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Investigation of Seepage Effect on River Dike’s Stability under Steady State and Transient Conditions

Authors:
  • Institute of GeoSciences. Polytechnic University of Tirana Albania

Abstract and Figures

World experiences reveal that catastrophic floods are posing a serious threat that comes not only from them as extreme events but also as the result of adaptation measures uncertainty, (i.e. dikes). In particularly old dikes constructed earliest at the north part of Albania, during the last floods demonstrated the high uncertainty and weak spots. In this paper, the significance of the seepage investigation and stability analysis are discussed. As a case study, parts of new dikes constructed last years along the Buna River located in north part of Albania are investigated. Filling materials for these dikes are represented mostly from silt and clay. Finite element method is used to perform seepage analysis while general limit equilibrium method is used to perform slope stability analysis. Both, seepage and slope stability analyses are done for three different scenarios: steady state, rapid filling, and rapid drawdown. Finally, it is concluded that silt-clay material used in these dike structure is posing serious uncertainty during seepage phenomenon by threatening the stability of entire dike structure especially during the transient condition (rapid filling and drawdown).
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POLLACK PERIODICA
An International Journal for Engineering and Information Sciences
DOI: 10.1556/606.2016.11.2.8
Vol. 11, No. 2, pp. 87–104 (2016)
www.akademiai.com
HU ISSN 1788–1994 © 2016 Akadémiai Kiadó, Budapest
INVESTIGATION OF SEEPAGE EFFECT ON RIVER
DIKE’S STABILITY UNDER STEADY STATE AND
TRANSIENT CONDITIONS
1
Alban KURIQI,
2
Mehmet ARDIÇLIOGLU,
3
Ylber MUCEKU
1
Illyrian Consulting Engineers sh.p.k, Rr. Andon Zako Cajupi, Nd. 14, H. 14, Ap. 2,
1019, Tirana, Albania, e-mail: albankuriqi@gmail.com
2
Civil Engineering Department, Engineering Faculty, Erciyes University, 38039, Kayseri,
Turkey, e-mail: mardic@erciyes.edu.tr
3
Institute of Geosciences, Energy, Water and Environment, Polytechnic University of Tirana,
Don Bosko nr.60, 1024, Tirana, Albania, e-mail: mucekuy@yahoo.com
Received 14 November 2015; accepted 22 March 2016
Abstract: World experiences reveal that catastrophic floods are posing a serious threat that
comes not only from them as extreme events but also as the result of adaptation measures
uncertainty, (i.e. dikes). In particularly old dikes constructed earliest at the north part of Albania,
during the last floods demonstrated the high uncertainty and weak spots. In this paper, the
significance of the seepage investigation and stability analysis are discussed. As a case study,
parts of new dikes constructed last years along the Buna River located in north part of Albania are
investigated. Filling materials for these dikes are represented mostly from silt and clay. Finite
element method is used to perform seepage analysis while general limit equilibrium method is
used to perform slope stability analysis. Both, seepage and slope stability analyses are done for
three different scenarios: steady state, rapid filling, and rapid drawdown. Finally, it is concluded
that silt-clay material used in these dike structure is posing serious uncertainty during seepage
phenomenon by threatening the stability of entire dike structure especially during the transient
condition (rapid filling and drawdown).
Keywords: Dike, Seepage, Slope Stability, Rapid Filling, Rapid Drawdown
1. Introduction
It is evident that considerable numbers of inhabitants are located closer to lakes,
shoreline, and rivers valley. In one point of view, establishment of high percentage of
inhabitants to the aforementioned locations has many opportunities but on the other
hand, these locations are vulnerable from different kinds of threat posed from extreme
events as increase of sea level, high magnitude of wave and flooding events.
