Page 61 - Buried Pipe Design

38 Chapter Two

TABLE 2.7 Minimum Depth of Cover for Fiberglass Pipe (AWWA M45-1996)
Condition Minimum cover, in
High-stiffness soils with crushed
rock and gravel with
15% sand
and 75% fines (soil stiffness
category SC1), with AASHTO-20 live load 24
Lower-stiffness soils (SC2 to 4) with
AASHTO-20 live loads 36
Use of hydrohammer for compaction 48
Other conditions Established by engineer
NOTE: The H-20 load assumes two 16,000-lb concentrated loads, one over the pipe,
the other 72 in away, corresponding to a 20-ton truck load.

The design of water piping for surface loads is provided in AWWA C101
(cast iron), AWWA C150, C151, and C600 (ductile iron), AWWA M11
(steel), AWWA M45 (fiberglass), and AWWA M23, C605, and C900 (PVC).
In all cases, the depth of cover is to be established by the engineer on the
basis of earth and surface load formulas to calculate the demand, and
pipe stress and deflection limits to calculate the capacity. Minimum
depth of cover is provided in AWWA M45 for fiberglass pipe (Table 2.7).
In civil engineering applications, the Handbook of Steel Drainage
and Highway Construction Products (American Iron and Steel
Institute) applies to low-pressure, large-D/t buried pipes. In this case,
the minimum cover for surface live loads is established on the basis of
experience. The minimum cover specified is one-eighth (D/8) for high-
way conduits, D/4 and D/5 for railway conduits, but not less than 12 in.
Deeper covers may be needed during construction for traffic of heavy
equipment.
Where surface loads are of an impact nature, such as the impact of
wheels on uneven roads, an impact factor is added to the surface load.
For gas and liquid pipelines, a minimum depth of cover is usually used
in place of detailed design analysis or encasement of the pipe.
Minimum depths of cover for ductile iron gas pipelines follow the rules
of AWWA C150. 7

Aircraft loads
Design live loads for modern airports may be very large. Airports are
often designed for wheel loads of aircraft which have not yet been
designed. Table 2.8 lists live loads for an aircraft loading of 180,000-lb
dual-tandem gear assembly.
In the design for live loads on pipe buried under runway pavement,
the impact factor is taken as 1.0. This is because the load is partially