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THE INFLUENCE OF POROSITY ON GEOMECHA-NICAL CHARACTERISTICS OF SNAIL SOIL IN THE LJUBLJANA MARSH
BOJAN ŽLENDER and LUDVIK TRAUNER
About the authors
Bojan Žlender
University of Maribor,
Faculty of Civil Engineering
Smetanova ulica 17, 2000 Maribor, Slovenia
E-mail: bojan.zlender@uni-mb.si
Ludvik Trauner
University of Maribor,
Faculty of Civil Engineering
Smetanova ulica 17, 2000 Maribor, Slovenia
E-mail: trauner@uni-mb.si
Abstract
This article focuses on mineralogical and physical characteristics of snail soil and their influence on parameter values of geomechanical characteristics. Snail soil, which got its name from fossil remains, is a typical layer observed in the Ljubljana marsh. It is distinctly porous, saturated and in a liquid consistency state. Snail soil was investigated for mineralogical and physical characteristics in the Laboratory of Soil Mechanics, Faculty of Civil Engineering of the University of Maribor. Mineral and chemical composition, visual appearance, specific surface and grain property were determined. Physical characteristics show that snail soil is saturated in nature, highly porous and almost liquid. Geomechanical characteristics were investigated for their interdependency on physical characteristics. A series of triaxial tests were performed on snail soil samples of different porosity, density and water content. Cylindrical samples of the height of 100 mm and the diameter of 50 mm were tested using three-axial testing apparatuses. The results of the tests show that interdependency exists between geomechanical characteristics and porosity. These relationships can be expressed as functions of density, porosity or water content. It is evident from the results that changes of the coefficient of permeability, the coefficient of consolidation, and the coefficient of volume compressibility are non-linear with respect to changes of porosity. Changes of mechanical parameters, such as Young's modulus, Poisson's ratio, and friction angle are indistinct and almost linear at lower changes of porosity.
Keywords
Snail soil, triaxial test, porosity, permeability, consolidation, Young's modulus, Poisson's ratio, friction angle
1 INTRODUCTION
This article focuses on mineralogical and physical characteristics of snail soil and their influence on parameter values of geomechanical characteristics. Snail soil, which got its name from fossil remains, is a typical layer observed in the Ljubljana marsh. It is distinctly porous, saturated and in a liquid consistency state.
The Ljubljana marsh is a wide tectonic sink which was formed two million years ago by a gradual depression of the area. The marsh is located in the south of Ljubljana, at the elevation of 287-290 m above the sea level, and covers the surface of 163 km2.
The Ljubljana marsh was inhabited already thousands of years ago. Archeological findings (fascine dwellings) date from the iron, copper, and bronze periods. Later, this marshy area was invaded by the Romans who were the first to start draining the land. The contribution of fascine dwellers from the Ljubljana marsh to a wider cultural space is also proven by the recent archeological finding, i.e. a wheel and the ax of a two-wheel vehicle, which dates from about 3200 A. D. For the present, this archeological finding is known to be the oldest of its type in the world.
Today, this area is almost completely drained and urbanized. Yet, the construction of traffic ways and tall buildings presents a great challenge to construction engineers due to the softness of layers below the surface.
Geological structure of the marsh is very interesting. Ground water is located immediately below the surface. The surface layer is composed of peat of the thickness of 1 m to 8 m. The dept of the peat is nowadays essentially smaller than in a past, due to the intensive excavations in
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B. ZLËNDËR & L. TRAUNER: THE INFLUENCe OF POROSITY ON GEOMECHANICAL CHARACTERISTICS OF SNAIL SOIL IN THE LJUBLJANA MARSH
a first half of the 20th century. Below the peat layer, there is a layer of snail soil of the thickness of a few meters at the borders to more than 10 m in the center of the marsh. The snail soil layer is distinctly porous, saturated with water and of a low-bearing capacity. There are clay and sandy-gravel layers below the snail soil layer. A layer of rocks starts at the depth of some ten meters.
Geological structure of the Ljubljana marsh has been studied by numerous experts. The oldest geological documents go back to the middle of the 19th century, when the first geological map was drawn of this region. Later, several other studies were performed. In the second half of the 20th century the researches were focused on geomechanical characteristics of the soil and detailed investigations of physical and mechanical characteristics of snail soil were performed. Strength parameters showing possible static loadings were also determined. To understand geomechanical characteristics of this marshy area it is essential to know rheological characteristics of snail soil. In 1979, in the framework of Trauner's doctoral thesis [1], slow triaxial tests were performed in the Laboratory of Soil Mechanics at the Faculty of Architecture, Geodesy and Civil Engineering of the University of Ljubljana. A similar investigation was later repeated in the Laboratory of Soil Mechanics (LSM), Faculty of Civil Engineering of the University of Maribor. A rheological model of snail soil was set which shows relationships between stresses, deformations and time [2]. The results of investigation were expressed with rheological dependencies, i.e. function relationships between rheological parameters and stress state and deformation, at void ratio e = 2.1.
An example of dependencies (Figure 1a) showing the relationship between shear stress t and effective normal stress a, with low value of friction angle q> = 21° and no cohesion c = 0.
10 20 30 40 50 60 70 80 90 100 110120 Effective stress cr' [kPa]
Figure 1a. Relationship between shear stress t and effective normal stress d.
Fig. 1b shows a typical result obtained for the octahedral strain eg depending on the octahedral shear stress t0 and effective octahedral stress state a' of snail soil.
0,07 r"l—i 0,06 0,05
S
£ 0,04 "8 0,03
■a
u 0,02 0,01 0,00
			E0 = I	0 (t»oV		= :	lOkPa		
			To	= 20J	Par-				
	Tn = 1	) kPa,		¿0 =	ler'se	c1			
									
