DFI Magazine September/October 2020 - 16

COVER

STORY

2020 OPA WINNER

Chase Center - Cast-in-Place Piles for Weak Soils

Northwest aerial view (credit: Jason O'Rear)

Chase Center opened in the San Francisco Bay Area in fall 2019 to
provide the home court of the Golden State Warriors. The 18,000
seats at the world-class sports and entertainment center also are
used for concerts by the likes of Metallica, Eric Clapton and Janet
Jackson. In addition, specialty stores and restaurants occupy the
facility on four city blocks in the Mission Bay district, which
evolved long ago from its shallow bay origin.
What's above ground at Chase emerged after a 30 ft (9.1 m)
excavation that was unprecedented in Mission Bay for its depth and
size. The 2-plus years of construction involving Langan
Engineering and Environmental Services and colleagues
addressed poor ground conditions that presented challenges for
designing foundations, supporting the excavation, dewatering and
maintaining a stable subgrade.

Subsurface Conditions
Site challenges stemmed from the area's origin as a shallow bay,
which was reclaimed from about 1860 to 1910 by importing
thousands of cubic yards of sand, rocks and dirt - much of it
debris from San Francisco's 1906 earthquake. Today, the subsurface
consists mainly of fill, Bay Mud, Colma Formation sand, Old Bay
Clay and Franciscan Complex bedrock, and has a significantly
variable profile across the site.

AUTHORS

The site fill extends 9 to 32 ft (2.7 to 9.8 m) as a thick, heterogeneous composition of loose to very dense sands and gravels, soft to
stiff clay, brick, rubble and rock - ranging from cobbles to boulders.
The thicker fill encountered adjacent to thinner fill was indicative of a
mud wave. Such waves can occur when heavy fill loads have been
placed atop weaker Bay Mud, causing a bearing-capacity failure. As
the Bay Mud fails, the fill sinks into the soil and Bay Mud pushes up
around the failure zone, causing the thick-and-thin fill profile. At the
same time, the mud mixes with fill, creating zones that include soft clay.
Bay Mud, a second layer about 2.5 to 45 ft (0.7 to 13.8 m) thick,
is a very soft to medium-stiff weak compressible marine clay. In contrast, the Colma Formation that forms the fourth layer is a mediumdense to very dense sand with varying fines content. Below the
Colma Formation is an overconsolidated, stiff to hard marine layer
known as Old Bay Clay, approximately 6 to 21 ft (1.8 to 6.4 m) thick.
Bedrock, consisting of serpentinite, greenstone, shale and claystone
of the Franciscan Complex, occurs from 33 to 130 ft (10 to 39.6 m)
deep. The bedrock is crushed to intensely fractured, soft to moderate
hard, and friable to weak, with deep to moderate weathering.
Groundwater, which is shallow, was typically encountered 6 to
12 ft (1.8 to 3.7 m) below the surface. With the site so close to San
Francisco Bay, the groundwater is influenced by tidal action,
creating additional construction difficulties.

Lori A. Simpson, P.E., G.E., Serena T. Jang, P.E., G.E., and Matthew C. Pepin, P.E.,
Langan Engineering and Environmental Services

16 * DEEP FOUNDATIONS * SEPT/OCT 2020

DFI Magazine September/October 2020

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