Periodically floods pose a serious threat to human life, valuable property and city
88 A. KURIQI, M. ARDIÇLIOGLU, Y. MUCEKU
Pollack Periodica 11, 2016, 2
infrastructure, [1], [2]. Construction of the adaptation and defenses systems is dating
very earlier in human being history. Adaptation systems are represented by structures as
dikes or embankment, which has two main purposes; to retain and diverting certain
amount of water that comes especially during flood events, [3], [4]. The first measures
to be adopted against these extreme events and to avoid drastic damages is related to
better conception of adaptation structures by considering different records from flood
events, [5]. In the past, construction of these kinds of structures generally has been
carried out by using simple procedures and equipment. In this context the older dikes or
embankment structures in generally, are presenting high index of risk related to failure,
especially during flood events, [6], [7]. The main factors that influence a dike failure
depends mostly from: type of materials (heterogeneity), used as filling material, not
sufficient compaction during the construction phase, which leads to the softening and
loosening of filling materials.
Adaptation measures against natural hazards [8] or those initiated by man-made are
one of the main challenges, going to the past it is obvious that a serious effort is taken
from people after they are affected especially from flood disasters. During the last
decades, floods probability of occurrence has increased in terms of the return period (i.e.
probability exceedance). So having sufficient information on earth structures and
specifically on dike performance is very important in order to have clear picture about
dike stability against different factor that cause failure of structure itself, [9], [10]. In
this paper, seepage effects on dike structures stability during steady state and transient
conditions are discussed. Additional factors that cause instability or in worse case total
failure of earthen dike structure in generally are: surface erosion, piping and seepage.
From these three factors, erosion can be consider as less threat factor that may cause
failure of dikes since it occurred at the outer part of the structure, so it can be controlled.
On the other hand, piping, which is considered as one of the main factors that cause
failure of earth structures all over the world, occurs due to the migration of small soil
particles into the coarse materials. This phenomenon takes place through the dam or
dike body, or under the structure foundation, [11], [12].
While the phenomenon of seepage that is discussed mostly in this paper represent
the uncontrolled saturation of filling materials inside the dike or earth structures in
generally. In this context, the stability of dikes is assessed for steady state and transient
condition. Steady state condition consists into the investigation of seepage effect on
dike stability when water level does not change. While transient condition is regarding
with phenomena known as rapid filling or rapid drawdown, which occurs during the
sudden change of water level (increments and decrements), which modifies flow
condition and effect directly soil properties inside the dike structure. Numerical
modeling based on finite element method is used to analyze the seepage for respective
conditions (i.e. steady state and transient). Finally, achieved results are compared for
both conditions and some main conclusion and recommendations are addressed to the
protection of dikes or embankment structures.
SEEPAGE EFFECT ON RIVER DIKE’S STABILITY 89
Pollack Periodica 11, 2016, 2
2. Structural failures
2.1. Dike breaching mechanisms
Failures of an earth structure involve the separation (rupture) of the embankment
material or its foundation depending on geotechnical conditions of the basement. This
type of failure is more prominent in large dams or embankment structures. However, it
is not exclusive to only large dams; similar occurrences happen on small earthen
embankments or dikes, [13], [14]. It is noticed that the failure of earth structures may be
caused due to different factor as construction activities, rainfall infiltration, also many
failures occur in natural soil slopes and excavated slopes. In many cases, stability
problems of embankments structures and dikes occur during the rapid filling of
reservoir or waterways and rapid drawdown, [15].
According to ‘Voorschrift Toetsen op Veiligheid’; National Protocol for Safety
Assessment of dikes in Netherlands [16], safety assessment of dikes should be done by
considering one of the 9 potential failure mechanisms: slide circle inside slope,
settlement, slide circle outside slope, piping, wave overtopping, erosion outside slope,
micro instability, softening and erosion foreland. Generally, natural soils are
characterized from high variability and heterogeneity, these characteristics as mentioned
above has a significant indication on physical-mechanical properties of the soil. One of
the main sources of heterogeneity is inherent spatial soil variability due to the different
depositional conditions and different loading histories, [17], [18], [19].
