Water retention properties of stiff silt
Retencijske lastnosti trdnih meljev
Barbara LIKAR1, Vikica KUK2 & Karmen FIFER BIZJAK1
1Barbara Likar, ZAG, Dimičeva ul. 12, SI-1000 Ljubljana, Slovenia; e-mail: barbara.likar@zag.si
2Vikica Kuk, Žegar 33, SI-3262 Prevorje, Slovenia; e-mail: vikica.kuk@gmail.com
1Fifer Bizjak Karmen, ZAG, Dimičeva ul. 12, SI-1000 Ljubljana, Slovenia; e-mail: karmen.fifer@zag.si
Prejeto / Received 9. 5. 2016; Sprejeto / Accepted 20. 4. 2017; Objavljeno na spletu / Published online 9.6.2017
Dedicated to Professor Mihael Ribičič on the occasion of his 70th birthday
Key words: stiff sandy silt, suction, Bishop - Wesley triaxial cell, water retention curve, potentiometer WP4C
Ključne besede: Trden peščeni melj, sukcija Bishop – Wesley triaksialna celica, retencijska krivulja, 
potenciometer WP4C
Abstract
Recent research into the behaviour of soils has shown that it is in fact much more complex than can be described 
by the mechanics of saturated soils. Nowadays the trend of investigations has shifted towards the unsaturated 
state. Despite the significant progress that has been made so far, there are still a lot of unanswered questions 
related to the behaviour of unsaturated soils. For this reason, in the field of geotechnics some new concepts are 
developed, which include the study of soil suction.
Most research into soil suction has involved clayey and silty material, whereas up until recently no data have 
been available about measurements in very stiff preconsolidated sandy silt. Very stiff preconsolidated sandy silt 
is typical of the Krško Basin, where it is planned that some very important geotechnical structures will be built, 
so that knowledge about the behaviour of such soils at increased or decreased water content is essential. 
Several different methods can be used for soil suction measurements. In the paper the results of measurements 
carried out on very stiff preconsolidated sandy silt in a Bishop - Wesley double-walled triaxial cell are presented 
and compared with the results of soil suction measurements performed by means of a potentiometer (WP4C). All 
the measurement results were evaluated taking into account already known results given in the literature, using 
the three most commonly used mathematical models. Until now a lot of papers dealing with suction measurements 
in normal consolidated and preconsolidated clay have been published. Measurements on very stiff preconsolidated 
sandy silt, as presented in this paper were not supported before.
Izvleček
Zadnje geomehanske raziskave zemljin kažejo na to, da je njihovo obnašanje veliko bolj kompleksno, kot se to 
lahko opiše z modeli za saturirane zemljine. Danes se trend raziskav nagiba v področje nesaturiranih materialov. 
Kljub velikim premikom na tem področju ostaja še vedno veliko vprašanj povezanih z obnašanjem materialov v 
nesaturiranem območju, zato so se v geotehniki uveljavili novi koncepti modelov, ki vključujejo študij sukcije.
Večina preiskav sukcije je osredotočenih na normalno konsolidirani glineni ali meljni material, medtem ko 
rezultatov preiskav v trdnem, prekonsolidiranem peščenem melju, po dosedaj znanih podatkih, ni objavljenih. 
Trdni, prekonsolidirani peščeni melj je tipičen za Krško kotlino, kjer se bodo gradili večji geotehnični objekti, in 
je poznavanje sukcije materiala ob povišanju vlage v njem bistvenega pomena. 
Za meritev sukcije je bilo uporabljenih več različnih metod. V članku so predstavljeni rezultati meritve sukcije 
trdnega peščenega melja v Bishop - Wesley dvostenski triosni celici in primerjava z rezultati meritev sukcije v 
potenciometru (WPC4). Rezultati so potrjeni z matematičnimi modeli, ki so v tovrstni literaturi najbolj poznani. 
Rezultati raziskav predstavljajo prispevek k poznavanju obnašanja prekonsolidiranega trdnega peščenega melja v 
nesaturiranih pogojih. 
GEOLOGIJA 60/1, 37-47, Ljubljana 2017
https://doi.org/10.5474/geologija.2017.003
© Author(s) 2017. CC Atribution 4.0 License
38 Barbara LIKAR, Vikica KUK & Karmen FIFER BIZJAK
Introduction
Recent research into the behaviour of soils has 
shown that it is in fact much more complex than 
can be described by the mechanics of saturated 
soils. The general field of soil mechanics can be 
sub-divided into that part which deals with sat-
urated soils and that part which deals with un-
saturated soils. Nowadays the trend of investiga-
tions has shifted towards the unsaturated state. 
