DESIGN, CONSTRUCTION AND PERFORMANCE OF GEOMEMBRANE SEALING
SYSTEMS IN EMBANKMENT DAMS
João Figueira
Abstract
The principal subject of this thesis is the geomembrane sealing systems on embankment dams. The evolution of
the knowledge is marked by the continuously deepening of the questions related to these systems, finishing the present work
with an effective design of a real case of an embankment dam.
It’s analysed general considerations on dams and their different types, basic concepts of geosynthetics and their
different applications, these knowledge has been obtained mostly on the last century with a certain emphasis in these last
decades due to pressures from various origins: the demographic explosion and its needs, environmental matters, new laws
and regulations and, even, new technologies. An effective dynamization of these solutions is needed as well as a correct
formation of the dam designers.
The geomembrane sealing system is demystified on this thesis as well as new concepts and procedures. All this
accumulated knowledge is put into practice in a real case of an embankment dam in Madeira Island, Portugal. The
preliminary design is done using the limit equilibrium method and then it’s made an analysis with finite elements method
(FEM). This last analysis consists in obtaining the stress-strain state of the structure of all the construction phases and during
the first filling of the reservoir, to ensure that the preliminary design is right. The FEM is also used to study the consequences
of defects on the geomembrane.
Based on all the information gathered and analysis made, one can conclude that GSS solutions are valid and
technically suitable to grant imperviousness to a dam.
Key-words: embankment dams, geossynthetics, geomembranes, waterproofing, dam project, leakage control
1. Introduction
Structures for retaining the water from natural
sources are quite common and it’s not a recent
technology. Throughout the history of mankind, dams
have been considered fundamental for the adaptation
process of the civilizations to the environment.
Recently, the use of dams has been greatly
increased especially because of the crescent need of
water and energy. These structures contain a lot of
environmental questions, politic decisions and economic
issues. The risk of rupture is a great concern and,
because of that, all the investment on the technological
development and safety control systems is needed and
justified.
Much effort has been done to optimize the design
and performance of dams, especially in embankment
dams. This optimization includes the system that grants
imperviousness to the dam. The present paper requires a
solid base of knowledge in the area of dam engineering
and to prevent the lack of it, it starts with some basic
aspects of this subject.
2. Embankment Dams
There is many types of dams, embankment dams
are defined for its complex structure and materials.
Represent 75% of the total dams in the world. They are
composed by earth, rock or mixtures, and some of them
must comprise a impervious curtain. Due to its nature,
they are characterized by its large base/height ratio and
large ductility and capacity to follow the deformation of
the foundation. This type of dams is recommended when
the foundation has a not so good performance and when
the vales are wide.
Despite the fact that the odds of failure of
embankment dams are decreasing, we have to learn
from the past to avoid the same mistakes. The main
causes of failure are: overtopping, internal erosion and
excessive flow and slope instability. The optimization of
its impervious curtain is integrated in the combat of the
second cause of failure and also influences the third one.
3. GSS Solutions
3.1 Introduction
Conventionally, this waterproof system was made
of clay, reinforced concrete, bituminous concrete or even
steel plates. There’s an alternative solution which has
1
been used for quite some time and consists of a
Geomembrane Sealing System - GSS, greatly developed
in the last three decades. The adequate design and
selection of the water tightness system is extremely
important and requires a full understanding of it.
3.2 Historical introduction
During the Second World War, polymers and
products made from polymers were created and
developed. They had been used to produce
geosynthetics and these products are used in a lot of
hydraulic and geotechnical structures. The use of
geosynthetics systems started after the war in drainage
canals and reservoirs. Due to its success and the
practical experience of these solutions, it allowed to gain
confidence and to implement it on another and larger
structures.
