Defined by the US Department of Homeland Security, Infrastructure Resiliency is the ability to reduce the magnitude and/or duration of disruptive events. The effectiveness of a resilient infrastructure or enterprise depends on its ability to anticipate, absorb, adapt to, and/or rapidly recover from a potentially disruptive event.
That being the case, seismic resiliency of buried pipes can be described as the ability to reduce the effects of seismic events on those pipes and the fluids in them.
Critical piping applications, such as oil and gas transmission lines, enjoy most of the seismic buried piping design studies and reports, as they should. Thoughts of a rupture invoke visions of hellish fireballs and untold environmental damage. Water transmission lines have only been the subject of studies in the last decade, and those studies start from what we know about oil and gas pipelines.
More cities are beginning to ask about seismic resiliency for less critical applications such as storm and sanitary sewers. There’s one problem – there are few papers and testing for owners, engineers, and manufacturers to lean on.
Instead, there exists powerful anecdotal studies like this one reviewing the successful performance of 16 long span buried culvert bridges located within a 30-mile radius of the epicenter of the 1994 Northridge earthquake in California.
There also exists Section 3.10.1 from the AASHTO LRFD Bridge Design Specifications, which states, “Seismic effects for box culverts and buried structures need not be considered, except where they cross active faults.”
Buried pipes for storm and sanitary sewer applications have withstood seismic conditions for many decades. Implementing the design approaches below will help to improve the seismic resiliency of these less critical pipelines.
TRANSIENT GROUND DEFORMATION
- Ground oscillation
- If necessary, use of a casing pipe can help relieve the carrier pipe of these transient loads.
PERMANENT GROUND DEFORMATION
- Surface Faulting
- Avoid locating pipelines along or across known active faults.
- Use softer backfill material 165 feet from each side of the fault line. This approach creates other structural concerns with flexible pipes and should be used cautiously.
- Oversize pipe trench width 50 feet for a length of 50 feet from the fault.
- Ground improvement to prevent landslides from occurring.
- Small aggregate sands, like those found in AASHTO A3 soils, in the presence of high ground water elevation are most susceptible to liquefaction. Crushed stone and a wider backfill zone will greatly improve the pipes resiliency.
While some pipe systems can also add increased pipe wall thickness to improve survivability, the largest factors in higher seismic resiliency reside in backfill materials and techniques. This has been borne out in study and practice.
Special thanks to Mark Taylor ([email protected]) for his knowledge and contribution for this piece.