Nieva P., Francisca F. (2007). “On the permeability of compacted and stabilized loessical silts in relation to liner system
regulations”. International Congress on Development, Environment and Natural Resources: Multi-level and Multi-scale
Sustainability, Cochabamba 11-13 Julio, Bolivia, pp. 69-77
On the permeability of compacted and stabilized loessical silts in relation to
liner system regulations
P. M. Nieva(1), F. M. Francisca(1,2)
(1)
National University of Cordoba, Civil and Environmental Department, Cordoba,
Argentina
(2)
National Research Council of Argentina (CONICET)
[email protected]
Keywords: permeability, montmorillonite-silt mixtures, landfills, loess
Abstract
Hydraulic conductivity (k) is the most important property of compacted soils to determine
their aptitude as construction material for liners. Most of liner system regulations specify
hydraulic conductivities lower than 1x10-7 cm/s for containment barriers in landfills. In
this work, the hydraulic conductivity of loessical silts was measured in two rigid-wall
permeameters: compaction-mold and oedometric-cell. Samples were tested at different
relative dry unit weights, and various montmorillonite contents. The effect of soil
structure on permeability is analyzed. Results show that only 15.9% of compacted silts
can attain hydraulic conductivities lower than that required by landfill guidelines.
Otherwise, loessical soils must be stabilized with clay to lower their hydraulic
conductivities up to acceptable values.
1. Introduction
Disposal of municipal solid waste (MSW) has significant impact on environment. The
most used technique to dispose wastes involves landfills, since this technique is a low
cost treatment. Other alternative treatments such as recycling, composting and
incinerating also involve a final landfilling. Even though this last technique can be
considered environmentally sustainable, in many developing countries landfill disposal
fails because most of available documents on landfilling are based on technologies and
practices suited only to conditions in industrialized nations [1].
Liners must have low hydraulic conductivity and high retention capacity of contaminants
[2]. Landfill leachate moves through the liner by means of two main mechanisms:
advection and diffusion. Diffusion prevails when the hydraulic conductivity is extremely
low; otherwise advection is the most relevant mass transfer mechanism, which in turn
depends on the hydraulic conductivity of the liner. In any case, the ability of compacted
soil liners to restrict the movement of water and contaminants can be determined by
monitoring its hydraulic conductivity [3].
Nieva P., Francisca F. (2007). “On the permeability of compacted and stabilized loessical silts in relation to liner system
regulations”. International Congress on Development, Environment and Natural Resources: Multi-level and Multi-scale
Sustainability, Cochabamba 11-13 Julio, Bolivia, pp. 69-77
Regulations for the design of liners vary according to the management of waste adopted
by different countries, and their economical development. Bouazza and Van Impe [4]
compiled minimum requirements for containment barriers in different countries. In most
of cases, the hydraulic conductivity and the thickness of liners depend on waste type.
Commonly, landfills for non-hazardous solid waste must have liners with hydraulic
conductivities lower than 1x10-7 cm/s and thickness larger than 0.6 m. Bentonite is
usually added to soil when the hydraulic conductivity of compacted specimens is lower
than this value [5,6].
Two different types of liners can be observed in landfills in Argentina. In most of cases,
liners involve either a layer of compacted soil or a double-liner system with compacted
soil and geomembrane. Usually, local soils are used as construction material for the
earthern liners. Soil deposits in the central and north-eastern of Argentina are composed
by loess formed by very fine sand, silt and clay particles lifted by wind and transported
by aeolian action [7,8,9]. Particle sizes of these sediments usually are: sand 1%-10%,
silt 50%-80%, and clay 2%-20% (by weight).
Hydraulic conductivity of natural soils depends on particle size, void ratio, specific
surface, degree of saturation and fluid properties. Soil structure, compaction energy and
tixotropy are also relevant properties in case of compacted soil [10,11]. The hydraulic
conductivity of Argentinean loessical silts has been extensively studied in the last
decade [12,13,14,15]. Francisca et al. [13] showed the effect of fluid type on the
permeability of natural and compacted loessical silts and concluded that it is extremely
difficult to achieve hydraulic conductivities lower than that required for the construction of
containment barriers.
