Slope Stability of Levees 659 IT IS SEEPAGE INDEED — A

Transcription

IT IS SEEPAGE INDEED — A SENSITIVITY STUDY ON SEEPAGE
AND SEEPAGE-INDUCED SLOPE STABILITY OF LEVEES
Khaled Chowdhury, PE, GE1
Sujan Punyamurthula, PhD., PE3
Richard Millet, PE, GE2
Gyeong-Taek Hong, PhD., PE4
Nichole Tollefson5
ABSTRACT
Typical failure modes observed in levees are caused by underseepage, through seepage,
slope instability, erosion, and overtopping. Overtopping and erosion are strongly
influenced by hydraulic conditions. For the remaining failure modes seepage is the major
contributor to multiple types of distress conditions in a levee during flood events.
Underseepage, which may cause piping and internal erosion of materials due to excessive
pore water pressure under the blanket layers, also negatively impacts slope stability by
reducing effective stresses in the foundation soils. Similarly, through seepage, which may
cause removal of materials from levee embankments due to piping through non-cohesive
soils, reduces the factor of safety against slope stability failure. Underseepage and
through seepage are dependent on multiple factors that include net head, embankment
and foundation stratigraphy, material types, levee geometry, hydraulic conductivity of the
embankment, blanket, and aquifer, contrast between blanket and aquifer hydraulic
conductivity, blanket thickness, and the presence or absence of a waterside blanket.
A sensitivity study was performed to evaluate different factors on steady state
underseepage and through seepage conditions on levees, and subsequently, the effects on
slope stability. The results of these seepage and stability studies were compared with
current United States Army Corps of Engineers (USACE) criteria (EM 1110-2-1913).
Different contributing factors were evaluated under both existing and remediated levee
conditions. If a reduction in a slope stability factor of safety is driven by seepage
conditions, seepage mitigation measures should also improve stability conditions.
Sensitivity study results indicate that slope stability factors of safety vary significantly for
a wide range of soil strength parameters as seepage conditions vary. However, this
variation in slope stability factors of safety is significantly reduced in levee sections
remediated for seepage conditions. Exceptions to these findings include levees founded
on soft organic soils or fissured highly plastic clay, where stability of an existing levee is
mainly dependent on the strengths of embankment and near surface foundation layers.
Due to the findings that levee seepage characteristics affect the majority of levee failure
modes (i.e. underseepage, through seepage, and seepage-induced slope stability
conditions), data collection efforts should focus more on collecting information on factors
that affect seepage conditions so that an effective mitigation measure can be designed.
1
Project Manager, URS Corporation, 2870 Gateway Oaks Ste 150, Sacramento CA 95833,
[email protected]
2
Vice President, URS Corporation, Sacramento, CA , [email protected]
3
Vice President and A/E Division Manager, URS Corporation, Sacramento, CA,
[email protected]
4
Senior Staff Engineer, URS Corporation, Sacramento, CA , [email protected]
5
Senior Staff Engineer, URS Corporation, Sacramento, CA , [email protected]
Slope Stability of Levees
659
INTRODUCTION
Seepage and slope stability are two major analyses used in geotechnical evaluations of
existing levees. There are many independent factors that affect steady state seepage and
slope stability conditions. However, as pore water pressure distributions in the
embankment and foundation layers are incorporated in the slope stability analyses,
seepage conditions have a major impact on slope stability.
A series of sensitivity study results showing effects of different contributing factors on
underseepage based on Blanket Theory can be found in USACE TM 3-424 (USACE,
1956) and Turnbull and Mansur (1961a and 1961b). In preparation for this paper, about
325 sets of seepage and slope stability analyses were performed to evaluate the effects of
different factors on steady-state underseepage and through-seepage conditions and
subsequently their effects on slope stability. Each set includes seepage and slope stability
analyses for a given set of conditions using SEEP/W and Slope/W software (GeoSlope,
2008 and 2009). A levee geometry section was prepared in accordance with the USACE
Sacramento District’s criteria for levees along major rivers (USACE, 2008). This study to
assess the sensitivity of factors of safety (slope instability) and exit gradients
(underseepage) to levee material and geometry was performed under five conditions:





Case 1: Embankment strength under a range of net head conditions
Case 2: Hydraulic conductivity contrast between an embankment and a blanket layer
for different drained strengths of the embankment under different net head conditions
Case 3: Hydraulic conductivity contrast between a blanket and an aquifer for a range
of drained strengths of the blanket layer under different net head conditions
Case 4: Blanket layer thickness for a range of drained strengths of the blanket layer
under different net head conditions
Case 5: Aquifer thickness for a range of drained strengths of the blanket layer under
different net head conditions

All these cases were evaluated without a fine-grained layer at the channel bottom, i.e.
with an aquifer exposed at the channel bottom. Cases 2 through 5 were also evaluated
with a fine-grained layer of constant thickness at the channel bottom. Analyses with a
fine-grained layer at the channel bottom were evaluated under two different boundary
conditions at the waterside vertical faces: no flow and total head conditions. Three sets of
analyses, which resulted in higher exit gradients (i) and phreatic surface breakouts and
lower factors of safety, were further evaluated under remediation conditions for seepage
improvement measures only.
SEEPAGE AND SEEPAGE INDUCED SLOPE STABILITY CONDITIONS
Factors affecting underseepage, through seepage, and slope stability conditions are
interrelated when geotechnical evaluations of a levee system progress from seepage
evaluation to slope stability evaluation. Turnbull and Mansur (1961a and 1961b) have
formulated the initial understanding of underseepage conditions of levees based on their
660
Innovative Dam and Levee Design and Construction
groun
ndbreaking investigation
i
ns on Mississsippi Valleyy levees durinng the early 1950s.
Findiings from theese early inv
vestigations were
w the bassis of Appenndix B in US
SACE
Engin
neering Man
nual (EM) 11
110-2-1913, which proviides mathem
matical analyyses of
underrseepage and
d subsurfacee pressure (allso known aas Blanket Thheory). Accoording to EM
M
1110-2-1913, thee factors affeecting the un
nderseepage (See Figure 1) are:








Net
N head on levee,
l
H
Thickness,
T
z and vertical hydraulic co
onductivity of top stratuum (blanket), Kbr and Kbll
Thickness,
T
d and horizon
ntal hydraulicc conductiviity of perviouus substratuum, Kf
Distance
D
from
m riverside levee toe to river,
r
L1
Base
B
width of levee and berm,
b
L2
Length
L
of fou
undation and
d top stratum
m (blanket) laandward of llevee toe, L3
Distance
D
from
m landside leevee toe to effective
e
seeppage exit, X3
Distance
D
from
m effective source
s
seepaage entry to rriverside levvee toe, X1
Figure 1. Illusstration of Paarameters Afffecting Undderseepage
(F
Figure B1 fro
om EM 11100-2-1913)
Curreently, there is
i no embank
kment throug
gh seepage aanalytical prrocedure avaailable in EM
M
1110-2-1913 for levee design
n and constru
uction. USA
ACE’s Engineer Technicaal Letter
L 1110-2-556
6) “Risk Bassed Analysiss in Geotechnnical Engineeering for Suupport of
(ETL
Plann
ning Studies”” (USACE, 2003) presen
nts determinnistic modelss used for prrobabilistic
analy
yses of throu
ugh seepage. In these detterministic m
models, the fo
following parrameters aree
identified as affeccting through seepage ev
valuations:






