May/June 2022 - 102

Table 2: Summary of Load Testing Durations
Activity
Install 3 instrumented test piles
Wait for grout to set
Compression Frame Set-Up and
Tear-Down
Compression Test - NYC
Compression Test - U.S. Std. of
Practice
Set-up and Tear-Down
Lateral Test
Figure 7: Pile head instrumentation
Challenges for Strain Gauge
Data Interpretation
Strain gauge interpretation is more
complicated for the pressure-grouted
piles because the exterior grout
diameter varies with depth and, as soil
is invariably mixed with the grout, the
strength and modulus vary. The
incremental rigidity (IR) method,
presented by Komurka in 2020, relies
on the change in test load divided by the
change in strain (ΔQ/Δε) and does not
require input of the pile cross-sectional
area. The IR method could have
eliminated the variable diameter issue.
However, fully mobilized skin friction
is required for the IR method, and only
the retests after the NYC procedure
moved the piles sufficiently. The effects
of residual loading made interpretation
of the retest data by the IR method
impossible.
Strain gage data were interpreted by
application of the commonly used
Tangent Modulus method, documented
extensively by Fellenius and others.
The exterior grout cylinder diameter
between each instrument location was
based on grout volume from the
installation logs. However, small
changes in assumed diameter can have
relatively large effects on calculated
load and unit skin friction with depth.
102 * DEEP FOUNDATIONS * MAY/JUNE 2022
Production Installation
Based on the test results, installation
criteria were developed that could
account for subsurface variation across
the site, which included a minimum
depth and a minimum of at least one of
three other factors: pile length within
the bearing layer as identified by a
minimum torque, a higher refusal
torque or a final grouting pressure
greater than 250 Psi (1.72 MPa).
Production installation of the
pressure-grouted helical piles started a
bit slowly, as is common for many
projects, with 3 to 4 piles (330 to 440 ft)
(100 to 130 m) installed per day.
Production was limited by grout
delivery to the on-site mixer. Pile
installation jumped around the site,
both to allow grout set-up before work
began on adjacent piles and because of
site availability. The first third of the
piles were installed without problems,
though drilling through the medium
dense sand in the 30 to 40 ft (9 to 12 m)
depth range was slightly slower, even
with use of the aggressive teeth on the
pile tip and leading helix, as shown in
Figure 8. Eventually, a handful of piles
met refusal in this sand layer, the result
of the material being densified by pile
installation and possibly migration of
grout from adjacent piles. Predrilling
to a depth of 40 ft (12 m) with a 16 in
(406 mm) continuous flight auger was
incorporated into production thereafter;
the cost of this predrilling was
much less than the delay, discussion
and rework costs associated with a few
noncompliant piles.
The list below summarizes final
production equipment and rates:
* Drive head carrier − CAT 349E
Excavator
* Drive head − APE HD150
* Grout pump − Scheltzke MPS 110v
D-C Automatic with 22 ton (200 KN)
cement silo
* Grout mix - neat cement 5 ksi
(34 MPa) mix with set retarder
* Ancillary machines for pile and
equipment handling - CAT 908
Loader & CAT 305 Excavator
* Piles with API threaded couplings
on 20 ft (6.1 m) sections
* Predri l l ing to 40 ft (12 m) −
Comacchio MC22 wi th 16 in
(406 mm) continuous flight auger
* Final production rate - 5 to 6 piles
(550 to 660 ft) (170 to 200 m) per day
Duration
Comments
Axial Compression Tests
2 days
1 week
1.5 day/test
3 day/test
0.5 day/test
First set-up can be concurrent with test pile
install
NYC Experience methodology - ASTM D1143-81
(R94) + NYC Code holds
ASTM D1143-07 (R13) (E18)
Lateral Tests
0.5 day/test
0.5 day/test
ASTM D3966
May/June 2022

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