Relining of either rigid or flexible culverts may be done either by slip lining or by installing a flexible liner. Slip lining is merely the insertion of a prefabricated pipe inside an existing pipe. The most common insertion pipes are either corrugated metal or plastic. Obviously, the cross-sectional area of the pipe will be reduced. This will likely affect the hydraulic capacity of the culvert. If a smooth plastic pipe is utilized, the velocity of the flow may be increased, in part offsetting the reduction in the capacity due to the decreased area. Should this be the case, the downstream end of the culvert should be investigated to prevent additional erosion. Regardless of the type of pipe inserted, the annular space between the inside of the existing pipe and the outside of the inserted pipe should be grouted. The final product is comparable to a new culvert, placed with little disruption to the traveling public.

Another method of relining consists of inserting a flexible tube inside the damaged pipe. The flexible tube will generally consist of a resin liner, which, after being inserted inside the subject pipe, is expanded to fit the full cross-section of the pipe. It spans irregularities such as joints that may have opened. Flexible reline pipes are available in a variety of materials such as HDPE, PVC, and thermal-set resins. They are installed using a variety of methods such as fold and form, and inversion processes. The products are specified and designed on a project-by-project basis. Many of the rehabili­tation products can be designed to provide additional structural supprt to the existing pipe. The manufacturer of the material should be consulted as part of the overall design process.

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