Despite the significant progress that has been 
made so far, there are still a lot of unanswered 
questions related to the behaviour of unsaturat-
ed soils. This refers in particular to the changes 
in volume and strength which occur in the soil. 
Under certain conditions soils may behave very 
unpredictably, either through a rapid increase 
in their volume (swelling) or by (structural) col-
lapse. For this reason, in the field of geotechnics 
some new concepts, which include the study of 
soil suction, are now being introduced.
Several methods for measuring soil suction 
are known, i.e. measurements with a stick ten-
siometer, with a potentiometer, with a pressure 
plate extractor and filter paper. These methods 
can provide reliable information at both high and 
very low suctions, whereas in the intermediate 
zone measurements can be unreliable.
This deficiency can be remedied by using of 
the Bishop - Wesley double-walled triaxial cell, 
which can provide very precise measurements of 
changes which occur in the volume of water and 
air in the pores.
In geotechnical engineering, soil suction is an 
important parameter which is used in the design 
and construction of buildings, roads, tunnels, hy-
dropower dams, and deep excavations, as well as 
for the construction of sealing layers and covers 
for surface waste deposits, and elsewhere. Suc-
tion also needs to be studied in the case of land-
slides. Numerous research projects have been 
performed, including geotechnical and environ-
mental aspects, in connection with waste dispos-
al and ground pollution. Special attention has 
been focused on the question of the swelling of 
soils after compaction into sealed layers for the 
covering of radioactive waste deposits (uMeDera 
et al., 1996; DelaGe, 2002).
Numerous authors have also studied the im-
pact of soil suction on the triggering of land-
slides in unsaturated soils (toFani et al., 2006; 
lu & GoDt, 2008; GoDt et al., 2009), some of them 
(nG et al., 2003; sPrinGMan, 2011) with the help 
of artificially caused precipitation.
In recent years quite a lot of research has 
been performed in Slovenia, too, in connection 
with various geotechnical structures (MačeK et 
al., 2006; PetKovšeK, 2006, MačeK, 2006). Based 
on the results of the laboratory investigations 
(PetKovšeK, 2006), the favourable impact of soil 
suction on the strength characteristics of ben-
tonite, very stiff clay (i.e. „Sivica“), and flysch 
material was determined. Apart from laborato-
ry tests, in situ soil suction measurements have 
been performed in Slovenia, since 2004, on the 
Lenart - Cogetinci - Vučja vas motorway sec-
tion, where an embankment of swelling clay was 
stabilized by lime (PetKovšeK, 2008) and the ef-
fect of soil suction on the stability of the large 
Slano blato landslide was examined by mačeK 
(2012). 
Until now lot of papers dealing with suction 
measurements in normal consolidated and pre-
consolidated clay have been published, but no 
such measurements on very stiff preconsolidated 
silt, as it is found in the Krško Basin, are men-
tioned. Since several important geotechnical 
structures will be built in this area, it was neces-
sary to carry out more detailed measurement of 
the soil suction occurring in this material. 
Background
The soil suction has been defined as the energy 
level of the pore water in soils (suescun Florez, 
2010). It is the result of the adsorption of water 
onto mineral grains, together with a capillary 
phenomenon and the osmotic effects of the salt in 
the pore water. It can also be labelled as a meas-
ure of the energy that attracts water into the 
structure of the soil, or keeps it there. It is per-
ceived as a tensile stress of the water in the soil, 
or as a negative pore water pressure (Marshall & 
holMes, 1988).
Three types of soil suction are known: matrix 
suction, osmotic suction, and total suction. Total 
soil suction is defined as the sum of the matrix 
suction and the osmotic suction, and is written 
as:
ψ = (ua – uw )+ π     (1)
Where: ψ is the total suction, (ua – uw) is the 
matrix suction, and π is the osmotic suction.
39Water retention properties of stiff silt
From Equation (1) it can be seen that the ma-
trix suction is defined as the difference between 
the pore pressure of air (ua) and the pore pres-
sure of water (uw). It is in fact defined as the en-
ergy which is needed to remove water from the 
soil matrix or from its structure, without any 
changes to the physical state of the water. Matrix 
suction occurs as a result of the capillary phe-
nomenon, the soil structure, and the adsorption 
of water onto the mineral grains (Bulut et al., 
2001). Osmotic suction (π) is a consequence of the 
concentration of salt dissolved in the water, and 
is independent of the pressure.
