Moser A.P., Folkman S. Buried Pipe Design
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TABLE 6.8 Earth Loads P
e
, Truck Loads P
t
, and Trench Loads P
v
, lb/in
2
Depth of 18-in pipe 20-in pipe 24-in pipe 30-in pipe 36-in pipe
cover, ft P
e
P
t
P
v
P
t
P
v
P
t
P
v
P
t
P
v
P
t
P
v
2.5 2.1 7.8 9.9 7.5 9.6 7.1 9.2 6.7 8.8 6.2 8.3
3 2.5 5.9 8.4 5.7 8.2 5.4 7.9 5.2 7.7 4.9 7.4
4 3.3 3.9 7.2 3.9 7.2 3.6 6.9 3.5 6.8 3.4 6.7
5 4.2 2.6 6.8 2.6 6.8 2.4 6.6 2.4 6.6 2.3 6.5
6 5.0 1.9 6.9 1.9 6.9 1.7 6.7 1.7 6.7 1.7 6.7
7 5.8 1.4 7.2 1.4 7.2 1.3 7.1 1.3 7.1 1.3 7.1
8 6.7 1.2 7.9 1.1 7.8 1.1 7.8 1.1 7.8 1.1 7.8
9 7.5 1.0 8.5 0.9 8.4 0.9 8.4 0.9 8.4 0.8 8.3
10 8.3 0.8 9.1 0.7 9.0 0.7 9.0 0.7 9.0 0.7 9.0
12 10.0 0.5 10.5 0.5 10.5 0.5 10.5 0.5 10.5 0.5 10.5
14 11.7 0.4 12.1 0.4 12.1 0.4 12.1 0.4 12.1 0.4 12.1
16 13.3 0.3 13.6 0.3 13.6 0.3 13.6 0.3 13.6 0.3 13.6
20 16.7 0.2 16.9 0.2 16.9 0.2 16.9 0.2 16.9 0.2 16.9
24 20.0 0.1 20.1 0.1 20.1 0.1 20.1 0.1 20.1 0.1 20.1
28 23.3 0.1 23.4 0.1 23.4 0.1 23.4 0.1 23.4 0.1 23.4
32 26.7 0.1 26.8 0.1 26.8 0.1 26.8 0.1 26.8 0.1 26.8
339
TABLE 6.8 Earth Loads P
e
, Truck Loads P
t
, and Trench Loads P
v
, lb/in
2
(Continued )
Depth of 42-in pipe 48-in pipe 54-in pipe 60-in pipe 64-in pipe
cover, ft P
e
P
t
P
v
P
t
P
v
P
t
P
v
P
t
P
v
P
t
P
v
2.5 2.1 5.8 7.9 5.4 7.5 5.0 7.1 4.8 6.9 4.5 6.6
3 2.5 4.6 7.1 4.4 6.9 4.1 6.6 3.9 6.4 3.8 6.3
4 3.3 3.3 6.6 3.1 6.4 3.0 6.3 2.9 6.2 2.8 6.1
5 4.2 2.3 6.5 2.2 6.4 2.1 6.3 2.1 6.3 2.1 6.3
6 5.0 1.7 6.7 1.6 6.6 1.6 6.6 1.6 6.6 1.5 6.5
7 5.8 1.3 7.1 1.2 7.0 1.2 7.0 1.2 7.0 1.2 7.0
8 6.7 1.0 7.7 1.0 7.7 1.0 7.7 1.0 7.7 1.0 7.7
9 7.5 0.8 8.3 0.8 8.3 0.8 8.3 0.8 8.3 0.8 8.3
10 8.3 0.7 9.0 0.7 9.0 0.7 9.0 0.7 9.0 0.7 9.0
12 10.0 0.5 10.5 0.5 10.5 0.5 10.5 0.5 10.5 0.5 10.5
14 11.7 0.4 12.1 0.4 12.1 0.4 12.1 0.4 12.1 0.4 12.1
16 13.3 0.3 13.6 0.3 13.6 0.3 13.6 0.3 13.6 0.3 13.6
20 16.7 0.2 16.9 0.2 16.9 0.2 16.9 0.2 16.9 0.2 16.9
24 20.0 0.1 20.1 0.1 20.1 0.1 20.1 0.1 20.1 0.1 20.1
28 23.3 0.1 23.4 0.1 23.4 0.1 23.4 0.1 23.4 0.1 23.4
32 26.7 0.1 26.8 0.1 26.8 0.1 26.8 0.1 26.8 0.1 26.8
SOURCE: Table 1 from AWWA C150.
