March/April 2022 - 85

In the second process we adjust the
mixing work, and typically observe that
the compressive strength increases
with the degree of mixing.
In the third process, the binder
content has been changed from to 12.5 to
3
20 lb/ft (200 to 320 kg/m ), keeping the
same slurry density and mixing work.
The latter condition could not be met
precisely, but the former tests can be
used to estimate corrected unconfined
compressive strength (UCS). Noting that
a 24% increase of BRN corresponded to a
13% increase of strength, we can reduce
the measured strength by 13% to obtain
about 0.33 ksi (2.29 MPa) instead of
measured 0.38 ksi (2.59 MPa). Consequently,
using more cement increases
the strength very effectively, as indicated
also by the BFP, and decreases the
total W/C ratio to 1.8 since the slurry
contains more cement than water
(W/C=0.692).
3
Complex Case Study
To understand how mixing decisions
can go wrong, I will recap a project in
Poland, which is comprised of DSM
works on six areas with organic soils
identified while constructing the A2
highway, connecting Warsaw with
Challenging A2 highway use of dry deep mixing
Berlin. The organic deposits, which
were up to 19.7 ft (6 m) deep, included
peat and gyttja, a fine-textured eutrophic
mud; the fine mud was very soft
when wet but stiff when dry.
The road embankments were designed
to be up to 11.5 ft (3.5 m) high and
131 ft (40 m) wide at the base level; they
were comprised of Limix columns 2 ft
(0.6 m) in diameter, covering 35% to 60%
of the treatment area of 6.7 acres (2.7 ha).
The anticipated settlement of the embankment
without GI was about 5.6 ft
(1.7 m), and 3.9 in (0.1 m) after installation
of the columns.
However, four months after embankment
formation, an excessive and
continuing settlement of 1.3 to 2.3 ft (0.4
to 0.7 m) was noted, and the column
strain reached about 10%. That was well
above the failure strain. In addition,
horizontal deformation reached 0.3 ft
(100 mm), and was also increasing.
A group of international experts
conducted a detailed root cause analysis
that included a Mind Map. Based on the
investigations, no deterioration nor
microbial processes were affecting the
mechanical parameters of the dry deep
mixing columns. However, it was
concluded that the inaccurate distribution
and mixing of binder had
resulted in lumps of binder in the
columns, preventing accurate hydration
of the cement and the following activation
of the slag. Consequently, the
binder added was not contributing to the
strength development to the same extent
as seen in laboratory-mixed samples.
Due to the foundation's anticipated
deterioration and limited time to complete
the highway, it was decided to
exchange all installed columns to alternative
technical solutions. The remediation
included soil exchange and
mineral fill compaction, installation of
precast piles and construction of a
concrete slab on top, and, on a small
scale, some wet DSM works.
Another Difficult Case
This is a second case where wet deep
soil mixing, conducted in 2001/2002,
went wrong. The owners of a modern
library in Cracow, which uses sliding
bookshelves, were very concerned
about possible differential building
settlements due to the presence of tertiary
marine clays in the subsoil. Therefore,
piling was initially considered,
which has been converted to GI with
wet DSM columns.
We had predicted that the building
would not settle more than 1 in (2.5 cm),
and we believed it was possible with
DSM. In fact, there were no settlements
at all, but a damaging heave instead,
reaching up to 0.4 in (10 mm) per month.
Initially we thought that the observed
heave was associated with the
swelling potential of the clay because
we had heavy rains during and after our
works, and the whole excavation pit was
nearly flooded with water. We started to
use steel ballast placed on foundations,
but this was completely ineffective.
Subsequently, we decided to lift all
pillars and place steel plates underneath,
so that they could be pulled out to
maintain deck level unaffected by
foundation heave. In addition, drainage
pipes were installed around and inside
the building to prevent watering of the
ground. Unfortunately, these attempts
DEEP FOUNDATIONS * MAR/APR 2022 * 85
March/April 2022

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