Field Methodologies: Detailed
Investigation
Andrew Simon
USDA-ARS National Sedimentation Laboratory,
Oxford, MS
andrew.simon@ars.usda.gov
Aims of this Section
•
To describe the methodologies and
instruments used to collect the necessary
data for bank-stability modeling.
Fundamental Processes Behind
Bank Stability
If we want to predict bank stability we need to quantify
the underlying processes controlled by force and
resistance to mass failure and hydraulic shear:
•
•
Bank shear strength (resistance to mass failure)
vs. Gravitational forces
Bank-toe erodibility (resistance to hydraulic erosion)
vs. Boundary shear stress
National Sedimentation Laboratory
Bank Profile and Stratigraphy
• Select critical bank geometry and survey profile to thalweg;
• Notate bank stratigraphy (including bank toe dimensions
and slope), dominant size class and layer thickness
from bank face or during augering; sample each layer
for particle-size distribution;
• Determine what techniques will be required to determine
critical shear stress and erodibility of the bank toe and
other layers;
Bank Shear Strength
Measuring Soil Strength
•
•
•
•
In-situ tests – Borehole shear test (BST)
Torvane – cohesion and friction combined
Shear vane (undrained clays only)
Laboratory test – shear box and triaxial cell
Iowa Borehole Shear Tester
Soil Strength Testing
80
70
60
50
40
30
20
10
0
y = 0.684x + 0.5
0
10
20
30
40
50
60
70
Normal Stress (KPa)
80
90
Shear Stress at Failure
(KPa)
Shear Stress at Failure
(KPa)
Shear Strength Envelope - Sand
c' = 0.5, ' = 34 degrees
100
Shear Strength Envelope - Clay
c' = 12.5, ' = 16 degrees
110
50
y = 0.296x + 12.5
40
30
20
10
0
0
10
20
30
40
50
60
70
Normal Stress (KPa)
80
90
100
110
Some “Ball Park” Figures
(based on more than 800 tests)
Soil Type
Statistic
Gravel*
Sand
Loam
Clay
c'
ca
'
(kPa) (kPa) (degrees)
- 0.0
36.0
g sat
(kN/m3)
20.0
75th percentile
5.8
1.0
32.3
19.1
Median
2.9
0.4
30.3
18.5
25th percentile
1.3
0.0
25.7
17.9
75th percentile
11.9
8.3
29.9
19.2
Median
8.4
4.3
26.6
18.0
25th percentile
4.6
2.2
16.7
17.4
75th percentile
18.0
12.6
26.4
18.3
Median
11.0
8.2
21.1
17.7
7.2
3.7
11.4
16.9
25th percentile
* From Selby (1982)
Measuring Pore-Water Pressure
•
•
Measure directly using
tensiometers and piezometers
Infer from water table height
mw = h.gw.
where
mw = pore water pressure (kPa);
h = head of water (m);
gw = unit weight water (kN/m3)
Measuring Matric Suction in the Field
• Auger to desired depth for BST testing
• Take undisturbed core with hammer sampler (take
second core sample for bulk unit weight)
• Insert digital tensionmeter
•Record readings every 15 sec for 6 – 10 minutes
Incorporating Suction in a Strength Test
Unsaturated Shear Test, Goodwin Creek Bend, MS
75
70
Ca = 22.7 kPa
’ = 0.37 = 20.3o
r2 = 0.99
Matric suction = 17kPa
65
SHEAR STRESS, IN kPa
60
55
50
45
40
Matric suction = 17kPa
At b of 14° this gives;
17 x (tan 14 °) = 4.2 kPa added
cohesion
Therefore;
35
30
25
20
15
10
c’ = 22.7 – 4.2 = 18.5 kPa
5
0
0
10
20
30
40
50
60
70
NORMAL STRESS, IN kPa
80
90
100
110
Hydraulic Erosion Processes
(Bank Toe)
Hydraulic Erosion Processes
•
•
•
•
Terms used in this section
Hydraulic shear stress – the force exerted by
water flowing over material, Pascals (1Pa=1N/m2)
Boundary shear stress to, critical shear stress tc,
excess shear stress te
Erodibility – amount of erosion per unit excess
shear stress, per unit time, m3/Pa/sec (m/sec)
Erosion rate – rate of bank retreat, m/sec
Shields Diagram by Particle Diameter
(For Non-Cohesive Materials)
Excludes
cohesives
Rule of Thumb for Uniform Sediments: tc (in Pa) = diameter (in mm)
Erosion Rate and Excess Shear Stress:
Cohesives
e = k (to- tc)
e = erosion rate (m/s)
Obtained from jet-test
k = erodibility coefficient (m3/N-s) device
to = boundary shear stress (Pa)
tc = critical shear stress (Pa)
(to-tc) = excess shear stress
Critical shear stress is the stress required to initiate
erosion.
Measuring Bank and Toe Erosion and
Erodibility (Cohesives)
• Jet test device scours a
hole in the bank or toe
and measures the shear
stress and erosion rate
• From this we calculate
critical (threshold)
shear stress and
erodibility coefficient, k
Measuring bank erodibility with the ARS
non-vertical jet test device
An Example:
Test 2, Hungerford Brook,
Rowell property, VT.