The first installation of a GSS in dams was in an
embankment dam in Italy. Contrada Sabetta dam was
built in 1959 with a covered system composed of a
double polyisobutylene geomembrane with 2,0 mm of
thickness as the only waterproofing system. After one
year, in 1960, the same technology was used in the
Dobsina dam in Slovakia with a 0,9 mm PVC
geomembrane in a covered system. In 1967, in France,
the water tightness of Miel dam was provided by a
covered geomembrane made of butylic rubber with 1,0
mm of thickness. Throughout the history it has been
achieved other important goals to the development and
rooting of this kind of technology. An internal system of
geosynthetics was firstly used on the spanish Odiel dam
in 1970. The first repair of a dam with a GSS was in
Czech Republic with a 0,9 mm PVC geomembrane on
the Obenice dam, in 1971. The first time an exposed
GSS was installed was in 1973 on the Banegon dam, in
France, with a 4 mm bituminous geomembrane. Other
great milestone achieved by this solution happened in
1997, when the first underwater installation was made in
the Lost Creek dam, in USA.
From the analysis of its evolution and the
completed projects, we can learn and optimize the
solution. We can take some conclusions of its history of
development like the choice of the best polymeric
material, its thickness and durability, some design details,
anchorage systems and failure mechanisms, etc.
The choice of the GSS is made from its availability,
personal experience, design specifications and available
information. We can see that the choice has also a
regional influence. For example, the use of bituminous
geomembranes is emphasized in France. Other types of
geomembranes, like elastomeric ones, fell into disuse
because of its inconvenient characteristics.
We can also take some conclusions about the
problems that some of these projects had with their GSS:
inadequate connections, deterioration due to wind action,
punching due to falling materials and misshapen
subgrade, localized deformation of the supporting soil
and the inevitable phenomenon of aging. If adequately
installed and explored, a GSS can have a good
performance for about 200 years (estimated values for
covered system in [1]), surpassing the lifetime usually
considered in dams design (100 years).
Because of that, it’s very important to make the
right choice and selection of the GSS. The installation
phase is also a crucial step to the lifetime expectancy of
the geosynthetic system.
Currently, the GSS technology is used in all types
of dams, new or existing ones. It’s a well-accepted
technique in all over the world. According to ICOLD data
[2] there’s more than 270 dams where the main
waterproofing system is made of a GSS, of which over
183 are embankment dams.
3.3 Design Criteria
A GSS design has to ensure safety and to prevent
failure or excessive changes on the stress-strain state of
the dam by the seepage phenomenon.. It has to
guarantee a good connection to the foundation and to the
concrete structures of the dam. It must have good
characteristics of flexibility to adapt to the dam
movements. For last and not least, the sliding
mechanism created by the layers of the system has to be
verified and prevented. If the friction force doesn’t
guarantee the stability, then it has to comprise an
anchorage systems.
A GSS is a system that has a geomembrane on it
for the purpose of granting imperviousness to the dam.
Due to its nature, the geomembranes are easily
damaged, so the system may comprise other
geosynthetics, like geotextiles to support and protect the
geomembrane, and geogrids, behind the watertight
element, to grant an adequate drainage.
3.3.1 Types of GSS
There’s three types of GSS [3]:
 Simple liner;
 Double liner;
2
 Composite liner.
The simple liner is comprised by just one
geomembrane. There’s many examples of it, it is most
used one. For example, the Miel dam from France
already presented. Jibiya dam in Nigeria, from 1989, has
a geocomposite consist of a PVC geomembrane
reinforced with a non-woven PP geotextile.
The double and composite liners are composed by
two consecutive elements of low permeability. The
composite liner consists on a set of a mineral component
and a synthetic component, frequently, a geomembrane
with a soil layer behind it with very low permeability, clay
for instance. The double one is a set of two synthetic
components like two consecutive geomembranes. These
type liners were created to minimize the possible leakage
from a hole or a defect on the more external
geomembrane.
These solutions have a problem with the water
entrapped between the two consecutive layers with low
permeability or inside the mineral layer from a possible
leakage. If a rapid drawdown of the reservoir level
occurs, the entrapped water can provoke excessive
pressure in the geomembrane and the materials above it.
It may cause the loss of stability of the system or the
uplifting of the system and the materials above it.
Nowadays, it’s common to put a drainage layer (possibly
a geogrid) between the systems or behind it. When the
solution is covered permanently and with enough weight,
this may not be a concern. These solutions guarantee a
redundancy of the system, improving its reliability.
3.3.2 Advantages of a GSS
GSS has been used many times now and its usage
is increasing due to its various advantages when
comparing it to the conventional solutions [3].
