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Publication Number:  FHWA-HRT-15-033    Date:  May 2015
Publication Number: FHWA-HRT-15-033
Date: May 2015

 

Scour in Cohesive Soils

CHAPTER 6. EROSION TESTING PROTOCOL AND RESULTS

All erosion tests in the ESTD were conducted by applying one or more of the seven combinations of flow, moving belt speed, and bed roughness that reproduced open channel flow conditions to each of the 17 soil types in table 10. Distilled water was used in the ESTD system.

For testing, each soil specimen was placed in an aluminum tray with sides extending up 0.5 inches (13 mm). The remaining 0.28 inches (7 mm) of the cylindrical soil sample is unconfined and available for erosion. The tray is fixed onto the sensor disk of the force gauge. Initially, the surface of the soil specimen is flush with the bed of the test channel. The sensor disk and soil specimen are elevated as erosion occurs to maintain this flush condition. Initial roughness of the soil specimen is carefully prepared so that it is similar to the bed roughness of test channel. When successful in matching the roughness, the initial average shear stress on the soil specimen should approximate the bed shear stress.

During each erosion test, the measured bed shear stress under each flow condition should be maintained as a constant. As erosion occurs, the measured shear stress would decrease if the specimen is held at a fixed position. Therefore, the specimen is automatically raised when the measured shear stress decreases below the target value so that the remaining surface of the soil sample is flush with the fixed bed.

Raising the specimen is controlled by a feedback loop on the force gauge. A shear stress range of 0.021 lbf/ft2 (1 Pa) is applied to determine whether the soil specimen should be elevated. Readings are checked every 2 s. If the shear stress dropped by more than the allowable amount, then the platform is elevated for an increment of time referred to as the elevation period. The elevation period varies depending on the erodibility of the soil specimen. Rapidly eroding soil specimens require longer elevation periods compared with more slowly eroding soil specimens. Because any protrusion or depression will introduce form drag and potentially alter the erosion patterns, these measures are important to reduce the potential for form drag.

A typical set of recorded instantaneous shear stress measurements is plotted in figure 39. The instantaneous shear stress fluctuated around the target bed shear stress of 0.31 lbf/ft2 (15 Pa). The figure also shows the elevation increases of the platform and soil sample. The soil elevation curve can be approximated with a linear function. The slope of the function, in units of depth versus time, represents one estimate of the erosion rate.

Each erosion test is typically run for approximately 1 h, but the time is adjusted depending on the erosion rate. The initial and final soil masses are measured. The mass loss is divided by the erosion time to calculate a mass erosion rate (mass per unit time). This mass erosion rate is divided by the surface area of the soil sample to derive a more useful mass erosion rate per unit area.

Water content of several soils was measured again after the 1-h test. Water content increased by less than 0.5 percent for soils prepared in the pugger mixer. The water content increased by approximately 2 percent for the compacted soils. Therefore, for soils prepared in the pugger mixer, the wet density did not change significantly. Because of this result, the total eroded soil volume can be calculated along with a depth-based erosion rate. This results in an erosion rate in depth per unit time.

In this graph, the abscissa represents time ranging from 0 to 800 s. There are two ordinate axes. The left ordinate axis represents shear stress ranging from 0 to 0.4 poundforce/square ft (0 to 19.2 Pa). The graph of shear stress shows a rapid climb from 0 to approximately 0.3 poundforce/square ft (0 to approximately 14.4 Pa) within 60 s. For the remainder of the test, the shear stress fluctuates between approximately 0.28 and 0.34 poundforce/square ft (13.4 and 16.3 Pa). A dashed line goes through the fluctuating shear stress representing the constant target shear stress of 0.31 poundforce/square ft (15 Pa) in the erosion test. The right ordinate axis represents soil level ranging from 0 to 0.4 inches (0 to 10.2 mm). The graph of soil level shows the gradual step function as the soil erodes from 0 for the first 60 s then goes up to approximately 0.18 inches (4.6 mm) at about 700 s. A best-fit line for soil level is shown.
1 inch = 25.4 mm.
1 lbf/ft2 = 47.8 Pa.
Figure 39. Graph. Example data recorded for sample with soil index 4

Erosion of soils prepared with the pugger mixer was in a form of soil lumps and cloudy matter. When bed shear stress was small, the primary erosion mechanism was the separation of soil lumps from the surface. The lumps were entrained from all areas of the soil surface, resulting in an irregular soil surface over time. As the bed shear stress increased, the cloudy matter appeared heavier. Figure 40 shows this change for soil sample 1W181. For the lower bed shear stress of 0.10 lbf/ft2 (4.9 Pa), there was no cloudiness (figure 40(a)). The soil was eroded in small lumps over the soil surface. When the bed shear stress was increased to 0.21 lbf/ft2 (10 Pa), increasing cloudiness was observed, and larger lumps of soil were entrained in the flow (figure 40(b)). When the bed shear stress was increased to 0.27 lbf/ft2 (13.1 Pa), the cloudiness increased, and the erosion rate increased. At this higher shear stress, shallow grooves appeared on the soil surface along the flow direction (figure 40(c)).

