DFI Magazine September/October 2020 - 19

One drilled shaft was installed and tested using bidirectional load
test methods, with pairs of strain gauges included to provide data in
each soil stratum. This test shaft was 48 in (122 cm) in diameter in
soil and 42 in (106.7 cm) in diameter in rock, with a 58 ft (17.7 m)
long rock socket. Sonic caliper testing and crosshole
sonic logging were performed. The shaft was loaded to 3,377 kips
(1,532 m ton/megagram), at which load the pile deflected 0.65 in
(1.7 cm) upward and 0.2 in (0.5 cm) downward. No downward failure was observed. Strain gauges embedded in the shaft were used to
determine frictional resistance in the various soil layers and in bedrock.
Twenty-five indicator ACIP piles were installed to evaluate the
ability of the equipment to install at least 10 ft (3 m) into bedrock.
The ACIP load test program consisted of 3 static compression, 3
static uplift and 10 dynamic load tests, all of which confirmed that
the planned 10 ft (3 m) of rock embedment was enough to support
the design loads.

Design and Construction
Based on test results, the foundations for production consisted of:
* 1,019 ACIP piles with 10 ft (3 m) of embedment in bedrock and
allowable axial compression capacities of 350 to 900 kips (159
to 408 m ton/ megagram) applied in eight zones based on the
thickness of various soil layers and the depth-to-bedrock
capacity. Pile lengths are 50 to 145 ft (44 m).

Installing drilled shafts and ACIP piles

* 314 drilled shafts that are 39 to 48 in (99 to 121.0 cm) in
diameter in soil and 36 to 42 in (91.4 to 106.7 cm) in diameter in
rock, with 12 to 65 ft (3.7 to 19.8 m) long rock sockets and
allowable axial compression capacities of 750 to 3,000 kips (340
to 1,361 m ton/megagram) applied in 12 capacity zones, based
on soil layer thickness, the depth to bedrock, and the rock-socket
length. Drilled shaft lengths are 45 to 170 ft (13.7 to 51.8 m).
The foundations were constructed over about four months using as
many as four foundation rigs working simultaneously each day.
Because of the extreme variation in subsurface conditions, the total
length of piles and embedment in bedrock had to be identified case

by case during construction. Langan developed a decision matrix
for each field representative to determine, in real time, the required
length and rock embedment to achieve the required capacity. The
team knew that foundation lengths could not be planned in
advance, and Malcolm Drilling was able to adjust cage length on the
spot as pile lengths were determined.

Support of Excavation
The design and construction of the tied-back CDSM cutoff wall was
constrained by shallow groundwater, poor fill and weak Bay Mud.
And, because of the variable soil conditions throughout the site,
lateral earth pressures were developed for design of the various
shoring systems in zones around the site, allowing the shoring to be
designed for specific conditions in each zone.
The perimeter shoring included 3,200 lft (0.98 lkm) of CDSM
wall. Over two-and-a-half months, the CDSM panels were installed
using a triple-axis soil-mix rig. The panels terminate in Bay Mud,
stiff clay or bedrock, depending on the depth of excavation and soil
conditions required in each part of the site to create groundwater
cutoff. A total of 730 soldier piles were installed from 25 to 75 ft
(7.6 to 22.9 m) of depth. Also, because of the various sizes of debris
found in the fill, extensive predrilling and pretrenching were
performed before the CDSM panels were installed.
Because Bay Mud could only provide low resistance, the lateral
restraint system was designed to avoid relying on the mud. The
system consisted of tiebacks installed at a very shallow angle (10
degrees) to remain in the fill, and tiebacks installed at a very steep
angle (up to 45 degrees) to dive through the Bay Mud and into the
dense sand of the Colma Formation - and sometimes into
bedrock. A total of 1,200 tiebacks were installed, with lengths of up
to 100 ft (30.5 m).
Depending on the depth of excavation and soil conditions
behind the wall, as many as three rows of tiebacks were installed.
Up to three rounds of post-grouting were performed in tiebacks
installed into the loose fill. Where the fill included soft clay (likely
from historical mud waves), additional tiebacks were needed to
develop sufficient restraint. Installation of tiebacks in the fill was
difficult during rising tides when significant groundwater intrusion
shot through the drilled hole, like a fire hose! To mitigate this
problem when possible, the schedule was adjusted to install
tiebacks that were near the bay during ebb tides.

Working Platform
Wet and weak fill and Bay Mud were exposed throughout most of
the bottom of excavation, where construction equipment would
apply pressures as high as 4,500 to 6,000 psf (215 to 287 kPa).
Recognizing that a stable and dry bottom was critical for the success
of deep foundation construction, a working platform was designed
and constructed using a decision matrix, depending on the
anticipated loads and actual site conditions. The working platform
consisted of layers of Tensar TriAx TX130S Geogrid and 3 to 4 ft
(0.9 to 1.2 m) of cement-treated fill or 3 to 6 ft (0.9 to 1.8 m) of
lime-treated Bay Mud. This working platform also served to resist
hydrostatic seepage during construction and helped maintain a
mostly dry bottom of excavation.
DEEP FOUNDATIONS * SEPT/OCT 2020 * 19

DFI Magazine September/October 2020

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