The first desirable characteristic of geomembranes
is the cost of it. These systems are more economical
than the traditional ones, unless the material of the
traditional solution like clay is very near the construction
site. Their supply, transport, storage and installation can
be optimized to make a very economical proposal.
Their permeability is much lower than the one
presented by the conventional materials. This
characteristic is very important in various applications,
like wastewater or contaminated water reservoirs. In
dams, this advantage is not so emphasized because the
priority is the structural and hydraulic safety and the
purpose of the dam. Geomembranes allow thinner
watertight systems without compromising their capacity
of waterproofing the structure.
Other great advantage of GSS is their capacity to
deform without tearing apart. Their elongations are large
enough to say that these systems are the recommended
ones when large deformations are expected. Their
flexibility allows the system to adapt perfectly to the
subgrade and its movements.
The systems with geosynthetics are so much
easier to deal with. They are not highly dependent of their
availability near the construction site. The characteristics
of traditional clay cores are dependent of the construction
quality; their permeability and durability vary with
experience and performance of its appliers. The
characteristics of geosynthetics are very much controlled
during fabrication and simplify a lot the construction.
They only demand special attention with their integrity
during its transport, storage and installation. The time of
construction can be reduced when using a GSS instead
of the traditional solutions. They present less constraints;
their installation can be made in accordance with other
parallel works and it’s not affected by the weather
conditions.
3.3.3 Location of the GSS
The GSS can be positioned in the dam adopting
several configurations. There are three kinds of positions:
 External system with a covered GSS;
 External system with an exposed GSS;
 Internal system.
The next picture seeks to catalog the different
possible arrangements of geosynthetic system (Figure 1).
All of them have some advantages and disadvantages,
but almost 90% of the GSS used in embankment dams
are positioned at the upstream face. Essentially this
configuration is the most used because of the following
reasons:




The vertical component of the water pressure
contributes to the stability of the dam;
It has less complications in the construction phase;
It allows visual inspection and maintenance on the
exposed solutions;
Their eventual repair or replacement is easier than
the internal system.
3
The chemical resistance is only put into analysis
when the retained liquid is other than water. Mechanical
resistance is the responsible characteristic for the choice
of the type of geomembrane. Normally the geomembrane
is usually axially tensioned and its axial resistance is the
most important characteristic of it. Proper design of the
various system forces is essential, from its installation to
its operation.
The durability is another feature to be
assessed. Its integrity is jeopardized in every phase, from
its production to its transportation, storage, installation
and usage. Their durability is evaluated by three
mechanisms: the loss of volatile particles, changes on its
structure and other effects (from vegetation, animals and
vandalism).
3.3.5 Stability Analysis
Figure 1: Possible configurations of a GSS solution.
But this system has some disadvantages too, like
the stability problem of the GSS and the need of an
adequate anchorage system. Uplift and damage from
debris and vandalism can affect an upstream system.
Comparing it with the internal system, it has a much
larger quantity of geomembrane. But the configuration
where the GSS is placed inside the dam is rare and
represents only 10 %.
3.3.4 Performance evaluation
The GSS must be evaluated by its material,
imperviousness, resistance and durability.
The type of polymeric material needs to be proper
assessed because different materials has different
behaviours. PEAD and PVC are the material most used
in this kind of application. They have a totally different
behaviour, but a PVC geomembrane presents a linear
elastic behaviour and has the ability to follow the
movements of the dam. PEAD has high crystallinity and,
by consequence, high rigidity. It has an elastoplastic
behaviour, not so much appreciated in dams. But PEAD
presents a larger mechanical resistance than the PVC
geomembranes.
The impervious capability of the geomembranes is
enough for this application and allows adopting thinner
solutions.
The stability of the system is analysed when we
have a GSS installed in the upstream face of the dam.
We have to ensure the safety against sliding and it must
be verified in two critical situations: when the reservoir is
empty and in a situation that where a rapid drawdown of
the reservoir level occurs. For the second situation we
have to guarantee a good anchorage system and, if
applicable, enough weight of the cover layer. Anyway, we
need to paid attention to the possible instability when
two impervious consecutive layers are present, as said
before.
For the first situation, we need to be more cautious.
