Page 37 - Buried Pipe Design

External Loads 15

TABLE 2.1 Approximate Values of Soil Unit Weight, Ratio of Lateral to Vertical
Earth Pressure, and Coefficient of Friction against Sides of Trench
Unit weight, Rankine’s ratio Coefficient of
Soil type lb/ft 3 K friction
Partially compacted
damp top soil 90 0.33 0.50
Saturated top soil 110 0.37 0.40
Partially compacted
damp clay 100 0.33 0.40
Saturated clay 120 0.37 0.30
Dry sand 100 0.33 0.50
Wet sand 120 0.33 0.50

were determined experimentally by Marston, and typical values are
given in Table 2.1.
Example Problem 2.1 What is the maximum load on a very rigid pipe in a
ditch excavated in sand? The pipe diameter (OD) is 18 in, the trench width
3
is 42 in, the depth of burial is 8 ft, and the soil unit weight is 120 lb/ft .
1. Determine C d . From Table 2.1 for sand, K K ′ 0.165.
H 8 ft 12 in
2.29
B d 42 in 1 ft
From Fig. 2.2, C d 1.6.
2. Calculate the load from Eq. (2.4):
12 2
42
2
W d C d B d 1.6 (120) 2352 lb/ft
Embankment conditions. Not all pipes are installed in ditches
(trenches); therefore, it is necessary to treat the problem of pipes
buried in embankments. An embankment is where the top of the pipe
is above the natural ground. Marston defined this type of installation
as a positive projecting conduit. Typical examples are railway and
highway culverts. Figure 2.4 shows two cases of positive projecting
conduits as proposed by Marston. In case I, the ground at the sides of
the pipe settles more than the top of the pipe. In case II, the top of the
pipe settles more than the soil at the sides of the pipe. Case I was
called the projection condition by Marston and is characterized by a
positive settlement ratio r sd , defined as (see Fig. 2.4)

)
( S + S − ( S + d )
r = m g f c
sd S
m
The shear forces are downward and cause a greater load on the buried
pipe for this case. Case II is called the ditch condition and is charac-
. The shear forces are directed
terized by a negative settlement ratio r sd
upwards in this case and result in a reduced load on the pipe.