January/February 2022 - 71
FEATURE ARTICLE
2011 Transit Center construction,
near Millennium Tower (left rear)
Soil Mixing Evolution in San Francisco
" San Francisco is 49 square miles
surrounded by reality. " So said Paul
Kantner of the local '60s icons, Jefferson
Airplane, and his words have still held
true. The rate, scale and complexity of
recent construction in this city has
been remarkable, even compared to the
Gold Rush or reconstruction after the
1906 earthquake. This article will
address developments in soil mixing in
San Francisco since 2011, and how the
marked settlement of a downtown high
rise has influenced this booming
construction market.
Local Soil Mixing
Deep soil mixing has gained prominence
in San Francisco for four reasons. First is
the regional concentration of expertise.
Japanese deep mixing technology was
introduced to the U.S. in 1987 by S.M.W.
Seiko. This pioneering company was
based in the East Bay and some of its
early employees still work at the eight
soil mixing contractors now operating in
the area.
Second, subsurface conditions here
are well suited to soil mixing methods.
The stratigraphy at city sites typically
comprises urban fill, dune sand, young
marine or marsh soils, Colma sand, and
Old Bay Clay. The fill and dune sand are
predominantly granular, forming wellblended
soil cement. Although the
young marine and marsh soils are
mainly cohesive, their low strength
allows for effective mixing, creating a
reasonably consistent product. The
stronger Colma formation, while
predominantly dense well-graded sand,
contains some silt and clay, which is
mixable using shaft and end mixing
AUTHOR Rob Jameson, Malcolm Drilling Company
DEEP FOUNDATIONS * JAN/FEB 2022 * 71
methods. Typically, groundwater is
within 15 to 20 ft (5 to 6 m) of grade,
requiring cutoff walls for deep excavations.
The Old Bay Clay unit, present at
approximately 100 ft (30 m) below the
downtown Transbay district, is an
effective groundwater control layer, and
cutoff walls are frequently keyed into
this strata, or at least constructed with a
deep toe within the Colma unit.
Third, the active seismicity of this
region generates geotechnical challenges
such as static settlement and
liquefaction. Soil mixing offers efficient
and cost-effective solutions for these
situations. Finally, market demand has
enabled contractors and consultants to
rapidly evolve their expertise, as each
successful project sets a new bar for
complex support of excavation in tight,
constrained urban sites. Although in
January/February 2022
Table of Contents for the Digital Edition of January/February 2022
Table of Contents
January/February 2022 - Intro
January/February 2022 - 1
January/February 2022 - 2
January/February 2022 - Table of Contents
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