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PIONEER PLACE BLOCK 50 DEVELOPMENT
PORTLAND, OR
Temporary Suspension Shoring Wall System (Patent Pending)
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Rouse Management Inc. (Owner/Developer) required a perimeter shoring wall adjacent to movement-sensitive light
rail tracks, deep utilities and overhead obstructions. They hired Golder Associates to solve the problem.
The lead designers for Golder Associates, Chris Wolschlag and John Byrne,
invented a cost-effective shoring system that could meet the challenges of congested below-grade urban construction.
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Drilling of Steeply-Inclined Tension and
Compression Elements for North Wall Lift 1
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PROJECT DATA AND REQUIREMENTS
- Located at a congested downtown city block in Portland, Oregon.
- Four-story above grade multi-purpose center.
- 226-ft by 232-ft excavation footprint for the basement retail center extends 16- to 26-ft outside of property lines, beneath adjacent sidewalks, curbs, and streets on the north, south, and east sides.
- Basement retail center extends 15- to 20-ft below adjacent street grades.
- Tri-County Metropolitan Authority (Tri-Met) light rail passenger trains travel immediately adjacent to the north and south walls.
- 30" and 36" diameter City of Portland combined sewers parallel north and south walls, and are located 14-ft below grade and 14-ft behind the walls.
- Subsurface consists of 15- to 20-ft of soft sandy silt and loose silty sand, underlain by dense to very dense sandy gravel.
CONSIDERATIONS FOR SHORING WALL SELECTION AND DESIGN
- Vertical drilling at wall line will require a power down of light rail DC overhead power lines.
- With passenger trains passing every 7 minutes, Tri-Met could only allow power down from 2:00 am to 6:00 am.
- Vertical soldier pile wall system economically prohibitive due to limited power down window.
- Tension anchors had to be steeply-inclined to pass under deep city combined sewers.
- Tri-Met required a wall movement criterion of "essentially zero" due to sensitivity of light rail train system.
- Relatively soft/loose soil conditions in upper 15- to 20-ft could result in short stand-up time and tendency for excessive wall movements.
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DESCRIPTION OF SHORING WALL SYSTEM
- We developed the suspension shoring wall system (patent pending) to address each of the constraints on the perimeter shoring.
- Steeply-inclined tension and compression ground anchors work together to suspend a shotcrete wall facing that transfers a net horizontal prestress load to the retained soil.
- Density of elements similar to soil nails to reduce individual anchor loads and facing requirements, and limit ground loss potential.
- Tension and compression elements detailed with unbonded lengths to transfer load beyond no-load zone as with conventional anchored walls.
- Design horizontal earth pressure and lockoff loads based on "at-rest" levels of apparent earth pressure rather than conventional active levels of apparent earth pressure.
- Shotcrete facing designed as two application system, comprised of flashcoat and final coat, to limit ground loss potential.
CONSTRUCTION & PERFORMANCE OF NORTH AND SOUTH SUSPENSION WALLS
- Active utilities and known obstructions were completely avoided.
- Stand-up time problems were mitigated by initial slot-cutting and flashcoating procedures.
- No perceptible lateral wall movements or vertical track movements based on the optical survey measurements, which had a +/- 0.06-inch accuracy.
- Shoring subcontractor bid for all three shoring walls was roughly 50 $/ft.2
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View of South Wall During
Construction of Shotcrete Lift #3
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PERFORMANCE OF EAST SOIL NAIL WALL
- As expected, the east wall, which was designed as a conventional soil nail wall, experienced a maximum lateral
movement of about ¾ of an inch, or 0.35% of the wall height. This demonstrated the movement potential of the soft
and loose sandy silt and silty sand, and the success of the suspension wall system along the north and south walls.
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View of Completed South
and East Shoring Walls
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