1 one dimensional seepage
TRANSCRIPT
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Priyantha Jayawickrama, Ph.D.
Associate Professor
One-Dimensional SeepagePermeability and Darcys Law
Texas Tech UniversityDepartment of Civil and Environmental Engineering
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CE 5321-001: Advanced Soil Engineering 2
References
Chapters 17, 18 and 19,
Lambe and Whitman, John Wiley
Chapter 17: One-dimensional flowChapter 18: Two-dimensional flow
Chapter 19: Soil Permeability and Filter
Requirements
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Permeability and Seepage
Why would geotechnical engineers be
concerned with this topic?
What applications come to mind?
What are the principles governing this
process?
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Subsurface Water
Vadose Zone
(zone of aeration)
Ground Surface
capill ary fr inge Water Table
(Phreatic Surface)
Zone of Phreatic Water
(zone of saturation)
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Artesian Conditions
Confined
sandstone
aquifer
Impermeable
shale
Non-flowing
well
Artesian pressure
surface
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Seepage Illustration:Loop 287, Lufkin, TX
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Roadbed Excavation in Sandy Soilk 1 x 10 -4cm/sec
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High Head: Dewatering Required(grid of French drains)
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Antilley Road Bridge, Abilene
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Antilley Road Bridge,Abilene
30-ft
100.1-ft92.3-ft
79.9-ft
61.6-ft
67.3-ft
61.4-ft
70.0-ft
Seepage location
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To Estimate Seepage Loss
Seepage through the body of the dam
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To Estimate Seepage LossSeepage through the body of the dam and the foundation soil
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To Estimate Seepage LossSeepage through the body of the dam and the foundation soil
Anisotropic Soil
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To Estimate Pore WaterPressures
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To Estimate Pore Water Pressures
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16
To Evaluate Quicksand Conditions
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17Dewatering Excavations
Other ApplicationsDewatering System Design
http://images.google.com/imgres?imgurl=http://www.godwinpumps.com/images/Wellpointphoto.jpg&imgrefurl=http://www.godwinpumps.com/wellpoint.html&h=253&w=400&sz=33&tbnid=LbTzrFXsfrcJ:&tbnh=75&tbnw=120&hl=en&start=10&prev=/images%3Fq%3Ddewatering%26svnum%3D10%26hl%3Den%26lr%3D%26sa%3DN -
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Other ApplicationsDrainage System Design
Pavement Drainage
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Drainage behind Retaining Walls
Other ApplicationsDrainage System Design
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Other ApplicationsDrainage System Design
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Other ApplicationsDrainage System Design
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H
b
d
w
1
2 3
4
P
Other ApplicationsDrainage System Design
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Other ApplicationsDrainage System Design
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Flow through soil and rockmaterial Soil and rock are porous
materials
Fluid flow takes place through
interconnected void spacesbetween particles and notthrough the particlesthemselves
No soil or rock material isstrictly impermeable
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Question
What causes flow of waterthrough soil?
Answer:
A difference in TOTAL HEAD
We will focus on Water Movement withinSaturated Soils!
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Total Head = pressure head +elevation head
soil
No Flow!
5ft 5ft
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soil
Flow from left to right!
5ft
9ft
Total Head = pressure head +elevation head
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soil
Flow from left to right!
Total Head = pressure head +elevation head
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5ft
5ft7ft
Total Head = pressure head +elevation head
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Comments re: Total Head
A differencein to tal headis required to cause
movement of water through soil.
Total head is measured in units of length;e.g., feet or meters; not psf, kPa, etc.
Darcys Law, governing fluid flow through a
porous medium, takes this one step further:
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Pressure Head, hp= pressure/w
Elevation Head, he= distance from
datum
Total head = hp+ he
Velocity head is negligibly small, e.g.at flux = 2ft/min, velocityhead=0.00002 ft.
Total Head
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How do you estimate flow rate ?
Flow rate will be controlled by Difference in total head between Point A & Point B
Distance between Point A & Point B
Type of Soil
5ft
5ft7ft
A
B
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Some Definitions
Quantity of Flow, Q Cu.ft
Flow Rate, q = Q/t Cu.ft/min
Flux, v = q/A Cu.ft/min/sq.ft = ft/min
5ft
5ft7ft
A
B
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Some Definitions
Hydraulic Gradient, i (no units)
L
h
L
HHi
AB
BA
5ft
5ft7ft
A
B
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Darcys Law
k= coefficient of permeability
i = hydraulic gradient (i < 5, laminar flow)
L
hi
A
tQ
A
qv
)/(
ikv
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Darcys Law; Applications
Elev. 0 ft
Elev. 30 ft
12ft
57ft
Confined Aquifer
Very low
permeable
soil
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Coefficient of Permeability
k= coefficient ofpermeability, or
hydraulic conductivity
k depends on the poresize in the soil
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Some typical k - values
uniform coarse sand .. k = 4 x 10 -1cm/sec
clean sand & gravel.. k = 1 x 10 -2cm/sec
silty sand . k = 1 x 10 -4cm/sec
sandy clay .. k = 5 x 10 -6cm/sec
Compacted clay .. k = 1 x 10 -7cm/sec
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Order of Magnitude ofk - values
Uniform coarse sand is1000 times more
permeable thansilty sand
Silty sand is 1000 times more permeable thanCompacted clay
Coarse Sand . Drainage Layers
Compacted Clay . Liner Material for Ponds
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Empirical relationships toestimate permeability, k
21010Dk
2155.3 Dk
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Grain Size Distribution Curves
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Head DistributionBelow Ground Water Table
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Head DistributionAbove Ground Water Table
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Calculation of Pressure Head inSeepage Conditions
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Calculation of Pressure Head inSeepage Conditions
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Calculation of Pressure Head inSeepage Conditions
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Calculation of Pressure Head inSeepage Conditions
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Oil Refinery Site, Kawasaki, Japan
Previously, for static ground water conditions
(Chapter 16, Lambe and Whitman)
Now modified for steady seepage conditions
See Handout
Calculation of Effective Stress underSeepage Conditions
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+4
-11
-21
-36
-51
C l l i f Eff i S d
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Calculation of Effective Stress underSeepage Conditions
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Calculating Seepage Force
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Calculating Seepage Force
This is what you havewhen upward seepage
is occurring
This is what you wouldhave had under staticwater conditions
The differencerepresents the energylost due to seepage
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Calculating Seepage Force, iw
L
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Consider:
Boundary water forces + Total weights
OR seepage forces + submergedweights
Seepage force/volume = iw
Force Equilibrium
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Evaluating Quick Conditions
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Evaluating Quick Sand Conditions
wstwv hqba
:00 sqandaWhen
wtv hbb )(
wwtv hb )(
0: vc andiiconditionsquickFor
w
sub
w
wt
c
cwwtb
hihb
)(,thereforeand)(0
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FOS against Quick SandConditions
w
sub
w
wt
c
cb
hi
)(
Factor of Safety against quicksand conditions, F
actual
c
i
iF