By Hugh B. Mickel, P.E.
| Monday, August 08, 2016 | 4001 Views |
I often receive questions on how to properly design a reline project, from assessment and hydraulics to structural design. I thought I would take time to discuss one involving the latter in particular.
QUESTION: I am doing a reline of a 60" diameter storm sewer. There are a lot of structural design methodologies being touted by various product manufacturers and sources. How do I know which ones are legitimate?
Apparently, you have done some research already! Yes, there are many design approaches and many that claim to be "fully structural " when in reality they are not. The answer to your question involves a number of aspects.
Purpose of the Relining Effort: If the goal is to correct an isolated problem then a review of the original pipe design method is warranted. Then the design loads intended to be carried by the pipe can be applied to the area of concern and a rehab method can be developed to handle those loads for the long term. This is sometimes easier said than done so read on for more helpful thoughts.
If however the goal is to provide a systemic long term solution that relies on the new pipe or lining system to perform as the original structure had but for another segment of time (75 years is a common service life goal for drainage and sewer pipelines) then the design method needs to be a proven and reliable one.
Drainage & Sewer Pipeline Design Historical Perspective: If a rehab method is trying to restore the original load capacity of the host pipe then the original design methodology and how it was constructed must be considered. For example, a rigid pipe design is quite different then a flexible pipe design. Within rigid pipe design, there must be considerations made on when design and construction occurred. This can help determine what design standard was used. Reinforced concrete pipe (RCP) design methods changed in the US during the 1990s but not all state DOTs adopted the new methodology immediately. Adaptations were also made during the conversion to Load and Resistance Factor Design (LRFD) methodology.
CMP designs have also evolved over time but not to the same extent. The American Concrete Pipe Association (ACPA) and the National Corrugated Steel Pipe Association (NCSPA), or manufacturers within those organizations can be valuable resources in this regard.
Of course, as built records and other documents pertaining to original design and construction are hugely valuable. For example, if CMP metal thicknesses or RCP wall thickness and reinforcement designs were pulled from state DOT standards, then those particular records are very helpful. Profile wall HDPE pipes have also undergone material and design methodology changes since they were introduced to the marketplace.
Structural Design: Height of cover, soil unit weight and design live load must be determined. If there is any possibility of voids around the host pipe or if the movement of groundwater or effluent can occur outside the host pipe, or if the backfill envelope or surrounding fill is compressible then the new pipe should be designed as if it is carrying the full load, like that of a direct bury installation. Frankly this is the smartest approach for any reline project: design the liner to carry the full load as if the structure is being constructed in a conventional manner. This takes away much of the guesswork. Then you can consult the AASHTO Standard Specification for Highway Bridges, Section 12 for various pipe design methods, included reinforced concrete pipe. I’ve personally witnessed a supplier of spray applied polyurea state that a 60 mil thick coating (0.060”) provided a structural lining for a large diameter culvert pipe. I was floored. I've also heard a supplier of spray on cementitious lining materials explain that a 1.75” thick lining on a 120” CMP would provide a fully structural lining. If either of these materials were applied prior to the loss of significant wall thickness, then I think they would definitely enhance the performance of a pipe. But once significant wall thickness has been lost, materials can’t be designed as a coating. They have to be designed to carry all applied loads. That includes the full soil column load and any live load pressures that reach the crown of the new pipe. So how is that accomplished?
The answer: There are published design methodologies that use a less than full load approach. These are commonly contained in appendices of ASTM specifications. Although these can be solid approaches, careful scrutiny is needed to potentially apply them to large span culverts, sanitary and storm sewers when any amount of wall section loss has occurred. Thin wall hoop compression theory does not apply to structures that are no longer a perfectly round hoop. It doesn't apply to diameters beyond 48 inch, or in situations where highway live loads are applied through cover heights that are less than 1/3 of the span. Likewise, the design approach in ASTM F1216 for cured in place plastic (CIPP) pipe doesn’t apply if significant deflection has occurred, and one could argue that if a rigid pipe has cracked or broken, that methodology no longer works by itself.
Also, all too often the specialty contractor or supplier of a lining material is engaged to provide the design. This can be very risky for an owner or the engineer. Likewise, contractor capability and product capability cannot be placed above engineering. Much of the nation’s transportation infrastructure was designed and built using methods that are known and proven. Current specifications and design standards from the American Association of State Highway and Transportation Officials (AASHTO) are a great resource. It is the bridge engineers within the state departments of transportation who make up the group that reviews and considers design methods for pipe products. When a design methodology has made it to Section 12 of the AASHTO Standard Specification for Highway Bridges (this is where the design methods for pipe related products are located), you can fully trust that it is legitimately structural. However, if a method is an adaptation of a limited research study, or it simply sounds too good to be true, be leery of it.