Hydraulic
H
conductivity
Hydraulic
H
graadient, i
Porosity,
P
n
Critical
C
stresss c (the sheear stress req
quired for floowing waterr to dislodge a soil
particle)
p
Particle
P
size, expressed as
a some repreesentative siizes such as D50 or D85
Friction
F
angle or anglee of repose
Slopee Stability of
o Levees
6661
In the deterministic models presented in ETL 1110-2-556, gradient, critical tractive
stress, and particle size are used to determine whether shear stresses induced by seepage
head loss are sufficient to dislodge soil particles. The model uses the gradient and
hydraulic conductivity to determine whether a seepage flow rate is sufficient to carry
away or transport particles once they have been dislodged. Grain size and pore size
information may also be used to determine whether soils, once dislodged, will continue
to move (i.e., piping) or be caught in adjacent soil particles (i.e., plugging). Very fine
sands and silt-sized particles are among the most through seepage-susceptible materials.
Slope stability problems are associated with a decrease in shear strength, or an increase in
shear stress, or both. According to Duncan and Wright (2005), the predominant processes
for a decrease in shear strength are:










Increase in pore water pressure (i.e., decrease in effective stress)
Development of cracks through the soil near the crest of the slope
Swelling (increase in void ratio) of clays, especially highly plastic and heavily
overconsolidated clays
Development of slickensides in clays (highly plastic clays) as result of shear on
distinct planes of slip
Decomposition of clayey rock fills
Creep under sustained loads
Leaching, resulting changes in chemical composition of pore water as water seeps
through the voids
Strain softening
Strength loss due to weathering (various physical, chemical, and biological
processes)
Cyclic loading causing liquefaction
Duncan and Wright (2005) also identify the mechanisms through which shear stress can
increase in a slope. These are






Increase of loads at the top of the slope
Water pressure in cracks at the top of the slope
Increase in soil weight due to increased water content
Excavation at the bottom of the slope
Drop in water level at the base of a slope during rapid drawdown conditions
Earthquake shaking
Based upon evaluation of these factors, it can be readily concluded that majority of the
factors affecting slope stability are related to pore water pressure.
In the analyses of levee stability under steady state conditions, the following factors have
major impacts:
662





