DFI Magazine September/October 2020 - 17

Construction Challenges
In addition to poor subsurface conditions, construction constraints
from the neighborhood included light-rail trains running along Third
Street on the west side of the site and a UCSF Medical Center hospital
kitty-corner from the site. Also, the neighborhood is a biotech hub
with nearby buildings that contain sensitive equipment. Therefore,
noise and vibrations were restricted to typical construction levels.
Because of the variable subsurface fill, soft clay and shallow
groundwater, the structure needed deep foundations that extended
into bedrock to gain adequate capacity. Most feasible was a cast-inplace deep foundation system that could be cased to maintain the
shaft openings in loose fill and soft clay. In addition, the foundation
contractor, Malcolm Drilling, had to adjust foundation lengths on the
fly to account for hardness and variations in the depth to bedrock.
Augered cast-in-place (ACIP) piles were the most feasible and
economical foundation type; but where loads of the arena core were
high and significant uplift resistance was required, foundations had
to reach far enough into rock to develop sufficient compression or
uplift capacity. Achieving the uplift capacity was particularly difficult
where bedrock was shallow and the overburden inadequate to
provide much uplift resistance without a significant rock socket. As a
result, Malcolm proposed a 'hybrid' foundation solution in which
drilled shafts with rock sockets were used about one quarter of the
time, when ACIP piles could not develop adequate capacity. This
value-engineered solution not only addressed uplift capacity
efficiently, but also ensured meeting a tight construction schedule
despite potential ground condition difficulties.

Excavation support from tied-back cutoff wall

The 30 ft (9.1 m) deep excavation - the deepest yet in Mission
Bay - extended into weak Bay Mud, requiring a shoring system
designed to provide adequate support. The shoring also had to
provide groundwater cutoff to maintain a dry excavation and
prevent the groundwater level outside the site from dropping.
Because of the compressible Bay Mud, lowering the groundwater
beyond the site's limits could have caused settlement of nearby
improvements-including the light-rail tracks. The excavation was
supported using a tied-back, cement deep soil-mixed (CDSM)
cutoff wall. Low resistance of the fill and Bay Mud, however, made
it difficult to adequately restrain the shoring.

Idealized profile of subsurface

DEEP FOUNDATIONS * SEPT/OCT 2020 * 17



DFI Magazine September/October 2020

Table of Contents for the Digital Edition of DFI Magazine September/October 2020

CONTENTS
CONTENTS 2
DFI Magazine September/October 2020 - Intro
DFI Magazine September/October 2020 - 1
DFI Magazine September/October 2020 - 2
DFI Magazine September/October 2020 - CONTENTS
DFI Magazine September/October 2020 - 4
DFI Magazine September/October 2020 - CONTENTS 2
DFI Magazine September/October 2020 - 6
DFI Magazine September/October 2020 - 7
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