July/August 2024 - 107

Mind the Gap:
Micropile Underpinning
of a Historic Structure
FEATURE ARTICLE
The completed high-rise
This article pertains to a 52-story
residential high-rise building in the
Ri t t enhouse Square sec t i on of
Philadelphia, a densely developed area
with mixed residential and commercial
buildings. The tower is surrounded by
Moravian Street along the north,
Walnut Street to the south, a residential
building to the east and a three-story
historic building to the west. It
measures 40 m (130 ft) long by 22.8 m
(75 ft) wide with a finished floor
elevation of 12 m (40 ft) and a full
basement at elevation 7.5 m (24.5 ft).
The historic building has a finished
floor elevation of 13.7 m (45 ft) and its
basement at elevation 10.7 m (35.3 ft). It
is supported on a 0.5 m (1.5 ft) thick
rubble wall with a bottom at elevation
10.6 m (35.0 ft). The tower mat extends
36 ft (11 m) below grade or about 28 ft
(8.5) below the existing historic building,
thereby requiring underpinning.
When fully loaded, the settlement of
the mat will cause the abutting
underpinning to also settle, which
would be detrimental to the historic
building. Therefore, an innovative
solution was necessary to isolate the
underpinning from the stresses
induced by the adjacent mat to
safeguard the historic structure.
Subsurface Investigation and
Characterization
A geotechnical investigation program
was carried out to characterize the
subsurface conditions and aid in the
analysis of the optimal foundation
system. It consisted of 17 test borings
with 3-9 m (10-30 ft) of rock coring, two
piezometers for long-term groundwater
monitoring and pumping tests.
The elastic property of the mat
subgrade material was of a particular
interest for settlement analysis. As
AUTHORS Kamil Nuzha, P.E., Jianchao Li, P.E., and Bashar Qubain, Ph.D., P.E., GeoStructures
DEEP FOUNDATIONS * JULY/AUG 2024 * 107
such, six pressuremeter tests were
carried out across the high-rise
footprint to accurately determine the
elastic moduli.
The Geologic Map of Pennsylvania
places the site within the Trenton
gravel formation, a very gravelly sand
interstratified with cross-bedded sand
and clay-silt beds. Underlying the
alluvial Trenton gravel deposits are
transitional weathered rock and the
bedrock of the Wissahickon formation,
which consists of oligoclase mica
schist, hornblende gneiss, augen
gneiss, and quartz and feldspar-rich
members exhibiting various degrees of
granitization.
In general , the strat ificat ion
consists of 2.1 m (7 ft) of loose to
medium dense urban fill, 12 m (39 ft) of
alluvial Trenton gravel deposits that
include a relatively thin 2.1 m (7 ft) high
plasticity clay and a thicker, 10 m (33 ft)

July/August 2024

Table of Contents for the Digital Edition of July/August 2024

TOC
July/August 2024 - Intro
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July/August 2024 - TOC
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