Every interface composing the GSS can contribute to the
sliding mechanism. There are several methods to
analyse the mechanism. In the present paper it’s used
the easier one and with good results, which is the limit
equilibrium analysis. The resistance mechanism is
composed by the friction forces mobilized in every
interface, as can be seen in the figure 2.
1 – Earthfill or Rockfill
2 – Geomembrane (GMB)
3 – Geotextile (GTX)
4 – Cover layer (CL)
Figure 2: Intervening forces in the sliding mechanism.
As so, the stability is ensured if the following
condition is verified:
4
𝑡𝑔 𝛿𝑖𝑛𝑡𝑒𝑟𝑓𝑎𝑐𝑒 > 𝑡𝑔 𝛽
where 𝛿𝑖𝑛𝑡𝑒𝑟𝑓𝑎𝑐𝑒 represents the friction angle of the
interface and 𝛽 the slope angle.
𝑇 cos 𝛽 = 𝜎𝑛 𝐿𝑅𝑂 tg 𝛿𝑈 + 𝜎𝑛 𝐿𝑅𝑂 tg 𝛿𝐿 + 0,5 (
1
1
2
2
2𝑇 sen 𝛽
𝐿𝑅𝑂
) 𝐿𝑅𝑂 tg 𝛿𝐿 −
( (𝛾𝐴𝑇 𝑑𝐴𝑇 ) + 𝜎𝑛 ) 𝐾𝐴 𝑑𝐴𝑇 + ( (𝛾𝐴𝑇 𝑑𝐴𝑇 ) + 𝜎𝑛 ) 𝐾𝑃 𝑑𝐴𝑇
The interface friction angle depends on many
different factors. For instance, the friction angle between
a geomembrane and a geotextile depends on the
manufacture process of them. They depend also on the
“in situ” conditions. Because of that, there are of different
values for these angles and in an application with this
dimension, a proper analysis of these interface friction
angles are needed specifically to this design.
When the stability condition is not verified, we need
an anchorage system to stabilize it. There is a lot of types
of anchorage: mechanical, chemical or even by loading.
The simplest one is the one where we prolong the GSS
penetrating the material of the dam. This anchorage can
be seen in the figure 3. The arrows represent the
intervening forces and the resistance forces mobilized
inside the anchorage by friction.
Figure 4: Anchorage design method.
This method is an approximate method that must
be used just in an preliminary design. Its results must be
verified by a more accurate analysis, like the one which
uses the finite element method.
Figure 3: Peripheral anchorage by prolonging the GSS.
To design the anchorage system we need to
estimate the tension in the GSS. The forces represented
in the figure 2, are the friction forces that are mobilized by
the sliding mechanism. To resist the sliding movement
and due to different friction angles, the GSS must
become tensioned. The tension is calculated by the
equilibrium on the geosynthetic, for instance, for the
geomembrane represented in the figure, its tension is
calculated by the difference between F3 and F4.
The anchorage design used in this paper is
demonstrated in the figure 3 and follows the design
method recommend by Koerner (2005). It will be adopted
an horizontal part characterized by its length, LRO, and a
vertical part, forming the anchor trench, with a length of
dAT. The calculation is an iterative process, conjugating
and criticizing the values of LRO and dAT. They are
obtained by the equilibrium of the horizontal forces.
∑ 𝐹𝑥 = 0 <=> 𝑇𝑥 = 𝐹𝑈𝜎 + 𝐹𝐿𝜎 + 𝐹𝐿𝑇 − 𝑃𝐴 + 𝑃𝑝
3.3.5 Construction details
This alternative solution has many advantages and
its becoming a very solid solution. But there are some
complications when dealing with a GSS. We have to
understand it all to judge and to design a solution like this
one. For instance, how the geomembranes are made and
how they are provided?
The production of geomembranes is made in two
ways: in fabrication and then transported in rolls or
panels to the construction site; or made “in situ” by
impregnation of a spray in a low permeability material.
The transportation of the former must be very careful to
avoid defects and all the material must adequately
labeled. Their storage has to be done in a way that
doesn’t cause any damage to it and prevent their
degradation, for example, protected from the UV
radiation.
