Concrete Blocks - Association of State Dam Safety Officials

ACBs –vs- Riprap

Angle **of** Repose

Major Differences

• Angle **of** repose

–Rock ~ 40 o

– ACBs ~ 80+ o

ACBs vs Riprap

42

40

Class 85L

42

40

38

38

36

36

34

34

32

32

30

30

28

28

Shear Stress (psf)

26

24

22

20

18

Class 40

L Series

26

24

22

20

18

Velocity (ft/sec)

16

16

14

12

Class 30S

14

12

10

10

8

8

6

6

4

4

2

2

0 2 4 6 8 10 12 14 16 18 20 22 24

Rock Size (ft)

Angle **of** Repose

Comparison **of** angle **of** repose

V=14, d=5

Rock size (ft)

4

3.5

3

2.5

2

1.5

1

0.5

0

Riprap

ACB

0 5 10 15 20 25 30 35 40

Side slope in degrees

Major Differences

• Angle **of** repose

–Rock ~ 40 o

– ACBs ~ 80+ o

• Range **of** sizes tested

– Rock – 2 to 12 inch

– ACBs – prototype

Major Differences

• Angle **of** repose

–Rock ~ 40 o

– ACBs ~ 80+ o

• Range **of** sizes tested

– Rock – 2 to 12 inch

– ACBs – prototype

• Effect **of** particle shape

– Inter-particle friction

Inter-Particle Friction

ACBs

Critical Shear Stress (psf)

60.0

50.0

40.0

30.0

20.0

10.0

0.0

0 50 100 150 200 250

Average Unit Weight (lbs)

ACBs

Critical Shear Stress (psf)

60.0

50.0

40.0

30.0

20.0

10.0

0.0

0 20 40 60 80 100

Average System Weight (psf)

ACBs

55

50

85L

Critical Shear Stress (psf)

45

40

35

30

25

20

15

10

30S

40L

40T

45L 50L

45

50T

50S

45S

55L

55

60T

55S

70L

75

70T

85

5

0

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90

Average System Weight (psf)

Tapered S - Open S - Closed Regular - Closed L - Open L - Closed

ACBs

Critical Shear Stress (psf)

55

50

L- Series

45

40

35

30

S- Series

25

Tapered

20

15

10

5

0

10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90

Average System Weight (psf)

Tapered S - Open S - Closed Regular - Closed L - Open L - Closed

Articulating **Concrete**

**Blocks**

ACB Testing Program

• CIRIA 1987

– “Design **of** Reinforced Grass Waterways

• FHWA 1988

– “Minimize Embankment **Dam**age During Overtopping Flow”

• FHWA 1989

– “Hydraulic Stability **of** Articulated **Concrete** Block

Revetment Systems During Overtopping Flow”

• St. Anthony Falls Lab, University **of** Minnesota 1995-2000

– Over 10 systems tested

• Colorado **State** University 1992-2012

– Over 65 systems tested

• Colorado Sate University 2007-Present

– Comprehensive analysis

Colorado **State** University

Engineering Research Center

Assessing Hydraulic Stability

• ASTM D7277 (2008)

– **State**-**of**-the-practice

for full-scale hydraulic

performance testing **of**

ACB systems

– Full-scale hydraulic

testing data

• Measured discharge

• Bed elevation along at

given stations

• Water surface elevation

at given stations

Hydraulic Conditions During Embankment Overtopping

Subcritical Flow

Increasing Velocity &

Decreasing Depth

Supercritical Flow

Increasing Velocity &

Decreasing Depth

Supercritical Flow

Constant Velocity &

Depth

Subcritical Flow

Turbulent Flow

Potential Subatmospheric Zone

Reservoir

Critical Depth

Theoretical Nappe

Pr**of**ile

Hydraulic Jump

on Slope

Normal Depth

Hydraulic Jump

At Toe **of** Slope

Tailwater

Embankment

System Installation

Finished Embankment

Prototype Flume Tests

Post Test Embankment Inspection

Loss **of** Intimate Contact

Inspect for Subgrade **Dam**age

Gully Formation?!

Hydraulic Conditions During Embankment Overtopping

Subcritical Flow

Increasing Velocity &

Decreasing Depth

Supercritical Flow

Increasing Velocity &

Decreasing Depth

Supercritical Flow

Constant Velocity &

Depth

Subcritical Flow

Turbulent Flow

Potential Subatmospheric Zone

Reservoir

Critical Depth

Theoretical Nappe

Pr**of**ile

Hydraulic Jump

on Slope

Normal Depth

Hydraulic Jump

At Toe **of** Slope

Tailwater

Embankment

Quantifying Performance Thresholds

Embankment Plot

First Hour Data

• ASTM D7276 (2008)

– **State**-**of**-the-practice

for hydraulic analysis

and system stability

analysis **of** articulating

concrete block systems

– Water surface pr**of**ile

(WSP) idealized to

compute shear stress

for non gradually

varied flow conditions

Elevation (ft)

104.00

102.00

100.00

98.00

96.00

94.00

92.00

0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00

Station (ft)