Height and slope inclinations
Total unit weight of soil layers
Effective stress shear strength parameters
Net head
Pore water pressure in steady state conditions
Whereas levee embankments subject to geotechnical evaluations usually range from 5
feet to 30 feet in height; these heights are small compared to dams or natural slopes,
which may range up to hundreds of feet in height. In the case of natural slopes and dams,
the imposed shear stress and shear strengths of an embankment and foundations, as well
as pore water pressure, are major contributors to slope stability. However, in case of a
levee embankment with moderate height, pore water pressure induced during steady state
flood conditions has significantly higher effect on slope stability.
CURRENT STATE OF GUIDELINES OR CRITERIA
FOR SEEPAGE AND STABILITY EVALUATIONS
Currently, seepage and slope stability of levees are evaluated in accordance with USACE
EM 1110-2-1913 and EM 1110-2-1902. The California Department of Water Resources
(DWR) has also developed Urban Levee Design Criteria (final draft, January 2012) for
California levees. These manuals and guidelines provide seepage and stability evaluation
criteria.
Underseepage may cause piping and erosion of materials from levee foundations due to
excessive pore water pressure under the blankets. Analytically, underseepage is
characterized by “Average Vertical Exit Gradient” at the toe of the levee through blanket
layer. Average vertical exit gradient is calculated as follows:
Average Vertical Exit Gradient =
Total Head Drop Across Blanket Layer
Thickness of Blanket Layer
Based upon USACE criteria, a calculated average vertical exit gradient value of 0.5 or
less at the toe of the levee at the design water surface is an acceptable criterion for
underseepage evaluations. An average vertical gradient value of 0.8 is identified as a
“Critical Exit Gradient,” which provides a factor of safety of 1 for a soil with unit weight
of 112.5 pcf. Considering the critical gradient value of 0.8, an average vertical exit
gradient of 0.5 provides a factor of safety of 1.6 for underseepage. However,
underseepage may occur at lower average vertical exit gradients. USACE’s TM 3-424,
Investigation of Underseepage and Its Control: Lower Mississippi Levees, describes a
relationship between a vertical exit gradient at a levee toe and the severity of
underseepage conditions observed during a 1950 flood in the Mississippi River. Table 1
summarizes these field observations.
663
Table 1. Average Vertical Exit Gradient vs. Seepage Conditions Trends
Observed During a 1950 Flood in the Mississippi River
Exit Gradient, i
Seepage Conditions
0 to 0.5
Light/No Seepage
0.2 to 0.6
0.4 to 0.7
0.5 to 0.8
Medium Seepage
Heavy Seepage
Sand Boils1
Source: ETL 1110-2-569 (Design Guidance for Levee Underseepage, May 2005)
1
The gradient required to cause sand boils varies considerably at the different sites, and relatively
low gradients were recorded near some sand boil areas. This may be due to the fact that at sites
where sand boils developed previous to the 1950 high water, only fairly low excess head may have
been required to reactivate boils in 1950, and as a relief of pressure occurs at the boil, readings of
piezometers near the boil may be somewhat lower than those farther from the boil (TM 3-424).
If a phreatic surface daylights on the landside of the slope of a levee under steady state
conditions, and embankment materials consist of low-plasticity soils (i.e., erodible soils),
it may indicate a potential for through seepage. Through seepage causes removal of
materials from levee embankments due to piping through low plasticity soils. Generally,
there is no specific criterion for through seepage in USACE design manuals.
According to USACE guidance for slope stability analyses of levees, static stability
analyses on a levee’s landside for long-term conditions assume steady state conditions. A
factor of safety of 1.4 or greater at design water surface elevations is considered
acceptable for steady state slope stability analyses. According to EM 1110-2-1902, pore
water pressures can be estimated from field measurements, hydrostatic pressure
computations for no-flow conditions, or steady-state seepage analyses techniques (i.e.,
via flow nets, finite element analyses, or finite difference analyses). Advances in seepage
and slope stability analysis software allow pore water pressure imports from steady state
seepage to stability analysis, and these analyses can be performed more easily than
before.
GEOLOGIC AND GEOMETRIC MODEL FOR SENSITIVITY ANALYSIS
An idealized geologic and geometric model was prepared to evaluate the sensitivity of
different factors on seepage conditions, and subsequently their effects on slope stability.
According to the USACE Sacramento District’s Geotechnical Levee Practice (USACE,
2008), “the minimum levee section should have a 3H:1V waterside slope, a minimum 20
feet wide crown for main line levees, major tributary levees, and bypass levees, a
minimum of 3H:1V landside slope, a minimum 20 feet landside easement, and a minimum
15 feet waterside easement. Existing levees with landside slopes as steep as 2H:1V may
be used in rehabilitation projects, if landside slope performance has been good.” Given
the USACE Sacramento District guideline and the purpose of evaluating the sensitivity of
different factors on seepage and seepage-induced slope stability, the following idealized
cross section was developed for the sensitivity analyses:
664
Figuree 2A. River with
w Levees on Both Siddes
Figure 2B. Idealized Seection withou
ut Fine-Graiined Blankett at Channel Bottom
Bottom
Figure 2C. Idealized Section with a Fine-Grainned Blanket at Channel B
Figure 2. Idealized
I
Seections and Boundary
B
Coonditions forr Sensitivity Study
Figurre 2A shows levee embaankments on both sides oof a channel.. An idealizeed section foor
analy
yses includess levee embaankment on one
o side exteending to thee middle of the channel..
As sh
hown on Figures 2B, thee idealized seection includdes a 50-foott-wide waterrside bench
and th
he aquifer laayer is conneected to wateer inflow froom the channnel. A no floow boundaryy
condiition at wateerside verticaal face was used
u
in sensiitivity analysses using secction shown
in Fig
gure 2B. To evaluate thee effects of fine-grained
fi
layer at the cchannel botttom,
Slopee Stability of
o Levees
6665
sensitivity Cases 2 through 5 were also evaluated with a waterside blanket of a constant
7.5-foot thick blanket at the bottom of the channel. An idealized section with a 7.5 feet
thick fine-grained blanket at the channel bottom is shown on Figure 2C. Two boundary
conditions (no flow and total head) at waterside vertical face were used in sensitivity
analyses using section shown in Figure 2C.
SENSITIVITY STUDY CASES
Case 1: Net Head and Embankment Strength
Case 1 was developed to evaluate seepage and stability conditions with increased net
head. The net head was varied to different ratios of the height of the levee. Slope stability
analyses were performed at each of these net head conditions with a range of drained
strength of the embankment. Case 1 sensitivity analyses were performed using a no flow
boundary condition at waterside vertical face. Table 2 summarizes seepage and stability
results and Figure 3 shows the average vertical exit gradient and factor of safety values
for different net head loading conditions.
As expected, these results indicate that with the increase of net head the average vertical
gradient increases and the factors of safety against slope stability decreases. The range in
slope stability factors of safety for a variation in embankment strength is narrow
compared to the steep increase in average vertical exit gradient.
Table 2. Summary of Case 1 Steady State Seepage and Slope Stability Results
Drained Shear Strength
Layer and
Thickness
(feet)
Soil Type
Net Head
0.08LEVEE
HEIGHT
(=1.5 feet)
0.21LEVEE
HEIGHT
(=3.75 feet)
0.42LEVEE
Embankment
Silty Clay
HEIGHT
18 feet
(=7.5 feet)
0.62LEVEE
HEIGHT
(=11.25 feet)
0.83LEVEE
HEIGHT (=15
feet)
Blanket
15 feet
Aquifer
20 feet
Aquiclude
20 feet
666
Hydraulic
Conductivity Horizontal, kh Cohesion
Intercept
(cm/sec)
(c') (psf)
1.0E-05
1.0E-05
1.0E-05
1.0E-05
1.0E-05
Effective
Friction
Angle, φ'
75
29
75
31
75
33
75
29
75
31
75
33
75
29
75
31
75
33
75
29
75
31
75
33
75
29
75
31
75
33
Silty Clay
1.0E-05
100
29
Sand
1.0E-03
0
33
Silty Clay
1.0E-05
150
31
Average Phreatic
Minimum
Surface
Vertical
Factor
Breakout
Exit
Safety under
Point
Gradient
Above Steady State
at
Landside Landside Conditions
Toe (feet)
Toe (i)
1.67
<0.1
0.0
1.75
1.80
1.63
0.15
0.5
1.68
1.72
1.51
0.30
