Throwback Thursday – Remember the Alaskan Way 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.
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