measuring erodibility of cohesive soils by gregory j. hanson usda-ars hydraulic engineering research...

Measuring Erodibility of Cohesive Soils

By Gregory J. HansonUSDA-ARS Hydraulic Engineering Research Unit

Overview

Importance of measuring erodibility.

Development of erodibility measurement apparatus

Description of Apparatus and method of testing

Analysis of results

Things to be aware of in testing and analysis

Conclusions

ESSENTIAL CRITERIA OF AN EROSION TEST

• BASED ON SOUND HYDRAULIC PRINCIPLES

• BASED ON KNOWLEDGE OF MATERIALS

• REPEATABLE AND CONSISTANT

• APPLICABLE IN CURRENT EQUATIONS

• APPLICABLE TO IN-SITU AND LAB TESTING

• SIMPLE, QUICK, AND INEXPENSIVE

Several methods are available for testing of soils.Wan and Fell (2004) place them in six categories:

1. Flume tests2. Jet erosion tests3. Rotating cylinder test4. Soil dispersion test5. Hole or crack tests6. Erosion Function Apparatus

Flume Tests

0 1 2 3 4 5 60

1

2

3

4

5

6

Head

cut M

igratio

n (m

)

Time (h)

dX/dt=0.83 m/h

Widening Discharge

Time (minutes)

0 100 200 300 400 500 600

He

ad

cu

t M

igra

tio

n (

m)

0

2

4

6

8

10

Run 4 - wc%=9.20 d = 1.68 g/cm3

Ar = 5.4 m/hr

Run 6 - wc% = 14.40 d = 1.79 g/cm3

Ar = 0.15 m/hr

High Compactive Effort

C om pactive E ffort~ 4000 ft-lb /ft3

W C % = 9.2%

W C % = 14.4%

Surface Detachment Surface Detachment

dY/dtdY/dt

Headcut Migration

dX/dt

Several methods are available for testing of soils.Wan and Fell (2004) place them in six categories:

1. Flume tests2. Jet erosion tests3. Rotating cylinder test4. Soil dispersion test5. Hole or crack tests6. Erosion Function Apparatus

A non-submerged jet test was used as a relative measure of erosion resistance .








?

?

Velocity

Time (min)

0 50 100 150 200 250 300

Sco

ur D

epth

(m

)

0.0

0.1

0.2

6.5 m/s4.8 m/s4.0 m/s3.4 m/s2.6 m/s1.7 m/s

Velocity

Uo(t/t1)-0.931(m/s)

10-5 10-4 10-3 10-2 10-1

Ds/

t (m

/s)

10-6

10-5

10-4

10-3

6.5 m/s

4.0 m/s

1.7 m/s2.6 m/s

3.4 m/s3.0 m/s

4.8 m/s

Y=0.015(X)

Ji (ASTM STD

D5852-95)







• SIMPLE, QUICK, AND INEXPENSIVE?

Erodibility Measurement

e, (Pa)

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Er,

(cm

/s)

0

2

4

6

8

10

Surface Detachment Surface Detachment

dY/dtdY/dt dd

dY/dt = EdY/dt = Err = = kkdd((ee – – cc))aa

H eadW ater S urface

O rig inal Bed

J

JJ

p

ie

P otentia lC ore

D iffusedJet

S couredB ed Jet C enterline

do

Stress D istribution

2

i

poi J

J

for J i > Jp

- J

Jk =

dt

dJc2

p2

od

T*

10-3 10-2 10-1 100

H*

0.0

0.2

0.4

0.6

0.8

1.0

Function of T* for Hp*= 0.00

CL-ML

c = 0.74 Pa

kd

= 5.7 cm3/N-sJ

Overview




Analysis of results


Conclusions

Vertical jet apparatus:

PumpHead tankJet TubeSubmergence tankHosesPoint gagePressure gageValves

Submergence tank

Jet Tube (1m long by 5cm id and 6.4 cm od)

Jet tube holding brackets

Point gage

Head tank (1m long by 5 cm id and 6.4 cm od)

Air relief valve on jet tube

Drive hammer locations on submergence tank

Head tank mast holder

Head tank Mast

Multi Angle JET DeviceMulti Angle JET Device

Laboratory JET DeviceLaboratory JET Device

Adjustable head tank

Hoses

Gage point

Water Source

Jet tube

Nozzle

Deflector

Specimen

Lid

Submergence tank

““mini”JET Devicemini”JET Device

1) Layout location ofpump head tank, jet

apparatus, and hoses for convenience and safety of the operator.

Procedure for conducting JET


2) Once the site is located and layout is determined, drive the submergence tank into the soil surface.


3) After the tank is set, the jet tube and point gage are attached to the submergence tank.


