evaluation of belted strand retention fabric and ... · pdf filestudied silt fence using...

Evaluation of Belted Strand RetentionFabric and Conventional Type C Silt

Fence using ASTM standards

Dr. Mark RisseDr. Sidney Thompson

Xianben ZhuKeith Harris

Department of Biological and Agricultural Engineering

Introduction

Users of erosion and sediment control (ESC)products have difficulty of comparing theperformance of different products and techniques.Few methods available for evaluating differentproducts for approval by State agencies.Different test procedures make it hard to compareresults

Literature reviewSome standard tests available

WisDOT (Several Categories)ECTC and North American Stormwater andErosion Control Association are working onproviding test procedures and standards. Labsapproved for testing of erosion mats:

– Colorado State University– San Diego State University– E-Lab, American Exclesior Inc.– Texas Transportation Institute's Hydraulics, Sedimentation, and

Erosion Control Laboratory

• Not Many Procedures for testing Silt Fence

• Wyant(1980) conducted a comprehensive study onsilt fence, which led to development of ASTMD5141 (Filtering Efficiency and flow through)

• Both Kouwen(1990) and Barrett et al. (1995)studied silt fence using different procedures, bothconcluded that deposition from the large pondedvolume created by the fence was the mainmechanism for sediment removal.

• Thiesen(1992) suggested that the Apparent OpeningSize of the fabric determines the amount of storagecapacity of the fence.

• Thomas Carpenter and Joel Sprague creatednew procedures for testing the effectivenessof sediment retention device. Primarilylooking at installation practices.

• Most State agencies look at materialproperties including strength of fabric,opening size, and

flow rate.

What is BSRF?

• Woven geotextile.

• Biodegradable

• Innovative design

Objective

• To test the filtering efficiency and flow rate ofBelted Strand Retention Fencing (SiltSaver) andType C silt fence using ASTM standard D5141

• To evaluate the effectiveness of this new fencematerial.

• Not approved in Georgia due to low flow rate andnot meeting strength specifications.

Procedure 1

• Flume constructed

according to ASTM

standard. (85x125 cm)

• Flume set at a 8% slope

• 50 L of mixture added to

top.

• Collect all effluent.

Procedure• The first run sediment free water

• Second run was a 2890 ppm (standard) concentration ofsediment laden water.

• A 3rd run with a 5780 ppm (Double) concentration wasrun on the same fence.

• Total time of flow was recorded to 20 minutes.

• Subsamples of the sediment laden water and the filtratewere taken for analysis.

• Procedure was replicated three times for each fence.

• Three different soils, Sand, Clay, Silt Loam

Results: Flow Rate

• Equations in Standard report flow rate inm3/m2/min.

• Errors in equations which are beingcorrected by ASTM.

• We report both the standard measurementand simply L/min.

Clear Water Flow RateAverage Blank Flow

0.5120

0.4235

0.0000

0.1000

0.2000

0.3000

0.4000

0.5000

0.6000

BSRF Type C

Fence Type

F2

0 (

m3/m

2/m

in)

No significant difference

Flow Rate

Standard Concentration

4.20

1.37

9.58

1.93 2.001.19

0.00

2.00

4.00

6.00

8.00

10.00

12.00

Sand Silt Loam Clay Loam

Soil Type

Av

g fl

ow

ra

te (L

/min

)

BSRF

Type C


0.0022 0.0023

0.0470

0.0014 0.0016

0.1072

0.0000

0.0200

0.0400

0.0600

0.0800

0.1000

0.1200


Soil Type

F2

0 (

m3/m

2/m

in)

BSRF

Type C

Type C had higher flow rate with sediment.

Flow rate much lower on siltand clay.

Flow RateDouble Concentration

0.0015 0.0005 0.00050.00210.00150.0098

0.0000

0.0200

0.0400

0.0600

0.0800

0.1000

0.1200


Soil Type

F2

0 (m

3/m

2/m

in)

BSRF

Type C

Double Concentration

0.47 0.45

2.08

1.33 1.31

1.78

0.00

0.50

1.00

1.50

2.00

2.50


Soil Type

Av

g f

low

ra

te (

L/m

in)

BSRF

Type C

Flow rates lower at double conc.

Less difference betweensoil types.

Flow Rate Conclusions• The type C silt fence had higher average flow rates

for all soil types in both the standard (2890 ppm)and double (5780 ppm) concentrations but not theclear.

• BSRF had a more than 60% reduction in flow ratewhen running a double concentration after thestandard concentration for finer textured soilswhile the type C had less than 35% reduction.

• Sediment on the fence appeared to influence flowrate.

