ce-442 l-3 to 10 embankment dams.pdf
Post on 16-Jan-2016
25 Views
Preview:
TRANSCRIPT
EMBANKMENT DAMS
(Nurek earth and Rockfill dam, Tajikistan)
Contents
1) Introduction
2) Types of embankments dams
3) Causes of Failure
4) Design considerations
5) Seepage analysis and control
6) Stability analysis
7) Construction techniques
1) INTRODUCTION
• Most ancient type of embankments
• Can easily be constructed on earth foundations
• More susceptible to failure as compared to gravity dams
Hirakund dam (Composite structure of earth, concrete and masonry.)
4.8 Km long 60.0 m River Mahanadi Orrisa
Beas Dam or Pong Dam 133 m on the river Beas
Ramganga Dam (1962) 125.5 m Near Kalagarh, earth and rockfill
Tehri Dam 260.0 m, Earth and Rockfill
Narek Dam 300.0 m Earth dam built in late 1970s on the Vaksh river
Narek, Tajiskistan
SOME OF THE EMBANKMENT DAMS
2a) TYPES OF EARTHEN EMBANKMENT DAMS
1. Homogeneous dam
• constructed of a single material
• slope pitching is provided
• Such dams are of moderate and low heights
• Drainage filter is provided to arrest the phreatic line
(a) Homogenous earthen dam
with toe drain
(b) Homogenous earthen dam
with horizontal blanket
(c) Homogenous earthen dam
with inclined chimney connected
to horizontal blanket
2. Zoned embankment type
• Provided with a central impervious core which control the seepage
• Filter controls the piping action
• Outer zone gives stability to the central zone
• Pure clay is not suitable for core due to shrinkage and swelling
properties of pure clay. However, clay mixed with fine sand or fine gravel
are suitable.
• Silts or silty clays used as the satisfactorily central core material
• Freely draining materials such as coarse sands and gravels are used in
outer shell
(a) Zoned dam with central vertical clay
core and toe drain
(b) Clay core zoned dam with central
vertical core and chimney filter with
horizontal blanket
(c) Inclined clay core zoned dam with
chimney filter with horizontal blanket
3. Diaphragm type embankments
• Have a thin impervious core, which is surrounded by earth or rock fill
• Diaphragm made of impervious soil, cement, steel, timber etc. – control
seepage and tied to the bed rock or to a very impervious foundation
• If the thickness of the diaphragm at any elevation is less 10 m or less
than the height of the embankment above the corresponding elevation,
the dam is called diaphragm type.
2b) TYPES OF ROCKFILL EMBANKMENT DAMS
1. Central vertical clay core
2. Inclined clay core with drains
3. Decked with asphalt or concrete membrane on upstream face with drains
(a) Rock-fill dam with vertical clay core,
chimney filter and horizontal blanket
(b) Rock-fill dam with inclined clay core,
chimney drain and horizontal blanket
(c) Decked rock-fill dam with upstream
asphaltic or concrete membrane with
chimney drain and horizontal blanket.
The phreatic line is for the small
amount of water that leaks through
the cracks of the upstream membrane
3) CAUSES OF FAILURE OF EARTH DAMS
1. Hydraulic failures
2. Seepage failures, and
3. Structural failures
(i) Hydraulic Failures
(a) Overtopping of dam resulting in washout
- sufficient free board is required
(b) Erosion of upstream face by waves
breaking on the surface
- Stone pitching or riprap should be provided
(c) Cracking due to frost action
- frost in the upper portion of dam cause heaving and cracking of soil
- additional free board of the order of 1.5 m be provided
(e) Erosion of downstream toe
-Erosion due to cross-currents from
spillway-controlled by side walls of
spillway
- erosion due to tail water – controlled
by providing stone pitching or riprap
upto the height of tail water depth
(d) Erosion of downstream face by
impact of rain and resulting sheet flow –
gully erosion
- Controlled by filling the cuts, grassing
the slopes, providing proper berms
(ii) Seepage Failures
(a) Piping through foundations
due to highly permeable cavities or
fissures or strata of coarse sand or
gravel
(b) Piping through dam
flow channels develop due to faulty construction, insufficient
compaction, cracks developed in embankment due to
foundation settlement, shrinkage cracks, animal burrows etc.
