Storm Sewers: Required Pipe Class

These tables give the class of reinforced concrete pipe required for a storm sewer, entered by nominal diameter and by the height of fill measured over the top of the pipe. They are selection aids compiled against a state standard specification for road and bridge construction; the governing specification for a live project is published by the Illinois Department of Transportation and always takes precedence over anything reproduced here.

How to read the tables

The seven columns are installation Types 1 through 7, each defined by a band of fill height over the top of the pipe. Enter with the nominal diameter down the left, move across to the fill-height band that covers the design condition, and read the class.

Roman numerals — a standard ASTM C 76 class. Class I is the lightest, Class V the strongest; the class number corresponds to a defined D-load in the product standard.
Entries such as V-3160D — a Class V section designed to a specific D-load, in this case 3,160 pounds per linear foot per foot of internal diameter at the 0.01 in crack. Where the tabulated requirement exceeds standard Class V, the pipe is furnished as a designed D-load section under ASTM C 655 rather than a standard class.
NA — that combination of diameter and fill height is not covered by the table. A 10 in. storm sewer under more than the minimum cover is a case to design directly rather than select.

Note the shape of the table before using it. Reading down a column, the required class generally falls as diameter increases within a fill band, because the load a pipe attracts and the strength the class delivers do not scale together — D-load is expressed per foot of diameter. Reading across a row, the required class rises with cover, as expected. Both patterns are real, and an engineer who expects only the second will misread the table.

What the tables assume

A class table is a shortcut through a full load calculation, and it is only valid where its assumptions hold. Three matter most.

Bedding. The supporting strength of an installed pipe is its three-edge bearing strength multiplied by a load factor determined by the bedding and the installation type. Poorer bedding than the table assumes reduces supporting strength directly.
Live load. At shallow cover, wheel loads dominate. The lowest fill band exists precisely because a pipe under one to three feet of cover in a street is in a more demanding condition than the same pipe under ten feet.
Trench versus embankment. Earth load on a pipe in a narrow trench differs substantially from the same pipe under a positive projecting embankment. Where an installation departs from the assumed condition — unusual trench width, an imposed surcharge, multiple lines in one trench — the load should be calculated rather than selected. See load carrying capacity.

English units

Storm sewers — reinforced concrete pipe class required by diameter and fill height (English units)
Nominal diameter, in	Type 1 — Fill Height: Greater than 1'- 0" Not exceeding 3'	Type 2 — Fill Height: Greater than 3' Not exceeding 10'	Type 3 — Fill Heights: Greater than 10' Not exceeding 15'	Type 4 — Fill Height: Greater than 15' Not exceeding 20'	Type 5 — Fill Height: Greater than 20' Not exceeding 25'	Type 6 — Fill Height: Greater than 25' Not exceeding 30'	Type 7 — Fill Height: Greater than 30' Not exceeding 35'
10	NA	NA	NA	NA	NA	NA	NA
12	IV	III	IV	V	V-3160D	V-3790D	V-4000D
15	IV	III	IV	V	V-3080D	V-3390D	V-3575D
18	IV	III	IV	V	V	V-3115D	V-3300D
21	IV	III	IV	V	V	V	V-3100D
24	IV	III	IV	V	V	V	V
27	IV	III	IV	V	V	V	V
30	III	III	IV	V	V	V	V
33	III	III	IV	IV	V	V	V
36	III	III	IV	IV	V	V	V
42	II	III	IV	IV	V	V	V
48	II	III	IV	IV	V	V	V
54	II	III	IV	IV	V	V	V
60	I	II	IV	IV	V	V	V
66	I	II	III	IV	IV	V	V
72	I	II	III	IV	IV	V	V
78	I	II	III	IV	IV	V	V
84	I	II	III	IV	IV	V	V
90	I	II	III	IV	IV	V	V
96	I	II	III	IV	IV	V	V
102	I	II	III	IV	IV	V	V
108	I	II	III	IV	IV	V	V

Metric units

The metric table is the same selection expressed for nominal diameters in millimetres and fill heights in metres. Diameters correspond conventionally rather than exactly — 300 mm for 12 in., 450 mm for 18 in., 900 mm for 36 in. and so on — and D-load values are expressed in the metric equivalent, so a V-3160D entry in the English table appears as V-150D here.

Storm sewers — reinforced concrete pipe class required by diameter and fill height (metric units)
Nominal diameter, mm	Type 1 — Fill Height: Greater than 0.3 m Not exceeding 1 m	Type 2 — Fill Height: Greater than 1 m Not exceeding 3 m	Type 3 — Fill Heights: Greater than 3 m Not exceeding 4.5 m	Type 4 — Fill Height: Greater than 4.5 m Not exceeding 6 m	Type 5 — Fill Height: Greater than 6 m Not exceeding 7.5 m	Type 6 — Fill Height: Greater than 7.5 m Not exceeding 9 m	Type 7 — Fill Height: Greater than 9 m Not exceeding 10.5 m
250	NA	NA	NA	NA	NA	NA	NA
300	IV	III	IV	V	V-150D	V-180D	V-190D
375	IV	III	IV	V	V-145D	V-160D	V-170D
450	IV	III	IV	V	V	V-150D	V-160D
525	IV	III	IV	V	V	V	V-150D
600	IV	III	IV	V	V	V	V
675	IV	III	IV	V	V	V	V
750	III	III	IV	V	V	V	V
825	III	III	IV	IV	V	V	V
900	III	III	IV	IV	V	V	V
1050	II	III	IV	IV	V	V	V
1200	II	III	IV	IV	V	V	V
1350	II	III	IV	IV	V	V	V
1500	I	II	IV	IV	V	V	V
1650	I	II	III	IV	IV	V	V
1800	I	II	III	IV	IV	V	V
1950	I	II	III	IV	IV	V	V
2100	I	II	III	IV	IV	V	V
2250	I	II	III	IV	IV	V	V
2400	I	II	III	IV	IV	V	V
2550	I	II	III	IV	IV	V	V
2700	I	II	III	IV	IV	V	V

After class selection

Class settles the strength requirement only. The wall thickness and weight that follow from the diameter and wall type are given under dimensions and weights; joint selection is a separate decision driven by head condition; and for culvert rather than sewer service the corresponding tables are Table IA for circular pipe and Table IIB for elliptical and arch sections.