Before the effective installation, it’s necessary to
inspect the subgrade of the GSS, ensuring a firm
superficial layer, as smooth as possible without any
elements that could damage the geosynthetics.
5
The installation is usually made vertically along the
slope and then, if applicable, opened in its full extend of
width. Sometimes it’s made horizontally in cases where
we have berms along the upstream slope. It’s very
important to leave an adequate overlapping between rolls
or panels of geomembranes to make a good connection,
10 to 30 cm. As soon as the connections are made, it’s
used a loading system to prevent its uplift.
The connections of GSS are very important and
can determine their success. There are many types of
connections, Figure 5 presents some. The choice depends
on the type of geomembrane and the specific conditions
found “in situ”. It should be known that different types of
connections require different procedures and conditions.
The method must be chosen taking into account the
temperature, pressure and productivity.
restitution system and the lining of a natural water
course.
Its water feeding is going to be made by Ribeira do
Alecrim and Levada Velha do Paul, in a pressurized way.
When the full filling of the reservoir is done, the water is
going to be at elevation 1352.00 m. It’s going to have a
spillway and an intake structure at elevation 1330.45 m.
Pico da Urze has an impervious curtain made by
geosynthetics on its upstream face and reservoir.
It has been executed an investigation campaign
and a characterization analysis of the intervening soils. It
allowed concluding that the foundation is very
heterogeneous and not suitable for foundation of a dam.
They present high deformability. It was considered
sufficient a general excavation of 1.5 to 2.0 m.
4.1 Preliminary-design
Figure 5: Types of connections: a) by extrusion; b) by fusion; c)
by chemical fusion and d) by chemical adhesives.
If the final solution consists in a covered solution,
the cover layer must be placed carefully to avoid any
damage to the GSS.
4. Case study: Pico da Urze Dam
To put into practice all the information here
reunited, a recent project of a rockfill dam in Madeira
Island is analyzed, Portugal.
Pico da Urze dam is an embankment dam located
in Paul da Serra, Calheta Region in Madeira. It will have
a capacity of 1,021 hm3 and will be built to increase the
power production capacity of another dam, owned by
Empresa de Electricidade da Madeira, S.A. It will have a
maximum height of 31 m and the dam crest will be at
elevation 1354.00 m. Besides the construction of the
dam, it’s going to be built various support buildings , an
upstream dyke, an excavation of its reservoir, a
In the prelimnary design section, it’s made the
choice of the adopted GSS, the calculations of the cover
layer thickness and the stability analysis. As
recommended by ICOLD, a rockfill dam must have an
upstream inclination from 1:1.5 to 1:2 (V:H) and all the
study has been made with these inclinations and an
intermediate one, 1:1,75, as a sensitivity analysis. For
this case, it’s considered two types of GSS: solution A,
composed by a PVC geomembrane coupled with a nonwoven geotextile and, additionally, another geotextile
over the system only in the covered area; and solution B
composed by a geocomposite composed with two nonwoven geotextiles and a PVC geomembrane between
them. The adopted solution is going to be chosen by the
results of the stability analysis.
The cover layer has to be designed to be dense
enough to dissipate the energy of the reservoir’s waves,
strong enough to undertake the impact of the waves and
have the adequate durability to endure the exposure to
the atmospheric agents and to different reservoir’s levels.
The layer is going to be a conventional riprap layer
placed upon the geotextile in the upstream face of the
dam. Its thickness is calculated by the method presented
in Fell et al. (2005):
0,33
𝑟 = 𝑛 𝐾∆ (𝑊⁄𝛾𝑟 )
where r is the thickness of the riprap, n is the number of
sub layers considered, K is a shape coefficient, W the
weight of the riprap, in kN, and 𝛾𝑟 the riprap’s weight unit.
To determine the value of W, the value of wave’s
height must be calculated for the maximum wind velocity
6
expected. For that, it must calculate the effective fetch of
the reservoir, the wave’s significant height and period.
The table 1 shows the results for the different slope
inclinations considered.
Table 1: W and r values for the various slope inclinations.