Breach mechanism is classified as following: hydraulic failure, geo-hydraulic failure
and global static failure, [20]. Hydraulic failure generally consists on overtopping [21]
and wave scour on the dike structure, while geo-hydraulic failure is due to the seepage
process through the dike core or foundation, which leads into the erosion process
associated with transportation of significant materials amount. Whereas concerning to
the type of breach mechanisms, if two aforementioned types are induced mainly from
pressure forces as ice, water, and wind waves, while the third type of breach
mechanisms (i.e. global static failure) is due to gravity and pressure forces occurring as
local or total failure of the dike structure.
2.2. Seepage phenomenon
Analyzing seepage phenomenon occurred in: earth irrigation system, storages, ponds
and other earth structure as dikes or earth dams is very important task to be carry out in
order to avoid any threat that may come as result of any possible failure, [22]. Earliest,
attention has been given generally on estimation of seepage loose regarding especially
to channel system and storage dam, while nowadays is given attention both amount of
water loose and impact on environmental degradation, [23]. Identification and
estimation of seepage phenomenon are very important issue for design and construction
in particularly for the earth dams and dike structures, [24]. In order to decrease the path
of flow line, which represents the seepage phenomenon through the earth structure,
designers proposing to use one of the following measures: impervious core, upstream
clays and blanket or cut-off trench. Still, even significant advancement achieved in the
field of geotechnical engineering, excessive seepage under or through the structure,
90 A. KURIQI, M. ARDIÇLIOGLU, Y. MUCEKU
Pollack Periodica 11, 2016, 2
threatens seriously the integrity of the structure, [25], [26]. In unconfined zone
regarding the earth structures, seepage origin is linked also with the free surface of the
structure. Free surface at the mean time play two different roles: it behaves as path of
streamline and as boundary condition within the seepage network by hindering the
procedure of calculation, [27], [28]. As mentioned above during flood events, seepage
phenomena are one of the main factors that induce failure of dike structures. The
instantaneous increase and decrease of water level at the river dike during the floods,
induce a typical unstable seepage process, moreover in these conditions a significant
deformation of structure can occur, [29].
3. Problem statement
Floods are natural phenomena, which happening very often especially in the north-
west part of Albania [30], (Fig. 1). During floods, water occupy for hours in case of
flash floods, or days in case of long-term floods that occurs generally during the winter;
when rainfall period is longer or spring as result of snowmelt, [31].
Fig. 1. Flood plan map of Buna River in Shkodra city and surrounding areas,
located at north-west of Albania
An unacceptable high risk of flood characterized the land of the Lower Drini-Buna
River basin, this is due to significant geological changes that happen last 150 years ago.
Along the Buna River flow, adaptation measures are constructing continuously,
especially after floods of winter 2010. Despite new protection structures constructed
SEEPAGE EFFECT ON RIVER DIKE’S STABILITY 91
Pollack Periodica 11, 2016, 2
after winter 2010, in addition, there is done reinforcement of the existing dikes. The
elevation of the existing dikes on the left bank of the Buna is placed in accordance with
an international agreement between Albania and Montenegro. This level provides
protection against the calculated flow discharge 2% exceedance probability
(i.e. 1-50-years), [32]. Even if flow substantially in excess of the 100-year design flood
arrives at Shirqi, a weir is designed such that the excess flow, above the 50-year
discharge, is diverted out of the river and across Trushi Field towards Murtemza. The
Shirqi to Belajdikes and the Pentari to Pulaj dikes, when rehabilitated to internationally
agree levels, are protecting the south-west of the study area, including the town of
Velipoja for excess flow, 1 to 50-year exceedance probability. Since in the Buna River,
continuously is occurring rapid filling and drawdown, this process leads to seepage
phenomenon even during the construction phase. In addition, this may affect the
stability of protection structures, especially earth structures (i.e. dikes). In this case as
mentioned above, seepage and stability of dike analysis is conducted by considering
different scenarios: steady state, rapid filling, and rapid drawdown.