0.045
k
(cm3N-1s-1)
tc
Shear Stress,
Pa
0.040
Erosion rate (cm s-1)
Erosion Rate, cm s-1
From Relation between Shear Stress and
Erosion We Calculate tc and k
0.035
tc = 2.46 Pa
0.030
0.025
0.020
0.015
0.010
0.005
0.000
0.00
1.00
2.00
3.00
Shear stress (Pa)
4.00
5.00
Original Relation for Erodibility (k)
Erodibility, m3/N-s
k = x tc y = 0.2 tc -0.5
Where; tc = critical shear stress (Pa),
x, y = empirical constants
3
ERODIBILITY COEFFICIENT (k), IN cm /N-s
10
1
Hanson and
Simon (2001)
0.1
0.01
k = 0.09 tc -0.48
0.001
0.0001
0.01
0.1
1
10
CRITICAL SHEAR STRESS, IN Pa
100
1000
Distributions: Critical Shear Stress
100
90
80
PERCENTILE
70
60
Yalobusha River System
Kalamazoo River
James Creek
Shades Creek
Missouri River
Upper Truckee River
W. Iowa, E. Nebraska
N Fork Broad River
Tualatin River System
Tombigbee River
S Branch Buffalo River
All Data
50
40
30
20
10
0
0.1
1.0
10.0
CRITICAL SHEAR STRESS (Pa)
100.0
1000.0
Distributions: Erodibility Coefficient
100
90
80
PERCENTILE
70
60
50
Yalobusha River System
Kalamazoo River
James Creek
Shades Creek
Missouri River
Upper Truckee River
W. Iowa, E. Nebraska
N Fork Broad River
Tualatin River System
Tombigbee River
S Branch Buffalo River
All Data
40
30
20
10
0
0.001
0.010
0.100
1.000
ERODIBILITY COEFFICIENT (k)
10.000
100.000
Erodibility Relation: Yalobusha River
System, MS
1.E+02
y = 0.2447x-0.5898
R2 = 0.4394
ERODIBILITY COEFFICIENT (k)
1.E+01
1.E+00
1.E-01
1.E-02
1.E-03
1.E-04
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
CRITICAL SHEAR STRESS (Pa)
1.E+00
1.E+01
1.E+02
1.E+03
Erodibility Relation, Kalamazoo River, MI
100.00
-0.6096
y = 2.7075x
2
R = 0.5313
ERODIBILITY COEFFICIENT (k)
10.00
1.00
0.10
0.01
0.01
0.10
1.00
CRITICAL SHEAR STRESS (Pa)
10.00
100.00
Erodibility Relation: Shades Creek, AL
100
ERODIBILITY COEFFICIENT (k)
10
y = 5.011x-1.0463
R2 = 0.7122
1
0.1
0.01
0.001
0.01
0.1
1
10
CRITICAL SHEAR STRESS (Pa)
100
1000
Erodibility Relation: James Creek, MS
1.E+02
ERODIBILITY COEFFICIENT (k)
1.E+01
y = 1.0988x-0.5584
R2 = 0.4687
1.E+00
1.E-01
1.E-02
1.E-03
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
CRITICAL SHEAR STRESS (Pa)
1.E+01
1.E+02
1.E+03
Revised Erodibility Relation
1000.000
-0.8375
y = 1.61 x
r2 = 0.55
3
ERODIBILITY COEFFICIENT, IN (cm /N-s)
100.000
10.000
1.000
0.100
0.010
0.001
0.000
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
CRITICAL SHEAR STRESS, IN (Pa)
1.E+01
1.E+02
1.E+03
Cohesive Strength Meter (CSM)
The CSM consists of a water-filled chamber 30 mm in diameter that
is pushed into the sediment. The jet of water comes from a
downward directed nozzle in the chamber. The velocity of the jet is
increased systematically through each experiment. Bed erosion is
inferred from the drop in the transmission of infrared light across
the chamber caused by the suspension of sediment.
Example CSM Results
110
100
TRANSMISSION, IN PERCENT
90
80
70
60
50
40
30
20
tc = 11 Pa
10
0
0.0
10.0
20.0
30.0
40.0
50.0
SHEAR STRESS, IN Pa
60.0
70.0
80.0
90.0
Comparison of Methods: tc
100
PERCENTILE
80
Original Jet
"Mini" Jet
Cohesive Strength Meter
60
40
20
0
0.0001
0.001
0.01
0.1
1
10
CRITICAL SHEAR STRESS, IN PASCALS
100
Comparison of Methods: k
100100
90
80 80
60 60
PERCENTILE
PERCENTILE
70
Original Jet
"Mini" Jet
Large jet
Mini jet
50
40 40
30
20 20
10
0 0
0.01
0.01
0.10
0.1
1.00
1
ERODIBILITY COEFFICIENT (k)
10.00
10
ERODIBILITY COEFFICIENT, IN cm3/N-s
100.00