July/August 2024 - 109

historic building will subsequently be
dragged down the same amount as the
mat settlement. The west wall of the
historic building will, however, settle
less than the east wall due to the
diminishing mat influence over that
distance. Consequently, the differential
settlement between the east and west
walls of the historic building will result
in an intolerable angular distortion that
will likely damage the building.
Due to the combination of the
heavily loaded high-rise and the
sensitivity of the historic building
abutting it, detailed finite element
method (FEM) modeling was necessary
to predict the amount of settlement and
study the impact of the mat settlement
on the historic building. SIGMA/W, a
2D finite element analysis program,
was used to model the geotechnical
section of the site that was based on the
results of the field exploration and
laboratory tests.
In addition to the tower mat, the
model included the existing historic
building to the west and another nearby
building to the east. The FEM model
simulated the different construction
stages starting with the initial stress
state representing the bearing pressure
below the existing buildings. The
different stages of excavation were
then carried out to the bottom of the
mat level. During the excavation stages,
the underpinning and its tiebacks were
added to the model. Finally, the mat
was loaded with 670 kPa (14 ksf)
pressure in three steps. The linear
elastic parameters of the upper, less
critical soil layers were incorporated
into the model based on published data.
However, the pressuremeter tests were
essential in obtaining representative
values of the elastic modulus of the
very dense, sand and gravel stratum
that influences the settlement of the
mat. The resulting elastic modulus
averaged 95 MPa (2,000 ksf).
Settlement contours under the mat
had a fairly uniform settlement of 38
mm (1.5 in). The underpinning of the
east wall of the historic building
abutting the bottom of the mat settled
the same as the mat. Settlement of the
west wall of the historic building,
however, was predicted to be 13 mm
(0.50 in). Therefore, differential settlement
between the west and east
walls were in the range of 25 mm
(1.0 in), over a 6 m (20 ft) distance,
which corresponds to an angular
distortion of 1/240 - presumably high
for such a sensitive building. It is worth
noting that the settlement of the
building west of the historic building is
less than 6.5 mm (0.25 in), which is
considered tolerable.
Design Solution
To minimize the impact of the mat
settlement on the historic building, the
underpinning piers were supported by
rock-socketed micropiles. By utilizing
micropiles, once the mat and the
subgrade below the underpinning piers
start to settle, the micropiles will
simultaneously pick up the underpinning
load. Being rock-socketed,
the settlement of the micropileunderpinning
system will be limited to
Settlement contours under mat foundation and underpinning
DEEP FOUNDATIONS * JULY/AUG 2024 * 109
July/August 2024

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