January/February 2023 - 81

time, greater depths it could penetrate
and its ability to meet strict deformation
criteria.
Existing Site and Subsurface
Conditions
The project site is located within the
Hunters Point Shear Zone. The project
site is generally level at about elevation
+3 ft (1 m) and the groundwater level
was observed at a depth varying between
7-12 ft (2-4 m) below existing
grade, although the piezometric head in
the deeper soil strata is generally
higher than the unconfined near
surface groundwater level. In general,
the geologic units encountered at the
project site consisted of artificial fill
extending to a depth ranging from 1018
ft (3-5 m). The fill was underlain by
young Bay mud, then upper layer
sediments of interbedded sands and
clays, which was underlain by old Bay
clay, followed by older colluvium and
then the Franciscan complex.
Design Considerations for
Facility 610
The Facility 610 Anaerobic Digestion
structure consists of five digester tanks
that extend from a lower basement level
that is approximately 34 ft (10 m) below
grade to maximum height of approximately
65 ft (19 m) above grade. Because
the excavation is approximately 40 ft
(12 m) deep with numerous site constraints,
including two adjacent
railroads that could not be impacted by
the construction, a rigid shoring system
was required. Excavation extended
through the artificial fill and Bay mud
and bottomed out in the upper layer
sediments. To ensure bottom cutoff, the
perimeter shoring wall was extended
160 ft (49 m) below grade to penetrate
into the relatively impermeable old Bay
clay for a bottom seal.
Excavation Support System
The design and construction of the
shoring system navigated several site
constraints to ensure the constructability
of the tieback/bracing system
D-Wall installation
DEEP FOUNDATIONS * JAN/FEB 2023 * 81
The diaphragm wall (blue) and designed tieback locations (green) relative to existing pile foundations (red)
and the abi l i ty to per form the
excavation efficiently. The geometry of
the excavation would have been ideal
for an internally braced shoring system
to avoid possible tieback conflicts
beyond the structure. However, blockouts
through the cast-in-place digester
tanks were not allowed, and the size
and shape of the tanks would have
required excessively large spans for the
internal bracing that could not meet the
tight deformation criteria. Other
challenges included: high groundwater
levels (excessive drawdown outside the
excavation was not permitted), very
soft and compressible young Bay mud
and protecting the active rail lines to
the west. The support of excavation
system had to be coordinated with
existing and new adjacent structures
including an existing sewer vault and a
forest of piles for new structures and
improvements surrounding the Facility
610 excavation.

January/February 2023

Table of Contents for the Digital Edition of January/February 2023

TOC
January/February 2023 - Intro
January/February 2023 - 1
January/February 2023 - 2
January/February 2023 - TOC
January/February 2023 - 4
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