Geoscience Reference
In-Depth Information
•
Pipe under drains should have grades
≥
0.5% (Desirable
>
1%).
•
Minimum local Grades
=
0.25%.
Table 15.14
Sizing of perforated pipe underdrains.
Length
Diameter
25 m
100 mm
<
25 m-100 m
150 mm
100 m-150 m
200 mm
•
Outlets should have a maximum interval of 150m.
15.15 Subsurface drains based on soil types
•
The permeability of the soil determines the required subsurface drain spacing.
Table 15.15
Suggested depth and spacing of pipe underdrains for various soil types (Highway design
manual, 2001).
Soil class
Soil composition
Drain spacing
% Sand
% Silt
% Clay
1.0 m Deep
1.25 m Deep
1.50 m Deep
1.75 m Deep
Clean sand
80-100
0-20
0-20
35-45
45-60
-
-
Sandy loam
50-80
0-50
0-20
15-30
30-45
-
-
Loam
30-50
30-50
0-20
9-18
12-24
15-30
18-36
Clay loam
20-50
20-50
20-30
6-12
8-15
9-18
12-24
Sandy clay
50-70
0-20
30-50
4-9
6-12
8-15
9-18
Silty clay
0-20
50-70
30-50
3-8
4-9
6-12
8-15
Clay
0-50
0-50
30-100
4 (max)
6 (max)
8 (max)
12 (max)
•
Trench widths should be 300mm minimum.
•
Minimum depth below surface level
=
500mm in soils and 250mm in rock.
15.16 Open channel seepages
•
Earthen channels are classified as lined or unlined.
Table 15.16
Seepage rates for unlined channels (Typical data extracted from ANCID, 2001).
Type of material
Existing seepage rates (Litres/m
2
/day)
Clays and clay loams
75-150
Gravelly clays, silty and silty loams, fine to medium sand
150-300
Sandy loams, sandy soils with some rock
300-600
Gravelly soils
600-900
Very gravelly
900-1800
•
A seepage of 20 Litres/m
2
/day is the USBR Benchmark for a water-tight channel
with sealed joints.
•
Concrete linings are typically 75mm to 100mm thick.
•
Refer Section 17 for typical compacted earth linings.
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