If it is assumed that the osmotic suction is 
zero, then the change in total suction equals to 
the change in the matrix suction. 
ψ = (ua – uw )    (2)
Suction can be expressed per unit volume, 
mass or weight. The unit expressed per unit vol-
ume is given in pascals (Pa), bars (bar), or atmos-
pheres (atm).
The soil-water characteristic curve (SWCC)
The soil-water characteristic curve, some-
times referred to as a „water retention curve“, is 
a curve which describes the relationship between 
the suction and the water content, so that it pre-
dicts the behaviour of soil in the drying phase 
(i.e. desaturation) and the wetting phase (i.e. sat-
uration). The shape of the water retention curve 
depends on many factors, from the density of the 
soil, its adsorption capacity, the size of the pores 
in the soil, whether the soil is in a drying or wet-
ting phase, and also on the mathematical model 
used for the calculation of the retention curve. In 
most cases, the water retention curve, which cov-
ers a range between 0 and 106 kPa, has a charac-
teristic „S“ shape (FreDlunD & XinG, 1994). Since 
suction values can be very high, retention curves 
are usually presented as graphs on a logarithmic 
scale.
A typical water retention curve is shown in 
Figure 1, where three identifiable sections can be 
observed: the saturation zone, the desaturation 
zone, and the residual zone. Two changes in the 
slope can also be seen, one at the point of air en-
try and the other at the point of residual water 
content. The soil is completely saturated in the 
saturation zone, where the suction is too small 
to drain the pores. Between the air-entry value 
and the point of residual water content there is 
a desaturation zone. This is the zone where the 
water in the pores is increasingly displaced by 
air. Within this zone, the suction is higher than 
the suction at the air entry value (KayaDelen et 
al., 2007). The air entry value is the point where 
the large pores begin to be drained due to the 
increase of suction (FreDlunD et al., 2011), as air 
enters them. The residual zone is an area where 
the transport of water in the soil is only possible 
by means of water vapour. 
The water retention curves could represent 
the drying phase (desaturation) or the rewetting 
phase (saturation). In the paper the results for the 
drying phase will be presented. 
Mathematical models for water retention 
curves
Various authors have suggested different em-
pirical equations and mathematical models for 
water retention curves. They can be defined as a 
function of the suction depending on the degree 
of saturation, or on the normalized water content. 
The following models are most frequently used, 
in which the water retention curve is expressed 
as a function of the volumetric (i.e. normalized) 
water content.
BrooKs & corey (1964):
 (3)
where: a and n are the fitting parameters deter-
mined by the method of minimum square roots, 
�
𝜃𝜃(𝜓𝜓) = 𝜃𝜃𝑠𝑠𝑠𝑠𝑠𝑠                     𝜓𝜓 ˂ 𝜓𝜓𝑠𝑠𝑎𝑎
  𝜃𝜃(𝜓𝜓) = 𝜃𝜃𝑠𝑠𝑠𝑠𝑠𝑠 �
𝜓𝜓
𝑎𝑎
�
−𝑛𝑛
     𝜓𝜓 ≥  𝜓𝜓𝑠𝑠𝑎𝑎
          
Fig. 1. A typical retention curve (SWCC: soil-water chara-
cteristic curve) (after FreDlunD et al., 2011 and KayaDelen 
et al., 2007).
40 Barbara LIKAR, Vikica KUK & Karmen FIFER BIZJAK
ψ is the soil suction, θR is the residual water con-
tent, θsat is the volumetric water content at com-
plete saturation, and ψae is the air-entry value .
BrooKs and corey (1964) divided the equa-
tion into two parts. In the first part the suction is 
smaller than the air-entry value, whereas in the 
second part it is greater than the air entry value 
(FreDlunD et al, 2011).
FreDlunD & XinG (1995): 
(4)
where: θsat is the volumetric water content at com-
plete saturation, ψ is the soil suction, ψR is the soil 
suction at the residual water content, and a, n, 
and m are curve-fitting parameters. 
van Genuchten (1980):
  (5)
where: θR – is the residual volumetric water con-
tent, and a, n, and m are curve-fitting parame-
ters. 
