July/August 2023 - 96

Substructure-related matters,
included:
* The temporary shoring for the
historic structural components,
which consisted of steel frames and
micropiles, was designed to provide
lateral and vertical support while
allowing the demolition, excavation
and new construction.
* The temporary support of excavation
for the proposed below grade
foundation. The temporary sheeting
provided was a flexible, freedraining
system composed of steel
soldier beams and lagging that were
internally braced using steel rakers
bearing on concrete heel blocks
below the proposed foundation.
* Underpinning consisting of continuous
and alternating unreinforced
concrete piers with reinforced concrete
lintels and lagging in between
(in some cases, micropiles had to be
used to overcome the difficult
ground conditions encountered).
These piers were designed to be
integrated with both the temporary
shoring and the temporary sheeting,
as well as to support some additional
permanent loading coming from the
structural components of the future
development.
* The total cut height of the required
excavation varied, with a maximum
cut height of approximately 23 ft
(7 m) along the perimeter.
Geology and Ground
Conditions
The sheeting, shoring, underpinning
and micropile work was design-build.
The contractor designed and constructed
these components from start to
finish utilizing in-house engineers and
field crew, in collaboration with a
consulting engineer, John Turner, Ph.D.,
P.E., Dan Brown and Associates.
Performing this work as design-build
was a key component for the success of
this project given the difficult subsurface
conditions, and efficient com96
* DEEP FOUNDATIONS * JULY/AUG 2023
munication was maintained at all times
between field crew, design engineers
and other key players.
The geotechnical engineer of record
performed seven soil borings extending
50-70 ft (15-21 m) below the ground
surface level. Data collected from these
borings were used by the contractor to
estimate the parameters later used for
the design of the sheeting, shoring,
underpinning and micropile work:
* Four main soil layers were noted:
fill, alluvium, terrace formation and
weathered rock.
*
Fill was encountered in five of the
seven borings. Fill consisted predominantly
of sand and clay with
some gravel and construction
debris. The layer thickness was
observed to be 5 ft (1.5 m), except at
the north end where two borings
showed fill layers up to 23.5 ft (7 m)
in thickness at an old gas station.
* The alluvium layer consisted of very
soft to stiff clayey sand and occasionally
interbedding sandy clay and was
where groundwater was encountered.
This layer is where most of the
foundation work took place.
* The terrace formation layer consisted
of medium dense to very
dense poorly graded gravel and
poorly graded sand.
* The deepest layer encountered in
the borings was the weathered rock
layer. This layer was encountered in
six of the seven borings. All borings
that encountered weathered rock
were terminated within it.
* The foundations were to bear on the
alluvium deposits which were not
suitable for spread footings due to
the potential excess differential
settlement within the building
footprint.
* There were also concerns related to
groundwater within the below
grade/basement levels when
selecting the foundation for the
proposed building.
* The new foundation was a mat slab,
with thicknesses ranging from
36-48 in (915-1,220 mm), bearing
on the alluvium soils.
The Buildings
2100 14th St. NW
A two-story brick building was located
adjacent to the south side of the
development at the intersection of 14th
St. and V St. NW.
2100 14th St. NW
July/August 2023

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