PE, CCS, LEED AP
The city of Wayzata, Minn., on the north shore of Lake Minnetonka, is a thriving community just 12 miles west of downtown Minneapolis. The lake has been a centerpiece of the community from its start as a vacation hub, and now the city is known more for executive offices and as a regional retail center. A beach, parks and walking/biking trails are interspersed with mansions that spread from the lake shore up the hill toward downtown.
The Wayzata Bay Shopping Center was once the center of the town’s retail experience. Originally a wetland on Lake Street at the edge of the city’s downtown district, it was filled in 1964 to make room for the mall. The wetland was reduced from 14 acres to two acres, replaced with an impervious parking lot and roofs, and contained in two large pipes so streets and a parking lot could be constructed above.
Trouble from the Start
The mall showed signs of trouble from the beginning. Although the buildings were supported by driven timber and steel piles to maintain their grade and integrity, the parking lot and exterior utilities relied on soils for support. Utility breaks and repaving were regular occurrences at the mall due to the constant long-term settlement of underlying wetlands soils 35 feet thick below 25 feet of urban fill. In addition, site settlement was exacerbated in a vicious cycle of repaving that added weight, setting off subsequent short-term settlement.
Utility breaks and repaving were regular occurrences at the mall due to the constant settlement. Parking-lot settlement was exacerbated in a vicious cycle of repaving that added weight, setting off subsequent short-term settlement. In addition, the site continually experienced long-term settlement irrespective of new loads.
Remarkable as it may sound, roof drainage for most of the 90,000 square feet of flat roofs drained directly into extensive void space below the mall, where significant settlement eventually allowed a person to walk upright. Periodic flooding in this unintentional “crawl space” required a small boat to navigate it for inspections and repairs. Two pipes on the mall’s north and west sides were open to the space, intended to overflow roof water to the city sewer. However, evidence showed that they back-flowed into the space instead.
A roof-pipe collection system was finally installed to route roof flows to an exterior storm sewer on the south side. The roof drain was connected to the sewer with a flexible connection. However, because the site and sewer continued to settle, the connection periodically ruptured unnoticed, thereby allowing the space to flood again during the next storm. Sanitary sewer breaks also periodically leaked into the space, unnoticed for long periods of time.
To add to the troubles, another repellent liquid—petroleum contamination—was present in various areas. These and other conditions pushed the obsolete, declining retail complex to near blight.
Promenade Takes Its Place
Through the years, numerous developers proposed various plans for site redevelopment, but none advanced very far. Presbyterian Homes & Services began investigating the property in early 2007. In February 2007, the design team first met with the Minnehaha Creek Watershed District to begin project collaboration, and the city and other regulatory bodies shortly thereafter. In June 2008, the city approved a General Plan put forward for a mixed-use development, and the detailed planning began in earnest.
“The Promenade of Wayzata” is a three-phase mixed-use redevelopment that replaces the old mall with six distinctive blocks focused on pedestrians and community amenities, featuring realignments of problematic downtown street intersections, extensive walk and street snowmelt systems, and geothermal heating and cooling.
Phase 1 construction was substantially complete in July 2014 and consists of two building blocks dedicated to the senior living community. Folkestone Terrace North includes independent living, assisted living, memory care and skilled nursing, with a second building block, Folkestone Terrace West, for independent living.
Phase 2 began in late 2013 and consists of a building block for independent living, Folkestone Terrace South, another building block for market-rate condominiums and apartments, Regatta Wayzata Bay Residence, and a block for a park, The Great Lawn. Phase 2 is anticipated for completion in mid-2015. A future phase will complete the park and add a sixth and final building block, East Block, with a hotel and office. All buildings are programmed for retail components at street level and have underground parking.
Building a project on this site was foreboding—it’s essentially a “land bridge” because all buildings, boulevards and utilities require unconventional foundation systems (e.g., concrete-grade beams supported by driven-steel pipe piling).
