1
Hydro-mechanical characteristics of a polymer-enhanced
bentonite-sand mixture for landfill applications
S.S. Agus
Research Assistant and PhD Student, Laboratory of Soil Mechanics, Bauhaus-University
Weimar, Coudraystrasse 11C 99421 Weimar, Germany.
Y.F. Arifin
PhD Student, Laboratory of Soil Mechanics, Bauhaus-University Weimar, Coudraystrasse
11C 99421 Weimar, Germany.
T. Schanz
Professor, Laboratory of Soil Mechanics, Bauhaus-University Weimar, Coudraystrasse 11C
99421 Weimar, Germany.
Introduction
Since its development in the Netherlands, Trisoplast®, an innovative polymer-enhanced
bentonite-sand mixture (PEBSM), has been intensively used and proposed as mineral sealing
material in many countries including several European countries and Malaysia. Since the
material consists of sand, bentonite, and polymer (with a microbial degradation resistant
mineral), its advantageous properties such as sealing capability, coefficient of permeability,
water retention capability, and long-term chemical stability are contributed by its each
element. In this investigation, hydro-mechanical characteristics of the material pertaining to
its use in landfills were studied. These included suction and water retention characteristics,
permeability, swelling, compressibility, and shear strength properties of the material.
Basic and physico-chemical characteristics of the material
Basic properties of the PEBSM used were determined according to DIN standards (DIN,
1987) and ASTM standards (ASTM, 1997). The material has a liquid limit and plastic limit
water content of 73% and 19%, respectively. The shrinkage limit of the material was
measured and has a value of 12%. The specific gravity was measured using picnometer
method and it has a value of 2.67. An average specific surface area of 19.9 m2/g and a cation
exchange capacity of 10.8 meq/100g were reported in Boels et al. (2003). Standard proctor
compaction curve was determined based on ASTM standards (ASTM, 1997) and the curve is
shown in Fig. 1. Initial water contents and dry densities of specimens used in the investigation
are also indicated in the figure. Generally, specimens at three different compaction conditions;
namely, dry of optimum, optimum, and wet of optimum were used in each test. Suction
isolines that were measured using a chilled-mirror hygrometer are also drawn in the figure.
Fig. 2. depicts an environmental scanning electron microscopic (ESEM) photo of the material
where an interaction between the polymer and bentonite in the mixture is shown.
Hydraulic properties
The hydraulic properties of the PEBSM investigated in this study consisted of the water
retention behaviour (or the soil-water characteristic curve (SWCC)) and the saturated and
unsaturated coefficient of permeability. The SWCC of the compacted PEBSM were measured
using two techniques; namely, axis-translation technique (in pressure plate apparatus) and
vapour equilibrium technique (in desiccator with salt solution). The results obtained from
both methods were combined to establish a single SWCC for each specimen. The measured
International Workshop « Hydro-Physico-Mechanics of Landfills”
LIRIGM, Grenoble 1 University,France , 21-22 March 2005
2
SWCC (i.e., plotted as a relationship between degree of saturation and suction) as shown in
Fig. 3., indicates no significant influence of compaction conditions (i.e., water content and dry
density) on the SWCC. Generally, the saturation range (i.e., the range of suction where air
phase is occluded) of the material does not extent to a high value of suction and is limited to a
maximum suction of about 10 kPa (i.e., the air-entry value (sb)). The residual range (i.e., the
range of suction where water phase is discontinuous) starts at about 40 000 kPa suction.