A summary of the test matrix and results is provided in table 15 for the erosion tests for soils prepared with the pugger mixer. The test ID-PxWyyySz-beginning with "P" indicates that these runs were completed with soils prepared by the pugger mixer. The x signifies the soil index (1-6), the yyy signifies the water content with the decimal point removed, and the z signifies the bed shear stress applied in the erosion test. S1 through S7 represent, from lowest to highest, the seven shear stress conditions described previously. The shear values are shown in table 15.

The table also includes the running time for each test and the measured soil loss in mass per unit time. The computation of the mass erosion rate (per unit area) and depth erosion rate are also summarized in the table.

In this photo, a progression of erosion changes are seen. For the lower bed shear stress of 0.10 lbf/ft2 (4.9 Pa), there was no cloudiness (image a). The soil was eroded in small lumps over the soil surface. When the bed shear stress was increased to 0.21 lbf/ft2 (10 Pa), increasing cloudiness was observed, and larger lumps of soil were entrained in the flow (image b). When the bed shear stress was increased to 0.27 lbf/ft2 (13.1 Pa), the cloudiness increased, and the erosion rate increased. At this higher shear stress, shallow grooves appeared on the soil surface along the flow direction (image c).
In this photo, a progression of erosion changes are seen. For the lower bed shear stress of 0.10 lbf/ft2 (4.9 Pa), there was no cloudiness (image a). The soil was eroded in small lumps over the soil surface. When the bed shear stress was increased to 0.21 lbf/ft2 (10 Pa), increasing cloudiness was observed, and larger lumps of soil were entrained in the flow (image b). When the bed shear stress was increased to 0.27 lbf/ft2 (13.1 Pa), the cloudiness increased, and the erosion rate increased. At this higher shear stress, shallow grooves appeared on the soil surface along the flow direction (image c).
In this photo, a progression of erosion changes are seen. For the lower bed shear stress of 0.10 lbf/ft2 (4.9 Pa), there was no cloudiness (image a). The soil was eroded in small lumps over the soil surface. When the bed shear stress was increased to 0.21 lbf/ft2 (10 Pa), increasing cloudiness was observed, and larger lumps of soil were entrained in the flow (image b). When the bed shear stress was increased to 0.27 lbf/ft2 (13.1 Pa), the cloudiness increased, and the erosion rate increased. At this higher shear stress, shallow grooves appeared on the soil surface along the flow direction (image c).
Figure 40. Photo. Erosion soil sample 1W183 with increasing shear

Table 15. Erosion test matrix and results for soils prepared by pugger mixer.