(c) Seepage by the outer surface
of conduit, may lead to
progressive piping
(d) Sloughing of downstream toe
due to saturation of soil (pore
water pressure)
(iii) Structural Failures
(a) Foundation slide
due to weak foundation like soft soil
such as fine silt, soft clay etc.
Failure of upstream slope caused by
failure of foundation
Failure of downstream slope caused by
failure of foundation
(b) Failure of upstream face due to
sudden drawdown
(c) Failure of downstream face during
full reservoir operation also being
steep
(c) Excessive settlement of dam
foundation
4) DESIGN CONSIDERATIONS
(A) Free Board
(B) Top Width
(C) Upstream and Downstream Slopes
(D) Suitability of Soils for Construction of Earth Dams
(E) Design of Filters
(F) Slope Protection
(G) Seepage Control
(A) FREE BOARD
(distance between max. Reservoir level and top of he dam)
(i) Wave height by Moliter equation
Free board = 1.5 hw
(km/hr) velocity Wind (km); Fetch(m);
km 32 for F 032.0
km 32 for F 271.0763.0032.0 4/1
VFh
VFh
FVFh
w
w
w
(ii) US BUREAU OF RECLAMATION (USBR) Recommendations
Spillway type dam height Min. free board above MWL
Uncontrolled Any height 2-3 m
Controlled < 60 m 2.5 m above top of gate
Controlled > 60 m 3.0 m above top of gate
Additional 1.5 m for frost action.
(iii) Saville method (IS 10635: 1993)
Normal free board = Free board above the FRL
Minimum free board = Free board above the MWL
Procedure for Computation of Normal Free Board
(1) Effective Fetch
Draw a line AB with A on dam axis and B on FRL so as to cover the maximum
reservoir water spread area within 450 on either side of line AB
Draw 7 radials at 60 interval on each side of AB
Effective Fetch
cos
cosXFe
(2) Compute wind velocity on water
Read wind velocity on land from IS 875 for 50 year return period for the region
Wind velocity on water = αv × wind velocity on land
Fe (km) αv
1 1.1
2 1.16
4 1.24
6 1.27
8 1.30
>10 1.31
(3) Compute wave height
(m) (s); (m/s); (m); (m);
56.1
length wave
45.0
period waveand
0026.0
2
0.25
2
0.47
22
LTVFeh
TL
V
gF
V
gT
T
V
gF
V
gh
w
e
ew
Design wave height, hd = 1.67 hw
(4) Compute wave run up R on smooth surface from the following chart
correspond to L
R/h
d
Type of Pitching Roughness coefficient
Cement concrete surface 1.0
Flexible brick pitching 0.8
Hand place riprap
Laid flat 0.75
Laid with projection 0.60
Dumped riprap 0.50
Run up on rough surface = Run up on smooth surface × roughness coefficient
If corrected R < hd; adopt R = hd
Wave run up on the rough surface
(5) Wind set up computation
Wind set-up is the result of piling up of the water on one end of the reservoir on account
of the horizontal driving force of the blowing wind.
D
FSWind
62000
V upset
2
S (m); V (km/h); F(km); D(m)=Average depth of water along fetch length F
Free board = R+S
If free board < 2 m; adopt 2m
Top of dam = FRL+ Normal free board
Minimum Free Board at MWL
• Calculate effective fetch at MWL
• Consider ½ to 2/3 wind velocity on land for computation of hw
• Take hd = 1.27 hw
• Minimum free board => 1.5 m
(B) TOP WIDTH
Top width, A > 3 m
A =H/3+3 for low dams 15-20 m height (USBR)
)();(
36.3
Code Japanese
H3.6 A
m 150 for H USBR
3/1
1/3
mHmA
HA
unit SI Use 1.5H1.65 A
m 30 For H
2.0 H0.55 A
m 30 For H
1/3
1/2
H
H = height of dam (m) above stream bed
(C) UPSTREAM AND D/S SLOPES (Terzaghi’s Side slopes)
Types of material U/s (H:V) D/s (H:V)
Homogeneous well graded 2.5:1 2:1
Homogeneous coarse silt 3:1 2.5:1
Homogeneous silt clay
(a) H < 15 m 2.5:1 2:1
(b) H > 15 m 3:1 2.5:1
Sand or sand and gravel with a central clay core 3:1 2.5:1
Sand or sand and gravel with a RC diaphragm 2.5:1 2:1
G = Gravel; W = well graded; P = Poorly graded; C = clay; S = Sand
M = silt; O = Organic; Pt = highly organic soil; H = high compressibility;
I = Medium Compressibility; L= low compressibility
(D) SUITABILITY OF SOILS FOR CONSTRUCTION OF EARTH DAMS
(E) DESIGN OF FILTERS (IS Code 9429-1999)
The filter material used for drainage system shall satisfy the following criteria:
• Filter materials shall be more pervious than the base materials;
• Filter materials shall be of such gradation that particles of base material
do not totally migrate through to clog the voids in filter material; and
• Filter material should help in formation of natural graded layers in the
zone of base soil adjacent to the filter by readjustment of the particles
Determination of Gradation of base material
Category Percentage finer than 75 micron
1 > 85%
2 40-85%
3 15-39%
4 < 15%
Note: Wherever the base soil in categories 1, 2 and 3 contains particles larger than 4.75
mm, the percentage of particles passing 4.75 mm shall be adjusted to 100 percent.
