September/October 2021 - 104

minimum UCS or shear strength (with
limits on the number and distribution
of noncompliant samples), but not a
maximum strength (reflecting soil
heterogeneity).
Lesson 2: DM specifications can
become bloated by blind repetition of
specifications from previous projects
that are irrelevant and redundant. Such
specifications often cross the line between
performance and method specifications,
further complicating issues.
Lesson 3: Designers can spend
A double axis machine conducts deep mixing for buttresses in an LVP111 foundation (credit, Trevicos)
augmented by expert review of the daily
DM installation records. In this way, data
obtained from testing can be extrapolated
to represent the entire DM works.
Test specimens taken from cores
drilled at 28-days' age during the field
validation program and production
can be preserved for longer durations
to demonstrate that a 56-day or even
90-day design strength specification
has been achieved. This would allow
the cost-conscious designer to specify
a lower 28-day strength, eliminating
the cost of the additional binder. It is
often preferable to conservatively
estimate strength gain with time
during design, and to establish the
specified strength in terms of the 28day
strength instead of the 56-day
strength because this can facilitate the
contractor's staging operations.
The FHWA design manual recommends
that the contractor propose
locations for wet grab sampling while
considering input from the owner/
engineer, and that the frequency be
guided by the following: " One wet grab
sample (one selected depth at one
location) should be retrieved every two
production days or for every 500 m of 3
treated soil, whichever produces the
higher sampling frequency. " FHWA's
manual, which has been reshared at
www.geoinstitute.org/geotechtools,
104 * DEEP FOUNDATIONS * SEPT/OCT 2021
expands this to require that " Sample
locations shall be distributed uniformly
both laterally and vertically within the
deep mixed zone. "
Many Lessons Learned
Published literature is replete with
DMM project successes and innovative
applications. Absent are criticisms and
failures, likely for legal reasons. The
following are a few authors' observations
of shortcomings in U.S. design and
construction of contemporary deep
mixing projects:
Lesson 1: Designers often specify
unjustified and sometimes unrealistic
performance criteria, resulting later in
unnecessary contractual issues due to
noncompliance. For example, unless
there is a valid design or quality assurance
need for a specific compressive
strength of a seepage barrier, the
requirement should be eliminated, deemphasized
or given a wide range as an
acceptance criterion. Rather than
specifying an absolute maximum
permeability limit, consideration should
be given to adopting the probabilistic
approach for strength specification
advocated by Filz and Navin (2010),
particularly if and when permeability is
measured on a large number of specimens
recovered from core samples. For
DMM applications involving structural
support, it is appropriate to specify a
months and even years evaluating a
project and preparing contract documents,
drawings and specifications.
However, a fraction of this time is made
available for contractors to bid the job.
This " back-end acceleration " hampers
the contractor (who may lack the
opportunity to fully understand project
nuances), and the owner (who, for the
same reasons, may receive an unbalanced,
unrealistic bid that leads to a
contractual/financial dispute). In addition
to giving bidders more time, owners
should ensure that they provide the
Geotechnical Data Report, the Geotechnical
Interpretative Report and the
Design Basis Report (each prepared by
the designer) to enhance bidders'
understanding.
Lesson 4: Designers should provide
field QA staff with a manual of engineering
considerations, with a brief description
of the project's purpose and goals.
The manual should relate the relevance
of each QA and QC item to the design
intent, allowing field personnel to fully
appreciate their individual tasks. Field
personnel should also undergo training
on the nuances of the DMM employed.
Lesson 5: Whereas QC is the contractor's
responsibility, as-built quality
problems rapidly become the owner's
problems through construction claims,
delays and remediation of defects.
Specifications written to ensure quality
throughout construction must be
emphasized by demanding timely contractor
submittals and prompt review of
test results by the owner's representative.

http://www.geoinstitute.org/geotechtools

September/October 2021

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