When a culvert or storm sewer reaches the end of its useful service life, the option to excavate and replace is commonplace.
However, having the additional option to reline the existing structure to provide a structural solution can save significant time and money.
Identifying Methods of Failure
One of the first steps in deciding a solution is to identify the failure mode of the original or “host” pipe, as well as trying to determine if it is sized correctly. It is not always possible for a full hydraulic study to be done on every reline but if the structure is undersized from its original design due to increased runoff, this may have caused the early or premature deterioration of the culvert, and a reduction in effective flow area may cause future issues.
Is the invert of the pipe deteriorated, yet the host pipe still has good overall shape? This may indicate that a higher abrasion rate over many years has worn down the invert. It may also be indicative that for many years culverts were installed with only a 50-year or less design life. In many of these cases, galvanized steel culverts have been installed, and the original design life has been exceeded. This can be addressed in the type of slip line product selected, and is still a good candidate for reline. Many other products such as Aluminized Type 2 Steel, Aluminum, Polymeric Steel, as well as PVC and HDPE have an expected service life of 75 to 100 years.
Has the pipe shaped changed, and if so why? Has a joint separated causing loss of backfill or has additional fill been added to the culvert exceeding the original design parameters? This may limit the solutions available for reline, however it does not rule it out.
Is there flow outside the host pipe that will cause loss of backfill? This may cause cracks or settlement in the pavement above. This can be addressed many times with improved inlet protection, as well as filling the void spaces while relining.
It is important to address any cavities in the surrounding backfill that, in many cases, can be handled with a well-thought-out grouting plan. Lightweight cellular grout can often travel or be injected into these void spaces and replace the washed out backfill and support around the host pipe.
But When Is it Too Late?
Inspecting the current conditions of the existing pipe is a crucial step to determining whether or not a reline solution is an option. Full corrosion or structural failure can make it an unsafe work space and inspection and review by a qualified professional must be taken into consideration when a reline may require entry into the existing pipe, and when its failure would compromise public safety. What constitutes full corrosion or structural failure is a debatable topic, but care must be taken when working underground.
A compromised structure can be mitigated by doing one or more of the following:
- Remove live load traffic directly over the work area – This may not be possible under major roadways
- Inject grout into any voids created by the moving structure and its soil envelope – if re-mining work is expected, the grouting can be done with excavatable cellular grout.
- Brace/strut the interior to prevent further movement.
With pipes that have corroded inverts, only in some cases can sliplining still be done safely, but concern must be taken not to destroy the liner pipe or disturb the host causing further damage or collapse. In several cases, a horseshoe-shaped arch with a properly designed cross-strut at its base that remains from one side of the invert corrosion (around the top to the other side) can be enhanced to provide the safe working environment needed.
The use of two-flange tunnel liner plate is also commonly used in a structurally compromised structure. Tunnel liner plates, either in steel or aluminum, can be used entirely to reline a structure or in conjunction with other products where the tunnel liner plate is just used for the most severe portion of the reline. Once grouted in place, this technique will give the contractor a safe working environment.
If the corrosion line extends up the side walls, this can be more challenging. Short segments of welded on steel flat stock or pipes can allow the loads to be bridged across a corrosion line. These can be welded or secured to the host pipe, as needed.
Any of these techniques need to be detailed and reviewed by qualified individuals, but it’s not rocket science.
Lastly, don’t forget about the hydraulics. Hydraulic performance expectations must be considered in conjunction with decisions around what can physically be built. It’s surprising how many inlet control culverts get lined with a smooth product or process in the spirit of simplistic Manning’s equation capacity comparisons when Manning’s ‘n’ values before and after have no impact on the actual capacity of most culverts. Inlet control and outlet control flow regimes must be checked, and the one producing the highest headwater depth governs how it is operating.
In summary, there really is no clear-cut answer to this question. Ultimately, an inspection of the existing pipe and a clearly outlined reline plan must be considered, along with a review of hydraulic performance before and after the relining work is done. Construction-wise, the ultimate deciding factor should be the safety of those involved in the reline work. If hydraulic goals can’t be reached with a safe construction plan, replacement is probably the best option.