July/August 2024 - 18

The soil nail was considered complete when structural grout
was observed exiting the drill hole.
For design verification, four 6 in (152 mm) diameter holes
were drilled through two footings left behind from the
previously demolished C Wing. These holes were filled with
structural grout and a steel core bar, allowed to cure for three
days and pull tested to failure to determine the ultimate groutto-concrete
bond stress. The test set-up included a hydraulic
jack with a center hole and dial indicators. The core bar was
incrementally loaded until pull-out failure was achieved. The
ultimate bond stress between the existing concrete and grout
was calculated to be about 350 Psi (2.4 MPa). To verify the
actual diameter cut by carbide drill bit in the dense glacial till,
sacrificial grouted holes were drilled, grouted, cured and
exhumed. These 10 ft (3 m) deep holes were drilled vertically
using a hollow bar, 5 in (127 mm) diameter bit. After being
excavated out and washed off, the diameter of these grout
columns was measured on average to be about 8 in (203 mm).
At the D Wing, using a design footing load of 3.2 kip/lft (47
kN/m), it was determined that a 6 in (152 mm) diameter, 19 in
(480 mm) long core hole through the footing and filled
completely with structural grout was adequate to engage the
underpinning micropile.
Completed composite underpinning on D Wing
Ten micropiles spaced 4.5 ft (1.4 m) on-center were
installed along the remaining portion of the D Wing. The
micropiles were drilled 30 ft (9 m) deep on a 5 degree batter.
Once these micropiles were installed, the upper level system
was ready to be installed. However, due to the change in site
conditions along the B Wing, approval of the soldier pile wall
design was still pending. As a result, the soil nail wall could
not be installed in the intended continuous manner, so the
upper level wall was temporarily terminated midway
between the B and D wings.
With one slight exception, the installation of the soil nail
wall along the D Wing proceeded without incident. During the
installation of the first lift of shotcrete, water generated from
washing out the ready-mix concrete truck (staged at the top of
the wall) traveled down the backside of the soil nail wall
causing a portion of the wall to sluff off. The damaged area was
quickly repaired the following day and diversion measures
were put in place at the top of the wall to prevent surface runoff
water from getting behind the soil nail wall again.
Teeth marks left from the Cat 349E excavator's bucket
were indicative of the effort required to excavate the dense
glacial till. The positive side was that the dense till allowed
the excavation to be cut close to vertical. During the
18 * DEEP FOUNDATIONS * JULY/AUG 2024

July/August 2024

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

TOC
July/August 2024 - Intro
July/August 2024 - 1
July/August 2024 - 2
July/August 2024 - TOC
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