Presentation on design of geotextile filters

1. Geotextile Filters: No Design Required?
Michael D. Harney
University of Washington

2. Why Use a Geotextile?
Geotextile filters offer
• Comparable performance
• Improved economy
• Consistent properties
• Ease of placement

3. Applications
• Filters around trench and edge drains
Trench Drain Installation, 1

4. Applications
• Filters around trench and edge drains
Trench Drain Installation, 2

5. Applications
• Filters around trench and edge drains
Trench Drain Installation, 3

6. Applications
• Filters around trench and edge drains
A Geocomposite Trench Drain

7. Applications
• Filters around trench and edge drains
Edge Drain Installation

8. Applications
• Drains for retaining walls and abutments
Panel Drain Installation

9. Applications
• Drains for retaining walls and abutments
Panel Drain
Schematic

10. Applications
• Wraps for slotted or jointed drain and well
pipes

11. Applications
• Interceptor, toe drains and surface drains

12. Applications
• Chimney and toe drains for earth dams and
levees

13. Mechanism
A geotextile serves as a catalyst to
promote the upstream soil to generate
its own internal filter.

14. Design Req’d?
Must perform the same functions as a
graded granular filter:
• Retain the soil (retention)
• Allow water to pass (permeability)
• Function for life of structure (clogging)

15. Design
Most Widely Used Criteria:
FHWA GTx Filter Design Criteria
(Christopher and Holtz, 1985)

16. Level of Design Req’d
1. Assess project’s nature:
• A critical or less critical project?
2. Assess severity of hydraulic conditions:
• Severe or less severe?

17. Criticality
Critical Nature of the Project
Item Critical Less Critical
Risk of loss of HIGH NONE
life / damage
Repair cost vs Very Great Equal to or
installation cost Less
Evidence of None Apparent
clogging

18. Severity
Severity of the Conditions
Item Severe Less Severe
Soil to be Gap-Graded; Well-Graded;
Drained pipable; Uniform
dispersive
Hydraulic HIGH Low
Gradient
Flow Conditions Dynamic; Steady State
Cyclic or
Pulsating

19. Filter Design
Retention Criteria
AOS or O95  B D85
Sands (< 50% passing No. 200 sieve):
Cu B
<2 1
2 Cu4 0.5Cu
4< Cu<8 8/Cu
8 1

20. Filter Design
Retention Criteria (cont’d.)
AOS or O95  B D85
Silts or Clays (> 50% passing No. 200 sieve):
Geotextile Type B
woven 1
nonwoven 1.8
For both AOS  3mm

21. Filter Design
Retention Criteria (cont’d.)
AOS or O95  B D85
For dynamic, pulsating or cyclic flow:
Use B = 0.5

22. Filter Design
Permeability / Permittivity Criteria
Permeability:
Project Nature Criterion
Less Critical kgeotextile  ksoil
Critical kgeotextile  10ksoil

23. Filter Design
Permeability / Permittivity Criteria
(cont’d.)
Permittivity:   kgeotextile / tgeotextile
% passing No. 200 Criterion
<15%  0.5 s-1
15% to 50%  0.2 s-1
>50%  0.1 s-1

24. Filter Design
Clogging Resistance
For less critical / less severe conditions:
O95 (geotextile)  3 D15 (soil)
AND:
• For nonwovens: ngeotextile  50%
• For wovens: Percent Open Area  4%
For critical / severe conditions:
• Perform a FILTRATION TEST with on-site
soils and candidate geotextile.

25. Conventional Clogging Tests
• Existing tests:
– Gradient ratio (GR) test (ASTM D 5101)
– Hydraulic conductivity ratio (HCR) test
(ASTM D 5567)
• GR and HCR have disadvantages

26. The Flexible Wall Gradient
Ratio Test (FWGR)
• A gradient ratio test using a flex wall
permeameter.
• Overcomes GR and HCR
disadvantages
• Biggest advantage: TIME REQUIRED

27. Filter Design
Survivability and Endurance
Survivability
• Must be sure that the geotextile can
survive the construction process.
Endurance
• Biological / chemical clogging
• Ultraviolet Degradation

28. Other Selection Considerations
1. For graded gravels and coarse sands: very
open monofilament wovens  high flow rates
permitted; low risk of blinding
2. Sands and Gravels, <20% Fines: Open
monofilament wovens, needlepunched
nonwovens  reduce the risk of blinding
3. 20% to 60% fines: Filtration tests are
recommended

29. Other Selection Considerations
(cont’d)
4. >60% Fines: Heavy weight, needlepunched
NWs and heat-bonded NWs work best—fines
will not pass.
5. Gap-Graded, Cohesionless: Consider a
uniform sand filter above a very open
geotextile
6. Silt w/ sand seams: Consider a heavy (thick)
NP NW so water may flow laterally through
GTx, if it becomes locally clogged.

30. Considerations for
Geocomposites
1. Compaction directly against the filter
should be avoided. Consider using
clean granular backfill against the drain.
2. Careful overlapping required at joints to
prevent infiltration.
3. Geotextile should extend beyond the
ends of the drain

31. Helpful References
Design Procedure:
• Geosynthetic Engineering; Holtz, et al.
• Designing w/ Geosynthetics; Koerner
Available Products and Specs
• GFR Specifier’s Guide, 2002 Ed.

32. Solid Advice
―All geotextiles will work in some
applications, but no one geotextile will
work in all applications‖
--The Late Allan Haliburton
Geosynthetics Pioneer

33. No Cookie-Cutter Designs!