4. Materials and methods
4.1. Materials
During the construction phase of dikes placed along the Buna River, raw materials
used for construction the dikes structures are represented mostly from silt-clay. Detailed
data information about materials shown below (Table I) are obtained after in situ
testing.
Table I
Materials properties used for dike construction
Type of
Construction
Materials
Parameters Quantity
Notations Symbols
Silt-Clay
Hydraulic Conductivity (m/h) K
saturation
0.0036
Volumetric Water Content
(m
3
/m
3
)
θ
w
0.08
Saturated Volumetric Water
Content (m
3
/m
3
)
Θ
s
0.55
Residual Water Content
(m
3
/m
3
)
Θ
r
0.18
Minimum Suction (kPa S
min
0.01
Maximum Suction (kPa) S
max
1000
Unit Weight (kN/m
3
)
γ
17
Cohesion (kPa) C 15
Friction Angel (degree)
'
φ
20
Curve Fitting Parameter n 1.3
Curve Fitting Parameter (cm
-
) α 0.78
92 A. KURIQI, M. ARDIÇLIOGLU, Y. MUCEKU
Pollack Periodica 11, 2016, 2
Volumetric water content is predicted by using Van Genuchten method as given in
Eq. (1):
m
n
rs
rw
a
+
+
+=
Ψ
ΘΘ
ΘΘ
1
, (1)
where,
w
Θ
is the volumetric water content;
s
Θ
is the saturated volumetric water
content;
r
Θ
is the residual water content;
Ψ
is the negative pore-water pressure; and
m
n
a
,
,
are curve fitting parameters. According to Van Genuchten method, curve-fitting
parameters are estimated based on volumetric water content of soil. Therefore, the best
point to estimate and evaluate the curve fitting parameters is the middle point between
residuals and saturated water content, [33], [34].
The slope of curve can be estimated by using Eq. (2) as following:
( )
p
p
rs
p
d
d
S
Ψ
Θ
ΘΘ
log
1+
=
, (2)
where,
p
Θ
is the volumetric water content at middle point;
p
Ψ
is the matric suction at
middle point. While parameters a and m are estimated as following by Eq. (3)
and Eq. (4):
)8.0exp(1
p
Sm =
, (3)
(
)
m
a
=
1
21
12
1
Ψ
. (4)
Based on Van Genuchten method, hydraulic conductivity of soil can be estimated
by Eq. (5);
(
)
( )
2
1
1
11
m
n
m
nn
sw
a
aa
kk
Ψ
ΨΨ
+
+
=
, (5)
where,
w
k
is the hydraulic conductivity; and
s
k
is the saturated hydraulic conductivity.
Relation between matric suction, volumetric water content and conductivity are
presented below, (Fig. 2). Within the increase of matric suction, volume water content
SEEPAGE EFFECT ON RIVER DIKE’S STABILITY 93
Pollack Periodica 11, 2016, 2
and conductivity are decreasing because of compaction and consolidation process,
which leads to increase of soil density, [35].
a) b)
Fig. 2. Graphical representation of a) volumetric water content vs. matric suction;
b) conductivity vs. matric suction
4.2. Methodology
There are many numerical methods and models used to investigate seepage
behavior. These common methods are represented by finite difference, boundary
elements, finite volume and finite element methods. From all of these methods, finite
elements methods are the most powerful and wide range of use relating to different
engineering problems, [36]. In this study, seepage and slope stability problems are
investigated for two different stages, respectively: steady state and transient condition.
For this purposes ‘Seep/W’ package program that is developed by Geo-Slope
International Ltd, which is based on Finite Element Methods (FEM) is used.