(a) Minimum D15 (f)
D15 (f) 5D15 (b) 0.1mm
(b) Maximum D15 (f)
D15 (f) should not be less than 0.2mm
Base soil category Criteria
1 D15 (f) ≤ 9D85 (b)
≥ 0.2mm
2 D15 (f) ≤ 0.7 mm
3 D15 (f) ≤ (40-A)/(40-15)*(4D85 (b)-0.7 mm)+0.7 mm
4 D15 (f) ≤ 4D85 (b)
A is the percent passing the 75 micron sieve after re-grading
(c) To minimize segregation, filters should have relatively uniform grading. D90
(f) should be less than 20 mm- to minimize segregation. Limit of D10(f) and
D90(f) are given below
D10 (f) (min) mm D90 (f) max (mm)
< 0.5 20
0.5-1.0 25
1.0-2.0 30
2.0-5.0 40
5.0-10 50
10-50 60
Filters should have a maximum particle size of 75 mm. Material passing the
75 micron sieve shall not exceed 5 percent.
Example (Two layer of filters)
FINE FILTER
Particle size gradation of base material is given below:
As size of material is more than 4.75 mm, it is re-graded by a factor 100/88)
From the particle size distribution graph
D15(b) = 0.0022 mm;
D85(b) = 1.5 mm
Fine Filter
Percentage finer than 75 micron is 33%, thus the category of base material is III
Minimum D15 (f)
D15 (f) 5D15 (b)=5*0.0022=0.011 which is less than 0.1mm
Adopt D15 (f) = 0.1 mm
Maximum D15 (f)
A = 33%
D15 (f) ≤ (40-33)/(40-15)*(4*1.5-0.7+0.7
= 2.2 mm which is greater than 0.2 mm OK
Adopt maximum particle size of 75 mm and material passing the 75 micron
sieve =5 %
From the graph
Lower limit D10(f) = 0.09 mm correspondingly D90(f) = 20 mm
Upper limit D10(f) = 0.5 mm correspondingly D90(f) = 25 mm
However, the available material is in the range of curve 5 & 6, as shown in the graph.
Percentage finer than 75 micron is less than 15%, thus the category of base
material is IV
Minimum D15 (f) – based on upper limit of fine filter (Curve 6)
D15 (f) 5D15 (b)=5*1=5 mm which in more than 0.1 mm
Adopt D15 (f) = 5 mm
Maximum D15 (f) - based on lower limit of fine filter (Curve 5)
D15 (f) ≤4D85(b)=4*3=12 mm which is greater than 0.2 mm OK
Adopt maximum particle size of 75 mm and material passing the 75 micron
sieve =5 %
From the graph
Lower limit D10(f) = 1 mm correspondingly D90(f) = 30 mm
Upper limit D10(f) = 1 mm correspondingly D90(f) = 30 mm
COARSE FILTER
Coarse Filter
DESIGN OF FILTER
IS Code 9429-1980
(i) D15 of filter/D85 of base < 5
(ii) 4 < D15 of filter/D15 of base < 20
(iii) D50 of filter/D50 of base < 25
(iv) Gradation curve of filter should be nearly parallel to the gradation curve of
base material
0
10
20
30
40
50
60
70
80
90
0.01 0.1 1 10 100 1000
Perc
enta
ge f
iner
Grain size (mm)
Base material F1 F2
Sand layer filter (F1)
d15 = 0.78 mm
d50 = 1.90 mm
d85 = 3.91 mm
Gravel layer filter (F2)
d15 = 8.75 mm
d50 = 21.24 mm
d85 = 43.74 mm
Base material (B)
d15 = 0.07 mm
d50 = 0.17 mm
d85 = 0.35 mm
Example
(F) SLOPE PROTECTION
(a) PROTECTION OF UPSTREAM SLOPE
Upstream protection is required against the wave action. The dumped rock
riprap is preferred type of protection.