<1®							specii specii	len 1 nen 2	
							specii - failurt	aen 3	
0 10 20 30 40 50 60 70 80 90 100 Effective octahedral stress a0r [kPa]
Figure 1b. Octahedral strain eg vs. shear stress Tg for effective octahedral stress state a'.
Fig. 1c shows a typical result obtained for the octahedral shear strain yg depending on the octahedral shear stress t„ and effective octahedral stress state a' of snail soil.
0,03
5 10 15 20 25 30 35 Octahedral shear stress x0 [kPa]
Figure 1c. Octahedral shear strain yg vs. octahedral shear stress Tg for different effective octahedral stress state ag'.
Three years ago, the testing of snail soil was repeated and upgraded with the investigation of mineralogical and physical characteristics, as well as of geomechanical characteristics depending on physical characteristics. This article briefly presents researches performed and the influence of snail soil porosity on geomechanical characteristics.
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B. ZLËNDËR & L. TRAUNER: THE INFLUENCe OF POROSITY ON GEOMECHANICAL CHARACTERISTICS OF SNAIL SOIL IN THE LJUBLJANA MARSH
2 CHflRflCTeRISTICS OF SNAIL SOIL
A set of samples were taken on the southwest region of the Ljubljana marsh to conduct this research. Sampling was performed in the region of 3 m x 3 m at the depth of 3 m, so the ground was excavated to the depth of 2,5 m and a thin wall tube sampler (of the internal diameter of 100 mm) was forced into the ground. Samples of the diameter of 100 mm and the height of 300 mm were immediately packed after sampling. The ground water level was 1m under the surface in the region of sampling.
2.1 MINERAL COMPOSITION
The mineral composition (Table 1) was determined in the Laboratory of Geological Survey of Slovenia. The samples were scanned by X-ray diffraction technique (XRD) using a Philips PW 3710 diffractometer, a goniometer 1820, with an automatic divergence slit and a curved graphite monochromator, operating at 40 kV, 30 mA with CuK radiation and an Ni filter.
a
Table 1. Mineral composition of snail soil samples.
Mineral composition	Weight portion (%)
Calcite	87
Kaolinite	7
Muscovite	4
Quartz	3
2.2 CHEMICAL COMPOSITION
Chemical composition of the snail soil (Table 2) was determined at Centrum for electronic microscopy in University of Maribor. The scanning electron microscope SIRION which is equipped with the energy dispersive spectrometer EDS Oxford INCA 350, was used. The latter allows qualitative and quantitative micro chemical point and plane analyses, as well as qualitative linear analysis and determination of surface element distribution. Elements from beryllium to uranium can be analyzed.
Table 2. Chemical composition of snail soil.
Element	Average value (%)	Standard deviation (%)
C	12.15	1.49
O	48.78	1.95
Mg	0.52	0.09
Al	2.90	1.14
Si	5.34	2.25
K	0.80	0.25
Ca	29.51	5.17
2.3 VISUAL APPEARANCE
Visual appearance of snail soil samples was tested with the environmental scanning electron microscope QUANTA 200 3D, at Centrum for electronic microscopy in University of Maribor. The electron microscope is equipped with a system of double jets, i.e. electronic and ionic. The microscope is denoted with the word "environmental" or with the marking "ESEM" because it can be used at different pressures and at 100 % humidity.
The pressure in the chamber determines the types of samples to be observed.
Three operating principles are possible:
-	High vacuum - It allows to observe conductive and non-conductive samples covered with a conductive coating (Au, C, Ag).
-	Low vacuum - It allows to observe conductive and non-conductive samples without prior preparation.