After an array of investigations, preliminary designs and collaborative meetings with regulatory authorities, the project goal formed to mimic native stormwater conditions when the site was a wetland. Recreating these conditions greatly exceeds regulations to match existing conditions.
This approach demanded a significant infiltration component to reduce volume and was a formidable task considering several facts:
• Voluminous volumes—the proposed site isn’t reducing impervious surface and therefore requires large expansive volumes of airspace integral to stormwater systems for attenuating storms.
• Impermeable mush—site soils have literally no integrity to support storm systems and are subject to significant long-term consolidation. Soils also are subject to significant short-term consolidation, even for minor raises in grade, and soils could bounce if cut. Except for deep native sands, soils have virtually no infiltration potential.
• Cramped quarters—from a vertical perspective, stormwater systems have to be located below streets in the narrow space between the bottom of substantive street structural systems (e.g., post-tension slabs, deep precast double tees) and above a shallow water table. In addition, any pipe discharges have to be elevated, because adjacent city storm sewers were perpetually full of water from the adjacent lake level, which was only a few feet below grade. This level bounced higher during storms due to flows from extensive offsite regional areas of upland neighborhoods.
• Laterally limited labyrinth—from a lateral perspective, stormwater systems must fit between dense arrays of driven piles, and far enough from buildings to limit seepage risks.
The geology underlying the Promenade of Wayzata is like a turbulent sea. A designer for the project stated, “I’ve never seen anything like it in over two decades working in the Midwest—one boring is clean sand 14 feet below grade, and another boring 50 feet away is 17 feet deeper for the same sand stratum.” In fact, sketches and graphs of site hydrogeology resemble a roiling ocean all tossed about.
Borings were utilized exclusively instead of test pits to gather geotechnical information due to the deep depth of the sand stratum, shallow water table and need to maintain uninterrupted traffic in the existing mall parking lot. Borings in promising areas for locating the infiltration system were continuously sampled well into the sand stratum to help identify veins of impermeable soils, if present. Piezometers also were installed across the site to gather groundwater information, including depths, flow directions and hydraulic conductivity testing.
The site consists of an unconfined surficial groundwater table averaging about five feet below grade within 8-24 feet of urban fill over 8-30 feet of clayey swamp deposits (“mush”). All this is in the way of reaching an underlying thick sand stratum critical for meeting stormwater-infiltration goals. To complicate things further, piezometric data indicated semi-confined groundwater in the sand stratum having a separate water table slightly elevated above the unconfined water table, both bouncing more than two feet between the lowest and highest readings as recorded within a period of several years.
Big Holes, Filtration Basins and a Pond
The design team settled on three primary systems to overcome the stormy sea of goals and constraints: 1) an infiltration basin, 2) a filtration basin and 3) a pond. Although these stormwater systems sound simple enough, and are traditional engineering, they’re far from ordinary.
The infiltration basin is located below a street, where it turns 90 degrees between three buildings. An under-street filtration system detains and filters runoff from about five acres of building roofs and boulevards, and an under-building pond manages and treats project stormwater runoff from two acres of walks, streets and vegetated areas.
The Engel Street Infiltration Basin is the most significant of the three primary stormwater systems and consists of a large infiltration area and attached detention vault. Excavated through the urban fill and swamp deposits to native clean sands, the infiltration area measures roughly 200 feet wide by 300 feet long with a depth ranging between 14-31 feet deep—a big hole. Although this depth is at, and arguably beyond, the limits of feasibility due to the shallow water table, it’s relatively shallow compared to other areas of the site.
The infiltration basin was equipped with 1,000 LF of 48-inch-diameter perforated Aluminized Steel Type 2 CMP from Contech Engineered Solutions. The maximum feasible infiltration area came up short for storage capacity to reach project goals. Therefore, a pile-supported 40- by 80-foot precast vault is connected to the infiltration pipe network to provide additional storage capacity. Little long-term settlement is expected below the vault, because the grade is a cut of more than several feet, so the floor is a concrete slab on grade.