DUMPED STONE RIPRAP (Singh & Varhney 2004)
Design of the dumped stone riprap is related to the criteria for the selection of
rock size and thickness of the rip rap layer directly to the design wave height.
(a) For embankment slopes 2:1 to 4:1 dumped riprap shall meet the following
criteria:
Placing riprap with hydraulic excavators
(Hand-placed riprap)
(b) Riprap shall be well graded from a maximum size at least 1.5 times the
average rock size to 2.5 cm spalls suitable to fill voids.
(c) Rip rap blanket shall extend to at least 2.4 m below the lowest low water.
(d) Filter shall be provided between the riprap and embankment to meet the
following criteria:
No filter is required if embankment material meets the above requirements for
the D85 size.
Thickness of riprap layer should be at least 1.5 times the size of the average
(D50) rock of weight W50.
HAND PLACED RIPRAP – labor cost high, now rarely being used
SOIL-CEMENT SLOPE PROTECTION
• Provided, if suitable rock for riprap is unavailable at the site.
• Consisted of a series of approximately horizontal layers of soil-cement
compacted in stair-step fashion up the embankment slope. The layer is
usually 2 to 3 m wide, compacted 15 cm vertical thickness.
• The most efficient construction 100 % of the soil should pass the 50 mm
sieve, at least 55% should pass the 4.75 mm sieve and between 5 and 35 %
should pass the 75 micron sieve.
• The cement content varies from about 7 to 15 % by volume of soil-cement.
2 to 3 m
Thickness 15 cm
U/s face of dam
(b) PROTECTION OF DOWNSTREAM SLOPE
Needed against erosion by rain-water and sometimes by wind also. If d/s
slope is rock – no protection required.
Turfing is provided.
Horizontal drain be provided at suitable interval and be joined with vertical
drain
5a) SEEPAGE THEORY
EquationLaplace--- 0
; 0 equation Continuity
;
Kh soil isotropicfor
;
2
2
2
2
y
h
x
h
y
v
x
u
y
hKv
x
hKu
yv
xu
x
y
y
y
yx
Ky
h
x
h
K
y
h
x
h
K
y
hK
x
h
y
v
x
u
y
hKv
x
hKu
y
2
2
2
2
x
2
2
2
2
x
2
2
2
2
x
Kxx' ----- 0
'
yieldsK
x'x of nSubsitutio
(1) 0 K
0 K
; 0 equation Continuity
;
soil isentropicfor
(a) Seepage through isotropic soil
f
d
yx
y
f
d
f
d
d
N N
HKKq
Kxx' x'-y with scale on dam draw the -
soil isentropicFor
N N
HK
qqflow seepage Total
channelsflow ofNumber N
drop ofNumber N
N
HK
y) (asH Kyx
HKKiAq AB, acrossflow
xK
x
(b) Seepage through isentropic soil
(2) Eq.from discharge seeepage thecompute andflownet draw the then
Kxx' x'-y with scale on dam draw the soil, isentropicfor Thus
(2) N N
HKKq
H
y)' (asy K
x'
H
Kxx' asy
x
Hq AB, acrossflow
y
f
d
yx
x
y
x
d
yx
y
x
x
x
K
NKK
x
K
K
KK
Phreatic lines in earth dams
Determination of phreatic line is required for
(a) Drawing flow net
(b) Estimating pore pressure
(c) Determination of saturation of downstream slope etc.