The purpose of this work is to compare the hydraulic conductivity of compacted loessical
silts with that required by landfill system regulations. Permeability test were performed in
compacted silts specimens using two rigid-wall permeameters: compaction-mold and
oedometer-cell. Obtained results are compared with those reported in literature and with
international regulations to discuss the aptitude of loess as construction material for
liners. Finally, the influence of clay fraction on hydraulic conductivity of loessical silts is
determined by testing silt-montmorillonite mixtures.
2. Materials and methods
Permeability test were performed in compacted loessical silts and compacted mixtures
of silt and sodium montmorillonite (Minarmco® trademark). Table 1 shows relevant
physical properties of these two soils. Loess samples were recovered from three
different locations in Cordoba State in the central area of Argentina. The main
geotechnical and physical properties of these sediments have been reported by Teruggi
[7], Rocca [16], Moll and Rocca [17], Francisca and Redolfi [18].
Different specimens were prepared at different relative dry unit weight (γrel), which is
defined in this work as:
Nieva P., Francisca F. (2007). “On the permeability of compacted and stabilized loessical silts in relation to liner system
regulations”. International Congress on Development, Environment and Natural Resources: Multi-level and Multi-scale
Sustainability, Cochabamba 11-13 Julio, Bolivia, pp. 69-77
γ rel =
γd
(1)
γ d (max)
where γd = dry unit weight and γd(max) = maximum dry unit weight obtained from the
standard Proctor test (ASTM D698).
The effect of clay content on the permeability of loessical silts was determined by testing
compacted silt-montmorillonite specimens. Tested samples were prepared with 2.5%
and 5.0% of montmorillonite and compacted at the 95% and 100% of the maximum dry
density.
Distilled water was used as permeant fluid in all cases. Permeability test were performed
in the compaction-mold and oedometer-cell permeameters by following the falling head
test procedure (ASTM D5856). Hydraulic gradient ranged between 1 and 10. Specimens
were permeated for a period of 5 to 10 day in order to obtain constant degree of
saturation and hydraulic conductivity. Figure 1 shows a schematic representation of test
procedure (a) and a picture of the used compaction-mold permeameter (b).
Table 1. Physical properties of tested soils
Property
Liquid limit, %
Plastic index, %
Passing sieve 200, %
Particles finner than 2 μm, %
Specific gravity
Loessical silt
27
2.8
96
4
2.67
Na-Montmorillonite
301
231
100
80
2.71
Standard
ASTM D4318
ASTM D4318
ASTM D422
ASTM D422
ASTM D1118
Pipette
Compaction-mold permeameter
Oulet port
Inlet port
(a)
(b)
Fig. 1: a) test devices, b) compaction-mold permeameter
Nieva P., Francisca F. (2007). “On the permeability of compacted and stabilized loessical silts in relation to liner system
regulations”. International Congress on Development, Environment and Natural Resources: Multi-level and Multi-scale
Sustainability, Cochabamba 11-13 Julio, Bolivia, pp. 69-77
3. Test results
Figure 2 shows the hydraulic conductivity of compacted loessical silts measured in the
compaction-mold and oedometer-cell permeameters. The k determined in similar
loessical soils by different authors [12,13,14,15] are also included, for undisturbed and
compacted condition. The permissible limit of 1x10-7 cm/s is shown as a reference value.
Hydraulic conductivities measured in the oedometer-cell permeameter were lower than
those measured in the compaction-mold permeameter. This result can be attributed to
the higher dry unit weight of the samples prepared in the oedometer cell (γd = 17.3 to
17.7 kN/m3) respect to the specimens tested in the compaction mold (γd = 16 to 16.8
kN/m3).
Figure 3 shows the influence of bentonite content and dry unit weight on the hydraulic
conductivity of loessical silts. Typical hydraulic conductivities measured in undisturbed
specimens are also shown. All samples were tested in the oedometer cell permeameter.