340
Steel and Ductile Iron Flexible Pipe Products 341
TABLE 6.9 Design Values for Standard Laying Conditions
Laying Bedding
condition
*
Description E¢ angle, deg K
b
K
s
Type 1
†
Flat-bottom trench.
‡
Loose backfill. 150 30 0.235 0.108
Type 2 Flat-bottom trench. Backfill lightly 300 45 0.210 0.105
consolidated to centerline of pipe.
Type 3 Pipe bedded in 4-in-minimum loose 400 60 0.189 0.103
soil.
§
Backfill lightly consolidated
to top of pipe.
Type 4 Pipe bedded in sand, gravel, or 500 90 0.16 0.096
crushed stone to depth of one-
eighth pipe diameter, 4 in mini-
mum. Backfill compacted to top of
pipe (approximately 80 percent
standard Proctor, AASHTO T-99)
¶
Type 5 Pipe bedded to its centerline in 700 150 0.128 0.085
compacted granular material, 4 in
minimum under pipe. Compacted
granular or select
§
material to top
of pipe (approximately 90 percent
standard Proctor, AASHTO T-99)
¶
*
See Fig. 6.1.
†
For pipe 30 in and larger, consideration should be given to the use of laying conditions other
than type 1.
‡
Flat bottom is defined as “undisturbed earth.”
§
Loose soil or select material is defined as “native soil excavated from the trench, free of
rocks, foreign material, and frozen earth.”
¶
AASHTO T-99, Standard Method of Test for the Moisture Density Relations of Soils Using
a 5.5-lb (2.5-kg) Rammer and a 12-in (305-mm) Drop. Available from the American Associa-
tion of State Highway and Transportation Officials, 444 N. Capital St. NW, Washington, DC
20001.
SOURCE: Table 2 from AWWA C150.
TABLE 6.10 Allowances for Casting Tolerance
Size, in Casting allowance, in
3–8 0.05
10–12 0.06
14–42 0.07
48 0.08
54 0.09
SOURCE: Table 3 from AWWA C150.
2. Design for internal pressure:
P
i
2.0 (working pressure 100 lb/in
2
surge allowance)
If anticipated surge pressures are greater than 100 lb/in
2
,which results
from instantaneous stoppage of a column of water moving at 2 ft/s, then the
actual anticipated pressures must be used.
P
i
2.0 (200 100) 600 lb/in
2
t 0.23 in
Net thickness t for internal pressure is 0.23 in.
3. Select net thickness and add allowances. The larger of the thick-
nesses is given by the design for trench load, step 1, and 0.25 in is
selected.
Net thickness 0.25 in
Service allowance 0.08 in
Minimum thickness 0.33 in
Casting tolerance 0.07 in
(see Table 6.10)
Total calculated thickness 0.40 in
4. Select the standard thickness and class. The total calculated thick-
ness of 0.40 in is nearest to 0.42 pressure class 250, in Table 6.12. Therefore,
pressure class 250 is selected.
Testing of ductile-iron pipe
A significant result of research and testing of buried flexible pipes is
the identification of performance limits. Tradi tionally, design is a
two-step process: (1) the conceiving of a device or system and (2) the
600 (32.00)
2 (42,000)
P
i
D
2S
342 Chapter Six
TABLE 6.11 Reduction Factors R for Truck Load Calculations
Depth of cover, ft
4 4–7 7–10 10
Size, in Reduction factor
3–12 1.00 1 1.00 1.00
14 0.92 1.00 1.00 1.00
16 0.88 0.95 1.00 1.00
18 0.85 0.90 1.00 1.00
20 0.83 0.90 0.95 1.00
24–30 0.81 0.85 0.95 1.00
36–64 0.80 0.85 0.90 1.00
SOURCE: Table 4 from AWWA C150.
predicting of performance of this system to determine whether it will
accomplish the purpose for which it was conceived. Any inability to
achieve this objective is called failure. Most designers visualize failure
as a sudden, calamitous, or catastrophic deformation such as a break
or a collapse. In the case of a buried pipe, failure could be the rupture
of a pipe due to internal pressure, but it could also be the deformation of
Steel and Ductile Iron Flexible Pipe Products 343
TABLE 6.12 Nominal Thicknesses for Standard Pressure Classes of
Ductile Iron Pipe
Outside
Pressure class
Size, diameter,
in in
150 200 250 300 350
Nominal thickness, in
3 3.96 — — — — 0.25
*
4 4.80 — — — — 0.25
*
6 6.90 — — — — 0.25
*
8 9.05 — — — — 0.25
*
10 11.10 — — — — 0.26
12 13.20 — — — — 0.28
14 15.30 — — 0.28 0.30 0.31
16 17.40 — — 0.30 0.32 0.34
18 19.50 — — 0.31 0.34 0.36
20 21.60 — — 0.33 0.36 0.38
24 25.80 — 0.33 0.37 0.40 0.43
30 32.00 0.34 0.38 0.42 0.45 0.49
36 38.30 0.38 0.42 0.47 0.51 0.56
42 44.50 0.41 0.47 0.52 0.57 0.63
48 50.80 0.46 0.52 0.58 0.64 0.70
54 57.56 0.51 0.58 0.65 0.72 0.79
60 61.61 0.54 0.61 0.68 0.76 0.83
64 65.67 0.56 0.64 0.72 0.80 0.87
NOTE: To convert inches (in) to millimeters (mm), multiply by 25.4.
