January/February 2022 - 74

have been excavated to over 70 ft (20 m)
in the Transbay District, with shoring
walls extending deeper than 100 ft
(30 m); this depth allowed for structural
embedment and sealing into the clay
unit for groundwater control. Sometimes,
walls have exceeded 130 ft (40 m)
to seal off and depressurize confined
aquifers for blowout mitigation, and for
basal heave control in the underlying
clay layers. Bedrock is too deep to
contribute strength or stability.
Although traditional SOE schemes
in San Francisco used the dense Colma
sand to provide toe stability for shoring
walls, the current, deeper excavations
effectively remove this competent soil,
and walls embed into the weaker,
underlying clay. This increases the
potential for basal heave effects that
escalate the already significant ground
movements predicted for these deep
excavations. The CSM method has
provided an effective solution to stiffen
SOE systems by allowing thicker walls
to accommodate larger, heavier steel
sections than in traditional multi-axis
soil mixing layouts. Also, buttress
elements constructed perpendicular to
the SOE wall can further stiffen shoring
systems to manage deformation and
effects on adjacent properties.
Project Example
Malcolm Drilling constructed SOE and
deep foundations for Oceanwide Center,
an L-shaped site that has a 61-level
tower and a 53-level tower, over
multilevel basements. The complex site
footprint wraps around six adjacent
structures, and required an SOE scheme
that could allow tower excavation to
progress without schedule constraints.
Overall, the mass excavation
extended to 72 ft (22 m) depth,
penetrating into the top of the Old Bay
Clay. This uncharacteristic depth was
further complicated by a thick sand lens
within the underlying clay soils that
required cut-off and depressurization.
Due to the large depth and extensive
removal of the Colma, the excavation
had a relatively low factor of safety
against basal heave.
The selected SOE approach was an
internally braced, combined shoring
and cutoff wall. Using a steerable,
instrumented CSM mixing tool ,
The 72 ft (22 m) deep Oceanwide Center basement excavation
Malcolm constructed a 140 ft (43 m)
deep, 3.3 ft (1 m) thick, continuous perimeter
wall. To minimize deformation
effects, heavy W30 and W33 steel
sections were placed at 4 ft (1.2 m)
centers around the site perimeter.
Structural beam lengths and other
factors meant " over-the-hole " mechanical
splices were employed during
wet set placement of the reinforcement.
Two of the six structures adjacent to
Oceanwide were high-rise towers
founded on piles driven into the Colma
formation. The planned excavation
depth extended below pile tip of those
structures. To further stiffen the SOE
wall at these critical locations, soil
cement buttresses were installed
perpendicular to the perimeter alignment.
The buttresses were constructed
from original site grade by overlapping
nominally reinforced soil cement
panels end to end. They were removed
in lifts as excavation progressed and
bracing levels were installed, with the
remaining length below final subgrade
acting as a shear-key to resist lateral
movement and basal heave. Finite
element modelling of the shoring
74 * DEEP FOUNDATIONS * JAN/FEB 2022
January/February 2022

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