Determination of consolidation parameters of a sludge using the “Seepage Induced
Consolidation Test”
La détermination des paramètres de consolidation d’une boue utilisant le “Seepage Induced
Consolidation Test”
Barbetti Luca
Laboratory of Geotechnics – Ghent University
ABSTRACT
The author is involved in a scientific research project at the Ghent University –Laboratory of Geotechnics, funded by the Flemish
Environmental Technology platform (MIP). 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.
This paper will discuss in more detail the “seepage induced consolidation test”. Indeed, this setup will be used to determine
consolidation and hydraulic conductivity parameters for the very soft soil and has a great potential to reduce the consolidation period,
while measuring accurately the changes in void ratio and permeability for a large variety of very small effective stresses.
The analysis of the test data is based on the large strain consolidation theory using the SICTA programme (Abu-Heyleh et
Znidarcic, 1992) with solver engine in a spread sheet frame. In the newly developed experimental set-up it was tried to improve the
reliability and repeatability of the testing method as well as to automate all testing steps.
The new equipment and test analysis are shown in this paper together with some preliminary results on a reference kaolin clay
material.
RÉSUMÉ
Le but de cette recherche scientifique au labo de Géotechnique de l’université de Gand est d’essayer de modifier le comportement
de consolidation de matériaux de dragage d’origine minérale et de manipuler éventuellement la rigidité de ces matériaux, par
l’utilisation des additifs de floculation. Cette investigation a été réalisée dans le cadre d’un programme de recherche exécuté grâce au
support de la direction MIP en Flandre. Cette contribution présente en détail la méthode “seepage induced consolidation test”. Ce type
d’essai est utilisé pour déterminer les paramètres de consolidation et la conductivité hydraulique d’un sol mou, limitant le temps de
consolidation et de mesure de la variation des vides et de la perméabilité sous des contraintes effectives très faibles. L’analyse des
résultats démarre de la théorie de consolidation à grande déformation et elle se présente à l’aide du programme numérique SICTA
programme (Abu-Heyleh et Znidarcic, 1992) avec un solveur moteur en tableur. Dans ce nouveau type d’essai développé, la
fiabilité, la reproductibilité des résultats et l’automatisation de l’essai ont été améliorées.
Le nouvel équipement est présenté dans cette contribution avec un exemple d’interprétation préliminaire des résultats d’un matériel
de référence comme l’argile kaoline
Keywords : SIC test, large strain consolidation, sludge, kaolin
1
INTRODUCTION
There are a number of experimental setups proposed in the
literature that attempts to predict the consolidation
characteristics and parameters for the high water content soils
(Imai, 1979; Znidarcic and Liu, 1989). These experimental
setups, known as “Seepage Induced Consolidation Tests”, have
a number of advantages:
- compressibility and hydraulic conductivity relationships
are obtained from a single test;
- relationships are obtained for a single hydraulic gradient;
- test is well suited for measurements in the low effectivestress range and
- the hydraulic consolidation test generally reduces the total
testing time for highly compressible materials.
Based on the original concept by Imai (1979) and on the test
setup proposed by Znidarcic and Liu (1989), it was tried with
our work to improve the reliability and the repeatability of the
results and to automate all test setup.
2
2.1
EXPERIMENAL SETUP
Test equipment
The seepage induced consolidation setup developed at the
Laboratory of Geotechnics of Ghent University may be divided
in the following elements as showed in figure 1: a differential
pressure (DT), a linear-potentiometer device (LVDT), a flow
pump, a load air-pressure controlled frame (LC), a data
acquisition system and a local water cell.
The cylindrical soil sample is located within a transparent
acrylic cylinder PMMA with a diameter of 90 mm and
maximum height of 250 mm. The sample sits between two
perforated POM plates, that together with two filter papers for
clay (to avoid clogging of the plates and flushing out of the
particles clay), represent the drained boundary conditions of the
sample.
The bottom of the cell and the perforated bottom plate have
to be saturated with water before starting the test in order to
avoid presence of air bubbles that can give problems of
pressures readings during the test.
A differential transducer DT is used to measure differential
pressures in the range 0-30 kPa. The DT has two pressure ports
in the cell (at top and at bottom), the generated pore water
pressures inside the sample are measured in the time giving also
an index of the consolidation degree process.
The flow pump system is a Harvard Apparatus Model
pump33 and it is composed by a drive motor, two syringes
(internal diameter of 16.9 mm) and 4 automatic switch valves
that control the water flow. The motor is controlled digitally,
allowing the selection of its speed and movement in both
directions.
Figure 1. Seepage induced consolidation test setup – Laboratory of
Geotechnics, Ghent University
The cell is also provide by a plunger that is leaned on the
perforated plastic top plate, the porous plate together with the
weight of the plunger apply circa 0.6 kPa on the sample, this
small initial load is used to avoid piping on the sample caused
mostly by the pump rates and by the soft initial state of the soil.
A load air-controlled frame is used to apply stresses though the
plunger with a maximum of 150 kg, corresponding to a pressure
of 250-200 kPa on the sample.
