July/August 2024 - 110

about 6.5 mm (0.25 in), which maintains
the angular distortion of the historic
building within tolerable limits. Installation
constraints dictated that the
micropile drill rig cannot be closer than
0.5 m (1.6 ft) from the face of the underpinning.
That is, the micropile needed to
be installed below the mat foundation.
As such, a pile cap was necessary to
transfer the underpinning load to the
micropiles. Furthermore, an isolation
gap was needed to account for the mat
settlement and prevent transferring
such a heavy load to the micropiles,
which would undoubtedly crush them.
To be on the safe side, it was decided to
maintain a gap of 76 mm (3 in) between
the micropile cap and the mat-double
the FEM model calculated settlement. To
properly create this gap, a three-sided
masonry wall with footing encapsulating
each pile cap was constructed.
The geotechnical design details of the
micropiles consisted of a 200 mm (8 in)
diameter by 4.5 m (15 ft) long bonded
section with 345 kPa (50 Psi) allowable
bond strength. The pile caps were 1 m
(3.3 ft) wide by 1.5 m (4.9 ft) high and
were doweled into the underpinning
piers. Due to the depth of excavation, the
micropile-underpinning system was
designed to support the entire building
through bearing of the east wall with
surcharge from the west wall, resulting
in a total load of 800 kN (180 kip) per pile.
Steel plates (25 mm or 1 in thick) cover
the enclosures to create the 76 mm (3 in)
gap above the micropile cap. A separation
of 50 mm (2 in) was also maintained
between the masonry walls and the
micropile cap by filling it with sand. The
mat foundation was further isolated
from the building wall with a white
geomembrane bond breaker to provide
Due to the depth of excavation, the micropile-underpinning
system was designed to support the entire building through
bearing of the east wall with surcharge from the west wall,
resulting in a total load of 800 kN (180 kip) per pile.
free movement of the mat and masonry
walls relative to the micropile caps. The
bond breaker was extended to the top of
the mat concrete pour on all sides.
Conclusions
An optimal mat foundation system was
made possible by an innovative micropile
underpinning of an adjacent historic
structure. A gap that was purposely set
above the micropile caps allowed the
mat settlement to take place without
overstressing or compromising the
micropiles. This ultimately safeguarded
the adjacent historic building. The mat
foundation proved to be the most costeffective
system. More importantly, the
use of a mat reduced the construction
schedule by about two months, resulting
in significant savings for the project.
The new building has been completed
almost two years ago and the adjacent
historic building has been functioning
well without any signs of distress.
Kamil Nuzha, P.E., is a project manager at
GeoStructures with more than 10 years of
professional experience in geotechnical field
explorations, foundation analysis and design,
and construction quality control for a variety
of projects. Nuzha has designed shallow foundations,
caissons, micropiles, timber piles,
sheet piles, tiebacks and various retaining
walls, and has experience in finite element
method geotechnical applications using GEOSLOPE
and Midas GTS computer software.
Jianchao Li, P.E., is a principal at
GeoStructures with 30 years of professional
experience in geotechnical investigations
and designs including soil-structure
interaction. Li's experience encompasses a
wide range of commercial development and
industrial and institutional projects as well
as transportation and bridge projects for
PennDOT, PA Turnpike and other agencies.
Bashar S. Qubain, Ph.D., P.E., is president
Enclosures around micropile caps to create a gap below the mat
110 * DEEP FOUNDATIONS * JULY/AUG 2024
and chief engineer at GeoStructures. With
more than 30 years of professional experience,
a strong academic background in structural
mechanics and geotechnical engineering,
Qubain brings unique insight into foundation
analysis and soil-structure interaction.
July/August 2024

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