January/February 2022 - 73

whose construction from 2011 through
2018 included a secant pile buttress
designed to limit deformation of
Millennium Tower during excavation
of the adjacent 60 ft (18 m) deep station.
The buttress was keyed into the 250 ft
(75 m) deep Franciscan bedrock, with
installation costs exceeding $50 million
(€44 million).
In 2016, it was publicly disclosed
that Millennium Tower had settled 16 in
(40 cm) in 7 years, compared to the
predicted 20-year settlement of only
5.5 in (14 cm). The Transit Authority
reported that 10 in (25 cm) of that
settlement had occurred in the two
years before their adjacent work started;
by 2018, Millennium's total settlement
was 18 in (45 cm). The residents, city,
developer, Transit Authority and others
initiated legal claims and counterclaims.
Settlements were reached without
allocation or admission of fault. The
Millennium Tower fallout has still
impacted most contractors and consultants
in San Francisco. In particular,
the project highlighted the potential
scale of liabilities surrounding deep
foundation and excavation support
work. Some repair schemes that were
considered had estimated costs on the
order of $300 million (€260 million).
Heightened sensitivity to potential
construction impacts on nearby structures
has also occurred nationally,
further emphasized by the recent
Champlain Towers collapse in South
Florida. One outcome is an increased
focus on contractual risk management
for all parties, notably the design
professionals and specialist subcontractors,
as has certainly occurred in
San Francisco construction " PostMillennium. "
Another
related impact of the
Millennium has been an increase in
regulation of geotechnical aspects of
construction by San Francisco's Department
of Building Inspection (DBI). In
2017, Interim Guidelines for Structural,
Geotechnical and Seismic Hazard
Design Review for Tall Buildings were
issued, and then formal ized in
November 2018.
Although permitting previously
relied solely on geotechnical engineer of
record recommendations, the new
guidelines require geotechnical peer
review of projects over 240 ft (75 m)
high. This requirement also applies to
buildings constructed in seismic zones
with soft soils, on sites overlying soft or
compressible soils or on any other sites
as directed by DBI. Specific considerations
the peer reviewer addresses
include ground improvement, effects of
dewatering on site and nearby, and
effects of construction-related activities
on neighboring foundations.
In essence, Millennium has increased
the focus on liability and risk
transfer between parties engaged in
geotechnical construction and created a
new requirement for third-party peer
review of support of excavation (SOE)
and bearing support for critical projects.
This means that design-build projects
require earlier contractor engagement to
determine the optimal construction
approach and steer projects through
permit approvals. Often, the peer review
requires enhanced analysis, including
finite element modelling of the geotechnical
construction process, supplemental
to the soil structure interaction
studies that are conventionally run for
completed structures under seismic
loading. In consequence, complex
projects now often have lead times of 6
to 18 months to break ground after a geotechnical
construction team is selected.
These changes have afforded
specialty contractors with an increased
role in project development and
participation in evaluating applicable
solutions for risks. Another upside is
that contractors are more frequently
selected based on expertise, rather than
solely on a low bid and potential
willingness to accept challenging
contract terms.
Post-Recession Challenges
Construction of 350 Mission Tower
kicked off the new, post-recession era in
San Francisco. Its 50 ft (15 m) deep
basement excavation was supported
with internally braced CSM walls that
extended through the Colma sands into
the Old Bay Clay. Since this project's
successful completion, increasingly
deep, complex excavations have
occurred in direct proximity to
established structures. Several sites
A cutter soil mix tool
DEEP FOUNDATIONS * JAN/FEB 2022 * 73
January/February 2022

Table of Contents for the Digital Edition of January/February 2022