W (kN)
r (m)
1:1.5
0.188
0.212
Inclination
1:1.75
0.161
0.201
Table 2: Data used for the stability analysis
Transition layer:
AT (kN/m3) 18
38 Ka 0.328
AT (°)
30,4 Kp 3.049
'AT (°)
The different values of the friction angles
presented in table 2 are divided by 1.25, as EC7
recommends in the design approach 1, combination 2, of
the GEO limit states. The considered interfaces are
between the support layer and the geotextile (GTX/SL),
between the smooth PVC geomembrane and the
geotextile (GTX/GMB) and the geotextile with the cover
layer (CL/GTX). These values came from bibliography. So,
the respective tests must be carried out specifically for
each case..
Firstly, the
stability was checked and the
conclusions are presented in table 3 for the different
slopes.
Table 3: Verification of the stability condition.
Inclination
1:1.5
1:1.75
1:2
°
33.69
29.74
26.57
 < ’GTX/SL
No
No
No
F1
F2=F3
F4=F5
1:1,5
1:1.75
1:2
2.85
2.98
3.07
1.93
2.02
2.08
2.33
2.43
2.51
 < 'GTX/GMB
No
No
No
F1-F2
TGTX
(kN/m)
0.92
0.96
0.99
TGTX
(kN)
29.13
33.99
38.84
F3-F4
Tgeocomposite
(kN/m)
-0.40
-0.41
-0.43
Tgeocomposite
(kN)
-12.61
-14.72
-16.82
Table 5: Tension forces of solution B.
For the stability analysis, it has been considered
the parameters displayed at Table 2.
Cover layer:
r (kN/m3) 22
r (m)
0.35
W (kN/m2) 7.7
Inclination
1:2
0.141
0.193
As the area of the reservoir is small, the thickness
needed of the riprap is also small, but there’s a minimum
value. The same method allows an estimation of its
dimension and the maximum diameter found is 0.30 m.
So, the average thickness adopted is 0.35 m.
Friction angles: '
GTX/SL (°) 25 20
GTX/GMB (°) 21 16.8
CL/GTX (°) 30 24
Table 4: Tension forces of solution A.
 < 'CL/GTX
No
No
No
As the table presents, the stability is not ensured
just by the intervening friction forces. So an anchorage
system is needed. The tension force calculations are
made for solutions A and B and are shown in tables 4
and 5.
Inclination
F1
F2=F3
1:1,5
1:1,75
1:2
2,85
2,98
3,07
2,33
2,43
2,51
F1-F2
Tgeocomposite
(kN/m)
0,52
0,54
0,56
Tgeocomposite (kN)
16,52
19,27
22,02
For these values of the tension forces, it was
performed a design analysis to determine the length of
the anchorage system. At the same time, it has been
made a sensibility analysis of it, changing the inclination,
the thickness of the cover layer in the anchorage zone,
changing the solution and considering the case of a
possible anchorage at the middle of the upstream slope.
It should be noted that the negative tension force in
geocomposite of the solution A doesn’t mean that it’s
going to be in compression, it indicates that the friction
force is more than sufficient to ensure the stability.
Tables 6 to 8 resume the results obtained for the
the horizontal part of the anchorage length, LRO, with an
index A for solution A and B for solution B and an index 2
when it’s considered an anchorage at the slope middle.
Table 6: Values for the anchorages length in a slope with the
inclination of 1:1.5.
Cover
thickness
(m)
0.5
1
1.4
0.5
1
1.4
LROA (m)
LROA2 (m)
LROB (m)
LROB2 (m)
7.125
3.563
2.545
LROA
dATA
(m)
(m)
1
0.464
1
0.252
1
0.162
3.563
1.781
1.272
LROA2
dATA2
(m)
(m)
1
0.231
1
0.083
0.5
0.083
3.178
1.589
1.135
LROB
dATB
(m)
(m)
1
0.236
1
0.076
0.5 0.082
1.589
0.794
0.567
LROB2
dATB2
(m)
(m)
1
0.073
0.5
0.039
0.5
0.009
Table 7: Values for the anchorages lengths in a slope with the
inclination of 1:1.75.