1.9
SWCC determination
1.8
Saturated permeability
tests
Oedometer tests
st = 500 kPa
1.6
st = 800 kPa
st = 2500 kPa
1.7
1.65
st = 1500 kPa
1.75
st = 5000 kPa
Dry density, ρ d (Mg/m3)
Proctor curve
determination
3
ρ d m ax = 1.827 Mg/m
1.85
to investigate the effect
of dry density on the
tensile strength
1.55
Sr = 100%
Swelling pressure
determination
Direct shear tests
Sr = 85%
wopt = 11.4%
1.5
Tensile strength tests
Sr = 15%
Sr = 30%
Sr = 45%
10
15
Sr = 60%
1.45
0
5
20
25
Water content, w (%)
Fig. 1. Standard proctor curve of the PEBSM
Fig.2. Bentonite-polymer interaction
The saturated coefficient of permeability of the material was measured using the constanthead flexible wall permeameter method (DIN, 1987 and ASTM, 1997). In order to prevent
volume change of the specimen during permeation, a confining pressure equal to the swelling
pressure of the specimen was applied in each test. Table 1 summarises the results of
permeability test performed in this study. The results indicate that the saturated coefficient of
permeability (ks) is not affected by the compaction conditions.
1.E-09
Unsaturated coeff. of permeability, k w (m/s)
100
Degree of saturation, Sr (%)
90
80
70
saturation
range
60
desaturation range
residual
range
50
dry
optimum
wet
bestfit-dry
bestfit-optimum
bestfit-wet
40
30
20
sb
10
sr
Sres
dry
1.E-10
optimum
1.E-11
wet
1.E-12
1.E-13
1.E-14
1.E-15
1.E-16
Range where liquid water transport may be
dominant
1.E-17
1.E-18
1.E-19
1.E-20
Suction corresponding to
residual degree of saturation
1.E-21
1.E-22
0
0.1
1
10
100
1000
10000
0.1
100000 1000000
1
10
Suction, s (kPa)
Fig. 3. SWCC of the PEBSM
100
1000
10000
100000 1000000
Suction, s (kPa)
Fig. 4. Permeability functions of the PEBSM
Table 1. Summary of saturated permeability test results
Specimen
Dry of optimum
Optimum
Wet of optimum
Initial water
content (%)
6.4
11.4
15.9
Initial dry
density (Mg/m3)
1.77
1.82
1.79
Initial total
suction (kPa)
3047
1278
774
International Workshop « Hydro-Physico-Mechanics of Landfills”
LIRIGM, Grenoble 1 University,France , 21-22 March 2005
Saturated coefficient
of permeability (m/s)
1.1x10-11
5.4x10-12
4.9x10-12
3
The unsaturated coefficient of permeability was computed from the SWCCs and ks values
using the statistical model proposed by Mualem (1976). The computation results are shown in
Fig. 4. It is thought that transport of water takes place in the liquid form when the soil has a
degree of saturation higher than the residual degree of saturation of the material. Beyond that,
the transport of water is dominated by transport in the vapour form.
Mechanical properties
The mechanical properties tested herein involved the determination of shear strength
parameters (i.e., effective cohesion (c') and effective angle of friction (φ')) and tensile strength
of the compacted PEBSM. The shear strength test was carried out in direct shear apparatus.
Several tests on saturated specimens and unsaturated specimens (i.e., by means of constant
water content tests) were performed in this study. The saturated shear strength parameters of
the material measured are summarised in Table 2. The shear strength of unsaturated
specimens can also be computed making use of the SWCC and the c' and φ' values for
saturated specimens using a method proposed by Vanapalli et al. (1996). The results of
computation of shear strength using the Vanapalli et al. (1996) model are presented in Fig. 5.
Table 2. Summary of direct shear test on the saturated specimens
Specimen
Dry of optimum
Optimum
Wet of optimum
Initial water
content
(%)
6.3
11.3
16.3
Initial dry
density
(Mg/m3)
1.77
1.83
1.77
Initial total
suction
(kPa)
3120
1295
746
Effective
cohesion
(kPa)
23.5
20.0
27.7
Effective
friction angle
(o )
37.0
40.7
36.1
The tensile strength of the compacted PEBSM was measured by means of constant water
content tests. Several specimens were tested including the specimens meant for investigating
the effect of density on tensile strength of the material (see Fig.1.). The results of shear
strength tests performed in this study are shown in Fig.6.