The relative dry unit weight (γrel) has high influence on k. Permeability increases abruptly
as consequence of small decreases of γrel.
Hydraulic conductivity decreases significantly with the montmorillonite content.
Specimens compacted at the maximum dry unit weight obtained with the standard
Proctor energy (γrel=100%) need around 2.5% of Na-montmorillonite to reach the
allowed permeability (1x10-7 cm/s).
Figure 4 shows the influence of the relative dry unit weight on the hydraulic conductivity
of loess. The same figure also shows the hydraulic conductivity and the corresponding
relative unit weight obtained by Nuñez et al. [12], Francisca et al. [13], Terzariol et al.
[14] and Aiassa et al. [15]. Obtained and reported values come together within two
apparent bounds. Typical differences between the hydraulic conductivities for the upper
and lower bounds are approximately one order of magnitude.
Hydraulic conductivity [cm/s]
1.0E-02
Test condition:
Undisturbed specimens
Compacted specimens
CM Compaction mold
OC Oedometer cell
1.0E-03
1.0E-04
1.0E-05
CM
1.0E-06
1.0E-07
EPA Limit
OC
Acceptable range
1.0E-08
Nuñez
Francisca
Terzariol
Aiassa
et al. [12]
et al. [13]
et al. [14]
et al. [15]
Fig. 2: Hydraulic conductivity of natural and compacted loessical silts
Nieva P., Francisca F. (2007). “On the permeability of compacted and stabilized loessical silts in relation to liner system
regulations”. International Congress on Development, Environment and Natural Resources: Multi-level and Multi-scale
Sustainability, Cochabamba 11-13 Julio, Bolivia, pp. 69-77
Hydraulic conductivity [cm/s]
Results shown in Figure 4 suggest a clear dependence between k and γrel. Note that
compaction energy of all tests have to be different, since at a given energy and dry unit
weight there are only two possible values of hydraulic conductivity, for the flocculated
and dispersed states [19]. Hence, the decrease of k can be attributed to the increase of
the dry unit weight and to the different soil structure (and compaction energy).
1.0E-04
Compacted γrel = 95 %
Compacted γrel = 100 %
1.0E-05
Undisturbed γrel = 74 %
1.0E-06
1.0E-07
1.0E-08
0
1
2
3
4
5
6
Montmorillonite content [%]
Fig. 3: Influence of montmorillonite content on the hydraulic conductivity of silt
specimens compacted at 95% and 100% relative dry unit weights
Hydraulic conductivity [cm/s]
1.0E-02
1.0E-03
Compaction mold
Oedometer
Nuñez et al. [12]
Francisca et al. [13]
Terzariol et al. [14]
Aiassa et al. [15]
equation (2)
1.0E-04
1.0E-05
equation (2)
1.0E-06
1.0E-07
1.0E-08
0.70
Maximum for
natural Loess
0.80
0.90
1.00
1.10
Relative dry unit weigth [%]
Fig. 4: Hydraulic conductivity of loessical silts measured in specimens with different
relative dry unit weights.
Nieva P., Francisca F. (2007). “On the permeability of compacted and stabilized loessical silts in relation to liner system
regulations”. International Congress on Development, Environment and Natural Resources: Multi-level and Multi-scale
Sustainability, Cochabamba 11-13 Julio, Bolivia, pp. 69-77
4. Analysis and discussion
Figure 2 shows that only few specimens have hydraulic conductivities lower than 1x10-7
cm/s. The hydraulic permeability of undisturbed loess specimens measured in the
oedometer cell falls between 2.0x10-4 cm/s and 9.4x10-6 cm/s. In case of compacted soil
samples, the measured conductivities range from 3.7x10-6 cm/s to 5.9x10-8 cm/s. From
these last two maximum and minimum values and assuming a Gaussian probabilistic
distribution for the hydraulic conductivity, the probability of having a compacted loessical
silt with k lower than 1x10-7 cm/s results 15.9%, for γrel between 95% and 105%.