*
Calculated thicknesses for these sizes and pressure ratings are less than
those shown above. (See Table 6.11 for actual calculated thicknesses.) Presently
these are the lowest nominal thicknesses available in these sizes.
Pressure classes are defined as the rated water working pressure of the pipe
in pounds per square inch (psi or lb/in
2
). The thicknesses shown are adequate for
the rated water working pressure plus a surge allowance of 100 lb/in
2
(689 kPa).
Calculations are based on a minimum yield strength in tension of 42,000 lb/in
2
(289,590 kPa) and 2.0 safety factor times the sum of working pressure and 100
lb/in
2
(689 kPa) surge allowance.
Thickness can be calculated for rated water working pressure and surges other
than the above by use of the formula shown in Sec. 4.1.2 of AWWA C150.
Ductile iron pipe is available for water working pressures greater than 350
lb/in
2
(2413 kPa).
Pipe is available with thicknesses greater than pressure class 350. See Table
6.13.
Lowest nominal thicknesses shown in Table 15.1 of ANSI/AWWA C115/A21.15
for threaded flanged pipe are still required.
Lowest nominal thicknesses shown in ANSI/AWWA C606 for pipe with
grooved and shouldered joints are still required.
SOURCE: Table 5 from AWWA C150.
TABLE 6.13 Diameter-Thickness Ratios for Laying Condition Type 2
*
Trench load P
v
, lb/in
2
Trench load P
v
, lb/in
2
Bending- Bending-
stress Deflection stress Deflection
design design D/t or D/t
1
†
design design D/t or D/t
1
†
6.29 6.18 170 9.70 7.94 120
6.34 6.19 169 9.79 8.01 119
6.39 6.21 168 9.89 8.08 118
6.44 6.23 167 9.99 8.16 117
6.50 6.25 166 10.09 8.23 116
6.55 6.26 165 10.19 8.31 115
6.60 6.28 164 10.29 8.40 114
6.66 6.30 163 10.40 8.48 113
6.71 6.32 162 10.51 8.57 112
6.77 6.34 161 10.62 8.66 111
6.82 6.37 160 10.73 8.76 110
6.88 6.39 159 10.84 8.86 109
6.94 6.41 158 10.96 8.96 108
6.99 6.43 157 11.08 9.07 107
7.05 6.46 156 11.21 9.18 106
7.11 6.48 155 11.33 9.29 105
7.17 6.50 154 11.46 9.41 104
7.23 6.53 153 11.59 9.54 103
7.29 6.56 152 11.73 9.67 102
7.35 6.58 151 11.87 9.80 101
7.42 6.61 150 12.01 9.94 100
7.48 6.64 149 12.16 10.09 99
7.54 6.67 148 12.31 10.24 98
7.61 6.70 147 12.46 10.40 97
7.67 6.73 146 12.62 10.56 96
7.74 6.76 145 12.79 10.73 95
7.80 6.79 144 12.96 10.91 94
7.87 6.83 143 13.13 11.10 93
7.94 6.86 142 13.31 11.29 92
8.01 6.89 141 13.49 11.50 91
8.08 6.93 140 13.68 11.71 90
8.15 6.97 139 13.88 11.94 89
8.22 7.01 138 14.08 12.17 88
8.29 7.05 137 14.30 12.42 87
8.37 7.09 136 14.51 12.67 86
8.44 7.13 135 14.74 12.94 85
8.52 7.17 134 14.97 13.22 84
8.59 7.22 133 15.21 13.52 83
8.67 7.26 132 15.46 13.83 82
8.75 7.31 131 15.72 14.16 81
8.83 7.36 130 15.99 14.50 80
8.91 7.41 129 16.28 14.86 79
8.99 7.46 128 16.57 15.24 78
9.07 7.51 127 16.87 15.64 77
9.16 7.57 126 17.19 16.06 76
9.25 7.63 125 17.52 16.51 75
9.33 7.69 124 17.86 16.98 74
9.42 7.75 123 18.22 17.48 73
9.51 7.81 122 18.59 18.00 72
9.60 7.87 121 18.98 18.56 71
NOTE: See p. 345 for footnotes.