To have accurate measurements of settlement and height
change of the sample it is used a linear-potentiometer, LRW2F-25-S, that measures deformation of the sample taking
readings between the top plate of the cell (zero-fix value) and
the settlement of the plunger (together with the sample) inside
the cell.
2.2
Testing procedure
The slurry is mixed at the desired density in a cylinder container
in the rotate table with a velocity of 10 rpm for circa one hour,
in the meantime the cell is prepared and all lines and the bottom
part are saturated with water, it is tried to avoid presence of air
bubbles in the close system that can cause errors in pressure
readings when the test is running.
The homogeneous slurry is poured into the cell; changes in
height of the sample are regularly registered in the time till it is
assumed that we reached the end of sedimentation and selfweight consolidation (constant height). At this point, a filterclay paper (to avoid losing of materials), the top perforated plate
and the plunger are sunk into the cell to create a small initial
overpressure (circa 0.6 kPa). This initial stress level is used to
avoid piping due to pump rates during the followings stages.
Consolidation and steady state are reached.
The second part of the test is defined by a seepage induced
consolidation through the pump: a flow rate is imposed across
the sample. The imposed seepage force speeds up the
consolidation process at a certain stress level during which the
sample consolidates and the induced pressure difference across
the sample increases. Measurements of changing in height and
differential pressures between the top and the bottom of the
sample are registered by the DT and LVDT, the process reaches
the steady state when these values stabilize. The selection of the
range of pump rates is closely related to the type of slurry and
its consolidation behaviour. The seepage consolidation through
the pump is normally performed in different steps, using 2-3
different pump rates, the increase of stress level can be
controlled by choosing higher flux rates.
From a certain stress level (~ 10 kPa), loads can be applied
using the loading frame. Measurements of change in height and
differential pressure are registered till a steady state. At the end
of each loading step, the pump is used to perform a permeability
test. The flux rate is chosen to be small enough to not induce
significant additional seepage stresses on the sample that should
be anyway counted in the real total stress applied.
Double initial drainage (from top and bottom of the cell) and
other data must be specified as input in the process of parameter
estimation for which the steady-state of the seepage induced
consolidation is simulated. The boundary conditions during the
test are:
- imposed flow rate by a pump, v, which is known from the
experiment; and
- free surface void ratio or the void ratio at zero effective
stress, eoo, which is measured performing simple free
sedimentation column test (d= 70 mm).
2.3
Methods of analysis
The test analysis is based on the one-dimensional finite
strain consolidation theory developed by Gibson et al. (1967).
The governing equation is given by:
⎡
⎤
d σ v ∂e ⎥ ∂e
d ⎛⎜ k ⎞⎟ ⎤⎥ ∂e ∂ ⎢
k
(G s − 1)⎡⎢ de
=
⎜ 1 + e ⎟ ⎥ ∂z ∂z ⎢ γ (1 + e ) de ∂z ⎥ ∂t
⎝
⎠⎦
⎣⎢
⎢ w
⎥
'
⎣
(1)
⎦
where z is the material coordinate positive downward and t is
time. At the permanent stage of seepage induced consolidation,
the equation becomes:
Figure 2. Seepage induced consolidation test setup – Laboratory of
Geotechnics, Ghent University
All sensors are connected to a computer through an Agilent
Data Logger controlled by a Labview program, a visual
programming language. This feature is important to have a
continuous check of the test and to determine the steady state
conditions.
(G s − 1)⎡⎢⎣ ded ⎛⎜⎝ 1 +k e ⎞⎟⎠ ⎤⎥⎦ dedz
d σ ' de ⎤
d ⎡⎢
k
v
⎥=0
dz ⎢ γ (1 + e ) de dz ⎥
⎣ w
⎦
since the void ratio is time independent.
(2)
This equation is equivalent to (Silva, 1999):
(
w
)
(3)
A=
from which one we can obtain:
z vγ
σ ' (z ) = σ ' + γ w (G s − 1)z + ∫ w (1 + e )dz
0
0
0 k
(4)
in this equation the first term represents the effective stress
given by the weight of the top POM plate and the plunger, the
second term represents the self-weight effective stress and the
last term represents the stress due to the seepage forces. Using
this equation, the effective stress at the bottom is given by:
σ'
hs v γ
w (1 + e )dz
= σ ' + γ w (G s − 1 )h s + ∫
0
0 k
bteor
W
d
G γ A
s w
(6)
(7)
To solve the one-dimensional finite strain consolidation theory,
it is considered that the steady state condition in the seepageinduced consolidation test is controlled by the material stress–
deformation relationships proposed by Liu and Znidarcic
(1991):
e = A (σ '+ z ) B
( e / e00 )
1/ B
C =
−1
k
eD
(10-11-12)
The aim of this paper is to show the starting test results proving
the efficiency of the test setup and the working of the
equipment.
The experimental setup therefore was initially tested using a
reference material, a Rotoclay HB Kaolin with a Gs=2.65. It
was prepared a slurry mixing kaolin with de-ionized water
without the addition of flocculants agents. The initial density
used for this test was 1.1 ton/m³.
