Figure 18. Sample design solution (issued as separate item).

system will be designed.

The first

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 ground

The design is tabulated in row 3,

slope of 2 feet in 75 is used as a

figure 17. For the line from building

first try.

With a 4-inch house sewer

C, the ground slope of 1 foot in 30

(the minimum size allowable) a full

was used.

Its design parallels that

flow of 127 gallons per minute is

of building B and is tabulated in row

obtained.

This is less than the

4.

actual

flow

and

therefore

is

unacceptable.

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

percent.

This means a 5.1-foot drop

been used.

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

about

1/2-foot

pipe

width)

below

invert of the outflow sewer at manhole

ground.

The sewer would start this

8 must be at least 2.5 feet below

low

and

probably

go

lower,

ground level or at or below the invert

necessitating much excavation.

Thus

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

smaller slope.

The design with a 6-

feet lower or 293.5 feet.

The sewer

inch sewer is shown in row 2, figure

is sketched on the solution drawing,

17.

The invert elevation at the

figure

18.

The

only

difference

building had to be chosen.

It was

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

elevation.

This will allow 2 feet of

any of the previous ones is a slope of

cover over the 1/2-foot pipe.

Thus

4.55 percent instead of the ground

the invert is 302 - 2.5 = 299.5 feet.

slope of 4.5 percent.

This slight

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.

An

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.

For the

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

available.

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.

This

placed at the same slope as the ground

will necessitate deep excavation.

As

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.

Another

there would not be sufficient cover in

solution is available, however.

That

the

center.

Two

solutions

are

is to drop the outlet of manhole 7 and

4-23