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. |
Host Pipe InsideDia. 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