November/December 2021 - 97

One limitation of the conventional
BDLT test is that the loading direction on
the foundation upper portion is different
from the actual superstructure loading
direction. Therefore, the foundation
upper portion elastic compression will
be smaller than the value had the top
compression loading been applied. This
limitation can be overcome by adding
the elast ic compression of the
foundation-measured displacement
when constructing an equivalent toploading
curve (ETL).
Top-Loaded Bidirectional Test
The TLBT method developed by the
author applies bidirectional loads to a
deep foundation element, using a
loading source located above the
foundation head. In the TLBT reusable
load assembly, loads are applied to the
foundation using the R-System, which
consists of two stacked steel plates
located at the geotechnical resistance
balance point or at the foundation base
connected to the load assembly via
vertical elements.
In the R-System, the shaft bearing
plate (SBP) transfers loads to the
foundation upper portion, and the base
bearing plate (BBP) transfers loads to
the foundation lower portion and the
foundation base. At the surface, above
the foundation head, a hydraulic jack is
located between the top load and
bottom load assembly (TLA and BLA).
The load assemblies are connected to
sleeved vertical elements referred to as
shaft mobilizer and base mobilizer bars
(SMB and BMB), which are consequently
connected to the R-System. As
the jack is pressurized and expanded at
the surface, the R-System plates are
separated, and the foundation is
bidirectionally loaded.
The TLBT arrangement overcomes
some of the construction challenges of
using a conventional BDLT. First, there
are no embedded hydraulic lines in the
new system, which reduces the risk of
losing the test foundation due to a
hydraulic fluid leak. Second, in the case
R-System prior to installation
of a mechanical failure on the jack
assembly, the issue can be addressed at
the surface, which also contributes to
lowering the risk of losing the test
foundation due to a jack assembly
failure. Third, and perhaps most
importantly for constructability, the
R-System provides the center-hole
spacing for a tremie line to pass through,
which avoids an impact on concrete
placement operations. This also lowers
the risk of creating soft-base conditions
or concrete integrity problems at or near
the foundation base.
The size, strength and quantity of
the vertical elements (SMBs and BMBs)
can be determined as a function of the
test load. These elements are connected
to the SBP and BBP through hex nuts,
allowing recovery of the mobilizer bars
after testing ends. With the TLA and
BLA also being reusable, the only
sacrificial element in the TLBT
Top-loaded bidirectional test arrangement
arrangement is thus the R-System.
Therefore, the TLBT might reduce user
costs, but more importantly, allows
more testing to be completed on a
project site. This could help optimize
designs and/or higher resistance
factors when projects are designed
under the load and resistance factor
design (LRFD) methodology.
The R-System is installed within the
rebar cage like the BDLT jack assembly
installation process. The main difference
is that the hydraulic jack and
related instrumentations are not within
the R-System. Once the R- system is
properly leveled, the rebar cage is
welded to the side of the plates
immediately above and below the
concrete break line, or approximately at
the contact line of the two stacked steel
plates.
Full-scale TLBT
For a proof of concept, a TLBT full-scale
arrangement was designed, planned,
installed, and tested in the summer of
2020 to observe a deep foundation
behavior under loading conditions
using the TLBT method. A 48 in (122 cm)
diameter drilled shaft was installed to
a maximum depth of 40 ft (12 m)
measured from the surface. The foundat
ion was instrumented using
vibratory wire strain gages (VWSG)
DEEP FOUNDATIONS * NOV/DEC 2021 * 97
November/December 2021

Table of Contents for the Digital Edition of November/December 2021
