Page 64 - Buried Pipe Design

40 Chapter Two

taken by the aircraft’s wings when the aircraft is landing. For taxiways,
aprons, and so on, an impact factor may be necessary (see Table 2.5).
The design engineer should seek current data available from the
Federal Aviation Administration.

Minimum soil cover
Figure 2.19 is copied from AISI graphs of vertical pressures on buried
pipes (Handbook of Steel Drainage and Highway Construction
2
Products ). As soil cover decreases, live load pressure on a buried pipe
increases. There exists a minimum height of soil cover. If the soil cover
is less than the minimum, the surface live load may damage the pipe.
Less obvious is a minimum height of soil cover for dead load (weight of
soil only). Each of these cases is discussed for rigid and flexible rings.
Only cohesionless soil is considered because vehicles are unable to
maneuver on poor soil such as wet cohesive soil.
Notation
A cross-sectional area of pipe wall per unit length of pipe
c distance from neutral surface of pipe wall cross section to most
remote fiber
D mean diameter of pipe
E modulus of elasticity of pipe material
H′ installed height of soil cover (see Fig. 2.24)
H rutted height of soil cover
I centroidal moment of inertia of pipe wall cross-sectional area
per unit length of pipe
M moment in wall due to ring deformation
P vertical soil pressure at level of top of pipe due to a surface
load distributed over a rectangular area
r mean radius of pipe
S compressive strength of pipe wall
T circumferential thrust in ring
W weight of a surface load
unit weight of soil
soil density in percent standard Proctor (AASHTO T-99, ASTM
D 698) for granular soil cover and embedment
y yield stress of pipe
ring compression stress