Concerning to the transient conditions, it consists in two different scenarios, rapid
filling, and rapid drawdown. Time of rapid filling is consider 12 hours, while a time of
rapid drawdown is consider 156 hours (i.e. 6.5 days). Concerning to the geometrical
configuration of dike, (Fig. 3) high of dike is 9 m including foundation, top of dike 5 m,
cross-section (width of the dike) is about 45 m and both slopes are 1:2.5. In addition in
Fig. 3 the Minimum Water Level (Min.W.L), the Normal Water Level (N.W.L) and the
Maximal Water Level (Max.W.L) are shown, respectively. The dike is discretized in
triangle mesh with dimension 0.3 m each of them.
94 A. KURIQI, M. ARDIÇLIOGLU, Y. MUCEKU
Pollack Periodica 11, 2016, 2
Fig. 3. Geometrical configuration of dike and discretization with triangle mesh elements
4.3. Governing equations
Seepage
Water flowing through the dike in both conditions (saturated/unsaturated), follows
the Darcy law Eq. (6).
ikq =
, (6)
where; q is the specific discharge; k is the hydraulic conductivity and i is the hydraulic
gradient. While two dimensional seepage flows through the soil for steady state is
derived by using Eq. (7) as following:
t
Q
y
H
k
yx
H
k
x
yx
=+
+
θ
, (7)
where, H is the total head;
x
k
is the hydraulic conductivity in the x-direction;
y
k
is the
hydraulic conductivity in the y-direction; Q is the flux; t is the time; and
θ
is the
volumetric water content. From this equation state is noticed that, amount of the rate of
flows changes in both directions (x, and y) by taking in account external applied flux is
equal with rate of change of the volumetric water content for certain time, [35]. For
steady state conditions, amount of water flux entering in certain volume of soil is equal
with the escaping water flux, whereas Eq. (7) reduced as below Eq. (8):
0=+
+
Q
y
H
k
yx
H
k
x
yx
. (8)
SEEPAGE EFFECT ON RIVER DIKE’S STABILITY 95
Pollack Periodica 11, 2016, 2
Concerning to the transient condition, two-dimensional flow through the soil is
described as following Eq. (9):
t
H
mQ
y
H
k
yx
H
k
x
wwyx
=+
+
γ
, (9)
where,
w
m
is the slope of water content;
w
γ
is the water content unit weight;
x
k
is the
hydraulic conductivity according to x-direction;
y
k
is the hydraulic conductivity
according to y-direction;
Q
applied boundary flux;
v
m
is slope of storage curve;
H
is
the total head and y-direction according y-axis.
According to Galerkin method of residual weighed to the governing differential
equation, the finite element equation for two-dimensional seepage can expressed by
(10) as following:
=
+
L
T
A
TT
dLqtdAdA NHNNHCBB
τλττ
, ,
)10(
where,
B
is the gradient matrix;
C
is the hydraulic conductivity matrix;
H
is the nodal
vector;
N
is the vector of interpolation function; q is the unit flux;
τ
is the thickens of
the mesh element; t is the time;
λ
is the storage parameter, for case of transient seepage
ww
m
γ
λ
=
; A is the designation for summation over the area of given element; and L is
the designation for summation over the edge of an element [37].
Since Seep/W package
of Geo-Slope International Ltd software, uses the backward differences method so Eq.
(10) can be written in simplified form as given Eq. (11):
(
)
011
HMQHMK
tt
+
=
+
, (11)
where
K
is the hydraulic conductivity matrix;
M
is the water storage matrix;
1
Q
is
the nodal flux vector at end of time increment;
1
H
is the head at the end of time
increment;
0
H
is the head at the start of time increment and
t
is the time increment.
