Pipe Materials Selection: The Customer Is Always Right

The customer is always right when it comes to pipe material selection for municipal water, stormwater, and sewer pipe selection; even when they are technically wrong. But problems arise more often when engineers and contractors do not adhere to relevant design and construction methods.

This is an observation that I have made over the years. The common theme from municipalities is their system has one dominant pipe material for operations consistency. The objective is to stock one material for repairs, new connections, and crew training.

Available new pipe materials for municipal installations include clay, ductile iron, steel, concrete reinforced of unreinforced, fiberglass, PVC, and HDPE.

AWWA recognizes that multiple materials may be properly designed and constructed as outlined in manuals for steel, PVC, HDPE, ductile iron, fiberglass, and concrete pipes.

The National Clay Pipe institute provides design guidance for clay pipe, and ACI for concrete pipe designs. Brick structures are labor intensive, costly and require trenching thus newer construction typically uses alternative modern materials.

Individual pipe materials have unique properties that age at different rates, have different design requirements, and require different maintenance. Pipe material property assessment must relate to surrounding soil, applied dead and live loads, and product loads such as liquids and vapors contained within the pipe, pipe operating pressure and cyclic loading.

Each of these features requires pipe engineering involving pipe material and their interaction with the surrounding loads and soil properties.

As demonstrated across the United States, different municipalities successfully select and install different materials that meet their preferences. There is no single pipe material works under all conditions.

The designer must consider the interaction between pipe properties, given loads, subsurface conditions, and local environmental conditions.

The contractor must have experience with successful installation of a selected pipe in the given local conditions.

The owner sets expectations and relies on the engineer and contractor skills to complete the project at a reasonable cost. Only when fatal flaw(s) at a specific site are recognized should an engineer raise a red flag regarding a site-specific issue that precludes use of a specific pipe material.

Pipe material selection should be a process of data gathering to make an informed decision. Data should at least resolve the following:

•What is the intended project purpose?
•How will the pipe barrels be joined to prevent leakage and provide
restraint?
• What connections will be required and how will they be installed?
• What is the usable cross-sectional area for delivery of the intended product?
• Who will be the Engineer of Record and do they have experience with this pipe material?
• Is there a lifecycle cost estimate?
• Is there consideration for future upgrades and redundancy?
• Does the route selection consider risks to safety, cost, environment, and schedule during construction and later maintenance?
• Can the pipe material be installed trench, trenchless, or both methods?
• What is the national and local history of the selected material and material options for the intended purpose?
• What are the selected material and material options mechanical properties and the environment parameters that define the properties?
• Do the mechanical properties change with environmental conditions: temperature, time, etc. within the expected environmental range for the project?
• What special construction conditions may be required to mitigate possible exposure to failures?
• What are the anticipated failure modes and will they be mitigated by design and/or construction?
• What inspection and engineering oversight will be needed for QA/QC for a Quality Project?

These questions require experience with construction using the selected material and knowledge of the failure modes other than overstressing of a specific material. Several possible failure modes for specific materials follow.

PVC can fail in a brittle manner under short term or impact loadings and flexible for uniform long-term loadings. Lower temperature increases the risk of brittle failure. Point loading at any temperature increases risk of brittle failure. Rapid crack growth may occur when longitudinal stress is elevated, but present-day material chemistry has reduced this type of failure. The pipe barrels are fused together by fusion welding. These welds are stronger than the pipe but can fail when welding criteria are not followed.

HDPE can fail in a ductile manner by unconstrained or constrained buckling under relatively low mechanical or soil loads compared to other pipe materials. The current HDPE resin is PE4710 which provides long term resistance to slow crack growth failures seen in previous resins. HDPE is a visco-elastic plastic material. It expands like a balloon when pressurized until it pops. Appropriate industry accepted factors of safety are used to prevent this failure mode. The pipe barrels are fused together by fusion welding. These welds are stronger than the pipe but can fail when welding criteria are not followed. HDPE resin is buoyant in water and will float in shallow trench installations if not adequately anchored.

Ductile Iron can fail when subjected to long-term corrosion and from failed joint seals. Internal corrosion causes tuberculation with subsequent long-term reduction in flow. Internal coatings can retard but not prevent internal corrosion. External corrosion can result from chemical attack by high reactive soils like marine clay or from anaerobic microbiological attack.

Concrete can fail when placed below groundwater including: joint seals either during construction or long-term joint separation thermal loads, progressive internal and external corrosion failure of tensile reinforcement, and concrete deterioration by internal sewer gas and/or external corrosive soil, road salt by leaching of cement paste. Angular backfill can abrade the concrete structure in shallow installations subjected to heavy cyclic loads such as railroads.

Clay can fail at the joints by leakage and is typically associated with construction. The short barrel lengths allow differential settlement between the barrels that can result in relatively large joint rotation.

Steel can fail by corrosion either with or without coating. Corrosion failures often involve passive or active corrosion protection systems that are not properly maintained. Both factory and field welds may fail. Some of the more common weld failures occur when the pipe is incorrectly specified for the intended use to save money.

Fiberglass can fail in a brittle manner by confined or unconfined buckling and by longitudinal bending of the barrel or rotation at joints. Under some loadings, fiberglass barrels experience shear failure along the length of the barrel when subjected to bending along trenchless bore paths.

Pipe selection should be made by the municipality based on lifecycle cost and the common pipe material in their system. There is no ‘silver bullet’ pipe material. Unless a pipe material has a fatal flaw for the intended use then a specific pipe material can be successfully designed and constructed. There is no substitute for experienced engineers and contractors.

Brian Dorwart, P.E., PG, BC-PW, M ASCE, is senior consultant with Brierley Associates Corp.