2. DRAINAGE BLANKET.

Figure 5 shows typical filter and drainage blanket

installations.

a. Permeability. Figure 6 (Reference 5, Subsurface Drainage of

Highways, by Barber) gives typical coefficients of permeability for clean,

coarse-grained drainage material and the effect of various percentages of

fines on permeability. Mixtures of about equal parts gravel with medium to

coarse sand have a permeability of approximately 1 fpm. Single sized, clean

gravel has a permeability exceeding 50 fpm. For approximate relationship of

permeability versus effective grain size D+10,, see Figure 1, Chapter 3.

b. Drainage Capacity. Estimate the quantity of water which can be

transmitted by a drainage blanket as follows:

q = k [multiplied by] i [multiplied by] A

quantity of flow, ft.3-/sec

where

q=

k=

permeability coefficient, ft/sec

i=

average gradient in flow direction, ft/ft

cross sectional area of blanket, ft.2-

A=

The gradient is limited by uplift pressures that may be tolerated at

the point farthest from the outlet of the drainage blanket. Increase

gradients and flow capacity of the blanket by providing closer spacing of

drain pipes within the blanket.

(1) Pressure Relief. See bottom panel of Figure 7 (Reference 6,

Seepage Requirements of Filters and Pervious Bases, by Cedergren) for

combinations of drain pipe spacing, drainage course thickness, and

permeability required for control of flow upward from an underlying aquifer

under an average vertical gradient of 0.4.

(2) Rate of Drainage. See the top panel of Figure 7 (Reference 5)

for time rate of drainage of water from a saturated base course beneath a

pavement. Effective porosity is the volume of drainable water in a unit

volume of soil. It ranges from 25 percent for a uniform material such as

medium to coarse sand, to 15 percent for a broadly graded sand-gravel

mixture.

c. Drainage Blanket Design.

The following guidelines should be

followed:

(1) Gradation.

Design in accordance with Figure 4.

(2) Thickness. Beneath, structures require a minimum of 12 inches

for each layer with a minimum thickness of 24 inches overall. If placed on

wet, yielding, uneven excavation surface and subject to construction

operation and traffic, minimum thickness shall be 36 inches overall.

7.1-275

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