Field Methodologies: Detailed Investigation
Andrew Simon USDA-ARS National Sedimentation Laboratory, Oxford, MS
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
Shear Strength Envelope - Sandc' = 0.5, ' = 34 degrees
y = 0.684x + 0.5
01020304050607080
0 10 20 30 40 50 60 70 80 90 100 110
Normal Stress (KPa)
She
ar S
tres
s at
Fai
lure
(K
Pa)
Shear Strength Envelope - Clayc' = 12.5, ' = 16 degrees
y = 0.296x + 12.5
0
10
20
30
40
50
0 10 20 30 40 50 60 70 80 90 100 110
Normal Stress (KPa)
She
ar S
tres
s at
Fai
lure
(K
Pa)
Some “Ball Park” Figures(based on more than 800 tests)
Soil Type Statisticc a
(kPa)c'
(kPa)
' (degrees)
gsat
(kN/m3)Gravel* - 0.0 36.0 20.0
Sand 75th percentile 5.8 1.0 32.3 19.1Median 2.9 0.4 30.3 18.525th percentile 1.3 0.0 25.7 17.9
Loam 75th percentile 11.9 8.3 29.9 19.2Median 8.4 4.3 26.6 18.025th percentile 4.6 2.2 16.7 17.4
Clay 75th percentile 18.0 12.6 26.4 18.3Median 11.0 8.2 21.1 17.725th percentile 7.2 3.7 11.4 16.9
From Selby (1982)*
Measuring Pore-Water Pressure• Measure directly using
tensiometers and piezometers• Infer from water table height
w = h.gw.
where w = 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
Unsaturated Shear Test, Goodwin Creek Bend, MS
Incorporating Suction in a Strength Test
Ca = 22.7 kPa’ = 0.37 = 20.3o
r2 = 0.99Matric suction = 17kPa
Matric suction = 17kPaAt b of 14° this gives;
17 x (tan 14 °) = 4.2 kPa added cohesionTherefore;
c’ = 22.7 – 4.2 = 18.5 kPa
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 o, critical shear stress c,
excess shear stress e
• 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: c (in Pa) = diameter (in mm)
Erosion Rate and Excess Shear Stress: Cohesives
= k (o- c)
= erosion rate (m/s)
k = erodibility coefficient (m3/N-s)
o = boundary shear stress (Pa)
c = critical shear stress (Pa)
(o-c) = excess shear stress
Critical shear stress is the stress required to initiate erosion.
Obtained from jet-test device
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
From Relation between Shear Stress and Erosion We Calculate c and k
Shear Stress, Pa
Ero
sion
Rat
e, c
m s-
1
c
k (cm3N-1s-1)
An Example:Test 2, Hungerford Brook, Rowell property, VT.
c = 2.46 Pa
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
0.040
0.045
0.00 1.00 2.00 3.00 4.00 5.00
Shear stress (Pa)
Ero
sion
rat
e (c
m s
-1)
Original Relation for Erodibility (k)
Erodibility, m3/N-s
k = x c y = 0.2 c
-0.5
Where; c = critical shear stress (Pa), x, y = empirical constants
CRITICAL SHEAR STRESS, IN Pa
0.01 0.1 1 10 100 1000
ERODIB
ILIT
Y C
OEFF
ICIE
NT (k), I
N cm
3 /N-s
0.0001
0.001
0.01
0.1
1
10
k = 0.09 c -0.48
44
Hanson and Simon (2001)
Distributions: Critical Shear Stress
0
10
20
30
40
50
60
70
80
90
100
0.1 1.0 10.0 100.0 1000.0
CRITICAL SHEAR STRESS (Pa)
PE
RC
EN
TIL
E
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
Distributions: Erodibility Coefficient
0
10
20
30
40
50
60
70
80
90
100
0.001 0.010 0.100 1.000 10.000 100.000
ERODIBILITY COEFFICIENT (k)
PE
RC
EN
TIL
E
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
Erodibility Relation: Yalobusha River System, MS
y = 0.2447x-0.5898
R2 = 0.4394
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
1.E+01
1.E+02
1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03
CRITICAL SHEAR STRESS (Pa)
ER
OD
IBIL
ITY
CO
EF
FIC
IEN
T (
k)
Erodibility Relation, Kalamazoo River, MI
y = 2.7075x-0.6096
R2 = 0.5313
0.01
0.10
1.00
10.00
100.00
0.01 0.10 1.00 10.00 100.00
CRITICAL SHEAR STRESS (Pa)
ER
OD
IBIL
ITY
CO
EF
FIC
IEN
T (
k)
Erodibility Relation: Shades Creek, AL
y = 5.011x-1.0463
R2 = 0.7122
0.001
0.01
0.1
1
10
100
0.01 0.1 1 10 100 1000
CRITICAL SHEAR STRESS (Pa)
ER
OD
IBIL
ITY
CO
EF
FIC
IEN
T (
k)
Erodibility Relation: James Creek, MS
y = 1.0988x-0.5584
R2 = 0.4687
1.E-03
1.E-02
1.E-01
1.E+00
1.E+01
1.E+02
1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03
CRITICAL SHEAR STRESS (Pa)
ER
OD
IBIL
ITY
CO
EF
FIC
IEN
T (
k)
Revised Erodibility Relation
y = 1.61 x-0.8375
r2 = 0.55
0.000
0.001
0.010
0.100
1.000
10.000
100.000
1000.000
1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03
CRITICAL SHEAR STRESS, IN (Pa)
ER
OD
IBIL
ITY
CO
EF
FIC
IEN
T, I
N (
cm3
/N-s
)
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
0
10
20
30
40
50
60
70
80
90
100
110
0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0
SHEAR STRESS, IN Pa
TR
AN
SM
ISS
ION
, IN
PE
RC
EN
T
c = 11 Pa
Comparison of Methods: c
CRITICAL SHEAR STRESS, IN PASCALS
0.0001 0.001 0.01 0.1 1 10 100
PE
RC
EN
TIL
E
0
20
40
60
80
100
Original Jet"Mini" JetCohesive Strength Meter
Comparison of Methods: k
0
10
20
30
40
50
60
70
80
90
100
0.01 0.10 1.00 10.00 100.00
ERODIBILITY COEFFICIENT (k)
PE
RC
EN
TIL
E
Large jet
Mini jet
ERODIBILITY COEFFICIENT, IN cm3/N-s
0.01 0.1 1 10
PE
RC
EN
TIL
E
0
20
40
60
80
100
Original Jet"Mini" Jet