Deflection Testing of Gravity Sewer Pipe
Deflection testing is usually performed
with a properly sized "go no-go" mandrel or sewer ball. For the purpose of deflection measurements the base inside pipe diameters without deflection are provided in the following table. The base inside diameter, or ID, is a pipe ID derived by subtracting a statistical tolerance package from the pipe's
average I.D. The tolerance package is defined
as the square root of the sum of the squared
standard manufacturing tolerances (OD,
wall thickness, and out or roundness).
The following table provides the base ID
and mandrel sizes for deflections of 5%
and 7.5% and for SDR35/PS46 and SDR26/PS115
pipe.
|
SDR35/PS46 |
SDR26/PS115 |
|
|
5%
Deflection |
7
1/2% Deflection |
|
5%
Deflection |
7
1/2%
Deflection |
Nominal
Size |
Base
ID |
Mandrel
Size |
Mandrel
Size |
Base
ID |
Mandrel
Size |
Mandrel
Size |
4" |
3.895 |
3.70 |
3.60 |
3.811 |
3.62 |
3.53 |
6" |
5.742 |
5.45 |
5.31 |
5.612 |
5.33 |
5.19 |
8" |
7.665 |
7.28 |
7.09 |
7.488 |
7.11 |
6.93 |
10" |
9.563 |
9.08 |
8.85 |
9.342 |
8.87 |
8.64 |
12" |
11.361 |
10.79 |
10.51 |
11.102 |
10.55 |
10.27 |
15" |
13.898 |
13.20 |
12.86 |
13.575 |
12.90 |
12.56 |
18" |
16.976 |
16.13 |
15.70 |
16.586 |
15.76 |
15.34 |
21" |
20.004 |
19.00 |
18.50 |
19.545 |
18.57 |
18.08 |
24" |
22.48 |
21.36 |
20.79 |
21.964 |
20.87 |
20.32 |
27" |
25.327 |
24.06 |
23.43 |
24.744 |
23.51 |
22.89 |
30" |
29.132 |
27.68 |
26.95 |
28.461 |
27.04 |
26.33 |
36" |
34.869 |
33.13 |
32.25 |
34.120 |
32.41 |
31.56 |
The maximum recommended installed deflection
is 7.5% at not less than 30 days following installation.
That deflection includes a safety factor of four
before any reverse curvature develops. It is
important that the inside of the sewer line be
cleaned thoroughly before deflection testing.
Debris or sediment in the pipe can impede the
passing of the mandrel and provide a false indication
of excessive deflection.
PVC pipe is classified as a flexible conduit.
It is because of this flexibility that PVC pipe
can be installed at burial depths in excess of
50 feet. A flexible pipe will deflect when placed
under a load. With Flexible conduit theory
vertical deflection of the pipe allows that soil
load to be transferred and supported by the strength
of the supporting soil on each side of
the pipe. As a result, the ability of PVC pipe
to withstand soil load without exceeding maximum
deflection limits is directly related to the
quality of the installation and compaction of
the embedment material around the pipe. (See
Uni-Bell PVC pipe association publication UNI-TR-1 "Deflection: The Pipe /Soil Mechanism" for more information.)
Deflection testing is not an issue when using
proper construction practices and inspection
during pipe installation and when using embedment
material which has been properly selected, placed,
and compacted. Locations with excessive deflection
should be repaired by re-bedding. As an alternate,
technology exists that utilizes a vibrating "rerounder" that
is pulled down the pipe to reshape the pipe and consolidate
the soil around it. This process can save time and
money in locations where excessive deflection is found.
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