Soil suction retention curves were in the pre-
sented paper calculated by using the described 
mathematical models. The function parameters 
were determined in Excel by using the Least 
Squares Method, and the retention curves which 
best fitted with the measured values were de-
fined. 
Experimental techniques
Several different methods can be used for soil 
suction measurements. In the paper the results of 
measurements carried out on very stiff sandy silt 
in a Bishop - Wesley double-walled triaxial cell 
are presented and compared with the results of 
soil suction measurements performed by means 
of a potentiometer (WP4C). 
Soil suction measurements using a Bishop-
Wesley double-walled triaxial cell
The double wall triaxial cell is an improved 
traditional triaxial cell. Its advantage lies in its 
double wall cell, which provides constant con-
ditions and thus enables the performance of ac-
curate measurement of the changes which occur 
in the sample‘s volume. In conventional triaxial 
systems water can enter into the saturated sam-
ple, or can be pumped out of it. At the same time 
the air pressure in the sample can be changed, 
which could influence to the volume expansion of 
the sample. 
On the other hand, in the double wall triaxi-
al cell, the same cell pressure inside and outside 
the cell ensures that the volume changes of the 
water in the inner cell is only the consequence 
of changes in the volume of the sample and not 
due to external influences (e.g. changes in the 
cell volume due to the water temperature). These 
changes lead to changes in the volume of water 
in the outer cell, which provides constant condi-
tions for the internal chamber.
The double wall triaxial cell ensures accurate 
measurement of matric suction within the range 
from 0 to 1500 kPa, depending on which porous 
plate is used.
The main parts of the double triaxial cell are 
(Fig. 2):
- a base pedestal on which the sample is placed,
- an inner cell and outer cell that are both 
cylindrical in shape and, after installation, are 
filled with water,
- a vertical force transducer together with a 
hydraulically powered system for vertical load-
ing,
- a pore pressure transducer,
- an axial displacement transducer,
- a pressure gauge in the lower chamber,
- 5 electromechanical volume pressure con-
trollers (VPC) for measuring and setting the air 
and water pressures and for measuring the vol-
ume changes,
- a coarse porous disk
- a porous plate with high air-entry value (a 
HAEPD: high air entry porous disk).
The entire system is computer-controlled, 
with corresponding acquisition and storage fa-
cilities.
The method of measurement of soil suction 
in the double-walled triaxial cell is designed in 
such a way that the sample is pressurized by air 
and water, i.e. by suction (Figure 3). The volume 
change at which equilibrium is established in the 
soil sample is measured. Depending on the vol-
ume change of the sample at certain controlled 
𝛩𝛩𝑛𝑛 =
𝜃𝜃 − 𝜃𝜃𝑅𝑅
𝜃𝜃𝑠𝑠𝑠𝑠𝑠𝑠 − 𝜃𝜃𝑅𝑅
=
1
�1 + �𝜓𝜓𝑎𝑎�
𝑛𝑛
�
𝑚𝑚 
𝜃𝜃 = �1 −
𝑙𝑙𝑙𝑙 �1 + 𝜓𝜓𝜓𝜓𝑅𝑅
�
𝑙𝑙𝑙𝑙 �1 + 10
6
𝜓𝜓𝑅𝑅
�
� ∙
⎣
⎢
⎢
⎢
⎡
𝜃𝜃𝑠𝑠𝑠𝑠𝑠𝑠
�𝑙𝑙𝑙𝑙 �𝑒𝑒 + �𝜓𝜓𝑎𝑎�
𝑛𝑛
��
𝑚𝑚
⎦
⎥
⎥
⎥
⎤
 
41Water retention properties of stiff silt
suction, information about the water content of 
the soil as a function of suction can be obtained 
by back analysis. Under controlled effective net 
stress state (suction stage) the change in the vol-
ume of the pore water and in the volume of the 
pore air are measured by volume measuring de-
vices. With the use of appropriate equations the 
change in the volume of the pore water is calcu-
lated as the result of suction.
When measuring suction using the Bishop - 
Wesley double-walled triaxial cell, the volume 
change of the pore water is calculated, for each 
of the suction stages, from the obtained data. 
The last measured level of the volume change 
for a given suction stage is taken into account in 
the calculations. When the values of the volume 
change are known for each suction stage, then the 
volumetric and gravimetric moisture content are 
calculated, as well as the degree of saturation. 
