January/February 2025 - 18

To satisfy all the criteria, the testing solution was to
perform bi-directional load tests at two production shaft
locations. The majority of the drilled shaft axial resistance
was designed to be obtained from side and base resistance in
the basalt rock. But there was also great curiosity as to what
sort of side resistance would be available in the permanently
cased portion of the shaft, e.g., casing to soil interface. As with
any bi-directional test, the decision where to place the load
cell assembly is always a challenge. It's a balancing act to
achieve the holy grail of resistance equilibrium above and
below the cell assembly.
Anticipating that the base resistance would be very large,
it was concluded by the design team to place the cell assembly
6 in (153 mm) above the shaft bottom. The bottom bearing
plate of the load cell assembly was 9 ft (2.7 m) in diameter,
where the shaft base diameter was 12.25 ft (3.7 m). This is
known as a " punch-out " style (or Chicago method) test but to
epic proportions. The advantage of course is that a larger unit
base resistance may be tested than if the load was applied to
the entire base area.
Another challenging undertaking is the jack assembly.
Plans and specifications required a test load of 48,000 kip
(213,500 kN). This would normally be accomplished by using
as few jacks as possible. But the widely seen supply chain
issues following the COVID-19 Pandemic required the use of
many smaller embedded jacks as larger jacks had prohibitive
lead times. Driving the load test was a complex array of
hydraulic pumps and manifolded valves to pressurize 12
individual embedded jacks.
Further enhancing the program, distributed fiber optic
strain sensing technology was incorporated. This is a
Load cell assembly with multiple jacks
promising emerging technology for pile testing because it
allows the strain profile to be recorded along the entire length
of the pile instead of only at discrete points. The cables were
installed and monitored alongside conventional sister bar
instrumentation. An astounding 4.2 mi (6.8 km) of hoses,
strain gage wires, optical fibers, crosshole sonic logging tubes,
thermal integrity profiling wires, telltale casings and rods
were installed on each test shaft.
Quality Assurance/Quality Control
Polymer support fluid was used during casing installation
and excavation. Extensive airlifting was performed resulting
in a full fluid column exchange volume. Sand content testing
was completed daily to monitor the suspended solids in the
fluid column until undetectable levels were measured,
greatly overachieving the specified criteria of <2%. The
extremely clean fluid column provided optimal conditions for
the sonar caliper to " see " the walls of the rock socket.
Verticality was exceptional with
only a 27 in (685 mm) centerline
offset over a 243 ft (74 m) depth.
Caliper measurements confirmed
the rock socket diameter was
slightly larger than the cutting
face of tooling.
Because the shaft design
heavily relied on base resistance
it was very important to have a
clean bottom of shaft. A shaft inspection device was specified
for all shafts to verify the shaft base was clean and provide the
confidence needed in the base resistance.
Along with detailed construction observations by project
designer Shannon & Wilson's full-time on-site geotechnical
engineer, post construction quality assurance was done with
thermal integrity profiling and crosshole sonic logging. While
thermal integrity profiling cables suffered high attrition during
(and after) the concrete placement, results were of limited use
for shaft acceptance. Reasons for these failures are still under
investigation in hopes of leading to the development of a more
robust cable-sensor string. Therein, crosshole sonic logging
was the primary nondestructive testing method for the shaft
integrity acceptance. Aside from performing some minor
chipping around the top of one shaft, all shafts had a concrete
rating of " acceptable. "
Load Testing Results
Winter 2022 in the Pacific Northwest included grey misty rainswept
December days with penetrating temperatures of
35-40°F (1-4°C). Testing was performed in accordance with
ASTM D8169-18, procedure A: Quick Test. The test concluded
when the shaft reached its nominal resistance in the upward
direction. The maximum applied load was 22,975 kip
(102,197 kN), acting in both directions, which produced 1.00 in
18 * DEEP FOUNDATIONS * JAN/FEB 2025
Sonar scan within the rock socket
January/February 2025

Table of Contents for the Digital Edition of January/February 2025