4.4. Slope stability
As it is mentioned above for both scenarios (i.e. steady state and transient
condition), slope stability of dikes is evaluated. Slope/W package based on general limit
equilibrium method that include key elements of other methods like Ordinary, Bishop,
Janbu and Spencer. In this study, Spencer method is used, which based on two factors
of safety equations. These factors of safety can be estimated in two different ways: with
respect to moment equilibrium Eq. (12) and horizontal force equilibrium Eq. (13),
governing equations are as following:
96 A. KURIQI, M. ARDIÇLIOGLU, Y. MUCEKU
Pollack Periodica 11, 2016, 2
(
)
[
]
DdNfWx
RNRc
F
m
±
+
=
φµββ
tan
, (12)
(
)
[
]
ωα
εφµβαβ
cossin
costancos
DW
Nc
F
f
+
=, (13)
while normal force is estimated as following Eq. (14 ):
( )
F
F
c
XXW
N
LR
φα
α
φαµβαβ
+
+
+
=tansin
cos
tansinsin
, (14)
where,
N
is the normal force at slice base;
X
R
is the inter-slice shear force;
X
L
is the
inter-slice shear force
on either side of slice
; F
m
is the factor of safety;
W
is the slice
weight;
β
is the inclination angel;
c'
is the soil cohesion;
φ
is the internal friction
angel;
α
is the inclination of the slice base;
D
is the concentrated point load and
R
is
the geometric parameter. Dike is analyzed in saturated and unsaturated condition [38],
described respectively by Eq. (15) and Eq. (16) as following:
(
)
φ
µ
σ
+
=
tan
wn
cS
, (15)
( ) ( )
r
s
r
wn
cS
θθ
θθ
φµµφµσ
αα
+
+
=tantan , (16)
where,
S
is the degree of saturation,
n
σ
is the normal stress;
α
µ
is the dynamic
viscosity of material;
w
µ
is the dynamic viscosity of water;
w
θ
is the volumetric water
content;
r
θ
is the residual volumetric water content and
s
θ
is the saturate volumetric
water content.
5. Results and discussions
5.1. Seepage analysis
Considering the parameters used as filling materials for construction of the dike,
respective analyses are performed. Soil parameters are introduced in the Seep/W model
in order to conduct respective seepage analysis. The hydrodynamics process that
occurred especially during the rapid filling and drawdown has the significant impact on
dike performance. Pore-water pressure and water flux per
x-
direction is investigated for
three different scenarios, (
Fig. 4
). Regarding to first scenario (i.e. steady state
condition), there is noticed a positive pore-water pressure about 35 kPa, that getting
SEEPAGE EFFECT ON RIVER DIKE’S STABILITY 97
Pollack Periodica 11, 2016, 2
increase up to a distance 12 m according to x-direction, then suddenly is decreasing up
to 22 according to x-direction, where negative pore-water pressure about -40 kPa is
observed as result of soil unsaturation at this part of dike.
Fig. 4. Seepage analysis, pore-water pressure for three different scenarios: steady state, rapid
filling and rapid drawdown
98 A. KURIQI, M. ARDIÇLIOGLU, Y. MUCEKU
Pollack Periodica 11, 2016, 2
While, maximum water flux about 1.10-4 m
3
/hr is reached at altitude 8.5 m same
distance (12 m) according to x-direction, after that it is decreasing until it becomes 0 at
25 m distance according to x-direction. Concerning to the second scenario (i.e. rapid
filling), there is noticed a higher pore-water pressure than the first scenario. Different
from the first scenario, in this case maximum pore-water pressure about 60 kPa is
reached for shorter distance (i.e. 8 m), then it is decreasing with a trend to a negative
pore-water pressure about -15 kPa at 18 m distance according to x-direction. While
maximum water flux about 2.10-4 m
3
/hr is reached at distance about 17 m after 12
hours, afterwards is decreasing until become zero at distance 25 m according to x-
direction, fluctuation of water flux for respective time is presented graphically (Fig. 4),
rapid filling. During the rapid drawdown, compare with the two aforementioned
scenarios, it is noticed significant reduction of pore-water pressure; whereas maximum
value of pore-water pressure about 15 kPa is reached at a distance 11 m, while drastic
reduction and negative pore-water pressure is occurred as result of water withdraw from
the soils and unsaturation process. Concerning to the water flux is decreasing
drastically, until it becomes 0 at distance 11 m after 156 hr of water withdraw at the
inner part of dike structure. Detailed representation of water flux is shown at a
respective graph (Fig. 4), rapid drawdown. Fluctuation of pore-water pressure and water
flux according to x-direction for the certain time is effected from soil parameter (i.e.