Soil suction measurements using 
a potentiometer (WP4C)
Soil suction was also measured by means of 
a WP4C (Fig. 4). This particular kind of potenti-
ometer is a device for measuring water potential 
using a chilled mirror. The device is easy to han-
dle, and measurements are carried out quickly 
and fairly accurately. The measurement range for 
suction is between 0 and 300 MPa, and its accu-
racy is ± 0.05 MPa over the measurement range 
between 0 and 5 MPa, and 1 % over the range be-
tween 5 and 300 MPa. The accuracy of the device 
also depends on proper calibration of the instru-
ment and on the ambient temperature at which 
measurements are performed. 
The potentiometer works by measuring suc-
tion on the principle of the relative humidity in 
the air. The specimen is inserted into the ma-
chine into the sealed chamber, which is equipped 
with sensors, a mirror, and a ventilator. The sen-
sors measure the temperature of the mirror, and 
Fig. 2. The Bishop-Wesley double-walled triaxial cell.
Fig. 3. Pressurizing the sample with an air pore pressure air 
(ua) and a water pore pressure (uw) (net effective stress con-
trolled stage).
Fig. 4. The WP4C potentiometer.
42 Barbara LIKAR, Vikica KUK & Karmen FIFER BIZJAK
the ambient temperature. The temperature of the 
mirror is precisely measured by means of a la-
ser beam, which operates on the principle of the 
Peltier effect, in which a photo-detector operates 
by precisely detecting the point at which con-
densation occurs on the mirror for the first time. 
The potentiometer directs the light beam onto 
the mirror so that it is reflects into the photo-de-
tector. When condensation occurs on the mirror, 
the photo-detector senses the change, and at the 
same time the thermocouple in the mirror re-
cords the temperature at which condensation has 
occurred. At the point of equilibrium the water 
potential of the air in the chamber is equal to the 
potential of the water in the sample. Soil suc-
tion is then determined according to Equation 10 
(WP4C, 2013).
    (10)
where: Ψ is the water potential, R is the gas con-
stant, T is temperature in the sample in Kelvin, M 
is thermolecular mass of the water, p is the pore 
pressure of the air, and po is the vapour pressure 
at the temperature of the sample.
Experimental results
The goal of the suction measurements was to 
determine the value of suction of very stiff precon-
solidated sandy silt in the Krško Basin, where it is 
expected that some large geotechnical structures 
will be built in the next few years. The suction 
measurements were performed in the above-men-
tioned Bishop-Wesley double-walled triaxial cell 
(sample V1) and potentiometer (sample V2).
Soil studied
The stiff sandy silt (sample V1) which was 
studied in the Bishop-Wesley double-walled tri-
axial cell was similar but not identical to the 
sandy silt (sample V2) which was used for the 
measurements in the potentiometer. However, 
using both measurement methods it was possible 
to compare data obtained by means of two differ-
ent suction measurement techniques.
The soil physical parameters are presented in 
Table 1. The results of the granulometric analy-
sis showed that samples V1 and V2 can be classi-
fied as sandy silt (saSi) according to the standard 
SIST EN ISO 14688-1, and silt (ML) according to 
the standard JUS U.B1.00.
Water retention curves
The measured data of the sandy silt sample V1 
are shown in Figure 5, where the retention curves 
were calculated by using the van Genuchten, Br-
ooks and Corey, and Fredlund and Xing. With the 
Bishop-Wesley double-walled triaxial cell the suc-
tion is measured in the volumetric water content.
From the retention curves it can be seen that 
the air-entry value for sandy silt V1 (ψae) occurs 
at a suction of 40 kPa. The suction which cor-
responds to the natural gravimetric water con-
tent of the sample (i.e. 27.7 %) amounts to about 
3-4 kPa which means that the sandy silt is at nat-
ural water content fully saturated. The slope of 
the curve in the middle section is steep, which 
means that most of the water in the sandy silt is 
bound in the capillaries, so that the material is 
able to release it at low values of suction.
𝛹𝛹 =
𝑅𝑅𝑅𝑅
𝑀𝑀
∗ 𝑙𝑙𝑙𝑙
𝑝𝑝
𝑝𝑝0
 
Table 1. The geomechanical characteristics of the sandy silt V1.
