A sandy silt with D85 = 0.10 mm is to be drained by a perforated drainage pipe surrounded by a 3/4 inch gravel and a filter fabric as shown in Figure 8.38. The estimated flow rate to this drain will be 5 gal/min per lineal foot (half of which is from each side), and the head loss through the fabric

Question

A sandy silt with [latex]D_{85} = 0.10 mm[/latex] is to be drained by a perforated drainage pipe surrounded by a 3/4 inch gravel and a filter fabric as shown in Figure 8.38. The estimated flow rate to this drain will be 5 gal/min per lineal foot (half of which is from each side), and the head loss through the fabric must not exceed 0.1 ft. Select an appropriate filter fabric from Table 8.3.

Product Designation	Thickness (mils)	Puncture Resistance (lb)	Apparent Opening Size (AOS)		Permittivity [latex](s^{-1})[/latex]
Product Designation	Thickness (mils)	Puncture Resistance (lb)	[latex]O_{95}[/latex] (mm)	Sieve Size	Permittivity [latex](s^{-1})[/latex]
TG 1000	155	170	0.150	100	0.6
TG 800	120	145	0.177	80	1.0
TG 750	100	115	0.177	80	1.1
TG 700	85	100	0.210	70	1.3
TG 650	75	95	0.210	70	1.6
TG 600	65	80	0.250	60	1.8
TG 550	55	70	0.300	50	1.9
TG 500	45	60	0.300	50	2.3
Evergreen is one of many manufacturers. Information on other products may be found in IFAI (1997) or by contacting the manufacturers.

Accepted Answer

Check maximum allowable apparent opening size (AOS).

[latex]O_{95} < (2 or 3) (D_{85})_{soil}[/latex]
< (2 or 3) (0.10 mm)
< 0.2 - 0.3 mm

Check minimum permittivity requirement.

[latex]Q=\left(5 gal/min\right) \left\lgroup\frac{1 ft^{3}/s}{449 gal/min} \right\rgroup =0.01 ft^{3}/s[/latex]

[latex]A = (0.5 ft + 1.0 ft + 0.5 ft) (1.0 ft) = 2.0 ft^{2} [/latex]

[latex]\frac{Q}{A} =\frac{0.01 ft^{3}/s}{2.0 ft^{2}} =0.005 ft/s[/latex]

minimum required [latex]\Psi =\frac{Q/A}{\Delta h} =\frac{0.005 ft/s}{0.1 ft} =0.05 s^{-1}[/latex]

Conclusion

Fabrics TG 750-TG 1000 definitely satisfy the AOS requirement, and the other fabrics marginally satisfy it. All of the fabrics satisfy the permittivity requirement by a wide margin. The costs of each are not given, so select the TG 1000 fabric, because it has the highest puncture strength, while still meeting the hydraulic requirements. However, if this fabric is too expensive or unavailable, the TG 800 or TG 750 also would be acceptable.

Occasionally, geotextile drains may be subject to soil clogging, which blocks the pores and prevents or severely restricts the flow of water. Although this is usually not a problem, the following conditions have been found to cause clogging (Koemer, 1998):

• Cohesionless sands and silts
• Gap-graded soils, and
• High hydraulic gradients

When all three of these conditions are present, it is better to use graded soil filters.

Clogging also can occur in soils where chemical precipitates tend to build up on the fabric, or when the groundwater contains a large concentration of biological matter (i.e., municipal landfill leachate).

Question 8.8: A sandy silt with D85 = 0.10 mm is to be drained by a perfor...

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