New Light Rail Station Waterproofing

At the University of Washington in Seattle, new subway station construction was underway for an overall extension of the University Link Light Rail (U-Link) that would provide access to the University’s campus, Medical Center, sports venues, and surrounding neighborhoods. The 3.15-mile light rail extension runs in twin-bored tunnels bringing the total length of the region’s light rail system to more than 19 miles. This underground subway station is about 780 feet long, 70 feet wide, and 80 feet deep with groundwater about 15 feet below the surface. The station is located only 50 yards from the existing Husky Football Stadium foundation, creating a significant construction challenge.

Building From the Top-Down

Top-down construction is a building process whereby heavily reinforced slab(s), with large openings within them, are cast on underlying earth acting as a form. Subsequently, level by level excavation equipment is lowered through these large openings and underlying soil is removed to the surface. These large floor slabs with opening are typically supported on previously constructed slurry walls.

Because of station proximity, groundwater control, and concerns regarding excavation impact on both Husky Stadium and adjacent roadways, a top-down construction approach was selected after much evaluation of these factors making project water control aspects more challenging.

Waterproofing Challenges

The original subway station waterproofing design detailed two very different techniques for water control. The station roof or lid implemented a PVC sheet membrane welded to PVC waterbar grids inlaid into the fresh concrete as it was being cast into place. Waterbar placement into fresh concrete at 20 to 30 foot rectangular grids is common for subway lid and wall construction. However, because the amount of large opening in the roof lid and the duration in which waterbars would be exposed to the environment, the concrete contractor was concerned about maintaining water tightness and wanted a different design approach at the lid. The sides of the station used slurry wall techniques which do not allow for exterior membrane application; therefore, an interior water management system was implemented in the design.

Design Build Experts Brought In

STRUCTURAL TECHNOLOGIES’ Moisture Control team was brought in to consult, redesign and modify the initial waterproofing design system for the station. PULLMAN and STRUCTURAL TECHNOLOGIES were contracted through the General Contractor to provide design-assist services (STRUCTURAL TECHNOLOGIES) and installation (PULLMAN) for waterproofing at various subway station locations.

Wall Construction Techniques at Station

Five-foot wide exterior walls were trenched out full depth through existing earth and filled with bentonite slurry. Steel reinforcing bar cages and metal plates were then placed into the trench with bentonite down to the platform level. Then, these slurry walls were filled with concrete displacing the bentonite slurry and leaving a reinforced concrete structural wall.

Lid and Slab Infill Work

After the slurry walls were set, the roof was cast with large openings. This process continued for a total of five levels below grade until finally completing excavation to the platform level. Spandrel beams brace the slurry walls as excavation proceeds downward. Mechanical, electrical and plumbing equipment is then lowered down the openings, upon completion. The openings, once used for access, are infilled with concrete and finishes begin.

Following lid slab concrete infill construction, the waterproofing process began. Team members assigned to this project had worked together on previous waterproofing projects throughout the country. They were experienced with materials and installation techniques key to quality and productivity for both the general contractor and owner.

Exterior Water Control Methods

Different waterproofing design and installation approaches were used at the roof and interior slurry walls. Elevator and escalator pit foundations attached to the exterior of the slurry walls were waterproofed by conventional below grade techniques. The roof system is a barrier membrane designed with redundancy in mind. A two-sided bentonite sheet was placed directly on the concrete roof slab substrate and covered with an Evaloy based PVC heat welded membrane to create a seamless top layer of waterproofing. The combination of composite bentonite and PVC sheet created a high performance, reliable, self-sealing system. Drainboard was placed over the composite sheet system to allow excess groundwater to drain away that filters through the earth cover.

Interior Water Control Methods

An interior water management system was used over the station slurry walls versus a standard barrier approach on the exterior. This negative side, multi-layer system included a dimple sheet drainage layer that channels water down to a drainage system along the floor edge. The dimple sheet layer is covered by a reinforced, heat welded polypropylene membrane to create a monomlythic top layer. This system is then covered by metal studs, drywall, and painted.

These membrane systems were used primarily because of their flexibility and high puncture and tear strengths. The membranes also have a high hydrocarbon resistance to chemicals while maintaining their flexibility, which was considered a big benefit for this specific application and allowed the waterproofing to be installed immediately after the roof and walls were cast.

PULLMAN and STRUCTURAL TECHNOLOGIES worked seamlessly together to improve the original design’s constructability and performance. This helped the general contractor stay on track with this fast-paced, unique, top-down approach to construction. The station began operation in early 2016 — months earlier than previously scheduled.