Cover
thinckness
(m)
0.5
1
1.4
0.5
1
1.4
LROA (m)
LROA2 (m)
LROB (m)
LROB2 (m)
8.986
4.493
3.209
LROA
dATA
(m)
(m)
1
0.566
1
0.332
1
0.227
4.493
2.247
1.605
4.046
2.023
1.445
LROB
dATB
(m)
(m)
1
0.311
1
0.129
0.5 0.121
2.023
1.011
0.722
LROB2
dATB2
(m)
(m)
1
0.122
1
0.002
0.5
0.029
LROA2
(m)
1
1
1
dATA2
(m)
0.299
0.130
0.066
7
Table 8: Values for the anchorages lengths in a slope with the
inclination of 1:1.2.
Cover
thickness
(m)
0.5
1
1.4
0.5
1
1.4
LROA (m)
LROA2 (m)
LROB (m)
LROB2 (m)
10.856
5.428
3.877
LROA
dATA
(m)
(m)
1
0.659
1
0.408
1
0.289
5.428
2.714
1.939
LROA2
dATA2
(m)
(m)
1
0.361
1
0.175
1
0.101
4.919
2.460
1.757
LROB
dATB
(m)
(m)
1
0.380
1
0.179
0.5 0.159
2.460
1.230
0.878
LROB2
dATB2
(m)
(m)
1
0.167
0.5
0.093
0.5
0.050
These results allow the following conclusions:




When the cover thickness is double, the anchorage
length is reduced to half;
The same happens when the tension is divided by
two due to the consideration of an anchorage at the
middle of the slope; the the anchorage length is
also only half of the previous case;
The lengths increase with the decrease of the
inclination of the upstream slope of the dam. This
occurs because as the inclination decreases, the
vertical component of the weight of the riprap layer
increases, increasing the intervening friction forces;
The length values of the solution B are smaller than
the values of the solution A, due to the level of
tension in the geosynthetics.
The solution to adopt depends on many
parameters, some of them analysed and some not
included in the present analysis. Because of that, some
possible solutions are pointed out. Due to the stability of
an eventual transition layer between the rockfill and the
GSS, the inclination of 1:1.5 mut be excluded. The ideal
inclination is 1:1.75, where the stability is assured and
the lengths are not too large as those determined for
inclination of 1:2.
The following solutions can be adopted:



Solution A with a cover thickness of 1.4 m and LRO
of 3.25 m;
Solution B with a cover thickness of 1 m and LRO of
2.1 m;
Solution B with a cover thickness of 1.4 m and LRO
of 1.5 m.
It should be noted that usually the anchorage cover
layer isn’t horizontal as the anchorage itself, so its
thickness isn´t constant. Because of that, these values
must be corrected and the length values increase about
1.2 m.
With the upstream face’s inclination and the
possible solutions defined, it is missing the stability of
the downstream slope and the excavation slope of the
reservoir. For that analysis, it was used the program
GeoStudio 2007 and performed a Slope/w analysis. The
results are shown in the igure 6 and figure 7,
respectively. The safety was verified for a 1:2 of
inclination of the downstream slope. For the slopes of
the reservoir, the inclination 1:3 was found adequate.
Figure 6: Downstream stability analysis for an inclination of 1:2.
Figure 7: Reservoir stability analysis for an inclination of 1:3.
4.2 Adopted design
After the preliminary calculations, it is designed
thecross section of the dam, adopting the solution A with
ananchorage horizontal length of 2 m (LRO) and a vertical
of 0,6 m (dAT). It was considered a maximum of 4 m for
the horizontal part of the anchorage. So an anchor trench
was necessary with a minimum depth of 0,60 m due to
the compaction process. The cross section is shown in
figure 8.
8
Figure 8: Cross section of the Pico da Urze dam.
4.3 Finite element analysis
As final analysis, it was performed a finite element
analysis to obtain the stress-strain state of the dam
during the construction and the operation. This study will
allow understanding the behaviour of the GSS and the
dam along all the construction phases and the first filling
of the reservoir. It will serve to support, or not, the
conclusions taken in the previous analysis.
Permeability
coeficient (m/s)
Matric Suction (kPa)
Besides that, it’s executed a seepage study to
understand the consequences of a defect on the
geomembrane. We'll try to link the consequences with
the characteristics of the defect, essentially, the influence
of the defect location and also the differences of various
dispersed defects and a larger concentrated one.