Runoff from about five acres of building roofs is directed to the detention vault before it passes through a filter-fabric pretreatment curtain wall to 1,000 lineal feet of perforated 48-inch-diameter corrugated metal pipe gallery, which distribute the runoff to the clean sands of the infiltration basin, thereby recharging the groundwater. A full 2.4-inch runoff event is fully infiltrated by the system, which translates to no runoff to Lake Minnetonka for 99 percent of all annual rainfall for five acres of mostly impervious surfaces.
The Mill Street Filtration Basin is 40 by 220 feet of similar construction, but the floor consists of engineered fill and a geotextile filter fabric that can be replaced. Below this sacrificial fabric is clean sand above butt-fusion welded, high-density polyethylene drain tile wrapped in another filter fabric and filter aggregate. An underlying clay barrier layer helps limit seepage toward adjacent building envelopes when the basin fills during storms.
The basin’s eastern third occurs over some of the worst soils onsite and results in a grade raise of about five feet—this area is the old mall “crawl space.” Therefore, the soils were surcharged to remove the primary consolidation associated with the grade increase. The basin’s western two-thirds resulted in a cut of about three feet and relatively smaller long-term settlement. On the basin’s west side, drain pipes connect to a pipe manifold rigidly connected to the pile-supported precast wall using a swiveling slip joint that allows the connection to maintain its integrity as the drain pipes settle with the soils through many years.
Runoff discharges onto riprap energy dissipation after being pretreated by two Contech Vortechs units that combine swirl concentration and flow controls into a shallow treatment unit that traps and retains trash, debris, sediment and hydrocarbons from stormwater runoff. Capturing sediment in the Vortechs units, where maintenance is relatively easy, was preferred over the larger, less-accessible filtration basin and pond.
The East Block Pond is the third primary stormwater system to be constructed with the project’s final, upcoming phase. The new pond manages and treats project stormwater runoff from two acres of walks, streets and vegetated areas of the East Block and The Great Lawn, after oil-grit unit pretreatment. It also manages and treats the overflow portion of larger, infrequent storms from the Engel Street Infiltration Basin. Finally, the pond manages and treats residential neighborhood runoff from about 10 acres that flows directly to the pond as well as another 10 acres flowing to the pond after passing through separate upslope ponds.
The reconstructed pond will be in generally the same location as the existing pond over some of the site’s worst soils, adjacent to the underground parking garage of the East Block building with a garage finish-floor elevation just more than a foot above the pond’s normal water level. Therefore, part of the criteria on which the pond shape is based includes virtually no filling of existing pond embankments, so consolidation is limited in the underlying soils. This helps maintain pond-liner integrity, thereby limiting leakage risks toward the immediately adjacent below-grade parking garage, especially during storms when the pond bounces several feet.
Assurance Against Settling
The site’s soils are anticipated to settle up to several feet through many years, thereby risking significant damage to the storm sewer. Conversely, possible upward-pushing perched water conditions (buoyant forces) could compromise storm-sewer integrity.
To solve the problem, the pipe is intermittently anchored to overlying post-tension street slabs with hangers, lightweight insulation board “backfill” was placed above the pipes, and poly sheeting and pea rock were placed on the sides and bottom. Virtually no weight bears down on pipes, and settling soils slip past. To address buoyant uplift forces and facilitate constructability, hangers are rigid and round reinforced concrete, matching the pipe diameter, and poured monolithically with the post-tension slab over the top of the pipe.
Given the extensive engineering, this new development that factors in an improved understanding of wetland dynamics, is expected to last much longer than the 50 years of its predecessor—turning a “mistake by the lake” into a vibrant mixed-use community that restores an economic engine while contributing to improved water quality.
James W. Tiggelaar, PE, CCS, LEED AP, is a professional engineer at LHB Inc. with 23 years’ experience in land development, with a focus on building and campus projects; e-mail: jim.tiggelaar at lhbcorp.com.
This article is a condensed version of a more comprehensive article highlighting the full array of civil engineering challenges and systems related to stormwater management at the project. To find out more about the project, go to www.lhbcorp.com or contact the author.