September/October 2021 - 103

are designed to resist specific design
loads and typically require higher
strengths, up to 350 psi (2.4 MPa) or
more. EuroSoilStab (2002) reports that
the strength of in situ material can be
20 to 50% of that of laboratory-mixed
specimens. To ensure compliance with
100 psi (0.7 MPa) as a minimum UCS,
for example, contractors must target
twice that value for wet grab tests and
bench scale tests and adjust the binder
content for the design mix accordingly.
Since cement cost strongly influences
the overall deep mixing installation
cost, overconservatism in specifying
strength can dramatically increase the
final mixing cost.
The value of 100 psi (0.7 MPa) has
some practical basis for seepage
barriers in that acceptance criteria are
often based on drilled core samples, and
soft walls are inherently more difficult
to core due to the expertise involved. A
more rational approach might be to
base strength compliance on postproduction
samples and use 200 psi
(1.4 MPa) as the minimum acceptance
criterion, with at least 90% of the
samples passing. Verification borings
and cores are used as the basis for
compliance with specifications on
homogeneity, continuity, and in situ
permeability results.
Permeability: Compliance with the
specified maximum permeability for
seepage barriers is best judged from the
results of in situ falling or rising head
tests. Verification boreholes drilled
with less than careful methods can
damage borehole walls, leading to in
situ permeability test values higher
than those obtained from laboratory
tests on pristine cast samples. In the
authors' opinion, permeability should
be judged using in situ testing as the
most representative of a seepage
barrier quality, delegating lab tests on
samples (wet grab or core) to only
serving as indicators of consistency.
Cutter soil mixing wheels in use (credit, Keller)
Criteria for judging the quality of a
DMM structure should be rated as
follows, listed by importance:
1. Geometric Dimensions (position,
depth, width, verticality and overlap
of panels) as determined by onboard
electronic records and field
measure.
2. Drift, with the FHWA Design Manual
recommending a maximum
drift of 1% in vertical alignment.
3. Homogeneity judged by visual
inspection and percentage recovery
of the cores and review of downhole
video records (with no particle more
than 3 in [76 mm] in any dimension).
4. Continuity, measured using angled
cores spanning panel overlaps to
confirm analyses of element verticality
measurements and calculations.
5. Permeability, as measured by in
situ falling head tests (if for a cutoff).
6. Strength, as measured from UCS
testing of postproduction samples,
with an appropriate reduction in
measured values to represent in situ
strength.
When deep mixing is used primarily to
provide some form of strength,
permeability is less of a design factor.
Instead, strength testing would become
second in importance in the above list.
This is a generalization of a quality
assistance/quality control (QA/QC)
program that would, of course, have to
be tailored to the specific application.
However, considering that many
specifications require 1%-3% of the
deep mixing product to undergo coring
or grab sampling during construction,
one must question the quality of the
remaining 97%-99% of the deep mixing
installation if repeated tests fail. The
2013 FHWA manual recommends that
2%-4% of deep mixing elements be
cored, with the larger percentage
applying to more important projects
and to smaller projects.
A combination of 100% QC coverage
and a relatively small amount of
verification coring and testing can
provide relatively high reliability that
the mixing is of good quality, provided
that QC and QA work is done well, with
high frequency observations of all
QC/QA activities by the owner/engineer.
It is imperative QC/QA efforts be
DEEP FOUNDATIONS * SEPT/OCT 2021 * 103
September/October 2021

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