In the first part of test there were performed three seepage
consolidation phase using three increasing values of pump rates,
2.62x10-07 m/s, 5.24x0-07 m/s and 1.05x10-06 m/s; effective
stresses, which were achieved in the sample, were in the range
0.5-3 kPa. The three green points, showed in figure 3 and 4,
represent values obtained by the seepage consolidation induced
from the bottom of the cell by the pump.
During the second phase the sample was loaded at two
different stresses, 10 and 20 kPa, the two red points in figure 3
and 4 represent values of void ratio and permeability measured
at these two values of effective stress.
(8)
and by the density–hydraulic conductivity relationship proposed
by Somogyi (1979):
k = Ce D
σv
PARTIAL RESULTS
3
where Wd is the dry wet of the sample and A is the area of the
sample cross the section.
Knowing the void ratio distribution, it is also possible to find
the sample height by the equation:
h
s
hteor = ∫ (1 + e ) dz
0
Z =
(5)
and Hs= sample solid height is defined as
h =
s
e 00
ZB
To have a certain accuracy on the curve obtained from this test,
it is needed to perform at least two or three seepage induced test
at different increasing values of flow rates and at least a couple
of loading test. Under a relatively large range of stress level
(0.1- 200 kPa), the soft sample is compressed into a uniform
layer at different steps. As it was shown, the measured
consolidation properties are the permeability, k, the effective
stress at a certain void ratio, σ’ and their corresponding void
ratio, e.
void ratio
γ
d σ ' (z )
k
k
v
−
G −1
s
1+ e
(1 + e ) dz
(9)
In these two equations e stands for void ratio and σ′ stands for
effective stress. The model parameters (A, B, C, D and Z) are
material characteristics used to predict consolidation behavior,
for all levels of stress, of soft soils which are determined from
the results of seepage induced consolidation test by using the
two compressibility-hydraulic conductivity equations and a
parameter estimation algorithm. The analysis is based on the
methods used in the SICTA program (Abu-Heyleh and
Znidarcic, 1992) but it is executed using a solver engine in a
spreadsheet program. This algorithm accomplishes numerical
simulation of a test performed on a soft soil sample and
minimizes the difference between the experimentally obtained
results and numerically calculated values. Using the
experimental results from the seepage induced consolidation
tests (eoo, height sample and differential pressure) and the
parameter estimation algorithm SICTA, the coefficient
parameters, A, B, C, D, and Z can be determined. These
parameters are needed to model the consolidation behaviour of
10
9
8
7
6
5
4
3
2
1
0
0,001
0,01
0,1
1
10
100
1000
effective stress (kPa)
Figure 3. Results from SIC test of compressibility curve of slurry
prepared with kaolin and de-ionized water at initial density of 1.1
ton/m³
void ratio
v =
high water content soils in the field under self-weight or
surcharge loading, therefore determination of realistic values for
them is crucial for the prediction of field behaviour.
These five coefficient parameters are determined as follows:
10
9
8
7
6
5
4
3
2
1
0
1,E-10
1,E-09
1,E-08
1,E-07
1,E-06
1,E-05
permeability (m/s)
Figure 4. Results from SIC test of permeability curve of slurry prepared
with kaolin and de-ionized water at initial density of 1.1 ton/m³
Figure 5 shows the five experimental points measured during
the test all fitting the values predicted using the SICTA
program and determining the five material coefficients using
the results from the SIC test.
particles and clay structures, and to define the effect of the mixing
method on the sludge, double porosity or homogeneous mixing of
the slurry.
ACKNOWLEDGEMENTS
measured values
0,12
predicted values
height (m)
0,1
0,08
0,06
0,04
0,02
0
0,1
1
10
100
effective stress bottom (kPa)
Figure 5. Results from SIC test of compressibility curve for slurry
prepared with kaolin and de-ionized water at initial density of 1.1
ton/m³
The ongoing research SIC tests results on minerals slurries
with addition of flocculating agents will be shown at this 17th
International Conference on Soil Mechanics and Geotechnical
Engineering in Alexandria in the paper “Determination of
consolidation parameters of dredging and industrial waste
sludge” (P.O. Van Impe, W.F. Van Impe; 2009).
4
CONCLUSIONS 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.
In our future research we will focus 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:
- use of different initial density of the slurry (slurries
mixed with flocculants at low 1.1 ton/m³ and high initial
density, 1.4 ton/m³);
- 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 xray 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
The author gratefully acknowledges Dr. Eng. P.O. Van Impe,
director of this research project and Prof. Dr. Eng W.F. Van Impe
as the director of the Laboratory of Geotechnics of Ghent
University.
The author is also thanking the Flemish Environmental
Technology Platform (MIP) for funding the research and to the
others research and industrial partners: the Flemish Institute for
Technological Research (VITO), the Division of Soil and Water
Management of the Catholic University of Leuven (KUL), the
College of Ostend-Bruges (KHBO), MWH, ENVISAN,
Rasenberg Milieu, DEME Environmental Contractors DEC,
Ghent Dredging, WVRB, Port of Antwerp, Agency for Maritime
and Coastal Services, Nyrstar and Tessenderlo Chemie.
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