Figure 5 shows schematically the estimated upper bound and lower bound of hydraulic
conductivity and the expected behavior at different levels of relative dry unit weights.
Proposed upper and lower bounds can be obtained as:
k = k∞ +
k0 − k ∞
⎛γ ⎞
1 + ⎜ rel
th ⎟
⎝ γ rel ⎠
(2)
th
γ rel
th
where γrel and γ rel
are relative unit weight and threshold relative unit weight (both in
percentage), and k∞ and k0 are the hydraulic conductivities at extremely high and natural
γrel respectively.
Dashed lines shown in Figure 4 defining the upper and lower boundaries were computed
from equation (¡Error! Vínculo no válido.). The upper bound was obtained with k∞u =1.6x10-7
th
cm/s k0u =1.6x10-4 cm/s and γ rel
= 94.5%. The lower bound was obtained with k∞l =5.0x10-8
th
cm/s k0l = 8.0x10-6 cm/s and γ rel
=92%. Note that all experimental values fall between
these two curves.
γrelth
Hydraulic conductivity (k)
I
u
k0
l
k0
Upper limit
II
III
A
B
Lower limit
D
C
u
k∞
E
F
Relative dry unit weigth (γrel)
Fig. 5: Identified zones and soil behavior
k∞l
Nieva P., Francisca F. (2007). “On the permeability of compacted and stabilized loessical silts in relation to liner system
regulations”. International Congress on Development, Environment and Natural Resources: Multi-level and Multi-scale
Sustainability, Cochabamba 11-13 Julio, Bolivia, pp. 69-77
Permeabilities k∞ and ko for the upper and lower bounds depend on soil type. In case of
loessical silts these values are expected to depend on clay content, as shown in Figure
3. Additionally, three clear zones are observed. Zone I is characterized by a small
decrease of k as γrel increases. Regardless of the increase of the dry unit weight, all
samples have flocculated structure within this zone (points A and D). At a given dry unit
weight, samples represented by point D have lower k and a less flocculated structure
than those corresponding to point A. After the threshold, within the zone II, very
significant reductions of k are observed. This behavior is attributed here to the decrease
of γd and to important changes in soil structure. Structures of specimens represented by
points B and E are more dispersed than those of points A and D respectively. Finally,
zone III gathers specimens with dispersed structure, where the slightly decrease of k
can be explained by the influence of the soil unit weight. In this zone, specimens F have
a more dispersed structure than specimens C. The lower k observed at the same γrel in
zones I to III can be attributed to the more dispersed structure of these specimens [19].
The efficacy in reducing k decreases after a given value of γrel (zone III). Hence, clay
must be added to loessical silts in order to achieve the hydraulic conductivity required by
most liner system regulations (1x10-7 cm/s).
5. Conclusions
This work focuses on the hydraulic conductivities of loessical soils. The effects of
relative dry unit weight, clay content and soil structure are analyzed. A new model is
proposed in order to define a range of possible values for the expected hydraulic
conductivity at a given relative dry unit weight. The main conclusions of this research
can be summarized as follow:
- A very large region in the central and north-eastern part of Argentina is covered
by loessical soil deposit. The hydraulic conductivity of compacted loessical silts
specimens falls between 3.7x10-6 cm/s and 5.9x10-8 cm/s. Only 15.9% of
compacted samples may have hydraulic conductivities lower than 1.0x10-7 cm/s.
- The addition of montmorillonite results a very effective treatment to reduce the
hydraulic conductivity of loessical silts.
- Hydraulic conductivities measured at different levels of relative dry unit weights
fall in a well defined range. A new model is proposed to simulate the upper and
lower limits and the influence of the relative dry unit weight on hydraulic
conductivity. Three characteristic zones were identified. Results show that soil
behavior in each particular zone can be associated to soil structure.
Acknowledgements
This work was partially funded by Agencia Cordoba Ciencia and MINARMCO S.A.. P.
Nieva thanks SECyT-UNC for the graduate fellowship.