Sample Water content Density
Dry 
density
Specific 
density Granulometric analysis 
Designation and 
unit
w
(%)
ρ (Mg/m3) ρd (Mg/m3)
ρs (Mg/
m3)
sand 
content 
(%)
silt 
content 
(%)
clay content (%)
Sandy silt
V1
27,2 1,93 1,58 2,80 33,3 63,8 2,5
Sandy silt
V2
25,3 1,99 1,53 2,76 42 57,8 0,2
43Water retention properties of stiff silt
Fig. 5. Comparison of the 
retention curves obtained 
by using the different 
models for the sandy silt 
sample V1.
Fig. 6. Comparison of the 
retention curves obtained 
by using the different 
models for the sandy silt 
sample V2.
The retention curve corresponding to the 
sandy silt of sample V2 was determined by using 
the potentiometer (Fig. 6). With the potentiometer 
suction is measured in the gravimetric water con-
tent. The residual moisture point occurs at almost 
zero water content, at a suction of about 500 kPa, 
whereas the air-entry point occurs at a suction 
of 60 kPa. The measured value for the suction at 
70 kPa and 22 % of gravimetric water content is 
questionable, since potentiometer measurements 
at suctions of up to 100 kPa are inaccurate. The 
slope of the retention curve is fairly steep. It can 
be seen that the moisture content decreases very 
rapidly, even at low values of suction. This indi-
cates that the water between the grains is bonded 
only in the capillaries, so that the material is able 
to release it at low values of suction. 
Discussion
Soil water retention curves were prepared on 
the basis of the models proposed by Brooks and 
Corey, Fredlund and Xing, and van Genuchten.
Based on the comparisons which were made 
between the different models, we tried to deter-
mine which water retention curve best fitted the 
measured values. From Figures 5 and 6 it can be 
seen that the measured values fit best when the 
Fredlund and Xing or van Genuchten model is 
used. In the case of the Brooks and Corey model, 
good agreement was observed with the measured 
values in the middle part of the curve, whereas at 
low suctions the fit was poorer. A sharp transi-
tion curve can be observed at the air-entry point, 
which makes identification of this point easy. 
44 Barbara LIKAR, Vikica KUK & Karmen FIFER BIZJAK
The sharp transition curve is due to the splitting 
of the model into two parts, where, in the first 
part of the equation, the moisture content is as-
sumed to be to fully saturated. For this reason 
in its initial part the curve is flat, whereas in the 
part from the air-entry point onwards it enters a 
sharp transition. In the case of the sandy silt V2 
the retention curves are located somewhat below 
the measured values.
Although each type of soil has its own kind of 
retention curve it is generally considered that var-
ious groups of soils have retention curves which 
are characteristic for them. The measurements 
results were also checked against data which can 
be found in the literature. Figure 7 shows the 
measured values of the retention curves for stiff 
sandy silt and a typical retention curve for sand 
and silt. The results of sandy silt V1 and V2 are 
shifted more in the direction towards retention 
curves which are typical for sand. 
The results were compared with the Sion 
(Geiser, 1999) silt which is by the structure the 
most similar material to the Krško valey sandy 
silt (Table 2). It has a lower percent of sand and 
the sample was reconstituted before the testing 
and the consolidated to the primary stress val-
ue. If we compare curves of the samples V1 and 
V2 with the results of the measurements of the 
Sion silt we can see that there are quite similar 
in spite of the fact that the samples V1 and V2 are 
preconsolidated (Fig. 8). The preconsolidation 
pressure was obtained by the pressuremeter and 
oedometer tests and it was found that the OCR is 
approximately 2. From the results we can con-
clude, that the presonsolidation ratio of samples 
V1 and V2 is not enough high to have an influence 
on the level of suction. Higher influence on the 
results have the sandy component in the silt.
Based on our experiences with testing of stiff 
silt material we could conclude that each of the 
Fig. 7. Comparison of the 
measured values with data 
from the literature (the re-
tention curves are accor-
ding to FreDlunD et al., 
2011).
Fig. 8. Comparison of the 
measured values of sandy 
silt (samples V1 and V2) 
with Sion silt and data 
from the literature (the re-
tention curve is according 
to FreDlunD et al., 2011).
45Water retention properties of stiff silt
described methods of measurement of soil suc-
tion has its advantages and disadvantages. From 
the point of view of accuracy, the Bishop - Wes-
ley double walled triaxial cell has a big advan-
tage, since very accurate measurements can be 
made of the changes occurring in the pore water 
volume and the air volume, and, as well as this, 
accurate measurements can be performed at low 
matric suction values. Additionally, the Bishop 
-Wesley double walled triaxial cell provides con-
stant conditions throughout the duration of the 
investigation, which reduces the possibility of 
errors due to the impact of the environment (e.g. 
temperature changes). The advantage of the cells 
lies also in the fact that measurements are made 
on only one of the samples, so that errors due to 
sample preparation can be avoided. However, in 
comparison with the potentiometer, the work is 
much more difficult and requires more knowl-
edge of how to handle with the device, and the 
measurements are also time-consuming.