Channel Lock 3 ' Test

106

104

102

100

98

96

94

Bed El

EGL

WSE

Meas. Bed

Pr ed. WSEL

Pr ed. EGL

Hour 1

Hour 2

Hour 3

Hour 4

92

90

0 5 10 15 20 25 30 35 40 45

St at i on ( f t )

Post Test Data Analysis

Flow Depth Calibration

2.40

2.20

2.00

Elevation (ft)

1.80

1.60

1.40

1.20

1.00

0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00

Station (ft)

Model Flow Depths

Measured Flow Depts

Post Test Data Analysis

Channel Lock 3 ' Test

106

104

102

100

98

96

Meas. Bed

Pr ed. WSEL

Pr ed. EGL

Hour 1

Hour 2

Hour 3

Hour 4

94

92

90

0 5 10 15 20 25 30 35 40 45

Stati on (f t)

Momentum Analysis

F x

∆(ρqV)

Unit discharge q

Sta.1

P 1

P 2

d 1 , V 1

d 2 , V 2

L

0

Wsin

Sta.2

Wcos

W

Angle

X - axis

0 = ½((d1+d2)sin + (1/L){ (½)d 1 2 - d 2 2 )cos - q 2 (1/d 2 – 1/d 1 ) }

Momentum Analysis

Test Results

20

18

Shear stress or velocity

16

14

12

10

8

6

4

Velocity, ft/s

Shear stress, lb/ft 2

Maximum

Velocity 19.3 ft/s

Maximum Shear

Stress 11.9 lb/ft 2

2

0

0 10 20 30 40 50 60

Station (ft)

DESIGN

METHODOLOGY

• Riprap and accepted erosion control methods

– Utilize both analytical and empirical relationships

– Both based on concept **of** critical values

• ACB Systems

– Quantify hydraulics

– Performance thresholds defined through testing

– Design techniques developed for application

**Safety** Factor Method

Based on considering the forces on a particle on a channel bed sloping at

an angle, , with the moment arms about the point **of** rotation, PR.

Discrete Particle

FM FM Wsin

M Wcos

M

L

Wcos

F L

M 4

F d

3

Wsin

M 2

W

M 1

PR

4 d 3 2 1

**Safety** Factor Method

F L

FLOW

l F'

4

L

F F' D

D

W S1

W S2

l 1

l 2

l 3

F R

Pivot Point

**of** Rotation

Overturning Forces

F D

& F L

= Drag & lift force

F' D

& F' L

= Additional drag & lift force

from block protruding above ACB matrix

Restraining Forces

F R

= Inter-block restraint

W S2

= Gravitiy force normal to slope

W S1

= Gravitiy force parallel to slope

Figure 1. Moment balance on an ACB at incipient failure

**Safety** Factor Method

Top **of** Bank

W s

a

W s

sin 1

W s

1

Bed **of** Channel

A. Channel cross section view

Figure 2. Three-dimensional view **of** a block on a channel side slope

with factor **of** safety variables defined

**Safety** Factor Method

A

Horisontal

Streamline

A'

0

d

F D WS sin 1

Block Projection

Once in

Motion

b

W S

sin 0

Toe **of** Slope

0

Vertical

B. View normal to plane **of** channel bank - original

Figure 2. Three-dimensional view **of** a block on a channel side slope

with factor **of** safety variables defined

SF and free body diagrams

currently utilized for:

•Block design

• Thickness extrapolations

• Slope adjustments

• Projection height analysis

Hydraulic Conditions During Embankment Overtopping

Subcritical Flow

Increasing Velocity &

Decreasing Depth

Supercritical Flow

Increasing Velocity &

Decreasing Depth

Supercritical Flow

Constant Velocity &

Depth

Subcritical Flow

Turbulent Flow

Potential Subatmospheric Zone

Reservoir

Critical Depth

Theoretical Nappe

Pr**of**ile

Hydraulic Jump

on Slope

Normal Depth

Hydraulic Jump

At Toe **of** Slope

Tailwater

Embankment

Manufacturer’s Disclaimer:

“The success or failure **of** any

engineering design is the

responsibility **of** the engineer **of**

record and not that **of** the

manufacturer **of** the revetment

system, its agents or

representatives.”

ASTM Standards for ACBs

• Hydraulic Modeling **of** ACBs

• Design **of** ACB Systems

• Manufacture **of** ACBs

• Installation **of** ACB Systems

1/2 Inch

FLOW

FLOW

1/2 Inch

ACB mats should abut each other & fastened …

Zippering UltraFlex Mats

Inadequate Spillway Capacity

~40% PMF ….

Shavers Creek **Dam**, PA

Approved ACB Systems:

Armorflex 50-T (Open Cell)

Petraflex H-916 (Open Cell)

50’

Existing Embankment Section

Drain Reservoir

Topsoil & Seed

Install ACBs

Install Drain Material (OGS)

Resurface Top **of** **Dam**

**Concrete** Cut**of**f Wall

Flatten Downstream Slope

Strip Downstream Face **of** **Dam**

Install Filter/Drain

Material

Flow

Detail At Top **of** **Dam**

Detail At Toe **of** **Dam**

Finished Grading, ACBs

Shavers Creek **Dam**