-	ESEM tape - It allows to observe all samples, also wet and greasy ones, and in-situ processes (hydration, dissolution ...).
In observing the samples of snail soil, all three principles were used. Photographs were taken of damp samples and different blow ups were made; photographs of dry samples (Figure 2a), minerals in crystallized form, and remains of micro-organisms in the snail soil (Figure 2b) were also developed.
Figure 2a. A dry sample of snail soil (ESEM).
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B. ZLËNDËR & L. TRAUNER: THE INFLUENCe OF POROSITY ON GEOMECHANICAL CHARACTERISTICS OF SNAIL SOIL IN THE LJUBLJANA MARSH
m '
Figure 3. Particle size distribution of snail soil.
Figure 2b. Remain of micro-organism in snail soil, a dry sample (ESEM).
2.4 SPECIFIC SURFACE
Specific surface of grains is the surface of grains per unit mass. It is expressed in square meters per gram of dry substance (m2/g). Specific surface of snail soil was determined at Chemical Institute Ljubljana, with the five-point BET method with adsorption of liquid nitrogen of 99.9% cleanliness and the temperature of 770 K.
The measurements were performed using the automatic TriStar 3000 gas adsorption analyzer, produced by Micromeritics Instrument Corporation, Norcross, U.S.A. The results of the test showed that snail soil has the specific surface of 5.03 ± 0.03 m2/g.
2.5 PARTICLE SIZC DISTRIBUTION
Grain property of the snail soil sample was determined using the Fritsch Laser Particle Sizer Analysette 22 at the laboratory of Geological Survey of Slovenia. The results of the grain property analysis show (Fig. 3) that this snail soil falls within the range of silt (MH) with respect to granulometrical structure.
2.6 PHYSICAL CHARACTERISTICS
Physical characteristics (Table 3) show that snail soil is saturated in nature, highly porous and almost liquid.
Table 3. Physical characteristics of snail soil.
Soil property	Symbol	Unit	Value
Water content	w	%	75
Plastic limit	wP	%	37
Liquid limit	WL	%	60
Plasticity index	IP	%	23
Consistency index	I c	-	-0.65
Liquidity index	IL	-	1.65
Density of solid	Ps	g/cm3	2.62
Unit weight	Y	kN/m3	15.5
Dry unit weight	Yd	kN/m3	8.8
Degree of saturation	S r	%	100
Void ratio	e	-	2.1
2.7 COMPRESSIBILITY AND CONSOLIDATION
The investigation of compressibility was performed in LSM. Snail soil is markedly compressible; its volume undergoes great changes already when loaded with small changes of stress. Triaxial consolidation tests were performed at the effective stress changes a'0.
Consolidation parameters of snail soil in nature are given in Table 4. The values of consolidation parameters change with lower porosity, the parameters can be expressed as functions of porosity.
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B. ZLËNDËR & L. TRAUNER: THE INFLUËNCË OF POROSITY ON GEOMECHANICAL CHARACTERISTICS OF SNAIL SOIL IN THE LJUBLJANA MARSH
Table 4. Consolidation parameters (void ratio e = 2.1).
Soil property	Symbol	Unit	Value
Consolidation coefficient	c V	m2/year	2,8 ^ 3,2
Coefficient of volume	m V	kPa-1	1 ^ 1,2 •lO-3
compressibility			
Coefficient of soil permeability	k	m/s	2 ^ 5 •lO-9
Secondary compression	C as		O.OO2
ratio	C ae		0.006
2.8 STRENGTH AND DEFORMABILITY
Strength parameters were determined in a series of triaxial tests. Strength parameters of snail soil in nature are given in Table 5. Values of parameters can also change with lower porosity; they can be expressed as functions of porosity.
Table 5. Consolidation parameters (void ratio e = 2.1).
Soil property	Symbol	Unit	Value
Consolidation coefficient	c	kPa	0
Friction angle	P	O	21
Constrained modulus	M c	kPa	800 ^ 1000
Poisson's ratio	V	-	0.4
			