Complete investigations in a Bishop - Wes-
ley double-walled triaxial cell, from the sample 
saturation and consolidation to the measure-
ments of soil suction may last as long as several 
months, whereas entire measurements in the po-
tentiometer take no longer than a week or two. 
Apart from making possible faster performance 
measurements, handling of the potentiometer is 
much simpler. The disadvantage of the device lies 
in the fact that the potentiometer does not give 
good results in the low suction range, and it is 
also very important to reduce the effect of tem-
perature changes on the soil suction results. The 
disadvantage of performing measurements with 
the potentiometer is that the measurements are 
made on a small amount of the sample having an 
unknown density, which has a considerable ef-
fect on the soil suction values obtained. A small 
amount of a sample can also increases the possi-
bility for making errors in the measurements. At 
the same time this might be an advantage, since 
there is no need for a large amount of the sample 
for the performance of the measurements.
From the geotechnical point of view, the au-
thors were particularly interested in the matric 
suction, which has an effect on the stress - strain 
properties of soils. Matric soil suction can be pre-
cisely measured only in a double-walled triaxial 
cell, whereas in measurement performed with a 
potentiometer only the total suction is measured, 
assuming that the osmotic suction is zero.
Conclusions
So far, most research of soil suction has been 
performed on clay and silt material, whereas 
measurements of the soil suction of stiff precon-
solidated sandy silt have not been performed yet. 
Very stiff silt is typical of the Krško Basin where, 
in the future, some very important geotechnical 
structures will be built. In particular, knowledge 
is needed about the behaviour of such soil at in-
creased or decreased water content is essential.
In the paper the results of measurements that 
were carried out on very stiff sandy silt in a Bish-
op - Wesley double-walled triaxial cell for the 
very stiff preconsolidated sandy silt are present-
ed, and compared with the results of soil suction 
measurements performed with a potentiometer 
(WP4C). Both methods were presented in detail. 
The results of the measurements were evaluated 
on the basis of already known results that have 
been presented in the literature, using the three 
most commonly used mathematical models which 
are used for the definition of retention curves.
On the basis of the results of the laboratory 
tests of unsaturated soils, with an emphasis on 
measurements of suction in a Bishop - Wesley 
double-walled triaxial cell and using a poten-
tiometer, it can be concluded that both of these 
methods of measurements provide useable re-
sults. The obtained results are in a good agree-
ment with the results which are already known 
from the literature, and the models used to de-
scribe the soil retention curve fit the measured 
values very well.
Table 2. The geomechanical characteristics of the Sion silt.
Sample Liquid limit Specific density Granulometric analysis 
Designation 
and unit
wl
(%)
ρs (Mg/m3)
sand 
content 
(%)
silt 
content 
(%)
clay 
content 
(%)
Sion silt 25,4 2,74 20 72 8
46 Barbara LIKAR, Vikica KUK & Karmen FIFER BIZJAK
The results were compared with the Sion silt 
(Geiser, 1999) which is by the structure the most 
similar material to the Krško valey stiff sandy 
silt. From the comparison it can be concluded, 
that the presonsolidation ratio of samples V1 and 
V2 is not enough high to have an influence on the 
level of suction. Higher influence has a presence 
of the sandy component in the silt material.
The disadvantages of most methods for meas-
uring soil suction in the laboratory lie in the in-
accurate and unreliable measurements which 
occur in the low suction range, i.e. up to about 
100 kPa. According to the obtained results, the 
Bishop - Wesley triaxial cell can provide accu-
rate measurement in this range. The specially de-
signed double-walled cell enables accurate meas-
urements of the volume changes which occur in 
the water and air in the pores, without being af-
fected by outside effects (e.g. changes in temper-
ature) The absolute advantage of this cell lies in 
the fact that the suction is measured only a single 
sample, so that the effect of errors in the prepa-
ration of the sample, or of its non-homogeneity, 
can be reduced. 
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WP4C 2013: Dew Point PotentiaMeter. Operator’s 
Manual. Decagon Device, Inc..