March/April 2023 - 74

This high blow count zone includes
gravel, clay and sand in addition to
occasional boulders. The groundwater
table is roughly at the AlluviumPotomac
interface and varies throughout
the year.
The site geology for the project made
for a challenging shoring design and
installation. The abrasiveness of the
Potomac Formation sands and gravels
resulted in excessive wear to drill
tooling. Cobbles and boulders obstructed
drilling and installation of structural
components of the shoring. As the
excavation progressed, loose sands and
silts were prone to running during
placement of lagging. Groundwater
management was difficult due to the
fine grained soils present limiting the
effectiveness of dewatering efforts.
Three Adjacent Sensitive and
Historic Structures
The House of the Temple is very
sensitive to any movement from
construction activities because the
northwest corner of the structure was
damaged in a 2011 earthquake. Even
though the House of the Temple is
located outside the influence of the
excavation, construction-related vibrations
were not permitted. In addition to
vibration impacts, there were concerns
that differential settlement due to
dewatering at the project site would
negatively impact the structure.
The carriage house is a one-story
brick structure with no continuous
footing as is typical for similar structures
built in the 1800s. The carriage
house was located approximately 8 ft
(2.5 m) away from the 40 ft (12 m) deep
shoring excavation. Project plans
New apartments adjacent to existing buildings
detailed incorporation of the carriage
house into the SRA building with
interior steel façade reinforcement and
new footings constructed to support
three additional floor levels. Existing
cracks in the carriage house were
documented prior to the start of any
construction work and monitored
during construction.
A four-story brick townhouse
(Campbell townhouse) to the south of
the project site was built in 1925 with a
one-story below grade basement in
addition to three stories above grade.
The Campbell townhouse is a privatelyowned
structure located across a 15 ft
(4.5 m) wide public alley. It was
assumed that the Campbell townhouse
foundation consisted of an unreinforced
concrete footing as is typical for similar
structures in the area. Interior preconstruction
surveys indicated the
townhouse was in good condition
without major cracks or deterioration
prior to the start of construction. The
exterior of the townhouse was brick
with minor cracks in the mortar joints.
B a s e d o n t i g h t s e t t l eme n t
thresholds for the carriage house and
Campbell townhouse, a movementbased
design was checked against the
typical apparent earth pressure
diagram design. A semi-empirical
method after Cording (1981) was used to
evaluate the excavation induced
movement of the shoring by evaluating
the relationship between the volume of
lateral wall displacement and the
soil/wall stiffness. This method
emphasizes the importance of the soil
modulus and the vertical distance
between lateral supports. Use of the
method leads to the conclusion that
stiff or dense ground will limit
movement of the shoring regardless of
its flexibility with the most important
factor for reducing movement being the
number of tiers of lateral support and
the distance between them. Adding an
additional tier of tiebacks or braces has
a greater influence on l imi t ing
movement of the shoring than reducing
the soldier pile spacing or using a larger
soldier pile section.
The movement-based design
Shoring Design
Cross section at carriage house
The temporary shoring chosen for this
project consisted of soldier piles, wood
lagging and tiebacks. This type of earth
retention is considered a flexible
shoring system, as compared to a stiffer
concrete secant pile or slurry wall.
Flexible systems allow for the small
amount of movement required for
active earth pressures to develop.
74 * DEEP FOUNDATIONS * MAR/APR 2023
method assumes good workmanship
and limited loss of ground during the
installation of the shoring. For the
movement-based design method to
p r o v i d e v a l i d r e s u l t s , s o und
construction procedures that limit loss
of ground and excessive ground
movements need to be used during
construction of the shoring. The
method of soldier pile installation can
greatly influence overall movements if
ground is lost during drilling or
vibrations densify soils during driving.
Lagging installation has the potential to
allow for ground loss if not performed
under control and with consideration of
soil conditions. The granular soils at
this project site generally had enough
silt and clay content for a 5 ft (1.5 m)
vertical lift to stand without unraveling.
Drilling tiebacks may result in ground
loss if positive pressure is not kept
inside the hole or if full depth casing is
not used and the drillhole collapses.
March/April 2023

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