3 POROSITY - GEOMECHANICAL PARAMETERS RELATIONSHIPS
The investigation of the influence of porosity on geo-mechanical characteristics was performed. Tests for determining mechanical properties were performed using triaxial apparatuses. The following steps were observed in testing: preparation of the sample, procedure on the apparatus, performance of the test, and interpretation of the obtained results. The investigation included drained and undrained stress oriented triaxial tests according to the following phases:
-	Saturation,
-	Consolidation,
-	Static loading.
In the first phase, saturation was tested by determining the coefficient B = du/da > 0.96 . This was a relatively short-term phase because of saturation in the nature.
The saturated sample was then consolidated at the selected effective isotropic consolidation stress a'3c and selected compression stress change Aac. The effective isotropic consolidation stress is expressed as a difference between the cell pressure ac and the back pressure ub.
Static loading was performed so that the sample was loaded with the selected compression stress da3c or the axial stress Aa = Aa .
a	z
Sixty-two triaxial tests were performed. The investigation was based on a series of tests in which the below conditions varied:
-	Void ratio e = 2.1 + 1.2 (Ae is calculated),
-	Initial effective pressures a0'=a'3c = 0, 50, 100, 150 kPa,
-	Variations of effective pressures Ad = 50, 100, 150 kPa,
-	Variations of axial pressures daz (depending on axial deformation).
The below pressures were measured during the test:
-	Cell pressure a3c (kPa),
-	Back pressure ub (kPa),
-	Pore water pressure uw (kPa),
-	Axial vertical stress in compression az (kPa),
-	Axial vertical and radial strain ez (-), er (-),
-	Volume deformation ev (-).
For undrained test ev (-)=0. Constants in the research are:
-	Specific surface,
-	Chemical composition,
-	Mineral composition,
-	Grain property,
-	Natural humidity, density and coefficient of porosity,
-	Liquid limit, plasticity index, consistency index,
-	Density of a solid mass,
-	Dry volume weight,
-	Saturation.
The following strength parameters were calculated: coefficients of permeability k (m/s), consolidation cv (m2/s) and volume compressibility mv (kPa-1), constrained modulus Mc (kPa), Young's modulus E (kPa) and Poisson's ratio v (-), or compression modulus K (kPa), and shear modulus G (kPa).
Standard oedometer tests and direct shear tests of snail soil were also performed.
Snail soil has the void ratio e = 2.1 in nature, which is an extremely high value. Values of mechanical parameters change with lowering the void ratio.
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Porosity (void ratio e) is in linear correlation with water content w. Density (p) or unit weight (y) is in nonlinear correlation with void ratio e.
1 Yw	Dr + (Sr • g/100) m
w =--e = -w- ■ e y=—r-—---Y (1)
Dr	Ys	r	1 + e	()
where:
yw, ys unit weight of water and solids, Dr relative density of soil, Sr=1 degree of saturation.
Figures 4 show the correlation between void ratio e and water content w, and the correlation between void ratio e and density p. Therefore, the snail soil was in liquid or plastic consistency, for different tests.
K = B ■ ft (3)
where:
K hydraulic conductivity,
effective porosity, B, n are constants.
The relationship of the coefficient of permeability k vs. void ratio e for all tests is:
k(e) = B1 ■en (4)
where
e	void ratio
B1, n1 are coefficients.
			| plastic |	| liquid |		0
- wp=37 e=1 p =1,85				< O		>
			« >			
		O >		Wl=60 e=1,6		
				P=1,65		
						
