Impact of flocculating agents on stress-strain
behaviour of natural mineral sludges
Luca Barbetti
The author is involved in a scientific research project at the Laboratory of
Geotechnics of Ghent University, funded by the Flemish Environmental
100Technology Platform (MIP). On this research project they are participating
also several universities, research institutes, contractors, design offices and big
industries as VITO, KUL, KHBO Oostende, LabMet, MWH, Rasenberg Milieu,
DEC, Envisan, Ghent Dredging, WVRB, MOW- Maritieme Toegang, Gementelijk
Havenbedrijf Antwerpen, Nyrstar and Tessenderloo Chemie.
The aim of this research is to possibly modify the consolidation behaviour of
dredged material of mineral origin, as well as to look after a possible stiffness
increase of the slurries, by adding flocculants.
MATERIALS AND METHODS
This project aims at using additives to optimise the behaviour of sludge
material. This optimization can be related to different aspects of the behaviour of
the material as sedimentation, consolidation, final density, final shear strength,
leachate production, stabilisation of organic and inorganic pollutants. Not all
aspects will be completely compatible between each other. In most cases, in the
industry, the main choice of a flocculants is based on optimizing sedimentation
behaviour, this based on very rudimentary tests (shaking beaker test, sieve
permeation test...). A correct optimization should be based on the full large strain
consolidation behaviour. To reach this, we try to obtain large strain parameters
(permeability and compressibility function) and shear and stiffness property for
sludges that have been altered using different types of additives, in this case
flocculants.
The impact of the technique used to add flocculants is also important in the
study of the consolidation behaviour. There were identified two kinds of possible
mixing methods in-site those were try to simulate in laboratory scale: the “inline
mixing” method of the flocculants with a sludge of initial relative low density (1.051.1 ton/m³) try to simulate the turbulent mixing that we can have in pipe-lines and
dumping methods, and the “in-situ” mixing method of a flocculants with a sludge
at a pre-consolidated stage (density of 1.2-1.3 ton/m³) try to simulate different kind
of injection methods that we can have directly in a real sludge storage pond.
Looking at the large number of tests to be performed and on the several type
of soils and flocculants at disposal, it was chosen to focus on the most interesting
sludge materials. The main part of the research has been done on the material
from the Ieper-Ijzer canal, that presents the most difficulty to be consolidate and
which has been adapted using the Praestol 650BC (cationic) flocculants. A whole
range of tests has been performed on the reference material : Seepage Induced
Consolidation cell test (SIC), CRS test, standard oedometer, big diameter
oedometer, simple shear test,… The basic materials have also been
characterised : grain size distribution, atterberg limits, chemical analysis of
pollutants.
PRELIMINARY RESULTS
The most important tool during our research work was the SIC test. This test
allows to determine the full compressibility and permeability functions
simultaneously over a large range of stress.
Unfortunately, the tests take quite a long
time (6 to 8 weeks) which leads to
disturbances in the measurements due to
gas formation. A simple drawing of the
test set-up is plotted here in figure 1,
while some typical results from the first
series of test are shown in figure 2 and 3.
Tests performed till now showed that the
modified materials show an increased of
permeability, till quite high densities. It is
also clear that the in situ mixing method
has a larger impact than the in-line
mixing. This is of course related to the
procedure of the in situ mixing and the
creation of the dual porosity structure. It
is necessary to repeat these tests on
samples which have been mixed with an Figure 1. Seepage induced consolidation test
actual existing mixing technique on a setup – Laboratory of Geotechnics
larger scale to confirm the results.
To avoid some problems with the gas formation, it was decided to perform
some faster oedometer compression tests. These have the disadvantage that
they do not allow direct measurement of permeability, instead we get values for
the consolidation coefficient, which is a direct indication of the velocity of
consolidation (combining permeability and compressibility). To this extent, we
have performed both small and larger scale oedometer tests.
All results so far indicate that the values of the consolidation coefficient cv are
indeed higher with the treated material (with slightly higher values for the in situ
mixed material as compared to the in-line mixing situation) which shows the
beneficial effect of the use of the flocculants.
