Page 278 - Buried Pipe Design

Rigid Pipe Products 249

airborne asbestos dust. Manufacturer’s safety procedures should be
followed.
The asbestos-cement-silica composite achieves a remarkably high
2
tensile strength of up to 4000 lb/in . This high strength is directly
attributable to the asbestos fiber reinforcement. AC pipe is durable
and has been operating in pipelines for more than 45 years. AC pipes
are not as prone to impact damage as some rigid pipes; nevertheless,
care should be taken in handling. When excavating to make connec-
tions to or in repairing AC pipes, care must be taken to prevent the
backhoe bucket from damaging the line. For water systems, AC pipes
are available for both transmission and distribution systems. They are
also available for various specialty applications.

Example 5.1—Gravity storm sewer A 36-in-diameter storm sewer line is to be
installed. It passes through a small hill which requires a trench 20 ft deep.
Native material, a silty sand with clay, will be used for final backfill. The
trench width at the top of the pipe is not to exceed 7 ft. Calculate the mini-
mum strength of asbestos-cement pipe for both B and C bedding. Also, what
strength will be required for a possible “worst case” if the trench width
exceeds the transition width and only C bedding is achieved? Groundwater
is 10 ft below the surface. (See Chap. 2 for design criteria.)
1. Determine the earth load (ditch condition)

H 20
2.86 (from Fig. 2.2, C d 1.9)
B d 7
K 0.150 clayey sand
2
3
2
W d C d B d 1.9 (120 lb/ft ) (7) 11,172 lb/ft
3
where unit weight of soil (assume 120 lb/ft ) and B d trench width
at the top of the pipe.
2. Determine the earth load (if transition width exceeded)
H 20 B d
6.67 from Fig. 2.6, 2.65
B c 3 B c

Assume r sd p 0.75. Then

B d
B d (transition) B c 2.65 (3) 7.95 ft ≈ 8.0
B c
2
2
W d (transition) C d B d 1.9 (120) (8) 14,592 lb/ft
3. Determine the class of pipe required for B bedding, with 7-ft trench width,

Load factor (LF) for B bedding 1.9 (see Table 3.2)