Suspended Solids (Ss) and FilterEfficiency (Fe)

• Standard states that only Filtering Efficiency should

be reported.

• We report Efficiency and Suspended Solids

concentration and Turbidity of effluent.

• With low slope, considerable amounts of sediment

settled out prior to even reaching the silt fence.

Suspended Solids in Effluent


73.3

166.7163.0

608.7

509.3

139.3

0.0

100.0

200.0

300.0

400.0

500.0

600.0

700.0


Soil Type

Av

g.

Su

sp

en

de

d S

oli

ds

(p

pm

)

BSRF

Type C


76.792.3

300.7

161.3

46.0

365.7

0.0

100.0

200.0

300.0

400.0

500.0

600.0

700.0


Soil Type

Av

g.

Su

sp

en

de

d S

oli

ds

(p

pm

)

BSRF

Type C


94.4

97.396.8

87.3

89.6

98.4

75.0

80.0

85.0

90.0

95.0

100.0


Soil Type

Fil

ter

eff

icie

nc

y

BSRF

Type C


97.5

94.295.294.4

78.9

82.4

75.0

80.0

85.0

90.0

95.0

100.0


Soil Type

Fil

ter

eff

icie

nc

y

BSRF

Type C

Filter Efficiency

Turbidity


43.3

92.7

138.3

77.0

452.7

359.3

0.0

50.0

100.0

150.0

200.0

250.0

300.0

350.0

400.0

450.0

500.0


Soil Type

Av

g.

turb

idit

y (

NT

U's

)

BSRF

Type C


43.3

167.0

220.7

77.7 83.2

25.5

0.0

50.0

100.0

150.0

200.0

250.0

300.0

350.0

400.0

450.0

500.0


Soil Type

Av

g.

turb

idit

y (

NT

U's

)

BSRF

Type C

Results for Filtering Efficiency• The BSRF proved more efficient at removing

suspended solids and turbidity for all soil types atboth the standard and double concentration runs.

• BSRF reduced suspended solids values 2 to 3 timeslower than the type C fence.

• BSRF retained its filtering efficiency for the doubleconcentration while the

type C lost 12 to 15% of

its efficiency on the finer

textured soils.

• Turbidity results mirrored

the Suspended Solids data.

• Both fence materials had

high filtering efficiencies.

Modifications to Standard• Since Filtering efficiency was high and significant

settling occurred prior to the fence, an additional test

was conducted with the flume set at a 58% slope.

• We also wanted to examine the influence of higher

hydraulic heads on the various fence materials since

this would be important to field applications.

• Same procedures were used.

• Three replicates of the silt loam soil were conducted

for each fence material.

1.941.95

3.59

1.57

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

Standard Double

Concentration

BSRF

Type C

Flow Rate @ 58% slope

• The higher relief resulted in a greater flow rates for bothfence materials. The BSRF fence under high reliefconditions exhibited higher flow rates at both the standardand double concentrations.

447.40474.23

289.87

860.07

0.00

100.00

200.00

300.00

400.00

500.00

600.00

700.00

800.00

900.00

1000.00

Standard Double

Concentration

BSRF

Type C

84.5283.59

89.97

70.24

60.00

65.00

70.00

75.00

80.00

85.00

90.00

95.00

Standard Double

Concentration

BSRF

Type C

BSRF was more efficientthan the type C though thedifferences were lesspronounced then 8 % slope.

Performance @ 58%

Suspended solidswere higher and efficiencies lower for both fences.

One additional test of effectiveness

• Flume is expensive toconstruct.

• We tested a new apparatusthat could be used to testsimilar properties.

• 5L of water with samesediment conc.

Results

• Flow rates were similar between BSRF (0.27L/min) and Type C (0.22 L/min) although theType C had a slightly higher clear water flowrate (22 to 19 L/min).

• BSRF had a higher filtering efficiency (95%to 88%) and percent reduction in turbidity(82% to 60%) than type C fence materials.

• These results were comparable to the flumetest results at 8%.

OK, so it filters better and flows at comparablerates, Is it strong enough though?

Typical Response

30 degree sidewalls, 4 ft post spacing

Hydrostatic load of 8 inches

Finite Element grid: 2 inches

Modeling of Loads on fence

Field verification of model

Model Validation

Strength Conclusions• BSRF fence withstood loads that would normally

be encountered in the field.

• While the fabric did not meet GA DOT specs fortensile strength or deflection, the design appearsadequate.

• Biodegradable design offers environmental andsafety benefits that should be considered.

• Modeling led to design improvements.

Questions??

Thanks to SiltSaver, Inc. for providing funds.

Material is not approved for use in Georgia by GA SWCC.

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