system will be designed.
as the pipe slope.
This gives an
sewers to be designed will be the
acceptable velocity in a 4-inch pipe.
house connections using figure 14.
The invert elevation is chosen 2.4
feet below the ground at building B.
a. Building A to manhole (MH) 8.
This gives 2 feet of cover over the 4-
The values are tabulated in row 1 of
inch (approximately .4 feet) sewer.
the data sheet (fig 17).
The design is tabulated in row 3,
slope of 2 feet in 75 is used as a
figure 17. For the line from building
With a 4-inch house sewer
C, the ground slope of 1 foot in 30
(the minimum size allowable) a full
Its design parallels that
flow of 127 gallons per minute is
of building B and is tabulated in row
This is less than the
The two choices now
c. Connections to MH 7. The design
available are to increase the slope
of the lateral between MH 8 and MH 7
and keep the 4-inch pipe size, or
is shown in row 5. Note that in this
increase the pipe diameter.
To get a
length of pipe infiltration can no
200 gallons per minute full flow in a
longer be ignored.
A figure of 4
4-inch sewer the slope must be 6.8
gallons per minute per 1,000 feet has
This means a 5.1-foot drop
The ground slope of 1
over the 75 feet. This would put the
percent is used and the minimum sewer
invert at manhole 8 about 5-1/2 feet
size of 6 inches is tried. The actual
(3.1 slope difference + 2 cover +
velocity is 2.83 feet per second. The
invert of the outflow sewer at manhole
The sewer would start this
8 must be at least 2.5 feet below
ground level or at or below the invert
necessitating much excavation.
of the incoming sewer, whichever is
the better choice is to go to a 6-inch
lower. Therefore, 297.5 is used. The
house sewer which will allow a much
invert of the sewer at manhole 7 is 4
The design with a 6-
feet lower or 293.5 feet.
inch sewer is shown in row 2, figure
is sketched on the solution drawing,
The invert elevation at the
building had to be chosen.
between the method used in the design
made to be 2-1/2 feet below the ground
of the lateral from MH 7-1 to MH 7 and
This will allow 2 feet of
any of the previous ones is a slope of
cover over the 1/2-foot pipe.
4.55 percent instead of the ground
the invert is 302 - 2.5 = 299.5 feet.
slope of 4.5 percent.
The invert elevation at manhole 8 is
increase is due to going from 4-inch
then 299.5 less than the 2-foot drop
to 6-inch pipe at manhole 7-1.
in the pipe or 297.5 feet.
elevation of 302.6 less a ground drop
of 9 feet cannot be used because this
b. Connections to MH 7-1.
would not provide 2.5 feet of ground
line from building B to MH 7-1, the
above the invert at the end of the
ground slope of 1 foot in 40 is tried
lateral. The data is shown in row 6.
Figure 18. Sample design solution (issued as separate item).
d. Main from MH 7 to MH 4. It can
One would be to start as
be seen from the profile that the
above but increase the slope so that
sewer between MH 7 and MH 6 cannot be
sufficient cover can be had.
placed at the same slope as the ground
will necessitate deep excavation.
starting at the elevation of the inlet
an example, the dotted line in the
invert at MH 7.
If this were done
solution could be followed.
there would not be sufficient cover in
solution is available, however.
is to drop the outlet of manhole 7 and