(a) For homogeneous dam with a horizontal filter
Shape of phreatic line is parabolic except near its junction with the upstream face
Casagrande AB=0.3HB
y
bHbS
SbHb
Sxyx
SFD
FDxyx
dx
dy K
KiAq
flow Seepage
Apoint At
directrix tofocus from distance length focal ,
22
22
22
22
KSq
SxSSxS
SKq
SxS
SS
SxS
SxSxSxy
2
2
22
222
22
22
2
1
2
1
dx
dy
2 as
(b) For homogeneous dam without filter
• Focal point F is located at the lowest point of downstream slope
• Portion KF is discharge face and shall be fully saturated
Casagrande general graphical solution
Angle of discharge face
(degree)
30 0.36
60 0.32
90 0.26
120 0.18
135 0.14
150 0.10
180 0
aa
a
400
180 aaaMay also be calculated by
known thusequation,parabola from known is aaa
(a) > 900
(b) = 1800
a =0
a
C
Directrix
a+a =S-(a+a)cos(180-)
a+a=S/(1-cos )
D
Directrix
(c) = 900
5b) SEEPAGE CONTROL MEASURES IN EMBANKMENT
DAM AND FOUNDATION
Basic requirements for the design of an earth or rockfill dam is to ensure safety
against internal erosion, piping and excessive pore pressure in the dam.
The seepage of reservoir water through the body of the dam or at the
interfaces of the dam with the foundation or abutment creates two main
problems, apart from causing excessive water loss and thereby reducing
usable storage of reservoir:
1. Seepage force causing excessive water loss
2. Piping
Seepage control and drainage features - adopted for the embankment
dam
Impervious core
Inclined/vertical filter with horizontal filter
Network of inner longitudinal drain and cross drains
Horizontal filter
Transition zones/transition filters
Intermediate filters
Rock toe
Toe drain
Relief wells
Upstream Impervious Blanket
Section of homogenous dam showing seepage control features
Section of zoned dam showing seepage control features
Inclined/Vertical Filter
Inclined or vertical filter abutting downstream face of either impervious core or
downstream transition zone is provided to collect seepage emerging out of
core/transition zone and thereby keeping the downstream shell relatively dry.
Horizontal Filter
It collects the seepage from the inclined/vertical filter or from the body of the dam,
in the absence of inclined/vertical filter, and carries it to toe drain.
The horizontal filter may extend from 25 to 100% of the distance from d/s toe to
the centre line of the dam. From practical considerations, a minimum
thickness of 1.0 m is desirable.
Graded filter be provided.
Inner Longitudinal and Inner Cross Drains
When the filter material is not available in the required quantity at reasonable cost,
a network of inner longitudinal and inner cross drains is preferred to
inclined/vertical filters and horizontal filters. This type of drainage feature is
generally adopted for small dams, where the quantity of seepage to be drained
away is comparatively small.
Transition Zones and Transition Filters
Transition zones/filters in earth and rockfill dams in the upstream and
downstream shells are necessary, when the specified gradation criterion is not
satisfied between two adjacent zones. They help to minimize failure by internal
piping, cracking, etc, that may develop in the core or by migration of fines from
the core material.
The filter material used for drainage system shall satisfy the following criteria:
a) Filter materials shall be more pervious than the base materials;
b) Filter materials shall be of such gradation that particles of base material do
not totally migrate through to clog the voids in filter material; and
c) Filter material should help in formation of natural graded layers in the zone
of base soil adjacent to the filter by readjustment of particles.
Horizontal Filters at Intermediate Levels
Horizontal filter layers at intermediate levels are sometimes provided in
upstream and downstream shells, to reduce pore pressures during
construction and sudden drawdown condition and also after prolonged
rainfall.
These filter layers should not be connected with inclined or vertical filters. A
minimum space of 2.0 m or more, should be kept between the face of
inclined/vertical filter and downstream intermediate filter
Horizontal intermediate filters
Rock Toe
The principal function of the rock toe is to provide drainage. It also protects the
lower part of the downstream slope of an earth dam from tail water erosion.
The top level of the rock toe/pitching should be kept above the maximum tail
water level (TWL). In the reach where the ground level at the dam toe is above
the maximum tail water level, only conventional pitching should be adopted.
The top of such pitching should be kept 1.0 m above the top of horizontal filter,
or stripped level, whichever is higher.