344
a pipe due to external soil pressure. However, only under rare combi-
nation of extenuating circumstances does a buried pipe ever collapse.
So failure needs to be defined. The problem is to identify the perfor-
mance limits. Just how much ring deflection is tolerable? Just how
much cracking of the mortar lining may be permitted? And so on.
Design of buried pipes should be based on performance limits that
have been identified in actual tests. This includes the identification of
pertinent fundamental variables. It includes the interrelationship of
these variables as determined from the actual tests (see Fig. 6.47).
From experience, the following parameters are found to be most
pertinent:
1. The D/t ratio or ring flexibility where D mean diameter of the
ring and t wall thickness (pipe or bell).
2. A measure of soil compressibility E¢ or , where E¢ soil stiffness—
the most important soil property—and density of soil (percent
of standard)—the single most important determinant of E¢.
Steel and Ductile Iron Flexible Pipe Products 345
TABLE 6.13 Diameter-Thickness Ratios for Laying Condition Type 2
*
(Continued )
Trench load P
v
, lb/in
2
Trench load P
v
, lb/in
2
Bending- Bending-
stress Deflection stress Deflection
design design D/t or D/t
1
†
design design D/t or D/t
1
†
19.39 19.14 70 33.84 44.09 50
19.82 19.77 69 35.08 46.56 49
20.27 20.43 68 36.41 49.26 48
20.73 21.13 67 37.83 52.19 47
21.23 21.87 66 39.34 55.40 46
21.74 22.67 65 40.96 58.89 45
22.28 23.51 64 42.70 62.73 44
22.85 24.41 63 44.57 66.93 43
23.45 25.37 62 46.57 71.56 42
24.07 26.39 61 48.73 76.66 41
24.74 27.49 60 51.06 82.29 40
25.43 28.66 59 53.57 88.54 39
26.17 29.91 58 56.30 95.48 38
26.95 31.26 57 59.25 103.21 37
27.77 32.71 56 62.46 111.85 36
28.64 34.26 55 65.96 121.54 35
29.56 35.93 54 69.79 132.44 34
30.53 37.74 53 73.98 144.74 33
31.57 39.69 52 78.57 158.68 32
32.67 41.80 51 83.64 174.54 31
NOTE: To convert pounds per square inch (lb/in
2
) to kilopascals (kPa), multiply by 6.895.
*
E¢ 300 lb/in
2
; K
b
0.210; K
x
0.105.
†
The D/t or D/t
1
for the tabulated P
v
nearest to the calculated P
v
is selected; when the cal-
culated P
v
is halfway between two tabulated values, the smaller of D/t or D/t
1
should be
used.
SOURCE: Table 8 from AWWA C150.
TABLE 6.14 Thickness for Earth Load plus Truck Load
Laying condition
Type 1 Type 2 Type 3 Type 4 Type 5
Total Total Total Total Total
Depth of calculated Use calculated Use calculated Use calculated Use calculated Use
Size, cover,
*
thickness,
†
pressure thickness,
†
pressure thickness,
†
pressure thickness,
†
pressure thickness,
†
pressure
inft in class in class in class in class in class
10 2.5 — — 0.25 350 0.23 350 0.20 350 0.18 350
3 0.26 350 0.24 350 0.22 350 0.19 350 0.18 350
4 0.25 350 0.22 350 0.21 350 0.19 350 0.17 350
5 0.24 350 0.22 350 0.20 350 0.18 350 0.17 350
6 0.24 350 0.22 350 0.20 350 0.18 350 0.17 350
7 0.24 350 0.22 350 0.20 350 0.19 350 0.17 350
8 0.25 350 0.22 350 0.21 350 0.19 350 0.17 350
9 0.25 350 0.23 350 0.21 350 0.19 350 0.17 350
10 0.26 350 0.23 350 0.21 350 0.19 350 0.17 350
12 — — 0.24 350 0.22 350 0.20 350 0.18 350
