Design Earth Dams

8/6/2019 Design Earth Dams

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Design of Earth Dams

Earth Dam Components


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Design Requirements

Types of Earth Dams


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Other Types of Dams Concrete

Roller Compacted concrete

Debris flows

Choice of Type Depends onAvailability of materials

Trucking increases cost

Homogeneous Earth Dam

Lots of low K material Till

Zoned

Limited core material

Rock available

Upstream construction

Use mine tailings as a construction material


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Foundation Requirements1. Strong foundation

Minimal differential settlement

No bearing capacity shear failure

Sand/Gravel or rock

2. Low hydraulic conductivity

Silt and/or Clay, non-fractured rock

Since 1 and 2 are often incompatible we need todo foundation treatment

Foundation Treatment Soil

Cut-offs walls

Sheet pile walls

Slurry trench

Impervious upstream blanket

Removal and replacement

Densification

Zone grouting

Rock

Removal and replacement of upper fractured rock Grouting

Impervious upstream blanket


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Hydraulic Efficiency Install upstream and down stream

peizometers

Efficiencies greater than 90% have beenattained

h

h

lossheadTotal

barrieracrosslossHeadE

H

==

Foundation

Treatment

(Mitchell)


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Foundation

Treatment

(Mitchell)

Foundation PreparationWant smooth transition to minimize

negative skin friction and soil arching asthat can led to hydraulic fracturing as well

as core cracking


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Foundation Preparation

Foundation Distortion

h

h

F

hL

L

B

B

=+

+

= 12

2

5.0

2

2

Hmvv =

Shear Strain () = v-h 1%


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Hydraulic FracturingOccurs when the porewater pressure is equalto the minimum embankment effective stress

whhvhHEHKK =>==

where

K~0.5 for compacted materials

Eh is the tensile modulus obtained from a triaxial extension testh = horizontal strain

Linear and Plastic Deformations


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Soil Arching Terzaghi 1943

Soil Arching Terzaghi Trap Door

Note significant reduction is vertical stress


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Soil Arching Terzaghi Trap Door

Soil Arching


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Approximation of

Vertical Stress in DAMS

( )

=

L

ZK

ve

K

L

tan2

1tan2

Where L is the width of the valley

K=constant ~1

Z is depth from dam crest

Approximation of

Vertical Stress in DAMS

tan2

L

v =

If H>L then z/L >1 and K=1


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Hydraulic Fracturingif

reservoirfullforw

H

2

L

h

tan~Kwherevh

K

Hydraulic Fracturingthen

HLwhen0'there2

2when0'

h=

==

w

wh

note

HL

Note: hydraulic fracturing will occur when L isapproximately equal to H

A few dams exists where average L < average H


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Design of Cutoffs Must prevent hydraulic fracturing

h>porewater pressure

Then Lcutoff> Hcutoff

Lcutoff1

Hcutoff

1

1

1

v = Hdam

Foundation Preparation1. To prevent hydraulic fracturing must:

Have no vertical shear boundaries that cancreate differential settlement

Smooth profile along base of dam

Cord Length greater than the height of dam

Cutoffs with cord lengths greater than cutoffheight

2. Hydraulic conductivity that will allow water to

pond behind the dam3. Strong enough to minimize distortional strains

to less than 1 percent


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Design of Zoned DamsRock fill will be much stiffer than core. Ascore settles arching will occur in the core.

Design of Zoned DamsTo prevent core arching and hydraulic

fracturing we need:

1. wide enough core

2. need shear layer between core and shellto prevent internal core cracking


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How wide of core do we need?

tWD

GZ

Z

c

core

v 2

1+=

where:

W is the width of core

Z is depth below crest

core is unit weight of core

G is the shear modulus of filter material

t is width of filter material

Dc is confined modulus of core = 1/mv

Can Geotextiles Prevent ShearNote t is very small for geotextiles thismakes very large

Resulting is significant arching incorecreating core cracking and hydraulicfracturing.

To prevent hydraulic fracturing need

tWD

GZ

c

2

35.0

2

Design Earth Dams

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