July/August 2023 - 97

Challenge and Solutions: The
north-facing, existing 120 ft (36 m) long
wall needed to be underpinned since its
foundations were higher than the
proposed bottom of the new mat.
Continuous 2.5 ft (0.7 m) wide piers
were installed with bells to increase
their bearing surface and stability.
A total of 40 piers were installed
with a maximum depth of 10 ft (3 m).
The piers were then laterally braced by
using an alternating internal bracing
system composed of inclined rakers
bearing on concrete heel blocks below
the mat slab.
2108/2110 14th St. NW
This address featured two historic brick
facades that needed to be preserved:
one approximately 25 ft (7 m) in height
and the other 37 ft (11 m) in height.
2108/2110 14th St. NW
The scope of work included shoring
and underpinning the existing facades
that were to remain, with these
systems providing vertical and lateral
support for the existing facades, as well
as temporary support for the 17 ft (5 m)
deep excavation for the new below
grade parking structure.
Challenges: At the ground level,
both of the facades were mainly
composed of two large storefront
openings that used wooden lintels to
carry the vertical loads above and
transfer these into very narrow brick
piers. These piers were not structurally
stable enough to carry the full load of
the facade above once demolition of the
return walls occurred, so these also
needed to be shored and stabilized.
The shoring steel frames had to be
installed through the existing floors
and in hand-excavated-shored pits to
20 ft (6 m) below existing grade. The
frames had to be designed and installed
under rigorous consideration to avoid
undermining the existing buildings
that already exhibited precarious
conditions, and to avoid conflicting
with the critical structural components
of the proposed building.
Due to the high variability of the
subsurface conditions on the site, there
were also high possibi l i t ies of
encountering soft soils at the subgrade
elevation of the underpinning piers and
shoring concrete pads.
Solutions: After carefully analyzing
the temporary and future loading
conditions of these facades, the
cont rac tor determined that an
underpinning system, composed of
alternating concrete piers bearing on
the alluvium soils, with concrete lintels
and lagging in between, was sufficient
to provide temporary and permanent
vertical support for these facades. This
underpinning system was al so
designed to withstand the lateral earth
pressures to provide lateral support for
the excavation and construction of the
new underground garage.
The t emporary shor ing was
designed to provide stability and lateral
support for the existing facades. This
primarily consisted of:
* Vertical steel posts bearing on
concrete pads.
* A series of steel girders, runners and
needle beams.
* Steel braces and channels to
stabilize the facades.
Additional shoring was also required
for the narrow brick piers to stabilize
and protect these while demolition and
other construction activities took place.
DEEP FOUNDATIONS * JULY/AUG 2023 * 97
July/August 2023

Table of Contents for the Digital Edition of July/August 2023
