DFI Magazine March/April 2020 - 17

Pit Underpinning Limitations
Given the specific subsurface conditions under the Carnegie
Building, there were several drawbacks to using pit underpinning
to stabilize this structure. The geotechnical challenges included
the following:
Glacial Soils and Rock
The boring logs indicated decomposed rock that typically consisted
of stiff to very stiff clay and sand with varying amounts of silt, gravel
and rock fragments. Hand-excavation of these types of soils,
especially in the confined space under the building, would have been
extremely difficult and time consuming, in addition to the process of
creating underpinning pits being extremely slow and laborious.

Site limits

Prior to addressing this need during construction, a geotechnical
investigation was performed to inform the design of the building
foundations and the support of excavation. The study revealed a fill
layer from the ground surface to a depth of 4 ft to 12 ft (1.2 m to
3.6 m). Glacial soils were encountered in some test borings below the
fill from approximately 4 ft to 10 ft (1.2 m to 3.05 m) below ground
surface. In some test pits, the glacial soils contained boulders up to
4 ft (1.2 m) in size.
Beneath the fill and glacial soils, some borings encountered
cemented sand from approximately 7 ft to 20 ft (2.1 m to 6.1 m)
below existing grade, with the cemented sand overlaying a
decomposed rock layer. Cemented serpentinite rock with rockquality designation (RQD) values ranging from 27% to 75%
underlaid the decomposed rock from an El. of 7 ft to El. -1ft (2.1m
to -0.3 m) in the north lot, and from an El. of between 28 ft to 9 ft
(8.5 m and 2.7 m) in the south lot.
The engineer's initial shoring recommendation was to use
soldier pile and lagging walls with temporary anchors or rakers for
soil retention, and pit underpinning techniques for the Carnegie
Building and other buildings that abut the proposed excavation
sites. Conventional pit underpinning is a time-consuming, laborintensive process that involves constructing small wood-sheeted
pits beneath a foundation. Sheeted pits, typically 3 ft (1 m) wide,
are hand-excavated below the footing to a competent bearing
stratum. Once a pit is completed, it is concreted to within several
inches of the underside of the existing footing and allowed to cure
overnight. The gap is then dry-packed with stiff mortar to
accomplish load transfer. Pits are excavated and completed in a
strategic order to form a continuous wall.
Upon reviewing the existing foundation of the Carnegie
Building and the geotechnical information about glacial soils and
decomposed rock underneath it, the contractor proposed an active
soil nail wall approach to underpinning this structure. This
technique would allow for excavation immediately adjacent to the
building while assisting with the overall construction schedule and
meeting the project performance requirements.

Bouldery Conditions
The presence of boulders under the building would have been
extremely consequential to the pit underpinning pier installation
process. The general contractor encountered a significant number of
boulders in the overburden materials. If boulders were found to be
fully or partially in the excavation path of an underpinning pier, they
would have needed to be removed manually in manageable pieces
since there was no way of accessing such a deep pit with an excavator
or other track-mounted equipment. Moreover, during the removal
process, there would have been a high risk of a boulder falling into
the excavation area, creating a large void directly under the building.
This could have resulted in differential building settlement.

Boulder protruding from beneath the building

Irregular Foundation Elements
The irregular foundations discovered under the building in the test
pit explorations performed during the geotechnical investigation
would have further complicated the construction of a traditional
pit underpinning system. Several test pits were dug adjacent to the
Carnegie Building that indicated that the foundation type varied
along the limits of excavation. For example, a test pit on the
northern facade of the building showed the foundation to be made
DEEP FOUNDATIONS * MAR/APR 2020 * 17

DFI Magazine March/April 2020

Table of Contents for the Digital Edition of DFI Magazine March/April 2020