1.8
1.55
1.59
1.37
0.45
3.6
1.40
1.44
1.20
0.61
6.3
1.23
1.26
Constant for all Case 1 Analyses
Fig
gure 3. Casee 1 Average Vertical
V
Exit Gradient (ii) and Factorrs of Safety (FOS) Plot
Case 2: Hydraullic Conducttivity Contrast Between
n Embankm
ment and Blanket
Case 2 was develloped to evalluate hydrau
ulic conductiivity contrasst between ann
embaankment and
d a blanket laayer. The em
mbankment’ss hydraulic cconductivity was varied
for a range of em
mbankment materials
m
with
h different ddrained strenngths and hyddraulic
uctivity valu
ues. The blan
nket, aquiferr, and aquicluude conditioons were keppt constant
condu
for alll cases. Table 3 summarrizes seepagee and stabiliity results annd Figure 4 ((4A to 4C)
show
ws the averag
ge vertical ex
xit gradient and
a factor off safety values for differeent net headd
condiitions with variable
v
hydrraulic condu
uctivity contrrasts betweenn embankmeent and
blank
ket.
Slopee Stability of
o Levees
6667
Layer and
Thickness (feet)
Soil Type
Case ID and
Contrasting
Soil Layers
Net Head
0.28LEVEE
HEIGHT
Case 2-I
(=5 feet)
Compacted
0.56LEVEE
Compacted
Clay
HEIGHT
Clay
Embankment
(=10 feet)
over Silty
Clay Blanket 0.83LEVEE
HEIGHT
(=15 feet)
Drained Shear
Average Phreatic Minimum
Strength
Surface
Vertical
Factor
Ratio of
Breakout
Exit
Safety
Horizontal
Point
Gradient
under
Hydraulic
Above
at
Steady
Conductivity of
Landside
Landside
State
Hydraulic
the Embankment
Toe (feet) Conditions
Toe (i)
Conductivity and Vertical
Horizontal,
Cohesion Effective
Hydraulic
kh (cm/sec)
Conductivity of Intercept Friction
(c') (psf) Angle, φ'
the Blanket
Case 2A: No blanket over channel
Kh(embankment)/
bottom with waterside (ws) no
Kv(blanket)
flow boundary
1.0E-06
0.4
125
32
0.28LEVEE
HEIGHT
(=5 feet)
Silty Clay
Embankment
18 feet
Case 2-II
Silty Clay
0.56LEVEE
Embankment HEIGHT
over Silty
(=10 feet)
Clay Blanket
0.83LEVEE
HEIGHT
(=15 feet)
1.0E-05
4
75
31
0.28LEVEE
HEIGHT
(=5 feet)
Case 2-III
Sandy Silt 0.56LEVEE
Sandy Silt Embankment HEIGHT
(=10 feet)
over Silty
Clay Blanket
0.83LEVEE
HEIGHT
(=15 feet)
1.0E-04
40
50
31
0.28LEVEE
HEIGHT
(=5 feet)
Case 2-IV
Silty Sand 0.56LEVEE
Silty Sand Embankment HEIGHT
over Silty
(=10 feet)
Clay Blanket
0.83LEVEE
HEIGHT
(=15 feet)
Blanket
15 feet
Aquifer
20 feet
Aquiclude
20 feet
668
1.0E-03
400
0
33
Silty Clay
1.0E-05
100
29
Silty Sand
1.0E-03
0
33
Silty Clay
1.0E-05
150
31
Average Phreatic
Vertical
Surface
Exit
Breakout
Gradient
Point
at
Above
Landside Landside
Toe (i)
Toe (feet)
Minimum
Factor
Safety
under
Steady
State
Conditions
Case 2B: 7.5 feet thick blanket
over channel bottom with ws no
flow boundary
(Case 2C: 7.5 feet thick blanket
over channel bottom with ws total
head boundary condition)
0.21
0.9
1.72
0.12
(0.19)
0.2
(0.9)
1.8
(1.74)
0.42
3.4
1.51
0.24
(0.38)
1.1
(2.9)
1.69
(1.55)
0.62
6.5
1.26
0.36
(0.57)
2.9
(5.9)
1.55
(1.32)
0.21
0.9
1.63
0.12
(0.19)
0.5
(0.9)
1.69
(1.64)
0.42
3.2
1.45
0.25
(0.38)
1.6
(2.7)
1.57
(1.47)
0.62
6.3
1.22
0.38
(0.58)
4.1
(5.9)
1.4
(1.25)
0.21
0.9
1.57
0.13
(0.19)
0.9
(0.9)
1.57
(1.56)
0.42
2.9
1.41
0.26
(0.39)
2.7
(2.9)
1.43
(1.41)
0.63
6.5
1.19
0.39
(0.58)
6.1
(6.3)
1.24
(1.21)
0.21
0.9
1.37
0.13
(0.19)
0.9
(0.9)
1.36
(1.36)
0.42
2.9
1.21
0.26
(0.39)
2.9
(2.9)
1.21
(1.20)
0.63
6.5
1.01
0.40
(0.58)
6.3
(6.5)
1.00
(1.00)
Constant for all Case 2 Analyses
Figure 4A. Case 2A
A – No Fine--Grained Blaanket over C
Channel Botttom
Fig
gure 4B. Casse 2B – 7.5 feet
f Thick Fiine-Grained Blanket oveer Channel B
Bottom with
Waterside No
N Flow Verrtical Face B
Boundary Coondition
Slopee Stability of
o Levees
6669
gure 4C. Casse 2C – 7.5 feet
f Thick Fiine-Grained Blanket oveer Channel B
Bottom with
Fig
Waterside
W
To
otal Head Vertical Face Boundary C
Condition
Fig
gure 4. Case 2 Average Vertical
V
Exitt Gradient (i)) and Factorrs of Safety ((FOS) Plots
Results for Case 2A indicate that the hyd
draulic conduuctivity of em
mbankment has little
was evaluatedd. Similarly,, the change
effectt on underseeepage condiitions in the range that w
in hydraulic cond
ductivity hass little effect on the heighht of the phrreatic surfacee breakout
abovee the landsid
de toe. Howeever, with th
he increase inn hydraulic cconductivityy values, the
quanttity of flow through
t
land
dside slope face
f
increasees. As the inccrease in hyddraulic
condu
uctivity valu
ue also indicaates transitio
ons from cohhesive fine-ggrained soilss to noncohessive coarse-g
grained soilss, the impactt in slope staability is maiinly due to a change in
the drrained streng
gth of emban
nkment soilss. For exampple, the diffeerence in sloppe stability
factorr of safety between a com
mpacted siltty clay (c’, φ’) embankm
ment and a sillty sand (φ’))
leveee is mainly due
d to the draained strengtths of the em
mbankment m
materials rathher than
underrseepage preessure and th
hrough seepaage phreatic surface breaakout.
s
an
nalyses weree evaluated uusing a consstant 7.5-foot-thick
Case 2B and 2C sensitivity
blank
ket above thee channel bo
ottom. Case 2B
2 results inndicate that tthe presence of finegrain
ned blanket overlying
o
thee channel bottom decreasses the averaage vertical exit gradient
when
n compared with
w Case 2A
A analyses reesults, proviided a no floow boundaryy condition iss
used on the waterrside verticaal face. Thesee improved sseepage connditions subssequently
ove the stabiility conditio
ons. Howeveer, as found oon Case 2C,, the positivee impact on
impro
670
Innova
ative Dam aand Levee D
Design and C
Constructioon
underseepage decreases if a total head boundary condition is used on the waterside
vertical face.
Case 3: Hydraulic Conductivity Contrast Between Blanket and Aquifer
Case 3 was developed to evaluate the hydraulic conductivity contrast between a blanket
and an aquifer layer. The hydraulic conductivity of the aquifer was varied with a constant
hydraulic conductivity of the blanket. Slope stability analyses were performed for each of
these cases with a range of drained strengths of the blanket layer. The embankment,
blanket, and aquiclude conditions were kept constant for all cases. Table 4 summarizes
seepage and stability results and Figure 5 (5A to 5D) shows the average vertical exit
gradient and factor of safety values for different net head conditions with variable
hydraulic conductivity contrasts between blanket and aquifer.
Results of Case 3A indicate that, with the increase in hydraulic conductivity contrasts
between blanket and aquifer (as indicated by the ratio between horizontal hydraulic
conductivity of aquifer, kh(aquifer) and vertical hydraulic conductivity of blanket, kv(blanket)),
the average vertical exit gradient increases and the phreatic surface breakout height
increases. These two conditions impact slope stability conditions by lowering factors of
safety.
Case 3B and 3C sensitivity analyses were evaluated using a constant 7.5-foot-thick
blanket overlying the channel bottom. Case 3B results indicate that the presence of a finegrained blanket overlying the channel bottom has a big impact on the average vertical
gradient at landside toe and the phreatic surface breakout, provided a no flow boundary
condition is used on the waterside vertical face. In these cases, the average vertical exit
gradient increases and then starts to drop (See Table 4), with a higher hydraulic
conductivity contrast between blanket and aquifer. However, as found on Case 3C, if a
total head boundary condition is used on the waterside vertical face, the results are closer
to the Case 3A conditions with exposed aquifer conditions at the channel bottom (i.e.,
there is little effect of the fine-grained blanket overlying the channel bottom).
671
Layer and
Thickness (feet)
Embankment
18 feet
Soil Type
Case ID and
Contrasting
Soil Layers
Net Head
Silty Clay
Hydraulic
Conductivity Horizontal,
kh (cm/sec)
Ratio of
Horizontal
Hydraulic
Conductivity of
the Aquifer and
Vertical
Hydraulic
Conductivity of
the Blanket
Kh(aquifer)
/Kv(blanket)
1.0E-05
0.28LEVEE
HEIGHT
(=5 feet)
Case 3-I
Silty Clay
Silty Clay Blanket over
Sandy Silt
Aquifer
0.56LEVEE
HEIGHT
(=10 feet)
1.0E-05
20
0.83LEVEE
HEIGHT
(=15 feet)
0.28LEVEE
HEIGHT
(=5 feet)
Case 3-II
Silty Clay
Silty Sand
Aquifer
Blanket
15 feet
0.56LEVEE
HEIGHT
(=10 feet)
1.0E-05
40
0.83LEVEE
HEIGHT
(=15 feet)
0.28LEVEE
HEIGHT
(=5 feet)
Case 3-III
Silty Clay
Silty Sand to
Sand Aquifer
0.56LEVEE
HEIGHT
(=10 feet)
1.0E-05
400
0.83LEVEE
HEIGHT
(=15 feet)
0.28LEVEE
HEIGHT
(=5 feet)
Case 3-IV
Silty Clay
Silty Clay
Blanket over
Sand Aquifer
0.56LEVEE
HEIGHT
(=10 feet)
1.0E-05
4000
0.83LEVEE
HEIGHT
(=15 feet)
Aquifer
20 feet
Aquiclude
20 feet
672
Sandy Silt
5.0E-05
SiltySand
1.0E-04
40
Silty Sand
to Sand
1.0E-03
400
Sand
1.0E-02
4000
Silty Clay
