Insituform® CIPP is designed as a structural product for its most common applications – gravity storm and sanitary sewer rehabilitation. As described in ASTM F 1216, there are two basic design methods. If the host sewer is capable of supporting soil and live loads, Insituform is designed to resist only the external buckling pressure exerted by groundwater. This design methodology is quite common because most sewers are generally circular and not in danger of collapsing. Also, once the CIPP is installed, the sewer is stabilized, and the surrounding backfill material is prevented from entering the sewer. Soil entering sewers leads to soil voids which is a principal cause of collapse.

In the second design method, all loads acting on the sewer are considered. These include soil, water, traffic, railroads, buildings and so on. The design for this case uses a model developed for new or direct bury sewers. It is generally agreed that this model is conservative, and eventually it will be replaced by a more accurate design technique leading to more cost-effective designs. However, because it is conservative, the chance of failure is very small.

Both of the above design cases have been extensively tested. The soil load model was tested in the large Utah State soil box with the results indicating that the design is conservative, as suspected. The Trenchless Technology Center (TTC) at Louisiana Tech tested the load case of external pressure due to groundwater. A design life of 50 to 100 years was substantiated. The most telling test, however, is the first Insituform® CIPP ever installed. This London CIPP was installed in 1971 and has been examined and sampled several times since. In all cases, the CIPP was performing well with physical properties in excess of current day standards.

The design models used, test results and 35 years of service all confirm that Insituform® CIPP for gravity sewers is a structural product.

For some pressure pipes, the Insituform Pressure Pipe Liner® (PPL) can be used to seal pressure pipes that are structurally sound but are leaking through open joints, cracks or pinholes. However, PPL will not increase the pressure capacity of the existing pipe nor will it support the internal pressure if the host pipe fails. In this case, Insituform® PPL is truly a liner, not a structural fix.