March/April 2022 - 84

As expected, for the mixing process
Mixing Process
lasting 72 sec in both cases, the BRN
value is higher in the second process
and amounts to 98 compared to 88.
However, this does not automatically
mean a better degree of mixing, because
in very cohesive soils, the first mixing
phase is decisive.
In a single cycle, the BRN is 70 in the
first phase, and only 35 in the second.
Consequently, when the initial penetration
rate is too fast, inclusions of
unmixed soil can occur, which remain
in the column and do not break up in
the subsequent mixing cycles.
In the field, other mixing parameters
are also important. Basically, the
following three parameters can be
adjusted on site:
(2) Mixing work
(1) The density of the slurry
(3) Binder content, which is the mass of
the binder divided by the volume of
treated ground
strengths depending on the initial soil
moisture. This dependence on the
initial water content of soil is not linear,
with a maximum strength for cohesive
soils that corresponds to the moisture
content slightly above the liquid limit.
Thus, mixing of soil that is too dry or too
wet is ineffective. Consequently, one of
the parameters to control closely is the
total water-to-cement ratio.
It is equally important to have a good
understanding of the mixing process
under field conditions. In Poland, in
addition to mixing the soil in one cycle,
multiple full-stroke mixing is often
used. A common belief is that perWet
mixing process filed test
forming a few mixing cycles is better
because it increases the blade rotation
number (BRN) and also allows the
material homogeneity to be improved
along the column length. However, this
assumption can be wrong, especially
with very cohesive soils.
For example, let us analyze the mixing
of a 3.3 ft (1 m) thick soil layer with a
paddle consisting of two mixing blades.
Adopting typical penetration and withdrawal
rates and rotational speeds, it is
possible to calculate the BRN values for
each mixing phase (marked blue on the
table for single cycle mixing and black
for mixing three times).
In principle, one could also adjust the
slurry flow rate, but this is rather impractical
for short columns; therefore,
we will keep this parameter constant, as
is typically done on field sites. Now we
can install four columns in the ground,
C1 to C4, and inspect how adjusting only
one parameter affects the soil mix
strength.
In the first process we just change
the slurry density. The use of a " thicker "
slurry leads to increased binder factor
in-place (BFP) because we pump less
volume into the ground. Consequently,
the strength increases, and the total
water-to-cement (W/C) ratio decreases,
also contributing to strength gain.
84 * DEEP FOUNDATIONS * MAR/APR 2022

March/April 2022

Table of Contents for the Digital Edition of March/April 2022

TOC
March/April 2022 - Intro
March/April 2022 - 1
March/April 2022 - 2
March/April 2022 - TOC
March/April 2022 - 4
March/April 2022 - 5
March/April 2022 - 6
March/April 2022 - 7
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