1.0E-05
Cohesion
Intercept
(c') (psf)
Effective
Friction
Angle, φ'
75
31
100
100
100
100
100
100
27
29
31
27
29
31
100
27
100
29
100
31
100
100
100
100
100
100
27
29
31
27
29
31
100
27
100
29
100
31
100
100
100
100
100
100
27
29
31
27
29
31
100
27
100
29
100
31
100
100
100
100
100
100
27
29
31
27
29
31
100
27
100
29
100
31
0
33
Average
Vertical
Exit
Gradient
at
Landside
Toe (i)
Phreatic
Surface
Breakout
Point
Above
Landside
Toe (feet)
Average
Minimum
Vertical
Factor Safety
Exit
under Steady Gradient
State
at
Conditions Landside
Toe (i)
bottom with waterside (ws) no flow
boundary
0.07
0.5
0.15
1.8
1.64
1.70
1.73
1.51
1.56
1.61
Phreatic
Minimum
Surface
Breakout Factor Safety
under Steady
Point
State
Above
Landside Conditions
Toe (feet)
Case 3B: 7.5 feet thick blanket over
channel bottom with ws no flow
boundary
(Case 3C: 7.5 feet thick blanket over
channel bottom with ws total head
boundary condition)
0.07
0.5
0.15
1.8
0.24
(0.24)
4.5
(4.5)
1.34
(1.34)
1.34
0.24
4.5
1.39
0.5
0.25
1.8
1.62
1.68
1.72
1.48
1.53
1.58
0.12
0.5
0.24
1.8
5.0
1.35
0.37
(0.39)
4.7
(5.0)
0.9
0.41
2.7
1.58
1.64
1.69
1.40
1.45
1.50
0.12
0.5
0.25
1.6
6.3
1.23
0.38
(0.58)
4.1
(5.9)
1.4
0.55
4.1
1.53
1.59
1.65
1.31
1.36
1.41
0.06
0.0
0.13
0.9
7.7
1.03
1.07
1.67
1.74
1.76
1.59
1.65
1.70
1.47
(1.04)
1.00
0.82
1.4
(1.26)
1.44
(1.29)
1.26
0.27
1.63
1.69
1.73
1.51
1.57
1.62
1.35
(1.22)
1.19
0.61
1.37
(1.35)
1.41
(1.39)
1.40
0.20
1.62
1.68
1.73
1.49
1.54
1.59
1.32
(1.31)
1.31
0.39
1.39
(1.39)
1.43
(1.43)
1.43
0.12
1.64
1.70
1.74
1.51
1.56
1.62
0.2
(0.79)
2.3
(7.4)
1.52
(1.07)
1.57
(1.10)
20
Shear strengh of aquifer and shear strength and hydraulic
conductivity of aquiclude constant for all cases
150
31
Slopee Stability of Levees
6773
Figure 5B: Case 3A-II, 3B-II, and 3C-II: Silty Clay Blanket over Silty Sand Aquifer
Figure 5A: Case 3A-I, 3B-I, and 3C-I: Silty Clay Blanket over Sandy Silt Aquifer
674
Innova
ative Dam aand Levee Design and Constructioon
Figure 5: Case 3 Average Vertical Exit Gradient (i) and Factors of Safety (FOS) Plots
Figure 5D: Case 3A-IV, 3B-IV, and 3C-IV: Silty Clay Blanket over Sand Aquifer
Figure 5C: Case 3A-III, 3B-III, and 3C-III: Silty Clay Blanket over Silty sand to Sand Aquifer
Case 4: Blanket Thickness
Case 4 was developed to evaluate the thickness of a fine-grained blanket layer. The
blanket thickness was varied while keeping the hydraulic conductivities of the
embankment, blanket, aquifer, and aquiclude constant. Slope stability analyses were
performed for each of these cases with a range of drained strengths of the blanket layer.
Table 5 presents a summary of the seepage and stability results and Figure 6 (6A to 6D)
shows the average vertical exit gradient and factor of safety values for different net head
conditions with variable blanket thickness and drained shear strengths of blanket.
Results of Case 4A indicate that with the increase in blanket thickness, the average
vertical exit gradient decreases. These conditions impact slope stability by increasing the
factors of safety. However, as the phreatic surface breakout remains almost the same in
all blanket thickness conditions, influence of the saturated condition of the embankment
on slope stability remains the same. This becomes more apparent with the increase in net
head on the levee, as the impact of phreatic surface breakout on slope stability increases
with increasing net head.
Case 4B and 4C sensitivity analyses were evaluated using a constant 7.5-foot-thick
blanket overlying the channel bottom. Case 4B results indicate that the presence of finegrained blanket overlying the channel bottom has a big impact on the average vertical
gradient at landside toe and the phreatic surface breakout, provided a no-flow boundary
condition is used on the waterside vertical face. In these cases, both the average vertical
exit gradient and the phreatic surface breakout decrease significantly compared to an
aquifer exposed at the channel bottom. However, as found on Case 4C, if a total head
boundary condition is used on the waterside vertical face, the results are closer to the
Case 4A conditions with exposed aquifer conditions at the channel bottom, (i.e., there is
little effect of the fine-grained blanket overlying the channel bottom).
675
Case ,ID, Layer
and Thickness
(feet)
Soil
Type
Net Head
Hydraulic
kh (cm/sec)
Average
Vertical Exit
Gradient at
Landside
Toe (i)
Cohesion
Intercept
(c') (psf)
Effective
Friction
Angle, φ'
Phreatic
Phreatic
Minimum
Minimum
Surface
Surface
Average
Factor
Breakout
Vertical Exit Breakout Factor Safety
Safety under Gradient at
under Steady
Point
Point
Steady State Landside
State
Above
Above
Conditions
Landside Conditions
Landside
Toe (i)
Toe (feet)
Toe (feet)
boundary
Embakment
(18 feet)
Silty
Clay
1.0E-05
75
31
1.0E-05
100
100
100
100
100
100
27
29
31
27
29
31
100
27
100
29
100
31
100
100
100
100
100
100
27
29
31
27
29
31
100
27
100
29
0.28LEVEE
HEIGHT
(=5 feet)
Case 4-I
Blanket
10 feet
0.56HEIGHT
0.66
MaxHEAD
0.56LEVEE
HEIGHT
(=10 feet)
0.83LEVEE
HEIGHT
(=15 feet)
0.28LEVEE
HEIGHT
(=5 feet)
0.56LEVEE
HEIGHT
(=10 feet)
Case 4-II
Blanket 15
feet
0.83HEIGHT
1.00MaxHEAD
1.0E-05
0.83LEVEE
HEIGHT
(=15 feet)
Silty
Clay
0.28LEVEE
HEIGHT
(=5 feet)
0.56LEVEE
HEIGHT
(=10 feet)
Case 4-III
Blanket
20 feet
1.11HEIGHT
1.33MaxHEAD
1.0E-05
0.83LEVEE
HEIGHT
(=15 feet)
0.28LEVEE
HEIGHT
(=5 feet)
0.56LEVEE
HEIGHT
(=10 feet)
Case 4-IV
Blanket
25 feet
1.39HEIGHT
1.67MaxHEAD
1.0E-05
0.83LEVEE
HEIGHT
(=15 feet)
0.9
0.59
3.2
1.56
1.62
1.67
1.36
1.42
1.46
0.88
6.3
1.12
0.21
0.9
0.42
3.2
1.58
1.63
1.69
1.40
1.45
1.49
0.62
6.3
1.22
31
1.25
1.59
1.65
1.70
1.42
1.47
1.52
100
27
100
29
0.16
0.9
0.32
2.7
0.48
6.3
1.25
100
31
1.29
27
29
31
27
29
31
1.59
1.65
1.70
1.43
1.48
1.53
100
27
100
29
100
31
1.0E-03
0
33
1.0E-05
150
31
1.6
0.54
(0.8)
3.8
(5.9)
0.12
0.5
0.25
1.6
0.38
(0.58)
4.1
(5.9)
0.09
0.5
0.19
1.4
0.29
(0.46)
4.1
(5.9)
0.08
0.5
0.16
1.8
0.24
(0.38)
4.1
(5.9)
1.22
100
100
100
100
100
100
Silty
Clay
0.35
1.18
27
29
31
27
29
31
Aquiclude
20 feet
0.5
1.14
100
Silty
Sand
0.17
1.08
100
100
100
100
100
100
Aquifer
20 feet
676
0.29
boundary
boundary condition)
0.13
0.9
0.27
2.7
1.24
0.40
5.9
1.27
1.31
1.63
1.69
1.74
1.51
1.56
1.61
1.35
(1.17)
1.40
(1.20)
1.44
(1.24)
1.63
1.69
1.74
1.52
1.57
1.62
1.35
(1.21)
1.40
(1.25)
1.44
(1.29)
1.63
1.70
1.73
1.51
1.57
1.62
1.35
(1.23)
1.40
(1.28)
1.44
(1.31)
1.63
1.70
1.74
1.51
1.57
1.62
1.35
(1.25)
1.40
(1.29)
1.44
(1.33)
Constant for all Case 4 analyses
6777
Figure 6B: Case 4A-II, 4B-II, and 4C-II: Blanket Thickness = 0.83 Levee Height
Figure 6A: Case 4A-I, 4B-I, and 4C-I: Blanket Thickness = 0.56 Levee Height
678
Innova
Figure 6: Case 4 Average Vertical Exit Gradient (i) and Factors of Safety (FOS) Plots
Fi
6D Case
C
4A
IV 4B-IV,
4B IV and
d 4C
IV Blanket
Bl k Thickness
Thi k
39 L
i h
Figure
6D:
4A-IV,
4C-IV:
= 11.39
Levee H
Height
Figure 6C: Case 4A-III, 4B-III, and 4C-III: Blanket Thickness = 1.11 Levee Height
CASE 5: AQUIFER THICKNESS
Case 5 was developed to evaluate the thickness of an aquifer. Aquifer thickness was
varied while keeping the hydraulic conductivities of the embankment, blanket, aquifer,
and aquiclude constant. Slope stability analyses were performed for each of these cases
with a range of drained strengths of the blanket layer. Table 6 presents a summary of the
seepage and stability results and Figure 7 (7A to 7E) shows the average vertical exit
gradient and factor of safety values for different net head conditions with variable aquifer
thickness.
Results of Case 5A indicate that with the increase in aquifer thickness and if the aquifer is
exposed at the channel bottom, both the average vertical exit gradient and phreatic
surface breakout increase. These conditions impact slope stability by decreasing factors
of safety.
Case 5A and 5B sensitivity analyses were evaluated using a constant 7.5-foot-thick
blanket overlying the channel bottom. Case 5A results indicate that the presence of finegrained blanket overlying the channel bottom has a big impact on the average vertical
gradient at landside toe and the phreatic surface breakout, provided a no-flow boundary
condition is used on the waterside vertical face. For example, the average vertical exit
gradient (i) drops from 0.4 (compared to 0.49 for case 5A) for an aquifer thickness of
0.33 times blanket thickness to 0.28 (compared to 0.70 for Case 5A) for an aquifer
thickness of 5.33 times blanket thickness. In these cases, both the average vertical exit
gradient and the phreatic surface breakout decrease significantly compared with an
aquifer exposed at the channel bottom. However, as found on Case 5C, if a total head
boundary condition is used on the waterside vertical face, the results are closer to the