Details of rock toe/pitching protection and toe drains are illustrated for various
combination of Tail Water Level (TWL) and stripped Ground Level (SGL).
1. Rock toe when TWL is higher than SGL
2. Pitching when TWL is higher than SGL
3. Rock toe + pitching when TWL is higher than rock toe
4. Pitching when SGL is above TWL
5. Pitching and lined toe drain
Height of rock toe is generally 30 to 40% of the reservoir head and gradation
of material should satisfy the filter criteria.
Toe Drain
Toe drain is provided at the downstream toe of the earth/rockfill dam to collect
seepage from the horizontal filter or inner cross drains, through the foundation as
well as the rain water falling on the face of the dam.
Closed toe drain
TWL
Relief Wells
To reduce the sub-stratum
uplift pressure d/s of the dam
to avoid boiling of sand and
piping
Generally spacing of well is
15 m c/c.
The well screen consists of
GI pipe of 10-15 cm dia.
Slotted with 5 mm to 50 mm
opening and covering about
10% circumference area of
the pipe.
Filter should meet the filter
criteria discussed earlier.
D85 filter > hole diameter
A typical relief well (all dimensions are in mm)
Positive Cut-off Trench
The positive cut-off trench consists of an impervious fill placed in a trench formed
by open excavation into an impervious stratum. Grouting of the contact zone of
the fill and the underlying strata constitutes an integral part of the positive cut-off.
Concrete Diaphragm
A single diaphragm or a double diaphragm may also be used for seepage
control.
Complete Partial
Grout Curtain
• Grouted cut-offs are produced by injection, within the zone assigned to the
cut-off, of the voids of the sediments with cement, clay, chemicals, or a
combination of these materials.
• Reduce permeability
• Approximate range of grain sizes that can be normally be grouted by different
types of grout material and mixture.
Types of grout Dia. of the material (mm) that can be grouted
Cement 0.5 - 1.4
Clay, cement, bentonite 0.3 – 0.5
Clay-chemical, bentonite chemical 0.2 - 0.4
Chemical 0.1 – 0.2
• Blanket grouting is done to a depth of 5-10 m through holes at spacing 3-5 m
• Curtain grouting is done to higher depth
Sheet Pile Cut-offs
Used in silty, sandy and fine gravel foundation, difficult to drive pile in boulders
Grout curtain
Upstream Impervious Blanket
Upstream impervious blanket is provided when a positive cut-off is too expensive.
Thickness 0.6 to 3 m. Effective control of exit gradients can generally be achieved
by a blanket length of about 5 times the head, combined with relief wells and
drainage trenches.
(A) Completely impervious blanket
f
d
f Zx
Hkq
Without blanket
f
d
f ZxL
Hkpq
With blanket
Substituting first Eq. into the second yield
dxp
pL
1
(B) Blanket for finite permeability (Bennet’s solution)
Total discharge qf at distance x from downstream end of blanket
dxZ
hkqdqqq
x
L b
bof
xL
Lboff
dxZ
hkdq
b
bb
Discharge through blanket of thickness Zb in elemental distance dx
x
hadx
hd
ZZk
kha
Z
h
Zk
k
dx
hd
Z
hk
dx
hdZk
dx
dq
Zdx
dhkqfoundationthefor
Z
hk
dx
dq
dx
dqAs
dxZ
hk
dx
d
dx
dq
dx
dq
bff
b
bff
b
b
bff
f
fff
b
b
f
fo
b
x
Lb
fof
2
2
2
22
2
2
2
2
a where
0
Bennet’s basic differential equation for a blanket of
finite permeability and constant thickness
o
ax
0
hh ;0t
ehh
blanket of length Infinite
xA ( x=0 at downstream of blanket) 0h ;t xA
(i)
b
bff
r
ax
r
f
r
ffff
k
ZZk
ax
aheahx
h
dx
dhAs
Zx
hkZ
dx
dhkq
1
i.e, blanket, infinite theas
discharge same thepasses which,x is imperviouscompletly equivalent of length Let the
0
r
Discharge Reduction (1-p)
Without blanket
With blanket
f
d
f Zx
Hkq
f
rd
f Zxx
Hkpq
rd
d
xx
xp
1e
1e
dx
dh
as
eehdx
dh
constant h eehh
blanket of length Finite
2ax
2ax
ax-ax
n
n
ax-ax