conductivity) and hydrodynamic processes that happening especially inside of the dike
structure. Concerning to last scenarios, during rapid filling cohesion of filling material
almost disappear so in these condition positive pore-water pressure is occurred while
during rapid drawdown as result withdraw of water from soil particles is noticed
decrease and afterwards negative pore-water pressure is occurred, (Fig. 5).
Fig. 5. Comparison of pore-water pressure occurred during steady state, rapid filling
and rapid drawdown fitted with respective aforementioned polynomial functions
In addition there the correlation between pore pressure and prolongation distance of
water flux is estimated along to x-direction. In all cases good correlation with a
determination coefficient R
2
>0.8 is noticed. The correlation between pore pressure and
SEEPAGE EFFECT ON RIVER DIKE’S STABILITY 99
Pollack Periodica 11, 2016, 2
prolongation distance is expressed through following polynomial equations, Eqs. (17),
(18), (19):
State)(Steady 8495.0,631.150625.20544.0
22
=++= RxxP
p
, (17)
Filling)
(Rapid 9539.0,099.577934.11343.0
22
=++= RxxP
p
, (18)
Down) Drow (Rapid 9848.0,0106.13814.31337.0
22
=+= RxxP
p
. (19)
If during the rapid filling dike structure maintains its stability as result of hydrostatic
pressure, on the other hand during rapid drawdown, dike structure is likely to fail its
stability as result of seepage process which induce migration of soil particles outside the
dike, [39].
5.2. Slope stability analysis
After seepage analysis, slope stability analysis is performed for each respective
scenario. During the slope stability analysis, shear strength of soils and factor of safety
is observed, (Fig. 6). Concerning to the first scenario (i.e. steady state condition), the
factor of safety is about 2.45; it remain constant. While shear strength is getting increase
about 36 kPa up to 16 m distance according to x-direction, after that it is decreasing
suddenly as the result of less consolidation of the outer side of the dike.
Regarding stability analysis for the second scenario (i.e. rapid filling); there is a
significant increase related to the factor of safety especially during the first hours of
rapid filling. Afterward, there is reached an equilibrium that corresponds to the factor of
safety about 5.7. The sudden increase of the factor of safety is due to the temporal
compaction of soils by the hydrostatic pressure. While shear strength is increasing up to
the distance about 18 m then it is suddenly decreasing as the result of the reduction of
soils strength parameter, in addition, shear strength in this scenario is lower than the
first scenario. Concerning to the third scenario (i.e. rapid drawdown), it is noticed a
significant decrease of a factor of safety comparing with the first and second scenario as
well. Differences of safety factors regarding third scenario are about 1.2% lower than
first scenario and 57% lower than a second scenario. While concerning the shear
strength it is increasing about 33 kPa up to 13 m distance according to x-direction, after
that it is decreasing suddenly.
In addition, shear strength comparing with the second scenario represent higher
value while comparing with the first scenario there is the significant reduction, (Fig. 7).
So, rapid change of water level at dikes structure generally lead to the degradation
and reduction of soil parameters, in this condition there is a higher threat of structure
failure. The correlation between shear strength and prolongation distance of the water
flux along x-direction is estimated. In all cases is noticed good correlation with a
determination coefficient R
2
>0.8.