Case 5A conditions with exposed aquifer conditions at the channel bottom (i.e., there is
little effect of the fine-grained blanket overlying the channel bottom).
679
Case ID, Layer,
and Thickness
(feet)
Embankment
18 feet
Blanket
15 feet
Case 5-I
Aquifer
Thickness
5 feet
0.33BLANKET
Soil Type
Net Head
Silty
Clay
Hydraulic
kh (cm/sec)
Aquifer
Strength
1.0E-05
1.0E-05
27
100
29
100
31
1.71
0.56LEVEE
HEIGHT
Silty
Sand to (=10 feet)
Sand
100
27
1.44
100
29
100
31
1.55
100
27
1.26
100
29
1.0E-03
c' = 0 psf
φ' = 330
Silty
Sand
Silty
Sand
Silty
Sand
Silty
Sand
680
Silty
Clay
0.49
2.5
5.4
1.66
1.50
1.30
31
100
27
100
29
100
31
1.70
0.56LEVEE
HEIGHT
(=10 feet)
100
27
1.42
100
29
100
31
1.52
100
27
1.22
100
29
100
31
0.28LEVEE
HEIGHT
(=5 feet)
100
27
100
29
100
31
1.69
0.56LEVEE
HEIGHT
(=10 feet)
100
27
1.39
100
29
100
31
1.49
100
27
1.18
100
29
1.0E-03
c' = 0 psf
φ' = 330
c' = 0 psf
φ' = 330
0.37
0.56
0.9
2.7
5.9
1.65
1.47
1.26
0.42
0.62
0.9
3.2
6.3
1.63
1.44
1.21
31
27
100
29
100
31
1.68
0.56LEVEE
HEIGHT
(=10 feet)
100
27
1.37
100
29
100
31
1.47
100
27
1.14
100
29
100
31
0.28LEVEE
HEIGHT
(=5 feet)
100
27
100
29
100
31
1.67
0.56LEVEE
HEIGHT
(=10 feet)
100
27
1.36
100
29
100
31
1.46
100
27
1.12
1.0E-03
c' = 0 psf
φ' = 330
100
1.0E-05
29
100
31
150
31
1.49
0.27
2.0
1.54
0.40
(0.45)
4.7
(5.2)
1.32
(1.29)
1.37
(1.33)
1.41
(1.37)
0.13
0.5
1.59
1.62
1.49
0.27
1.8
0.40
(0.51)
4.5
(5.4)
0.12
0.5
0.45
0.68
3.4
6.8
1.62
1.42
1.17
1.51
0.25
1.6
1.57
0.38
(0.58)
4.1
(5.9)
1.35
(1.21)
1.40
(1.25)
1.44
(1.29)
0.11
0.2
1.62
1.64
0.47
0.70
3.4
6.8
1.62
1.42
1.15
1.70
1.74
1.53
0.22
1.4
1.59
1.64
0.33
(0.65)
3.6
(6.5)
0.09
0.2
1.56
1.1
1.69
1.73
1.20
0.23
1.33
(1.25)
1.37
(1.30)
1.42
(1.34)
1.63
1.56
0.9
1.55
1.60
1.25
0.23
1.69
1.73
1.57
0.21
1.68
1.73
1.29
100
c' = 0 psf
φ' = 330
0.5
1.59
0.18
100
1.0E-03
1.62
0.13
1.34
0.28LEVEE
HEIGHT
(=5 feet)
0.83LEVEE
HEIGHT
(=15 feet)
Aquiclude
20 feet
0.32
0.7
100
1.0E-03
boundary
boundary condition)
1.60
0.16
0.28LEVEE
HEIGHT
(=5 feet)
0.83LEVEE
HEIGHT
(=15 feet)
Case 5-V
Aquifer
Thickness
80 feet
5.33BLANKET
31
100
0.83LEVEE
HEIGHT
(=15 feet)
Case 5-IV
Aquifer
Thickness
40 feet
2.66BLANKET
75
Case 5B: No blanket over channel
boundary
0.28LEVEE
HEIGHT
(=5 feet)
0.83LEVEE
HEIGHT
(=15 feet)
Case 5-III
Aquifer
Thickness
20 feet
1.33BLANKET
Effective
Friction
Angle, φ'
See Below See Below
0.83LEVEE
HEIGHT
(=15 feet)
Case 5-II
Aquifer
Thickness
10 feet
0.66BLANKET
Cohesion
Intercept (c')
(psf)
Average
Average
Phreatic
Minimum
Phreatic
Vertical
Vertical
Minimum
Surface
Factor
Surface
Exit
Exit
Factor
Breakout
Safety
Breakout
Gradient
Gradient
Safety under
Point Above
under
Point Above
at
at
Steady State
Landside Toe Steady State
Landside Toe
Landside
Landside
Conditions
(feet)
Conditions
(feet)
Toe (i)
Toe (i)
1.38
(1.16)
1.43
(1.20)
1.48
(1.24)
1.66
1.72
1.76
1.56
0.18
1.1
1.62
1.67
0.28
(0.72)
3.2
(7.0)
1.18
1.42
(1.11)
1.47
(1.14)
1.52
(1.17)
Constant for all Case 5 analyses
6881
Figure 7B: Case 5A-II, 5B-II, and 5C-II: Aquifer Thickness = 0.66 Blanket Thickness
Figure
7A:
5A-I,
5C-I:
= 00.33
Fi
7A Case
C
5A
I 5B-I,
5B I and
d 5C
I Aquifer
A if Thickness
Thi k
33 Blanket
Bl k Thickness
Thi k
682
Innova
Figure 7D: Case 5A-IV, 5B-IV, and 5C-IV: Aquifer Thickness = 2.66 Blanket Thickness
Figure 7C: Case 5A
5A-III,
III 5B-III,
5B III and 5C-III:
5C III: Aquifer Thickness = 11.33
33 Blanket Thickness
Figu
ure 7E. Case 5A-V, 5B-V
V, and 5C-VA
Average Verrtical Gradieent ( i) and F
FOS Plots foor
Aquiffer Thickness = 5.33Blannket Thickneess
Figure 7. Caase 5 Averag
ge Vertical E
Exit Gradiennt (i) and
Factors of Safety (FOS
S) Plots
Slopee Stability of
o Levees
6883
REMEDIAT
R
TION SECT
TIONS WIT
TH SEEPAG
GE MITIGA
ATION ME
EASURES
To ev
valuate the effects
e
of seeepage conditions on sloppe stability, tthree sectionns with higheer
underrseepage and
d phreatic su
urface breako
outs and low
wer factors of safety weree evaluated
with seepage mitigation meassures. Thesee three analysis sets are C
Case 3A withh Silty Clay
blank
ket over Sand
d aquifer, Caase 4A with a blanket thhickness of 00.56 times levee height,
and Case
C
5A with
h aquifer thicckness of 5.3
33 times blaanket thickneess. In the firrst 2 sectionns
(Casees 3A and 4A
A), a cutoff wall
w was useed as a seepaage mitigatioon measure, and in the
third case, a drain
ned seepage berm was used considerring thick aqquifer. In all these three
sectio
ons, improveed seepage conditions
c
also improvedd slope stability conditioons
signifficantly. Tab
bles 7 throug
gh 9 present the assumpttions and ressults of thesee three
comp
parisons and Figures 8 th
hrough 10 in
nclude the se epage and sttability outpputs from
SEEP
P/W and SLO
OPE/W.
Table 7. Assu
umptions and
d Results of Case 3A Secction with
Seepage Mitigation
M
M
Measure
An exam
mple of Case 3 with Seepage
S
Mitigation by using
Fully Pe
enetrating SB Cutoff Wall
Existing Conditio
on
Layer and
a
Thickn
ness
(feett)
Soil Type
H
Net Head
Hydraulic
Anisotrphy
Conductivity Ratio,
Horizontal,
kh/kv
kh (cm/sec)
Cohesion
Intercept,
(c') (psff)
Effective
Friction
Angle, φ'
Reme
ediated Condition
Phreatic
Ave
erage
M
Minimum
Surface
Average
Factor
Vertic
cal Exit Breakout
Vertical Exit
Point
Grad
dient at
Saffety under Gradient at
Above
Landside
Ste
eady State Landside
Landside Co
Toe (i)
oe (i)
onditions
To
Toe (feet)
Phreatic
Minimum
m
Surface
ety
Breakout Factor Safe
Point
under Steady
Above
State
Landside Condition
ns
Toe (feet)
No blanket over cha
annel bottom with ws no flow boundary
Embank
kment
Silty Clay
18 feet
Blank
ket
15 feet
Aquiffer
20 fee
et
Aquiclu
ude
20 feet
1.0E-05
4
75
31
1.0E-05
4
100
27
Sand
1.0E-02
4
0
33
Silty Clay
1.0E-05
4
150
31
Silty Clay
LEVEE
0.83L
HEIGHT
(=15 feet)
Rebuilt section above levee degrade fo
or cutoff wall:
Compacted Clay: kh = 10-6 cm/sec, kh/kv = 4, c'
c = 100 psf, φ' = 32°
Cuto
off Wall: kh = 10-7 cm/se
ec
0
0.82
7.7
1.00
<0.1
No
Breakout
1.58
Figuree 8. Seepagee and Stabilitty Results foor an Exampple of Case 33A
withoutt and with Seepage Mitiggation Meassure
684
Innova
Design and C
Constructioon
Table 8. Assu
umptions and
Seepage Mitigation
M
M
Measure
An exam
S
Mitigation by using
Fully Pe
enetrating SB Cutoff Wall
Existing Conditio
on
Layer and
a
Thickn
ness
(feett)
Soil Type
Net Head
H
Hydraulic
Anisotrphy
Conductivity Cohesion
Ratio,
Horizontal,
(
Intercept (c')
kh/kv
kh (cm/sec)
(psf)
Drained
Effective
Friction
Angle, φ'
Ave
erage
Vertic
cal Exit
Grad
dient at
Landside
oe (i)
To
Reme
ediated Condition
Phreatic
M
Minimum
Surface
Average
Vertical Exit
Factor
Breakout
Point
Above
Ste
eady State Landside
Toe (i)
Landside Co
onditions
Toe (feet)
Phreatic
Minimum
m
Surface
ety
Point
under Steady
Above
State
Landside Condition
ns
Toe (feet)
No blanket over cha
annel bottom with no flow ws boundary
Embank
kment
(18 fe
eet)
Silty
Clay
Blank
ket
10 fe
eet
0.56HEIGHT
0.66 Max
xHEAD
Silty
Clay
Aquiffer
20 feet
Aquiclu
ude
20 fe
eet
Silty
Sand
Silty
Clay
1.0E-05
4
75
31
Rebuilt section above
a
levee degrade fo
or cutoff wall:
Compacted Clay: kh = 10
C
1 -6 cm/sec, kh/kv = 4, c'
c = 100 psf, φ' = 32°
Cuto
off Wall: kh = 10-7 cm/se
ec
LEVEE
0.83L
HEIG
GHT
(=15 feet)
1.0E-05
4
100
27
1.0E-03
4
0
33
1.0E-05
4
150
31
0
0.88
6.3
1.08
0.21
No
Breakout
1.56
Figuree 9. Seepagee and Stabilitty Results foor an Exampple of Case 44A
Slopee Stability of
o Levees
6885
Table 9. Assu
umptions and
Seepage Mitigation
M
M
Measure
An exam
S
Mitigation by using
150 feet wide Seepage Berm
m
Existing Conditio
on
Hydraulic
Anisotrphy
Conductivity Ratio,
Horizontal,
kh/kv
kh (cm/sec)
Layer and
a
Thickn
ness
(feett)
Soil Type
Embank
kment
18 fe
eet
Silty
Clay
1.0E-05
Blank
ket
15 feet
Silty
Clay
Aquiffer
Thickn
ness
80 feet
5.33BLAN
NKET
Silty
Sand
Aquiclu
ude
20 fe
eet
Silty
Clay
Net Head
H
Cohesion
Intercept,
(c') (psff)
Effective
Friction
Angle, φ'
4
75
31
1.0E-05
4
100
27
1.0E-03
4
0
33
1.0E-05
4
150
31
Reme
ediated Condition
Phreatic
Ave
erage
Minimum
M
Average
Surface
Factor
Vertic
cal Exit Breakout
Vertical Exit
Point
Grad
dient at
Above
Landside
Ste
eady State Landside
Landside Co
Toe (i)
To
oe (i)
onditions
Toe (feet)
Phreatic
Minimum
m
Surface
ety
under Steady
Point
State
Above
Landside Condition
ns
Toe (feet)
No blanket over cha
annel bottom with ws n o flow boundary
LEVEE
0.83L
HEIG
GHT
(=15 feet)
Seepage Berm: kh = 10-3 cm/sec, kh/kv = 4
4, c' = 0, φ' = 32°;
0
0.70
6.8
1.13
0.23
No
(0.58 at
Breakout
Berm Toe)
1.74
Figuree 10. Seepage and Stabiliity Results ffor an Exampple of Case 55A
686
Innova
Design and C
Constructioon
CONCLUSIONS
The results of the sensitivity analyses performed under five different sensitivity cases can
be summarized as follows:

The hydraulic conductivity contrasts between embankment and blanket have low
impact on the average vertical exit gradient and phreatic surface breakout. In this
study, the embankment hydraulic conductivities were varied with a constant blanket
hydraulic conductivity. However, the quantity of flow increases with the coarsergrained embankment and steady state stability factor of safety drops due to low or no
cohesion and low effective stress in coarser-grained materials.

The hydraulic conductivity contrasts between blanket and aquifer have a large impact
on the average vertical exit gradient and phreatic surface breakout. In this study, the
aquifer hydraulic conductivities were varied with a constant blanket hydraulic
conductivity. The average vertical exit gradient and phreatic surface breakout
increase with increase in the ratio between horizontal hydraulic conductivity of the
aquifer, kh,blanket and the vertical hydraulic conductivity of the blanket, kv,blanket.

Thin fine-grained blanket may create an unsafe condition for underseepage. In this
study, blanket thicknesses were varied between 0.66 times maximum net head and
1.67 times maximum net head and the average vertical exit gradient dropped from an
unacceptable to acceptable range.

Average vertical exit gradients increase with increase in the aquifer thicknesses. In
this study, aquifer thicknesses were varied between 0.33 times blanket thickness and
5.33 times blanket thickness and the average vertical exit gradient increased from a
marginal range to an unacceptable range.