Figure 9: Permeability vs. matric suction curve for the rockfill.
4.3.1 Stress-strain analysis
For these analyses, the soil parameters considered
are the following:
Table 9: Parameters to the finite elements analysis.
Parameter
Material
model
Behaviour
Unit weight
Oedometric
Modulus
Poisson’s
coefficient
Internal
friction angle
Permeability
coefficient
MohrCoulomb
Drained
16
Transition
material
MohrCoulomb
Drained
18
MohrCoulomb
Drained
22
kN/m3
120 000
10 000
90 000
90 000
kN/m2

0.23
0.3
0.3
0.3
-

52
35
38
35
°

Figure 9
10-7
10-3
10-4
m/s
Name
Rockfill
Foundation
Type

MohrCoulomb
Drained
19
Eoed
Model
Riprap
Unit
-
The rockfill permeability cannot be represented by a
constant permeability coefficient, because it is totally dry
and when the geomembrane defect appears, so the flow
in the rockfill is dependent on the matric suction. To
make the simulation more realistic, it is adopted the
permeability vs. matric suction curve presented in figure
9.
For this analysis, it was used the PLAXIS program
to simulate the staged construction and to obtain the
stress and strains states of Pico da Urze dam. To turn
the simulation more realistic, the construction of the
rockfill was made in 17 phases of 2 m height each. The
results of the last stage of the rockfill construction are
shown in figure 10. The results of the maximum
displacements (1.048 m) along the critical vertical axis)
are shown in figure 11, where it can conclude that the
largest displacement occurred at the rockfill interface with
the foundation, emphasizing the high deformability of the
foundation. About the first filling of the reservoir, the total
deformation verified was at the bottom of the reservoir,
where the horizontal displacement was 0.73 m and
vertical displacement was 0.51 m.
Figure 10: Total incremental displacements of the final
construction stage.
9
Table 11: Flow results of the dispersion analysis
1360
Defect Elevation (m) Flow (l/s)
A+B+C
4.6813
A2
1351.4
3.0192
B2
1343.875
3.0342
C2
1335.22
2.3481
1330
1300
0
0,5
1
1,5
Figure 11: Total displacements of the dam along a vertical axis.
The GSS presents 0.60 m of total displacement
during the first filling and an additional tension force of
0.010 kN/m induced by its differential displacements.
This tension force increase can be neglected and it can
be concluded that the preliminary design is well done.
4.3.2 Seepage analysis
For this analysis, to study the seepage
phenomenon along defects on the geomembrane the
GeoStudios program with a SEEP/w analysis was used .
It was considered four defects (each one with 0.3 m2):
one on the upper side of the upstream slope (A), another
at the middle (B), the third at the bottom (C) and the last
at the bottom of the reservoir (D). To study the
consequences of the defects it has been determined the
flow in the vertical axis of the dam and results are
presented in table 10.
Table 10: Flow results of the defects.
Defect Elevation (m) Flow (l/s)
A
1351.4
1.6953
B
1343.875
1.6916
C
1335.22
1.2983
D
1329
0.000167
The results show that the permeability differences
between the intervening materials have a great influence.
The defect D presents perfectly the significant
imperviousness of the foundation. But there are other
factors that exert some influence on the results like the
length of the water path inside the dam (and its direction)
and the intensity of the water pressure.
To study the consequences of the dispersion of the
defects, it was simultaneously considered defects A, B
and C and then compared with three larger isolated
defects (0.9 m2): one at the upper part (A2), another at
the middle part (B3) and the third at the bottom part of
the upstream slope (C2). The results are presented in
table 11.
As expected, the flow is larger. The disperse
defects presents more severe consequences than the
others. Obviously, the best case is when the defects are
concentrated in a specific zone. By the results, it is better
in terms of performance and it is also better to repair just
a specific zone.
5. Conclusion
As conclusion, the GSS solutions are a valid,
technically suitable and cost-quality adequate solution to
grant imperviousness to a dam. Unfortunately, these
solutions are impregnated with lack of information and
experience that causes a high uncertainty and
unacceptance. It is of greater importance the continuous
investment on investigation, monitoring of existing
solutions and education.
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