14 — — 0.26 350 0.23 350 0.20 350 0.18 350
16 — — — — 0.24 350 0.21 350 0.18 350
20 — — — — — — 0.22 350 0.19 350
24 — — — — — — 0.24 350 0.19 350
28 — — — — — — 0.26 350 0.20 350
32 ————————0.21350
12 2.5 — — 0.27 350 0.25 350 0.21 350 0.19 350
3 0.28 350 0.25 350 0.23 350 0.20 350 0.18 350
4 0.26 350 0.24 350 0.22 350 0.19 350 0.18 350
5 0.26 350 0.23 350 0.21 350 0.19 350 0.18 350
6 0.26 350 0.23 350 0.21 350 0.19 350 0.18 350
7 0.26 350 0.23 350 0.21 350 0.19 350 0.18 350
8 0.27 350 0.24 350 0.22 350 0.20 350 0.18 350
9 0.27 350 0.24 350 0.22 350 0.20 350 0.18 350
10 0.28 350 0.25 350 0.23 350 0.20 350 0.18 350
12 — — 0.26 350 0.24 350 0.21 350 0.18 350
346
14 — — 0.28 350 0.25 350 0.21 350 0.19 350
16 — — — — 0.26 350 0.22 350 0.19 350
20 — — — — — — 0.24 350 0.20 350
24 — — — — — — 0.26 350 0.20 350
28 — — — — — — 0.28 350 0.21 350
32 ————————0.23350
14 2.50
‡‡
0.29 300 0.26 250 0.23 250 0.20 250
3 0.28 250 0.25 250 0.22 250 0.20 250
4 0.26 250 0.24 250 0.21 250 0.19 250
5 0.25 250 0.23 250 0.21 250 0.19 250
6 0.25 250 0.23 250 0.21 250 0.19 250
7 0.26 250 0.24 250 0.21 250 0.19 250
8 0.26 250 0.24 250 0.21 250 0.19 250
9 0.27 250 0.24 250 0.22 250 0.19 250
10 0.28 250 0.25 250 0.22 250 0.20 250
12 0.29 300 0.26 250 0.23 250 0.20 250
14 0.31 350 0.28 250 0.23 250 0.20 250
16 — — 0.29 300 0.24 250 0.21 250
20 — — — — 0.26 250 0.21 250
24 — — — — 0.29 300 0.22 250
28 ——————0.25250
32 ——————0.27250
16 2.50
‡‡
0.31 300 0.27 250 0.23 250 0.21 250
3 0.29 250 0.26 250 0.23 250 0.20 250
4 0.27 250 0.25 250 0.22 250 0.20 250
5 0.27 250 0.24 250 0.22 250 0.20 250
6 0.27 250 0.24 250 0.22 250 0.20 250
7 0.27 250 0.25 250 0.22 250 0.20 250
8 0.28 250 0.25 250 0.22 250 0.20 250
9 0.28 250 0.26 250 0.22 250 0.20 250
10 0.29 250 0.26 250 0.23 250 0.20 250
12 0.31 300 0.28 250 0.24 250 0.21 250
14 0.33 350 0.29 250 0.24 250 0.21 250
NOTE: See p. 353 for footnotes.
347
16 — — 0.31 300 0.25 250 0.21 250
20 — — 0.34 350 0.28 250 0.22 250
24 — — — — 0.30 250 0.24 250
28 — — 0.34 350 0.27 250
32 — — — — 0.29 250
18 2.50
‡‡
0.32 300 0.28 250 0.24 250 0.21 250
3 0.30 250 0.27 250 0.23 250 0.21 250
4 0.28 250 0.26 250 0.23 250 0.20 250
5 0.28 250 0.25 250 0.22 250 0.20 250
6 0.28 250 0.25 250 0.22 250 0.20 250
7 0.28 250 0.26 250 0.23 250 0.20 250
8 0.29 250 0.26 250 0.23 250 0.20 250
9 0.30 250 0.27 250 0.23 250 0.21 250
10 0.31 250 0.28 250 0.24 250 0.21 250
12 0.33 300 0.29 250 0.24 250 0.21 250
14 0.35 350 0.31 250 0.25 250 0.22 250
16 — — 0.33 300 0.26 250 0.22 250
20 — — — — 0.29 250 0.23 250
24 — — — — 0.32 300 0.26 250
28 — — — — 0.36 350 0.29 250
32 — — — — — — 0.32 300
20 2.50
‡‡
0.33 250 0.29 250 0.25 250 0.21 250
3 0.31 250 0.28 250 0.24 250 0.21 250
4 0.30 250 0.27 250 0.23 250 0.21 250
TABLE 6.14 Thickness for Earth Load plus Truck Load (Continued )
Laying condition
Type 1 Type 2 Type 3 Type 4 Type 5
Total Total Total Total Total
Depth of calculated Use calculated Use calculated Use calculated Use calculated Use
Size, cover,
*
thickness,
†
pressure thickness,
†
pressure thickness,
†
pressure thickness,
†
pressure thickness,
†
pressure
inft in class in class in class in class in class
348