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GENERAL CONSIDERATIONS
The design principles for flexible pipe are based on the following characteristics of flexible pipe:
- In buried flexible pipe, the bending stress of trench load is reduced by the lateral soil reaction developed as the pipe deflects under trench load and the sides of the pipe push outward against the side fill.
- Flexible pipe, initially deflected by trench loads, is rerounded by internal pressure, which thereby reduces the stress of trench load. Therefore, when a flexible pipe is subjected to external trench load and internal pressure in combination, the stresses of trench load and internal pressure are not additive, and the pipe may be designed for whichever stress requires the greater design thickness. This is contrary to rigid pipe design in which the simultaneous case of internal pressure and external load must be examined.
- Flexible pipe usually is required to carry smaller earth loads than more rigid pipe, because the flexible pipe, in deflecting under the earth load, transfers a part of this load to the side fill and alters the frictional forces in the backfill in such a way as to reduce the earth load to a value less than that imposed on a more rigid pipe.
DESIGN FOR INTERNAL PRESSURE - Design equation 
Where: 
DESIGN FOR BENDING - Design equation 
Where: 
DESIGN FOR DEFLECTION - Design equation 
Where: 
Tables for allowable operating pressure and design depths of cover for typical trench soil types are shown below.
INT ACIPCO International Ductile Iron Pipe - Design DUCTILE IRON PIPE ISO CLASSES
| DUCTILE IRON PIPE ISO CLASSES |
SIZE
(mm) |
NOMINAL THICKNESS (mm) |
| K7 |
K8 |
K9 |
| 100 |
- |
- |
6.1 |
| 150 |
- |
- |
6.3 |
| 200 |
- |
- |
6.4 |
| 250 |
- |
- |
6.8 |
| 300 |
- |
- |
7.2 |
| 350 |
- |
- |
7.7 |
| 400 |
- |
- |
8.1 |
| 450 |
- |
7.6 |
8.6 |
| 500 |
- |
8 |
9 |
| 600 |
7.7 |
8.8 |
9.9 |
| 700 |
8.4 |
9.6 |
10.8 |
| 800 |
9.1 |
10.4 |
11.7 |
| 900 |
9.8 |
11.2 |
12.6 |
| 1000 |
10.5 |
12 |
13.5 |
| 1200 |
11.9 |
13.6 |
15.3 |
| 1400 |
13.3 |
15.2 |
17.1 |
| 1500 |
14 |
16 |
18 |
| 1600 |
14.7 |
16.8 |
18.9 |
INT ACIPCO International Ductile Iron Pipe - Design DUCTILE IRON PIPE MAXIMUM ALLOWABLE OPERATING PRESSURE (BARS)
DUCTILE IRON PIPE
MAXIMUM ALLOWABLE
OPERATING PRESSURE (BARS) |
SIZE
(mm) |
ALLOWABLE
OPERATING
PRESSURE |
| K7 |
K8 |
K9 |
| 100 |
- |
- |
33 |
| 150 |
- |
- |
33 |
| 200 |
- |
- |
33 |
| 250 |
- |
- |
33 |
| 300 |
- |
- |
33 |
| 350 |
- |
- |
33 |
| 400 |
- |
- |
33 |
| 450 |
- |
35 |
40 |
| 500 |
- |
33 |
38 |
| 600 |
26 |
31 |
36 |
| 700 |
24 |
29 |
34 |
| 800 |
23 |
28 |
32 |
| 900 |
23 |
27 |
31 |
| 1000 |
22 |
26 |
30 |
| 1200 |
21 |
25 |
29 |
| 1400 |
20 |
24 |
28 |
| 1500 |
20 |
24 |
27 |
| 1600 |
20 |
24 |
27 |
Notes:
Operating pressures for 100mm - 400mm sizes are based upon a factory test of 1.5 times the operating pressure. Contact ACIPCO if higher pressures are required.
INT ACIPCO International Ductile Iron Pipe - Design DUCTILE IRON PIPE MINIMUM FACTORY TEST PRESSURE (BARS)
DUCTILE IRON PIPE
MINIMUM FACTORY TEST PRESSURE (BARS) |
SIZE
(mm) |
K7 |
K8 |
K9 |
| 100 |
- |
- |
50 |
| 150 |
- |
- |
50 |
| 200 |
- |
- |
50 |
| 250 |
- |
- |
50 |
| 300 |
- |
- |
50 |
| 350 |
- |
- |
50 |
| 400 |
- |
- |
50 |
| 450 |
- |
71 |
80 |
| 500 |
- |
68 |
76 |
| 600 |
55 |
62 |
70 |
| 700 |
51 |
59 |
60 |
| 800 |
49 |
56 |
60 |
| 900 |
47 |
53 |
60 |
| 1000 |
45 |
52 |
58 |
| 1200 |
40 |
40 |
40 |
| 1400 |
40 |
40 |
40 |
| 1500 |
40 |
40 |
40 |
| 1600 |
40 |
40 |
40 |
Notes:
Minimum factory test pressure for 450mm and larger pipe is based upon internal pressure required to produce stress in nominal pipe wall equivalent to 75% of the minimum specified yield strength of the material. In some sizes and pipe classes, test pressures are limited by the maximum allowable factory test pressure currently in ISO 2531.
 TYPICAL TRENCH -- Pipe placed on flat bottom trench. If rock or hardpan is encountered, a layer of loose soil should be used as a bedding. Bell holes are recommended.
TRENCH TYPE 1 Embedment -- dumped, less than 70% standard Proctor density.
TRENCH TYPE 2 Embedment -- lightly consolidated, 70%-80% standard Proctor density.
TRENCH TYPE 3 Embedment -- light compaction, greater than 80% standard Proctor density.
TRENCH TYPE 4 Embedment -- medium compaction, 85%-95% standard Proctor density.
TRENCH TYPE 5 Embedment -- high compaction, greater than 95% standard Proctor density. |
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