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

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