n
ax
x
h
a
r
r
f
d
f Zx
Hkq
f
rd
f Zxx
Hkpq
rd
d
xx
xp
Discharge Reduction (1-p)
Without blanket
With blanket
(ii)
1e
1efactor by reduces x length effective length, finitefor Thus
length infinite as same 1
length finitefor 1e
1e
2ax
2ax
r
2ax
2ax
axxas
ax
r
r
0
0.2
0.4
0.6
0.8
1
1.2
0 50 100 150 200 250 300
facto
r
x
This factor increase with increase of x, but rate of increase becomes
very slow after , therefore for design optimum value of 2 ax 2 ax
m 176 x optimum
0.008 a
cm/s 10
cm/s 105
5.1
20
5
3
b
f
b
f
k
k
mZ
mZ
cosWN sinWT
TRTRMd
6) STABILITY OF SLOPES
The slices method (Swedish slip circle method)
1. Locate the centre of the possible failure arc
2. Divide earth mass into 6 – 12 slices (vertical) of equal thickness
3. Numbered the slices
4. Compute the weight of slice taking into consideration dry and saturated weight of
soil
Total disturbing moment (R=radius of failure surface)
Resisting moment
RNLcMR
)tanˆ(
NRLc tanˆ
RNcL )tan(
R
d
R
M
MSF ..
T
NcL
tan
T
UNcLSF
tan)(..
Factor of safety against sliding
Under pore pressure
byW )0(2
111
byyW )(2
1212
byyWNNN)(
2
11
by
yyyWW NN )
2....( 121
N= No. of slices
Various materials, namely riprap, internal filters, rock toes etc. falling within the
sliding mass shall be considered to have the same properties as those of the
respective zones within which they are located.
Location of the centre of the critical slip circle (Fellenius, 1936)
P is obtained by knowing 21&
Slope 1 2
1:1 28 37
2:1 25 35
3:1 25 35
4:1 25 35
5:1 25 37
Few points
The most critical circle passes through the toe/heel of the slope when > 30, or
slope angle > 530, irrespective of .
The most critical circle intersects the slope in front of the toe or heel if < 30,
and slope angle < 530
Location of critical
circle for d/s slope
Location of critical
circle for u/s slope
1. For very small value of (<3o) and < 53o, the centre of the critical arc
likely to fall on a vertical line drawn through the centre of the slope.
2. Critical arc cannot penetrate the hard strata and will be tangent to it.
Stability of embankment dams should be checked for the
following conditions
1. Stability during and at the end of construction
2. Stability of d/s slope during steady seepage
3. U/s slope under sudden drawdown
4. Steady seepage with sustained rainfall for d/s slope
5. Stability of u/s and d/s slopes under earthquake condition
1. Stability during and at the end of construction
Embankment dam is normally compacted at 80 to 90% saturation that is
80 to 90 % of the pore space is filled by water and the rest by air bubble.
The compression of this water-air pore fluid under increasing load of
embankment causes build up of pore pressure.
For stability check, pore pressure is required
Hilf (1948) method: the induced pore pressure
w
wa
V
pu
p
VV
pu
02.0
and,V saturated becomes soil theand solution into goesair When
percent) (in soil embankment of eunit volum inpresent water pore of volumeV
percent) (in ionconsolidat ofstart before
embankment of eunit volum in pores soil inair free of volumeV
volumeembankment original of portiona as ncompressio embankment
pressure catmospheri Absolute
02.0
0
a
w
a
0
0
2. Stability of d/s slope during steady seepage
- Dam is fully saturated below phreatic line
- Can be solved using Effective stress method
T
UNcLSF
tan)(..
N = Wcos W use saturated unit weight for soil below phreatic line
U Estimated by drawing flow-net T = W sin
T
NcLSF
tan'..
5.1.. SF
Total stress method (IS 7894)
N’ = W cos W = based on submerged unit weight of soil
T = W sin W = based on saturated unit weight of soil
All zones of the dam and foundation lying below the tail water level, if any shall
be considered as buoyant (submerged) for both N’ & T.