There were also encounter some less positive aspects, in situ mixing creates a
dual porosity structure with lower shear strength at low stress levels. At higher
stress levels, this effect is minimal. In-line mixing induces higher creep
deformation at low stress levels. The relevant (long term) tests are still running,
so it cannot be specified if this behaviour is limited only to low stress levels.
Gas formation has a devastating impact on the long term behaviour of the
material, leading to a decrease in permeability and therefore prolonging the
consolidation time. In some cases, swell could be occurring due to the large
volume of gas produced.
Permeability curve
I- no floc
Compressibility curve
5
I- inline- 0,7% Pr650BC
5
I-in situ - 0,1% Pr650BC
4,5
I-in situ - 0,1% Pr650BC and sand
4,5
4
3,5
3
2,5
2
1,00E-08
I- no floc
I- inline- 0,7% Pr650BC
I-in situ - 0,1% Pr650BC
1,00E-09
1,00E-10
I-in situ - 0,1% Pr650BC and sand
4
void ratio
INTRODUCTION
Luca.Barbetti@UGent.be
barbettiluca@libero.it
void ratio
www.ugent.be
Supervisors: William F. Van Impe, Peter O. Van Impe
Laboratory of Geotechnics
3,5
3
2,5
2
10
permeability (m/s)
Figure 2. Permeability curve using the SIC test,
test on sludge from Ijzer Canal - Ieper, added
with Praestol650BC flocculants
100
eff stress (kPa)
Figure 3. Compressibility curve using the SIC
test, test on sludge from Ijzer Canal - Ieper,
added with Praestol650BC flocculants
CONCLUSION AND FUTURE WORKS
The first series of results presented in this paper shows as SIC test
equipment can be used in order to predict compressibility and hydraulic
conductivity behaviour of soft soils. The results showed an improvement of the
sludge treated with the cationic flocculants, improvement that can be related to
an increase of permeability of 2-3 times for a large range of effective stress,
between ~10 – 40 kPa.
In our future research we will continue on the study of soft clayey mineral
slurries, and try to understand if using additives have any impact on the
consolidation of the slurry and on its permeability in function of effective stress
and time. Variables of our studies will be: mixing methods of flocculants (in-line
mixing at low initial density, mechanical and injection mixing at high initial
density, low and high energy of mixing); type of flocculants (anionic, cationic
and non-ionic, artificial and natural) and its efficiency in function of the storage
time.
Together with SIC tests other standard and non standard tests will be
performed to determine shear strength development in presence of flocculants
at different stress level; the stiffness of the consolidating soil behaviour will be
studied, using bender elements. The samples used for the SIC tests will be
also subjected to x-ray tomography analysis, it will be possible to have nondestructive 2D and 3D images of the consolidated sample, algorithms analysis
to determine porosity, pore size distributions, particle sizes and orientation of
the structure of the slurry in presence or not of flocculants. These analyses will
help us to study the behaviour of the flocculants with the clay particles and
clay structures, and to define the effect of the mixing method on the sludge,
double porosity or homogeneous mixing of the slurry.
Some large scale test have been also designed to have the possibility to
compare the laboratory results with real case test.
References
(1)
Been, K., Sills, G.C., 1981. Self weight consolidation of soft soils: an
experimental and theoretical study. Geotechnique 31, (4) 519-535
(2)
Gibson, R.E., England, G.L., Hussey, M.J.L., 1967. The theory of one dimensional
consolidation of saturated
clays: I. Finite nonlinear consolidation of thin homogeneous
layers. Geotechnique 17 (3), 261–273.
(3)
Abu-Hejleh A.N., Znidarcic D., 1992. Consolidation characteristics determination for
phosphatic clays- Volume 1-2-3, Publication No. 02-084 104 University of Colorado
(4)
Silva, W.S., De Azevedo, R.F., 2001. Hydraulic consolidation test: equipment
description and testing analysis. University Federal of Vicosa, Department of Civil
Engineering, Vicosa, Brazil
(5) Znidarcic, D., Liu, J.C., 1989. Consolidation characteristics determination for dredged
materials. Proc., 22nd Annual Dredging Seminar, Ctr. For dredged Studies. Texas A&M
Univ., College Station, Tex, 45–65