Terrain assessment, drainage and erosion - Geotechnical Investigation and Design Tables - page 198

Table 15.20 Equivalent aggregate cross sections as a 100 mm OD corrugated plastic pipe (Forrester,
2001).

Drainage element Size Area (m 2 ) Comments/Permeability

Corrugated plastic pipe 100 mm, ID 85.33 mm 0.0057 Flow Q 2.7 Litres/sec:

piezometric gradient, i

= =

=

1%

20 mm aggregate 1.87 m * 1.87 m 3.5 k 0.075 m/s

=

14 mm aggregate 2.45 m * 2.45 m 6 k 0.045 m/s

=

10 mm aggregate 3.32 m * 3.32 m 11 k 0.025 m/s =

7 mm aggregate 4.24 m * 4.24 m 18 k 0.015 m/s =

5 mm aggregate 5.83 m * 5.83 m 34 k

=

0.008 m/s

15.21 Discharge capacity of stone filled drains
•

The aggregate size affects the flow capacity. Following seepage analysis, the
appropriate stone sizing may be adopted.

Table 15.21 Discharge capacity of 0.9 m * 0.6 m cross-section stone
filled drains (Cedergren, 1989).

Capacity (m 3 /s) Size of stone Slope

19 mm to 25 mm 0.01 200

0.001 20

9 mm to 12 mm 0.01 50

0.001 5

6 mm to 9 mm 0.01 10

0.001 1

15.22 Slopes for chimney drains
•

Chimney drains are used to cut of the horizontal flow paths through an earth
dam.

Table 15.22 Slope for chimney drains.

Drainage material Slope (1 Vertical : Horizontal)

Sand 1V : 1.75H

Gravel 1V : 1.5 H

Sand/Gravel 1V : 1.75H

Gravel wrapped in geotextile 1V : 1.5H

15.23 Drainage blankets
•

Drainage blankets are used below roads or earth dams.

•

The size should be based on the expected flow and length of the flow path.