1	1,2	1,4	1,6	1,8	2
Void ratio e [-]
We obtained the expression k = 4 ■ 10-11 ■ e5,5019 for a series of all tests (where e is void ratio). Fig. 5 shows the relationship of the coefficient of permeability vs. void ratio for all tests. Deviation of the results (R = 0.3) from the above function is high because the function does not include stress conditions or pore pressure gradients, respectively. However, a more detailed description of the coefficient of permeability can be made with the functions of void ratio and stress states a0' = 50, 100, 150 kPa or with pertaining pore pressure gradients.
			| plastic |	| liquid |		
						
	0					
		« : ❖ :			p=1,65	
	wp=37 e=1			>	«	
	p=1,85					* 1
1	1,2	1,4	1,6	1,8	2
Void ratio e [-]
Figure 4. The correlation between void ratio e and water content w, and correlation between void ratio e and density p.
3-1 COEFFICIENT OF PERMEABILITY
The coefficient of permeability is expressed with equation k = cv ■ mv ■ Yw (2)
Figure 5. The coefficient of permeability k vs. void ratio e.
1.E-10
The coefficient of permeability of snail soil in nature is k = 2 •10-9 m/s. The value of the coefficient of permeability decreases with decreasing void ratio e as shown in Fig. 5. The permeability can be expressed similar to a known expression of Ahuja et al. [3].
Figure 6. The coefficient of permeability k vs. water content w.
If the relationship of the coefficient of permeability k vs. void ratio e (or density p, or water content w) is
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B. ZLËNDËR & L. TRAUNER: THE INFLUËNCË OF POROSITY ON GEOMECHANICAL CHARACTERISTICS OF SNAIL SOIL IN THE LJUBLJANA MARSH
expressed in the logarithmic form, we can see that it is almost linear. Figure 6 shows this relationship as the coefficient of permeability k vs. water content w.
3.2 COEFFICIENT OF CONSOLIDATION
The coefficient of consolidation cv is expressed using equation from BS 1377 [4]
1,65-D2
A - f,nn
(5)
where:
1.E-03
t 6,E
11
a I"
8 O.BOO
	♦♦♦ -	
0 s		
: O « * % ^ ..........0« «...... «		O
0 O ...............	y = 9E-05x3'5005	
« 0		
1,200
1,400
1,600 Void ratio e [-]
Figure 8. The coefficient of volume compressibility mv vs. void ratio e.
D diameter of the specimen (mm), X coefficient which depends on the drainage,
1.E-03

time of primary consolidation.
The relationship of the coefficient of consolidation cv vs. void ratio e could be expressed similarly to the coefficient of permeability k. Fig. 7 shows the relationship of coefficient of consolidation c vs. void ratio e for all
v	c
tests. We obtained the expression cv = 3 -10-8- e1,464 (e is void ratio).
1.E-07 8.E-08 6.E-08 4.E-08
2.E-08
0.B-00
O	V 0	0		0	
	0 0 f				
		»0		>	
0 0 .......0...............«	0	•			
			y = 3E-08X1"644		
					