50
10000
reasonable range for application
range with doubt
45
Dry
Dry (investigation on density effect)
Optimum
1000
Tensile strength, t (kPa)
Shear strength, τ (kPa)
40
average recommended maximum shear
strength at 10 kPa net normal stress
(= 250 kPa)
100
Dry (computed)
Optimum (computed)
10
1
100
25
20
15
5
500 kPa suction
10
30
10
Wet (computed)
Constant water content test
1
Wet
35
1000
10000
100000
Suction, s (kPa)
Fig. 5. Shear strength function of the PEBSM
0
100
Suction at optimum
water content of
Trisoplast, st=1280 kPa
1000
10000
Suction, s (kPa)
Fig.6.Tensile strength function of the PEBSM
Coupled hydro-mechanical behaviour
The coupled hydro-mechanical characteristics of the PEBSM were tested by means of
swelling pressure and oedometer tests. Three types of swelling pressure tests were performed;
namely, constant volume, swell-load, and swell-under-load methods. Generally, the swellInternational Workshop « Hydro-Physico-Mechanics of Landfills”
LIRIGM, Grenoble 1 University,France , 21-22 March 2005
4
load method provides the maximum swelling pressure, except for the specimen compacted
wet of optimum. The following table summarises the results of three swelling pressure tests
performed on the PEBSM together with the swelling potential (at 7 kPa vertical stress).
Table 3. Summary of swelling potential and swelling pressures of the PEBSM
Specimen
Dry of optimum
Optimum
Wet of optimum
Initial
water
content
(%)
6.3
11.3
15.9
Initial dry
density
Swelling
potential
(Mg/m3)
1.77
1.83
1.79
(%)
8.4
10.4
5.5
Swelling pressure (kPa)
Constant
SwellSwellvolume
load
under-load
method
method
method
47
80
52
62
97
60
61
48
29
The oedometer tests on the compacted PEBSM were performed using conventional oedometer
cells. The results were interpreted using the rectangular hyperbola method (Sridharan et al.,
1987). The coefficient of consolidation (cv) of the PEBSM ranges between 10-9 m2/s and 10-7
m2/s for void ratio ranging from 0.4 to 0.65 irrespective of the compaction conditions. The
modulus of stiffness is found to vary from 400 to 100 000 kPa for the same range of void
ratio. The compression index (Cc) varies between 0.04 and 0.2 while the swelling index (Cs)
ranges from 0.005 to 0.02 without any pronounced effects of the compaction conditions.
Conclusion
Results of a study on the hydro-mechanical characterisation of a polymer-enhanced bentonitesand mixture (PEBSM) have been presented. The test results indicate that the hydromechanical characteristics of the PEBSM investigated herein (i.e., SWCC, saturated
coefficient of permeability, shear and tensile strengths, and consolidation behaviour), except
the swelling pressure and swelling potential are not significantly affected by the compaction
conditions.
References
ASTM (1997) Soil and Rock: dimension stone, geosynthetics. In 1997 Annual Book of
ASTM Standards, Vol. 04.08 and 04.09., ASTM, Philadelphia, PA.
Boels, D., Beest H., Sweers, H., and Groeneveld, P. (2003) Investigation of the functional
lifetime of Trisoplast in relation to chemical compositions of pore water solutions in barrier.
Alterra-rapport 528/JW/01-2003. Alterra, Green World Research, Wageningen, the
Netherlands.
DIN (1987) Baugrund und Grundwasser. Benenen und Beschreiben von Boden und Fels.
Deutsche Institut für Normung e.V., Beuth Verlag GmbH, Berlin.
Mualem, Y. (1976) A new model for predicting the hydraulic conductivity of unsaturated
porous media. Water Resources Research, 12: 513-522.
Sridharan, A., Murthy, N.S., and Prakash, K. (1987) Rectangular hyperbola method of
consolidation analysis. Géotechnique, 37(3): 335-368.
Vanapalli, S.K., Fredlund, D.G., Pufahl, D.E., and Clifton, A.W. (1996) Model for the
prediction of shear strength with respect to soil suction. Canadian Geotechnical Journal, 33:
379-392.
International Workshop « Hydro-Physico-Mechanics of Landfills”
LIRIGM, Grenoble 1 University,France , 21-22 March 2005