Nieva P., Francisca F. (2007). “On the permeability of compacted and stabilized loessical silts in relation to liner system
regulations”. International Congress on Development, Environment and Natural Resources: Multi-level and Multi-scale
Sustainability, Cochabamba 11-13 Julio, Bolivia, pp. 69-77
References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
G.M. Savage, L.F. Diaz, C.G. Golueke, C. Martone. Guidance for landfilling waste in
economically developing countries. U.S. Environmental Protection Agency (EPA),
EPA/600/SR-98/040, 1998
C. D. Shackelford, C. H. Benson. Soil liners and cover for landfills. Lecture notes for
Caterpillar Hemispheric series of short courses, CoHemis, Vol. 1, 1995.
M. C. McBrayer, M. Mauldon, E. C. Drumm, G. V. Wilson. Infiltration test on fractured
compacted clay. Journal of Geotechnical and Geoenvironmental Engineering, 123(5),
469-473, 1997.
A. Bouazza, W. F. Van Impe. Liners design for waste disposal sites. Environmental
Geology, 35(1), 41-54, 1998.
D. E. Daniel. Clay Liners. Geotechnical Practice for Waste Disposal, Chapter 7, 137163, Chapman & Hall, Great Britain, 1993.
M. H. Gleason, D. E. Daniel, G. R. Eykholt. Calcium and sodium bentonite for
hydraulic containment applications. Journal of Geotechnical and Geoenvironmental
Engineering, 123(5), 438-445, 1997.
Teruggi M. E. The nature and origin of Argentinean loess, Journal of Sed. Petrol. 27,
323-332. 1957
M. Iriondo. Models of aeolian silt deposition in the Upper Quaternary of South
America, Journal of South American Earth Science 10(1), 71-79, 1997
M.A. Zarate. Loess of southern South America, Quaternary Science Reviews 22(1819), 1987-2006, 2003.
J. K. Mitchell, D. R. Hooper, R. G. Campanella. Permeability of compacted clay.
Journal of the Mechanics and Foundations Division, 91(4), 41-65, 1965.
C. Benson, H. Zhai, X. Wang. Estimating the hydraulic conductivity o compacted clay
liners. Journal of Geotechnical Engineering, 120, 366-387, 1994
E. Nuñez, C. A. Micucci, O. A. Varde, A. J. Bolognesi, O. Moretto. Contribución al
conocimiento de los suelos loéssicos estabilizados de depósitos naturales. 2º
RAMSIF, Argentina, 1-29, 1970.
F. M. Francisca, G. A. Cuestas, V. A. Rinaldi. Estudio de permeabilidad en limos
loéssicos. Encuentro de Geotécnicos Argentinos, Córdoba-Argentina, 1-18, 1998.
R. Terzariol, E. Redolfi, R. Rocca, M. E. Zeballos. Unsaturated soil model to loessic
soils. 12th Panamerican Conference on Soil Mechanics and Geotechnical
Engineering, USA, 1317-1322, 2003.
G. Aiassa, M. E. Zeballos, R. Terzariol. Infiltración y conductividad hidráulica de
limos compactados. Utilización en barreras de contención. XXI Congresso Brasileiro
de Mecánica dos Solos e Engenharia Geotécnica, Curitiba-Brasil, 1-6, 2006.
Rocca R.J. Review of Engineering Properties of Loess. University of California,
Berkeley. Report CE 299, 1985
Moll L., Rocca R. Properties of Loess in the Center of Argentina. IX Panamerican
Conference on Soil Mechanics and Foundations Engineering. Chile, Vol. 1, 1991
Francisca F.M., Redolfi E.R. Parametric Analysis of the Deflections of Flexible Pipes
in Collapsible Soils, 12th Panamerican Conference on Soil Mechanics and
Geotechnical Engineering, Vol. 2, 2073-2079, 2003.
J. K. Mitchell. Fundamentals of soil behavior. Second Edition. John Wiley & Sons,
New York-US, 1993.