100 A. KURIQI, M. ARDIÇLIOGLU, Y. MUCEKU
Pollack Periodica 11, 2016, 2
Fig. 6. Slope stability analysis, factor of safety and shear strength for three different scenarios:
steady state, rapid filling and rapid drawdown
Fig. 7. Comparison of shear strength for three scenarios: steady state, rapid filling and rapid
drawdown fitted with respective aforementioned polynomial functions
SEEPAGE EFFECT ON RIVER DIKE’S STABILITY 101
Pollack Periodica 11, 2016, 2
The correlation between shear strength and prolongation distance i expressed through
following polynomial equations, Eqs. (20), (21), (22):
State)(Steady 8495.0,631.150625.20544.0
22
=++= Rxx
τ
, (20)
Filling), (Rapid 8869.0
,201.127574.23159.00162.00003.0
2
234
=
+++=
R
xxxx
τ
(21)
Down) Drow (Rapid 9539.0,937.141403.20606.0
22
=++= Rxx
τ
. (22)
6. Conclusions
In this paper seepage process and slope stability of dike, structure is investigated
through numerical modeling for different scenarios. Scenarios considered during
numerical modeling are as following: steady state, rapid filling and rapid drawdown.
The investigation of seepage process during the first scenario revealed that there is
certain water volume passing through the dike structure. In this case, there is no any
threat concerning the stability of dike, even that presence of water inside the dike
structure lead to a reduction of mechanical properties of soil. For second scenario,
safety factor is increased significantly up to 5.71 but the reduction of shear strength
indicates that increase of dike stability is just temporary phenomenon’s that happen as
result of temporary compaction of the filling materials by hydrostatic forces. Whereas
during the third scenario; stability of the dike structure is decreased compared with the
two first scenarios, safety factor achieved during this scenario is 2.43. The decreasing of
the factor of safety is revealed perceptibly by the significant reduction of soils
parameters like; shear strength as presented on the respective graph. In this case study is
noticed that during rapid filling Finally after detailed investigation of the seepage
phenomenon through the dike structure, it is conclude that internal and external
hydrodynamic processes and especially seepage is a serious threat that should be
considered not just during design process but also operation, even that materials used
for construction of the dike structure or earth structures in generally may have very
good parameters. In addition, designers should be aware what kind of filling materials
should be used for construction of earth structures and adaptation measures in case of
structure failure. Especially for high earth structures, using silt-clay as filling materials,
based on standards recommended may lead to the rapture of structure and then
catastrophic disaster. Also, proper riprap layers should be considering during the design
phase in order to increase the safety of dike structure and to prevent any possible failure.
102 A. KURIQI, M. ARDIÇLIOGLU, Y. MUCEKU
Pollack Periodica 11, 2016, 2
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The levee failure event occurred along the Foenna stream (Tuscany region, central Italy) on 1st January 2006 is investigated. The levee body vulnerability to seepage is first assessed through a procedure here proposed and based on a simple vulnerability index. In particular, the index is defined for dimensionless geometry of levee body and related to the probability of occurrence of the levee seepage through the identification of 'fragility curves'. The procedure can be applied for embankments whose soil hydraulic parameters are unknown or partly known. The obtained results indicate that the investigated levee, if intact, would not have to be vulnerable to seepage for the flood event occurred on January 2006 when, instead, the failure happened. The analysis shows that the failure was likely due to the presence of burrows at middle height of levee body riverside drawing the flow into the embankment triggering the piping phenomenon. This study indicates that to prevent the failure due to burrows presence, the levees must be actively maintained through both the burrows closing and the capture of wild animals. Finally, considering that scarce data are usually available for this type of events worldwide, this paper aims also to illustrate the collected data in terms of: maximum stage recorded at a gauged section close to the failure location, status of the levee, evolution of the failure event.
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On the basis of the generalized physical model of the riverbank, the experiments were conducted to study the mechanisms of riverbank failure under the coupling effect of seepage flow and river flow. The experimental setup was specially designed, as well as test point location, parameters and procedures, and the main influencing factors were analyzed affecting riverbank failure based on the failure types, the variations of pore water pressure and soil displacement. The results indicated that the coupling effect has different influences on the bank failure in three aspects: the failure type, the process and the extent. In addition, the river flow played a more important role than the seepage flow in the coupling effect on the bank failure.