Page 44 - Buried Pipe Design

22 Chapter Two

undisturbed soil. The Marston tunnel load equation is

W t C t B t ( B t 2C) (2.9)

where W is the load on the pipe in pounds per linear foot and is spe-
t
cific weight. The load coefficient C t is obtained in the same way that
C was determined (see Fig. 2.2). And B is the maximum tunnel
t
d
is the OD of the pipe. The coefficient
width; or if the pipe is jacked, B t
C is called the cohesion coefficient and is, dimensionally, force per unit
area (lb/ft ).
2
as well as C . This
Equation (2.3) can be used in calculating C t d
equation indicates that for very large values of H/B, C approaches a
t
limiting value of 1/(2K ′). Thus, for very deep tunnels, the load can be
.
closely estimated by using the value of 1/(2K ′) for C t
It is readily apparent that the theory for loads on pipes in tunnels or
being jacked through undisturbed soil is almost identical to the theory for
loads on pipes in trenches. The tunnel load will be somewhat less because
of the soil cohesion. It is also apparent from Eq. (2.9) that C is very impor-
tant in determining the load. Unfortunately, values of the coefficient C
have a wide range of variation even for similar soils. The value of C may
be determined by laboratory tests on undisturbed samples. Conservative
values of C should be used in design to account for possible saturation of
the soil. It has been suggested that about one-third of the laboratory
determined value should be used for design. The Water Pollution Control
Federation (WPCF) Manual of Practice, No. FD-5, recommends the use
6
of values given in Table 2.4 if reliable laboratory data are not available or
if such tests are impractical. It is also suggested that this coefficient be
taken as zero for any zone subjected to seasonal frost and cracking or loss
2C cannot be negative.
of strength because of saturation. The factor B t
Therefore, 2C cannot be larger than B .
t
Flexible pipe
A flexible pipe derives its soil-load-carrying capacity from its flexibility.
Under soil load, the pipe tends to deflect, thereby developing passive

TABLE 2.4 Recommended Safe Values of Cohesion C
Values of C
Material kPa lb/ft 2
Clay, very soft 2 40
Clay, medium 12 250
Clay, hard 50 1000
Sand, loose, dry 0 0
Sand, silty 5 100
Sand, dense 15 300