In the first
article, we discussed how different pipe renewal methods can improve the
flow characteristics of deteriorated pipe by smoothing over structural
defects, eliminating root growth, and reducing the friction coefficient, or
Manning's "n" value. In
this second article, we will compare the impact of partially deteriorated vs.
fully deteriorated pipe on liner thickness, which in turn affects pipe
capacity.
A key factor in determining the thickness for CIPP (cured-in-place pipe) liner
is the structural condition of the host pipe.
If the host pipe is deteriorated to the point it can no longer support
a load, then the liner must be sized to withstand all dead and live loads
anticipated on the pipe. In addition, if the host pipe is oval shaped due to
partial collapse, a thicker liner is required to compensate for the reduced
hoop stress.
Definitions
- Partially
Deteriorated Pipe: Existing
pipe has minor structural deficiencies, but can still support all loads.
- Fully
Deteriorated Pipe: Existing pipe is in
poor structural condition, and may not withstand existing loads.
- %
Pipe Ovality = 100 x (Max. Diameter – Min. Diameter)/Mean Diameter.
For partially deteriorated
pipe, liner thickness is independent of external loading, as these loads are
carried by the host pipe. For fully
deteriorated pipe, liner thickness design must consider soil depth,
live load, and ground water depth as well as % ovality.
In the example below, the resultant capacity increase for partially
and fully deteriorated pipe is compared for two ovality values: 0% and
10 %. Typically, ovality should not exceed 10-15% for CIPP liners.
As indicated in the table, the capacity increase ranges from 14% vs.
27%, depending on the host pipe condition.
8” Sewer Capacity
Increase with CIPP for Differing Host Pipe Conditions (Example)
|
Design
Conditions:
Soil
Depth = 15 ft.
Ground Water Depth = 10 ft.
Live Load = H20 (16,000 lbs)
|
Host
Pipe Inside Dia.
n=0.014
|
%
Ovality
|
Min.
Liner Thickness
|
Design
Liner Thickness
(1)
(2)
|
Lined
Pipe Inside Dia.
n=0.010
|
%
Capacity Increase
(3)
|
|
|
(in)
|
(%)
|
(mm)
|
(mm)
|
(in)
|
(%)
|
|
Partially
Deteriorated
|
8.00
|
0%
|
3.1
|
3.5
|
7.72
|
27%
|
|
Partially
Deteriorated
|
8.00
|
10%
|
4.1
|
4.5
|
7.65
|
24%
|
|
Fully
Deteriorated
|
8.00
|
0%
|
3.8
|
4.5
|
7.65
|
24%
|
|
Fully
Deteriorated
|
8.00
|
10%
|
6.9
|
7.5
|
7.41
|
14%
|
(1)
Source: www.insituform.com – Design Guide and Specification
(2)
Safety Factor = 2.0; Resin System Modulus = 300,000 psi
(3)
% Capacity Increase = (n existing/ n new) x (Dia
new / Dia existing)8/3
x 100