])1([ hnhhh rcww
3.1.. SF
3.0 U/s slope under sudden drawdown
Effective stress method
Pore pressure is computed using Bishop formula
hw = pore water pressure at a point
w = unit weight of water
hc = height of core material at the point
hr = height of shell material at the point
n = porosity of shell
h = drop in head under steady seepage condition at the point
h
Measured pore pressure in Alcova dam (USBR, Design of small dams)
Total stress method
Zones above phreatic line: All materials shall be considered as moist
Zones in drawdown range: For computing driving forces the core material and
non-free-draining material shall be considered as saturated and freely draining
material shall be considered as moist. For computing resisting forces, consider
submerged unit weight of soil.
Zones below drawdown level: All zones including foundation zone below the
drawdown level shall be considered as submerged for computing both the
driving and resisting forces.
4.0 Steady seepage with sustained rainfall for d/s slope (IS7894)
Shell and other material lying above the phreatic line shall be considered as
moist for calculating driving forces and buoyant for resisting forces.
Saturation of shell material shall be assumed as
• 50% if K < 10-4 cm/s
• 0% if K > 10-2 cm/s (in between assume linear variation)
F.S. > 1.3 (Use total shear stress method)
5.0 Stability of u/s and d/s slopes under earthquake condition
- follow the same procedure, however, increase T by Te = W cos .h
and decrease N by Nc = W sin .h
Thus T = (1+h) W cos
N = (1-h) W sin
F.S. > 1
h = horizontal earthquake coefficient.
7) CONSTRUCTION TECHNIQUES
Hydraulic Fill Dams
In this type of dams, the construction, excavation, transportation of the earth is
done by hydraulic methods. Outer edges of the embankments are kept slightly
higher than the middle portion of each layer. During construction, a mixture of
excavated materials in slurry condition is pumped and discharged at the edges.
This slurry of excavated materials and water consists of coarse and fine materials.
When it is discharged near the outer edges, the coarser materials settle first at the
edges, while the finer materials move to the middle and settle there. Fine particles
are deposited in the central portion to form a water tight central core. In this
method, compaction is not required.
A pool is created between the 'beaches'. The core level is always below the beach
level because the rate of sedimentation there is much slower.
The width of the core is controlled by the percentage of fines in the borrow soil and
the level of water in the core pool. At the start of each 1-2m lift, the level in the
core pool is raised to provide a width somewhat greater than the maximum limit of
core in the shell. A core zone with jagged edges is shown above.
Rolled fill method:
All fill material for the embankment should be placed in layers (or lifts) not
greater than 150mm thick.
The largest size particle should not be greater than 1/3rd the height of the lift,
that is, 50mm. Each layer should be thoroughly compacted before the next
layer is placed. A minimum of 6 passes to achieve the required compaction
effort is generally required by a suitable compaction machine
The compaction effort achieved should be on average 98% Standard
maximum Dry Density (MDD).
The minimum compaction effort should be 95% Standard MDD. If the range
of compaction effort varies throughout the dam, then it can lead to the dam
embankment settling to different degrees (differential settlement) causing the
embankment of the dam to crack. This may ultimately lead to leakage and
dam failure.
Rolled fill method (continued)
The material forming the embankment should be placed with sufficient
moisture to ensure proper compaction. The moisture content should be in the
range of –1% to + 3% of optimum moisture content (OMC). If the material is
too dry, water should be added. If the material is too wet it should be spread
and mixed.
Before each additional 150mm lift is added to the embankment, the
preceding lift should be scarified to ensure that the two lifts are properly
joined so that no natural paths for seepage are present that may result in
dam failure.
The construction operations fall into four principal groups of activities, relating
to
(1) Material source development: Activities involve the opening out of borrow
areas or quarries, including the installation of fixed plant, e.g. crushers,
conveyors, etc. Access and haulage roads are also constructed between the
various borrow areas and the embankment site, and excavation
and haulage plant is mobilized.
(2) Foundation preparation and construction: Activities, including river
diversion, can proceed concurrently with the development of the fill sources.
Topsoil and weathered surface drift deposits etc. are removed. In the case of
a soft, compressible foundation, strength can be enhanced.
(3) Fill construction and control: Control of placing is centred upon
supervision of water content, layer thickness and compaction procedure.
(4) Ancillary works construction: embraces the construction of spillway and
stilling basins, culverts or tunnels for outlet works etc.
top related