Installing a renewal liner in an existing sewer typically improves the flow characteristics by smoothing over structural defects, eliminating root growth, and reducing the friction coefficient, or Manning's "n" value. A best-case scenario design would improve the n value from 0.014 (typical for deteriorated VCP sewer) to 0.009 (typical for a new PVC or HDPE liner). This results in a 55% increase in flow capacity. However, this optimistic claim does not take into account liner thickness, liner pipe diameter, and the condition of the host pipe. The liner thickness requirements will vary with material used, renewal method, host pipe condition, severity of offset joints, soil depths, soil type, live load, and hydrostatic load. There are three (3) basic methods for trenchless renewal of deteriorated clay or concrete pipe: 1) Cured-in-place pipe (CIPP) liner, 2) Deform & reform, and 3) Slip line.
For circular pipes flowing full, Manning's equation can be simplified to compare the before and after pipe capacity as a % of existing capacity, as follows: % Existing Flow Capacity = (n existing/ n new) x (Dia new / Dia existing)8/3 x 100
For the example appearing in the table below, an "n" value of 0.010 has been used as a more realistic value for long-term design conditions. The liner thickness is based on typical values for pipe at 15 ft. pipe depth and a partially deteriorated host pipe. Applying these values to the table below gives an increase in pipe capacity ranging from -17% to +23%.
Typical Sewer Capacity Increase by Various Lining Methods
Rehabilitation Method Host Pipe Inside Dia. Host Pipe "n" Liner Pipe Outside Dia. Liner Thickness Renewal pipe "n" % Capacity Increase (in) (in) (%) CIPP 8.00 0.014 8.000 0.236 0.010 19% Deform & Reform 8.00 0.014 8.000 0.195 0.010 23% Slipline 8.00 0.014 7.125 0.274 0.010 -17%
In the second article, we will compare partially deteriorated and fully deteriorated liner design and it's impact on liner thickness and Manning's "n" for each rehabilitation method.