Throwback Thursday – Remember the Alaskan Way Viaduct?

BackgroundAlaska Viaduct

The 2001 Nisqually earthquake highlighted the fact

that the Alaskan way viaduct is nearing the end of its

useful life. After the earthquake, portions of the surface

street and a section of the viaduct settled over 5 inches

and were continuing to settle. Design experts have

examined replacing the central waterfront portion of

the viaduct and seawall with a variety of solutions that

include new elevated structures or surface roadways

or underground tunnels. The target date for removing

the viaduct is 2012, but before that, the settled portion

of the viaduct needed to be addressed.

Scope of Work

A new foundation was designed, using micropiles

and reinforced concrete around the outside of the

existing foundations of the four settled viaduct

columns. Micropiles were chosen because the drilling

equipment can work within the limited space beneath

the viaduct structure. A total of 88 micropiles with a

design load of 190 kips each were specified. The design

of the micropiles was left to the contractor.

Construction MethodSeawall placement

Before construction began, a test program was run to

prove the adequacy of the contractor’s micropile design.

NW Cascade installed two test micropiles and tested

them in both tension and compression. The testing was

specified to be to 167% of design load, but the tests were

run 250% of design load (475kips) without failure.

Northwest cascade installed the micropiles with a track

mounted Klemm 806-4 drill that sat on the existing pavement

and reached down into the shored excavation around each

column. The micropiles consisted of an 7” diameter steel

casing that was drilled down through approximately 30

feet of fill and soft wet sands, and another 30 feet into the

competent marine deposits. Grade 75 #20 Dywidag epoxy

coated threadbars were placed inside the casing and each

micropile was pressure grouted through the casing. In order

to ensure that the minimum embedment of 30 feet into marine

deposits was achieved, NWC used an unbonded length of 50

feet giving the micropile a total length of 80 ft. The micropiles

were capped with plates and a reinforced concrete cap that

attached to the sides and top of the existing footings with

epoxy dowels.

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