Some projects, like the Centre 425 building nearing completion in downtown Bellevue, Wash., require an extra dose of advance planning and creative thinking. Because soil anchor tiebacks could not be used along two sides of the Centre 425 site, an alternative method had to be found to support the excavation for this 16-story boutique office building’s eight subgrade parking levels and foundation.
The solution lay in rearranging the project pieces, as well as the order in which they were constructed. A unique approach to forming and excavation enabled working from the top down to first construct an L-shaped section of each level, which as each level was completed provided support to the exposed excavation walls, then working back up to ground level.
“The top-down approach was the most constructable and economical option,” said Todd Lee, executive vice president and director of operations for the project management group of Sellen Construction Co., the project’s Seattle-based general contractor. “It also allowed for the best construction schedule.”
Excavating one floor at a a time
To accomplish the top-down approach, engineers in Peri Formworks’ Woodland, Wash., office designed large table forms using standard components from the Peri Rail Climbing System (RCS). However, in this case, the tables were designed to be suspended by rods rather than being supported from below.
The shoring subcontractor, Malcolm Drilling, first installed traditional steel piles, wood lagging and tiebacks along two sides of the excavation. Concurrently, Malcolm installed a dozen 24-inch drilled steel caissons filled with concrete and extending 20 to 30 feet beyond the planned 80-foot excavation depth in a pattern covering the opposite side of the building footprint.
The caissons provided support for a temporary steel framework that was used to support about 13,000 square feet of table forms, representing about one- third of each parking level slab. After the framework was installed atop the caissons, Northwest Construction began excavating beneath the framework. In a mining-like approach, one hydraulic excavator advanced the face of the excavation while a bulldozer moved the dirt out from under the frame. A second excavator loaded it onto a conveyor that filled trucks waiting on the street. The total excavation amounted to 132,600 cubic yards of material.
Soil conditions on the site permitted somewhat more than 20 feet of unsupported excavation. When the first 20-foot-deep excavation had been completed, the table forms were moved into place and suspended from the steel framework.
The parking level slabs are typical 8-inch-thick, post-tensioned construction, using mostly 6,000 psi concrete. A higher strength mix was puddled in select areas where rapid strength gain was required. Concrete was pumped from street level.
The form tables themselves were sized to fill the space between the caissons, which ultimately became the columns supporting the structure. Forming around the caisson/columns was easily accomplished using Peri Variokit components, working with steel angles welded onto the columns. A wide, shallow beam along each caisson line accommodated drilled-in dowels and a closure pour.
“We prebuilt the table forms in our warehouse about 150 miles south of the job,” said Justin Lunday, a sales manager with Peri Formworks and one of the formwork designers. “Each form was about 14 feet wide, and they varied from about 40 feet to 80 feet in length.”
The caisson/column spacing was designed to minimize the number of holes that had to be left in the slab for the support rods to reach the lower levels. The table forms used standard Peri formwork components, such as its GT24 girders. But to achieve the higher capacity required by the longer spacing between rods, truss-like construction was used to build the RCS rails for the rail climbing system - in this case, a rail lowering system.
After the P1 slab was poured and another level had been excavated below it, the forms were lowered to the P2 level. This sequence continued on about a two-week cycle down through P7. The lowest level, P8, was placed ongrade at the bottom of the excavation.
At that point, crews dismantled and removed the table forms and placed the foundation for the concrete core. “We had to get the mat footing poured and get a couple lifts up on the core before we could start that first infill piece of the slab,” Lee said.
“We got to the point where we were turning a floor every three days,” Lee said. “So the core appeared to go much quicker than the slabs. But the timing worked so that once we got back up to the street level with the slabs, we were topping out the core, which then allowed us to start structural steel.”
Sellen used Peri Skydeck forms for the second set of parking level slabs coming up out of the hole. The cycle time, both going down and coming back up, was about two weeks per pour.
Preplanning and retrospection
Breaking the concrete placement for each parking level into two pours actually made good sense. With a 36,000-square-foot footprint, finishing an entire slab in a single day would have been quite challenging. But rearranging the placement order to go down one side and then come up the other solved the excavation support problem and also allowed the use of the somewhat more economical traditional approach coming up out of the excavation on the second half.
Adopting the two-stage top-down then back up construction sequence directly affected two other aspects of the project. Constructing the steel framework at ground level provided a real benefit early on. By topping it with a plywood deck, Sellen gained a large staging area in this busy urban setting that would not otherwise have been available.
The project sequencing also limited Sellen’s crane selection options. “We needed to get the crane installed during excavation, which didn’t allow us to put it inside the excavation,” said Lee. Putting it on the side of the street eliminated the possibility of using a hammerhead crane because it could not be freestanding and at the same time tall enough to miss the apartment building across the street. The solution: a luffing crane. The crane was jumped three times, as core construction progressed, to continue providing access to the farthest areas of the site some 200 feet away.
Construction of the parking levels reached street level in mid-March 2016, and the building topped out in mid-May. Completion is expected in fall 2016.