4. Set the head tank & mast on the submergence tank and set the head tank height.


5. Connect hoses from 1) water supply to head tank, 2) head tank to jet tube, and 3) head tank overflow.


6. Determine the height of the jet nozzle (orifice) Ji by taking point gage readings at the nozzle and initial soil surface reading at time zero. Also take a zero point gage reading at the deflector plate as a reference point. Enter the point gage readings on the data sheet. 7. Place the deflector plate in front of the jet nozzle and set the point gage against the plate. This closes off the nozzle. Initiate flow to the head tank and jet tube. This process should remove air from the hose between the jet tube and head tank. At the top of the jet tube is also an air release valve to remove air from the jet tube.

Pre-test filling of submergence tank.


8. Once the system is filled with water, set the point gage upstream of the jet nozzle at least 10 nozzle diameters to eliminate any flow disturbance from the point gage. The water then proceeds to impact the deflector plate and fill the submergence tank. 9. Once the submergence tank is filled, take an initial head reading by measuring the distance from the top of the head tank to the top of the water surface in the submergence tank or stream channel, whichever is higher. Then move the deflector plate out of the way of the orifice to begin testing. Record the time of test initiation and duration.

Measuring Head from surface to surface.


10.Take point gage readings of the bed at time intervals adjusted to the rate of erosion. Typical time intervals will be 30 sec to 1 min initially and will increase to 10 to 20 minutes depending on the rate of erosion. A set of 10 to 12 readings is recommended with a total test duration of up to 2 hours. The operator may find that it is necessary to feel the tip of the point gage touch the soil surface to avoid pushing the tip into the soil. This is often necessary in soft soils.

Recording Point Gage Readings.


Match the stress range of interest to the stress magnitude of the jet test. The initial stress is set by the controlling the height of the nozzle and the head on the jet, h.

head , h (m)

0.0 0.5 1.0 1.5 2.0 2.5 3.0

i, (P

a)

0

50

100

150

200

250 Jp/Ji

1

2

5

head, h (ft)

0 1 2 3 4 5 6 7 8 9 10

0

1

2

3

4

5

i , (lb/ft 2)

Overview




Analysis of results


Conclusions

JET DATA

DATE ######JET TEST

LOCATION Station 53 in flume OPERATOR gjh

ZERO POINT GAGE READING 1.222 TEST # 2

PRELIMINARY HEAD SETTING 87 PT GAGE RDG @ NOZZLE 1.263

NOZZLE DIAMETER (IN) 0.2505 NOZZLE HEIGHT (FT) 0.200

SCOUR DEPTH READINGS HEAD SETTING

TIME DIFF PT GAGE MAXIMUM TIME HEAD

(MIN) TIME READING DEPTH OF (MIN) (IN)

(MIN) (FT) SCOUR (FT)

0 0 1.063 0.000 0 87.00

10 10 1.032 0.031 10 87.00

20 10 1.023 0.040 20 87.00

30 10 1.014 0.049 30 87.00

40 10 0.999 0.064 40 87.00

50 10 0.990 0.073 50 87.00

60 10 0.977 0.086 60 87.00

70 10 0.974 0.089 70 87.00

80 10 0.973 0.090 80 87.00

0 5 10 15 20 25

Ds

(cm

)

0.0

0.1

0.2

0.3

0.4

Time (hrs)

Soil C

Two Step Process:

1. Determine critical tractive stress

2. Determine the erodibility coefficient

x

0 1 2 3 4 5 6 7 8

f

-7

-6

-5

-4

-3

-2

-1

0

1

2

A

fo

Asymptote

c= 2 Pa

T*

0.001 0.01 0.1 1

H*

0.0

0.2

0.4

0.6

0.8

1.0

FunctionData

J*

T*

0.001 0.01 0.1 1

H*

0.0

0.2

0.4

0.6

0.8

1.0

FunctionData

J*

kd = 1.2 cm3/N-s

In-situ and LaboratoryMeasurement of Erodibilityrr= = kkdd((ee--cc))

Overview




Analysis of results


Conclusions

Items to be aware of:

1. The operator should strive to maintain constant differential pressure during testing.

2. Depending on the condition of the soil, the operator may need to seal the inside perimeter of the submergence tank prior to testing.

3. If the soil material is soft the operator may need to make sure the point gage does not push down into the soil surface.

4. The operator may need to sweep material out from under the jet during testing.

5. The operator should test in the stress range of interest.

6. For homogeneous – isotropic soils, scour should decrease with time. Soils are not necessarily homogeneous or isotropic.

Sealing submergence tank if necessary.

Overview




Analysis of results


Conclusions

Conclusions:

Assessment of erodibility is essential for predicting material performance and erosion modeling.

The jet testing apparatus provides a method for directly testing the erodibility of soil materials and fits the sixessential criteria.

measuring erodibility of cohesive soils by gregory j. hanson usda-ars hydraulic engineering research...

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