					
1,200
1,600 Void ratio e [-]
1,800
Figure 7. The coefficient of consolidation cv vs. void ratio e.
3.3 COEFFICIENT OF VOLUME COMPRESSIBILITY
The coefficient of volume compressibility mv is expressed with equation
Ae
Ae
7 = ^ (6)
v (1 + e) - Act ' Act
Fig. 8 shows the relationship of the coefficient of volume compressibility mv vs. void ratio e for all tests. We obtained the expression mv = 9 ■ 10-5 ■ e3,5005 .
The logarithmic form of relationship of volume compressibility mv vs. water content w shows linear correlation log. mv = 3 ■ 10-10 ■ w3,5005.
Water content w [%]
Figure 9. The coefficient of volume compressibility my vs. water content w.
3.4 STRENGTH PARAMETERS
To determine the relationship of strength parameters vs. void ratio an insufficient number of tests were performed therefore the results are unreliable. Fig. 8 shows Young's modulus E vs. void ratio e. We can see that strength does not substantially increase with increasing density or decreasing porosity; a greater difference can be only seen at higher changes of density or porosity. Poisson's ratio is v = 0.4 at void ratio e = 2.0 and decreases for 0.03 at void ratio e = 1.4.
5000 4500 4000 3500 3000 2500 2000 1500 1000 500 0
					
					
					
			y = 7699,1X"31465		
					
					
					
					
					
					
1,2
1.6
Void ratio e [-]
1,8
Figure 10. Young's modulus E vs. void ratio e.
If the relationship of the Young's modulus E vs. water content w is expressed in the logarithmic form, we can see that it is almost linear (Fig. 10).
cv =
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B. ZLËNDËR & L. TRAUNER: THE INFLUËNCË OF POROSITY ON GEOMECHANICAL CHARACTERISTICS OF SNAIL SOIL IN THE LJUBLJANA MARSH
10000
S
Water content w [%]
ACKNOWLEDGMENTS
This research is being financed by the Slovene Ministry of Higher Education, Science and Technology. The financial support of DARS, the Slovene Motorway Company, and the company Prevent is also gratefully acknowledged.
Figure 11. Young's mo dulus E vs. water content w.
REFERENCES
The same is true for shear properties. Fig. 12 shows the relationship of friction angle q> vs. unit weight y. We can see that it increases almost linearly at lower changes of density or void ratio, respectively.
16	16,5
Unit weight y [%]
		0
		
		
		
		
		= O.OOx3 79
		
		
17,5
[1]	Trauner, L. (1982). Applicability of theory of elasticity for foundation design, Doctoral thesis, University of Ljubljana, 479 p., Ljubljana
[2]	Trauner, L. et al. (1982). Structure soil interaction, Research report, University of Maribor, 67 p., Maribor
[3]	Ahuja, L. R., et al. (1989). Evaluation of spatial distribution of hydraulic conductivity, using effective porosity data, Soil Science 148, p. 404-411
[4]	British Standards Institution (1999). Consolidation, BS 1377, Part 8, London
Figure 12. The friction angle f vs. unit weight y.
4 CONCLUSIONS
This article focuses on the investigation of snail soil and the research of its mineralogical and physical characteristics. Geomechanical characteristics were investigated for their dependence on physical characteristics. A series of triaxial tests of snail soil of different density, porosity and water content was performed. The results of the tests show that geomechanical characteristics depend on porosity.
The relationships were expressed as functions of porosity. It is evident from the results that changes of the coefficient of permeability, the coefficient of consolidation, and the coefficient of volume compressibility are nonlinear with respect to void ratio. Changes of mechanical parameters such as Young's modulus, Poisson's ratio and friction angle are indistinct and almost linear at lower changes of porosity.
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