The resulting range in slope stability factors of safety for a variation in embankment
strength (for this study: ’ range between 290 and 330 with a c’ = 75 psf) is narrow
compared to the steep increase in average vertical exit gradient with net head
increase.

The resulting range in slope stability factors of safety for a variation in blanket
strength (for this study: ’ range between 270 and 310 with a c’ = 100 psf) is narrow
compared to the steep increase in average vertical exit gradient with net head
increase.

Assumption of boundary condition of the waterside vertical face is important if a
fine-grained blanket is present over the channel bottom. A fine-grained blanket at the
channel bottom can positively impact the underseepage conditions by lowering the
average vertical exit gradient if a no-flow boundary condition is used on the
waterside vertical face. However, if a total head boundary condition is used on the
waterside vertical face, the positive effects of the presence of the fine-grained blanket
at the channel bottom reduce and the results are closer to aquifer exposed at the
channel bottom.

If the low factor of safety is driven by seepage forces, a seepage mitigation measure
should also improve the slope stability factor of safety. In this study, 3 sets of
687
analyses with high average vertical exit gradients and phreatic surface breakout were
analyzed by using seepage mitigation measures such as cutoff walls and seepage
berms. In all these cases, the improvement of the seepage conditions improved the
factor of safety even with the lower bound of the drained strength of the blanket.

In most cases, a reduction in slope stability is mainly due to adverse seepage
conditions. Therefore, improvement of seepage conditions also increases slope
stability. Exceptions to these conditions include levees founded on soft organic soils
or fissured highly plastic clay, where stability of an existing levee is mainly
dependent on the strengths of embankment and foundation layers, and where seepage
mitigation measures may not improve the stability conditions to an acceptable level.
In most cases, levees show seepage distress before they fail indicating a deteriorating
seepage conditions during flood. Therefore, it is very important to have a greater
understanding of the conditions affecting both underseepage and through seepage. Data
collection efforts should focus more on collecting information about factors that affect
seepage conditions so an effective mitigation measure can be designed for levees with
seepage and seepage-driven slope stability conditions.
ACKNOWLEDGEMENTS
The authors would like to thank Francke Walberg, PE of URS (Overland Park, Kansas)
and Derek Morley, PE of URS (Sacramento) for reviewing the paper and providing
valuable suggestions during preparation of this paper.
REFERENCES
California Department of Water Resources (CA DWR). 2012 . Urban Levee Design
Criteria Final Draft. January, 2012.
Cedergren, H.R. 1989. Seepage, Drainage, and Flow Nets. Third Edition. John Wiley &
Sons, Inc. New York, New York.
Duncan, J. M. and Wright, S. 2005. Soil Strength and Slope Stability. John Wiley &
Sons, Inc. Hoboken, New Jersey.
Geo-Slope International Ltd (Geo-Slope), 2008. Stability Modeling with SEEP/W 2007
Version: An Engineering Methodology. Fourth Edition Calgary, Alberta, Canada.
November.
Geo-Slope International Ltd (Geo-Slope), 2009. Seepage Modeling with SEEP/W 2007:
An Engineering Methodology. Calgary, Alberta, Canada. May.
Turnbull, W. and Mansur, C. 1961a. Design of Underseepage Control Measures for
Dams and Levees. American Society of Civil Engineers (ASCE) Transaction Paper No.
126(1).
688
Turnbull, W. and Mansur, C. 1961b. Underseepage and Its Control: A Symposium.
American Society of Civil Engineers (ASCE) Transaction Paper No. 3247.
United States Army Corps of Engineers (USACE). 2008. Geotechnical Levee Practice.
Sacramento District REFP10L0.
USACE. 1956. Investigation of Underseepage and Its Control: Lower Mississippi River
Levees. Technical Memorandum 3-424. Army Engineer Waterways Experiment Station,
Vickburg, Mississippi. October.
USACE. 2000. Engineering and Design – Design and Construction of Levees. EM 11102-1913.
USACE. 2003. Recommendations for Seepage Design Criteria, Evaluations, and Design
Practices, Report Prepared for the Sacramento District USACE. July.
USACE. 2003. Engineering and Design – Slope Stability. EM 1110-2-1902.
USACE. 2003. Risk-Based Analysis in Geotechnical Engineering for Support of Planning
Studies. ETL 1110-2-556.
USACE. 2005. Engineering and Design – Design Guidance for Levee Underseepage.
ETL 1110-2-569.
689

Slope Stability of Levees 659 IT IS SEEPAGE INDEED — A

Transcription

Similar documents

promise - theraineysisters

and spend the afternoon enjoying: And so much more!

New Jan Group Supply Service

BODYWORK WITH REBOZO Within tradition, the rebozo in México

Bili Blanket Phototherapy System

Blanket®

8.5x11_Office Leasing Brochur

Welding Blankets

FirSt light™ FirSt light