July/August 2023 - 80
system. Underpinning and supporting
16 ft (5 m) of soil with surcharge load
from the existing building foundations
would require two levels of anchored
support, depending on the location of
the traditional underpinning in addition
to multiple levels of staging to
install the underpinning.
Existing foundation elements also
posed several challenges. The existing
shallow foundations ranged from
3.5-7 ft (1-2 m) square foundations at
column locations, connected by 2.5 ft
(0.7 m) wide strip footings around the
existing building exterior. The bearing
pressures beneath the existing footings
ranged from 3.4-3.7 ksf (163-177 kN m2).
Typically, these bearing pressures
would be transferred directly to the
underpinning elements. With no immediate
access to the backside of the
footings, the underpinning contractor
would have to: (a) expand underpinning
pits deep into the slope to reach the back
of the footings, (b) excavate below the
basement level on the inside of the
building to reach the back of the existing
footings or (c) account for these bearing
pressures as additional lateral surcharges
on the underpinning. If these
foundation pressures acted upon traditional
underpinning as surcharges,
these loads would create the need for
significant additional lateral bracing
and underpinning elements.
Required underpinning locations (blue) and existing structures (green and yellow)
Excavation heights of up to approximately
16 ft (5 m), in addition to the
technical challenges described above,
also posed significant risk to the safety
of the workers, who would need to
work directly beneath existing concrete
or stone that could potentially
fall, or soil that could potentially cave
in, if the underpinning pit was not
prepared properly.
Geotechnical Background and
Recommendation
The ground investigation report outlined
the existing soils as loose to medium
dense fill material consisting of sands
and silty sands with varying amounts of
gravel overlying medium dense to very
Rebar and shotcrete assembled and installed on excavation day
dense silty sand with varying amounts
of gravel overlying fractured gneiss/
gneiss bedrock with rock quality designation
values ranging from 30-50%.
Additionally, based on historical
photos of the 1992 renovation, the base
course used beneath the existing
foundations was visually similar to
roadway base course.
There was no mention of underpinning
of existing foundations in the
geotechnical report. This report may
have been prepared before the proposed
foundation elevations were discussed to
be below the existing foundations. Initially,
the report called for earth support
to resist the lateral earth and surcharge
pressures and called for the proposed
walls to be designed for the lateral earth
and building surcharge pressures. Based
on discussions with the project structural
engineer, the proposed foundations
and walls were only designed for lateral
earth pressure, and therefore the
proposed underpinning was required to
be permanent to support the existing
building and prevent the existing
building from surcharging the proposed
renovation foundation walls. Initially,
due to the challenges traditional underpinning
typically faces and the added
permanent design requirement, an
active soil nail wall was proposed and
developed to support the lateral earth
and existing foundation surcharge
pressures. For simplicity in the design
80 * DEEP FOUNDATIONS * JULY/AUG 2023
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Table of Contents for the Digital Edition of July/August 2023
TOC
July/August 2023 - Intro
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July/August 2023 - TOC
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