What to Inspect When You’re Inspecting (Big Pipe Edition)
As engineers, we often focus on the design stage of a project’s life cycle. In many ways, this is a good thing. After all, we need to get the design correct. However, the most critical stage of a project is really construction, where our designs get implemented in the field.
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A failure to implement those designs correctly can lead to significant challenges down the road. The question is how can we, as design engineers, ensure proper focus is given to the construction inspection stage. When thinking about construction inspection:
- There are tasks that, when initiated early, can ease the burden later in the project life cycle;
- It is beneficial for the inspection team to know why design decisions were made so that they are better prepared to identify potential problems; and
- There are many times when how activities are completed in the field is as important, if not more so, that what activities are completed.
This article is not an all-inclusive guide to pipeline inspection. It is instead a brief overview, highlighting a few places to focus our attention in the design and construction of pipelines as it pertains to inspection activity. My intent is to highlight the importance of inspection in the overall infrastructure design and construction process that results in better project delivery for our customers.
When to Engage Inspection in the Project Life Cycle
When it comes to pipe (particularly large diameter or high-pressure pipe), the sooner we engage construction inspection in the project life cycle, the better value we receive in the end. As a project progresses from concept to design to construction, the project team loses the ability to influence cost as that cost to implement a change increases exponentially when you enter construction.
Accordingly, the potential complexity of a project should drive the engagement of construction inspection. Unfortunately, the first time an inspector sees the plans is usually at the pre-construction meeting…hopefully the inspector was invited! But, for complex projects, it is reasonable to engage inspectors in preliminary design or even concept development. There are times where early engagement may not be necessary (e.g. installation of an 8-inch waterline in the middle of a vacant parcel), but, more often than not, involving the inspectors early will benefit the project.
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Understanding Pipe Design to Know What is Happening
The structural design of pipe requires pages of calculations, and proper installation (to provide a functioning and serviceable product) requires adherence to detailed guidance. Without proper installation in accordance with the design assumptions, the pipe system will not perform as expected. And, installation quality and performance are much more in the hands of the contractor and inspector than the engineer.
An inspector should know if the project is using a commodity or engineered product, and this is not simply a matter of pipe size/diameter or material. Commodity products are generally purchased from a catalog with standard and conservative design assumptions. This could include ductile iron or PVC pipe. Alternatively, an engineered pipe such as concrete pressure pipe or steel pipe is designed with specific tolerances, assumptions, and criteria for depth of cover, dead load, live load, buoyancy, internal pressure, trench width, and sometimes even how the pipe can be safely shipped, stored, or handled.
For instance, one of the most critical aspects of pipe design is the embedment – that all important fill material in the pipe zone, from flowline to crown. Inspectors should think of engineered pipe as a composite system of trench, soil, embedment and the pipe itself. One element (the pipe) cannot function as designed without the proper treatment and installation of the other (the trench and embedment). It is truly a composite structural system.
By engaging the inspector early and emphasizing the importance of the trench width, material gradation, lift height, and compaction/consolidation methods, he or she can know with confidence what is happening with the pipe during construction. By communicating our design criteria with inspectors and emphasizing where compromises cannot be made in the field, we can minimize the risk of pipe failure. Knowing the why helps identify brewing problems. Addressing those potential problems early is a key to success in any construction effort.
When Means & Methods Matter
Additionally, the means and methods of pipe construction are critical to the life cycle of the installed product. The following are a few examples to consider.
- Handling: Many times, the loading conditions during pipe installation and pipe handling are different. We often see this with aerial crossings. Confirming the coating or lining is not damaged during handling loads may be necessary.
- Embedment: Remember, proper embedment is vital to the structural integrity of pipe. Therefore, how the contractor places and consolidates or compacts the embedment material may be more important than simply getting the gradation correct.
- Bolt Up: How often have you walked up to see someone with a three-foot-long cheater bar pounding away to tighten down bolts at a flange? A proper design has a specific bolt, nut, gasket, and pattern for how that connection is made. You should never allow a mixture of bolts to be used, and simply going around clockwise with a cheater bar will overload your gasket and almost surely lead to leaks in the future.
The guidebook for when means and methods must be considered would be long indeed, but from this simple list, it is clear that how things are done is critical.
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This article could easily be turned into a series of tips, tricks, and hints for pipeline inspection, and similar articles could be written for pavement or facility inspection. The point is not to outline every detail here, but rather to emphasize the important role that our inspection professionals have in providing high-quality infrastructure systems.
As a pipe engineer, I can get everything correct in my design, but it will not matter if the contractor uses the wrong embedment material, overstabs the pipe, doesn’t compact the material, or bolts up the connections improperly. We strive to get inspection personnel engaged as early as possible to understand what we have designed and, more importantly, why it matters. Only then can we form a truly effective project delivery team.
Justin Reeves, P.E., is vice president at Lockwood, Andrews & Newnam, Inc. (LAN), a national planning, engineering and program management firm.