Test ID

Soil Sample

Shear Stress, Pa

Running Time, s

Soil Loss, g

Mass Erosion Rate, g/s/m2

Depth Erosion Rate, mm/h

P1W156S3

1W156

4.9

3,763

3.58

0.30

0.50

P1W156S4

1W156

7.2

5,081

18.04

1.12

1.87

P1W156S5

1W156

10.0

3,512

30.14

2.71

4.53

P1W156S6

1W156

13.1

1,614

37.38

7.32

12.22

P1W156S7

1W156

15.0

1,199

40.29

10.62

17.73

P1W165S3

1W165

4.9

3,644

3.74

0.32

0.54

P1W165S4

1W165

7.2

3,340

1.58

0.15

0.25

P1W165S5

1W165

10.0

1,244

36.64

9.31

15.54

P1W165S6

1W165

13.1

1,331

39.74

9.43

15.75

P1W165S7

1W165

15.0

1,497

37.17

7.84

13.10

P1W181S2

1W181

3.6

7,842

14.14

0.57

0.98

P1W181S3

1W181

4.9

2,398

15.09

1.99

3.43

P1W181S5

1W181

10.0

1,115

40.28

11.41

19.72

P1W181S6

1W181

13.1

430

36.15

26.56

45.88

P2W147S3

2W147

4.9

3,627

1.38

0.12

0.20

P2W147S4

2W147

7.2

3,932

7.29

0.59

0.97

P2W147S5

2W147

10.0

4,323

16.60

1.21

2.01

P2W147S6

2W147

13.1

3,678

19.76

1.70

2.81

P2W147S7

2W147

15.0

3,875

31.95

2.60

4.31

P2W167S3

2W167

4.9

3,400

2.42

0.22

0.37

P2W167S4

2W167

7.2

3,804

11.70

0.97

1.59

P2W167S5

2W167

10.0

4,355

16.97

1.23

2.02

P2W167S6

2W167

13.1

1,964

39.03

6.28

10.29

P2W177S2

2W177

3.6

2,485

16.74

2.13

3.46

P2W177S3

2W177

4.9

1,785

30.26

5.36

8.71

P2W177S4

2W177

7.2

764

31.32

12.95

21.06

P2W177S5

2W177

10.0

628

31.87

16.03

26.07

P2W177S6

2W177

13.1

451

32.78

22.96

37.34

P3W160S3

3W160

4.9

3,935

0.64

0.05

0.09

P3W160S5

3W160

10.0

3,810

0.54

0.04

0.07

P3W160S6

3W160

13.1

1,755

15.43

2.78

4.63

P3W160S7

3W160

15.0

3,186

5.04

0.50

0.83

P3W180S3

3W180

4.9

3,379

3.90

0.36

0.60

P3W180S4

3W180

7.2

3,621

9.56

0.83

1.38

P3W180S5

3W180

10.0

3,538

15.67

1.40

2.31

P3W180S6

3W180

13.1

2,779

20.42

2.32

3.83

P3W180S7

3W180

15.0

2,014

20.14

3.16

5.21

P4W189S3

4W189

4.9

3,551

5.08

0.45

0.77

P4W189S5

4W189

10.0

3,758

20.60

1.73

2.97

P4W189S6

4W189

13.1

1,869

15.49

2.62

4.49

P4W189S7

4W189

15.0

3,790

38.90

3.24

5.56

P4W198S3

4W198

4.9

3,814

16.26

1.35

2.32

P4W198S5

4W198

10.0

1,262

34.76

8.70

15.02

P4W198S7

4W198

15.0

728

31.73

13.77

23.76

P4W217S2

4W217

3.6

3,502

8.31

0.75

1.29

P4W217S3

4W217

4.9

2,686

13.30

1.56

2.70

P4W217S4

4W217

7.2

1,090

34.91

10.12

17.45

P4W217S5

4W217

10.0

678

36.88

17.18

29.64

P4W217S6

4W217

13.1

613

34.39

17.72

30.57

P5W215S5

5W215

10.0

3,658

0.66

0.06

0.10

P5W215S6

5W215

13.1

3,565

2.23

0.20

0.34

P5W215S7

5W215

15.0

3,909

4.78

0.39

0.67

P5W231S4

5W231

7.2

3,646

7.80

0.68

1.20

P5W231S5

5W231

10.0

3,591

22.70

2.00

3.54

P5W231S6

5W231

13.1

2,782

26.36

2.99

5.31

P5W231S7

5W231

15.0

3,764

29.09

2.44

4.33

P5W248S3

5W248

4.9

3,816

4.82

0.40

0.71

P5W248S4

5W248

7.2

3,600

19.87

1.74

3.09

P5W248S5

5W248

10.0

2,946

32.82

3.52

6.23

P5W248S7

5W248

15.0

1,083

30.63

8.94

15.82

P6W192S4

6W192

7.2

3,716

0.33

0.03

0.05

P6W192S5

6W192

10.0

3,512

3.05

0.27

0.47

P6W192S6

6W192

13.1

3,135

2.55

0.26

0.44

P6W192S7

6W192

15.0

4,067

6.14

0.48

0.81

P6W200S5

6W200

10.0

3,503

4.34

0.39

0.68

P6W200S6

6W200

13.1

3,634

18.59

1.62

2.80

P6W200S7

6W200

15.0

3,513

32.14

2.89

5.01

P6W231S3

6W231

4.9

3,462

3.50

0.32

0.55

P6W231S4

6W231

7.2

3,372

18.17

1.70

2.94

P6W231S5

6W231

10.0

3,831

28.18

2.32

4.01

P6W231S6

6W231

13.1

2,730

29.31

3.39

5.86

P6W231S7

6W231

15.0

1,685

28.83

5.41

9.33

1 ft = 0.3 m.
1 inch = 25.4 mm.
1 lbf/ft2 = 47.8 Pa.
1 oz = 28.3 g.

As would be expected, erosion rate increases with increasing shear stress. Figure 41 illustrates the wide variation of erosion rates for different soil samples using the soil sample with the lowest water content from each soil index. The figure also illustrates that in some cases, the measured values did not always behave in a monotonically increasing manner. (See soil type 3W160.)

The results of the erosion tests on the soils prepared by compaction were not useful because of the problem of slaking discussed previously. Therefore, these results are not reported. It may be noted, however, that the erosion of these soils was also in a form of soil lumps. The lumps were much larger than those for the soils prepared with the pugger mixer. The erosion process also featured large-scale depressions (deep valleys) in the sample surfaces.

In this graph, the abscissa is shear stress ranging from 0 to 0.35 poundforce/square ft (0 to 16.8 Pa). The ordinate represents depth erosion rate ranging from 0 to 0.8 inches/h (0 to 20.3 mm/h). Going from least erodible to most erodible, the soil types shown are 5W215, 3W160, 6W192, 2W147, 4W189, and 1W156. The least erodible soil does show erosion rates greater than 0.03 inches/h (0.8 mm/h), while the most erodible soil eroded at 0.7 inches/h (17.8 mm/h) at a shear stress of 0.31 poundforce/square ft (14.8 Pa). The 3W160 soil appears to exhibit inconsistent behavior with erosion rates jumping up at a shear stress of 0.27 poundforce/square ft (12.9 Pa) but then declining significantly at a higher shear stress.
1 inch = 25.4 mm.
1 lbf/ft2 = 47.8 Pa.
Figure 41. Graph. Representative plots of erosion rate versus shear stress

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