September/October 2021 - 105

Lesson 6: Contractors can be
overconfident when assuming the
adequacy of a binder design based on
past experience, which can reduce
preconstruction field investigation and
laboratory testing for new projects.
Preconstruction test programs should
always be required to benefit the
contractor and owner. The FHWA
Design Manual recommends installing
at least three test elements using
different mixing parameters. These
programs benefit both parties in that
the deep mixing elements can be
installed as designed. The 2013 manual
recommends that any changes exceeding
10% of the previously approved mix
design be revalidated through laboratory
and field testing, which we fully
support.
Lesson 7: Coring and wet grab
sampling for the validation program
should be held to the same standards as
the production works.
Lesson 8: Fresh grout temperature is
usually restricted in specifications to
below 95˚F (35˚C) to minimize thermal
stresses and cracking. But this requirement
is taken from specifications for
concrete placement. Temperature control
is difficult to sustain in hot climates
without extraordinary measures. The
35˚C rule should always be applied to
deep mixing installed using continuous
trenching methods due to the monolithic
nature of the installation. Some
relaxation of this rule should be adopted,
though, for these projects installed using
a primary-secondary pattern of element
construction since there is some time for
heat to dissipate between these elements.
Lesson 9: Prehydration of bentonite is
often specified for mixes containing
bentonite. In some ground conditions
and with high cement factors, it has
been demonstrated that the compressive
strength and permeability at 28days'
curing time are unaffected by
adding nonhydrated bentonite. This
relatively new development has not
been proven to be universally applicable,
so adoption of nonhydrated bentonite
should be on a case-by-case basis,
with approval demonstrated by comparative
tests both in the laboratory and
field (using site-specific soils and binders).
Lesson 10: To minimize core and
borehole wall damage during drilling,
verification borings should be conducted
by experienced drillers using
state-of-the-practice techniques and
drilling rates not exceeding one ft/min.
Generally, triple-tube, wire-lined coring
produces the best results because the
drilling tools are more stable and the
core more insulated from drilling
disturbances. Specifications should
detail the handling, storage and transport
of core samples selected for
acceptance testing if the test results of
core samples are to be used for
acceptance. This particularly applies to
samples from remote locations, tested
offsite. Although such requirements
seemingly intrude on the contractor's
means and methods, noncompliant test
results for an otherwise acceptable deep
mixing product soon become the
owner's problem.
Lesson 11: Optical televiewer technology
allows video inspection in great
detail of the entire borehole length. This
enhances confident evaluation of
homogeneity, at least in the vertical
direction. Linear consistency of compressive
strength test results from postproduction
" wet grab " samples adds to
confidence. Careful and systematic
review of the daily production logs and
real-time construction information by
trained construction inspectors can
demonstrate consistency in means and
methods that lead to homogeneity and
continuity in the final product.
Lesson 12: It is difficult for some
contractors to successfully core lowstrength
mixed soils without damaging
the core, leading to a misrepresentation
of the compressive strength of the in situ
material. In such circumstances, more
effective and representative measurements
can be based on postproduction
samples, but only after applying a
correction factor of double the required
design strength.
Lesson 13: In situ falling head tests
should remain the acceptance criterion
for the specified maximum permeability
of a deep mixing seepage barrier.
The only value of laboratory permeability
tests on postproduction samples is
added confirmation of consistency.
Laboratory tests on core samples often
lack accuracy and are misrepresentative
due to sample microfissuring, especially
if hard gravels are incorporated into the
DM product.
Conclusion
By virtue of the many large and successful
projects completed over threeplus
decades, and the pro-active
business development efforts undertaken
by contractors and the various
trade associations supporting them,
DMM has reached a " mature " status in
the eyes of U.S. ground engineers. The
DMM market is vigorous and competitive,
and its flexibility is illustrated by its
ability to very quickly satisfy " emergency "
requests in an economical manner
while still providing a high-quality
product. The specialty contractors drive
innovation and development, but
Mother Nature and national cash flow
largely define market size and direction.
Donald Bruce, Ph.D., is president of Geosystems
based in Venetia, Pennsylvania. He is a former
chair of DFI's Micropile and Slurry Wall
committees, and served similar roles with ADSC
committees. Bruce has spent his entire
professional career in specialty geotechnical
construction processes as a researcher, teacher,
contractor and now consultant. He consults on
projects throughout the world and has authored
over 350 technical papers and 3 textbooks.
Pete Cali, Ph.D., is president of Sea Level
Engineering in Metairie, Louisiana. After a
career as a geotechnical engineer at the U.S. Army
Corps of Engineers and as an adjunct professor at
Tulane University, he is now engaged in
geotechnical engineering consulting.
DEEP FOUNDATIONS * SEPT/OCT 2021 * 105
September/October 2021

Table of Contents for the Digital Edition of September/October 2021
