That is definately getting into some touchy areas. I've worked quite a few pipelines with seismic considerations, as has been pointed out, probably the most famous is TAPS. But there are plenty of others. There are many considerations, not the least of which is will any of your pieline be in frozen ground? You need to get an expert in this.
For some basic guidance, pipeline fault crossings generally utilize three basic fault-crossing trench configurations. This will depend on what type of stress the pipeline will be in - strike/slip, compression or tension. A depth of soil cover not exceeding one meter is generally required for all special fault-crossing trench configurations. The 3 configurations used to mitigate the 3 types of stresses are: 1) sand shading, trapezoidal trench, and geotextile lining. The angle of the pipeline with regards to the fault is also critical as is the length the pipeline traverses the fault, sometimes a long length is best, sometimes short, depends.
Liquefaction may also be a consideration and that adds yet another layer of complications. I know in many of the pipeplines I've worked on, a higher yield pipe is used for the fault crossings.
The special construction measures generally must extend over the length of pipeline that might possibly experience transverse (horizontal or vertical) movement. The width of crossing, Wc, on each side of the pipe must be included in the length of special construction measures, because the surface expression of any future fault movements could occur anywhere within the crossing width. A depth of cover not exceeding one meter is normally specified for each fault-crossing zone and whatever depth is calculated, it should be maintained within reasonable tolerance over a length of crossing. Small variations in cover over short distances will not be detrimental, but significant or frequent deviations should be avoided.
There is allot that goes into the design, you must look at the seismicity of the pipeline route, check local tables and contour maps for peak acceleration, short period spectral accelerations, and l.0-second period spectral accelerations for various applicable event return periods.
If frozen soil is a possibility, then the performance of a buried pipeline at fault crossings depends on maintaining a relatively low-strength soil medium around the pipeline in the zone of soil failure. What happens is during wintertime temperature extremes at the higher elevations, the soil will freeze from the surface downward, sometimes to a depth that will serve as a nearly rigid encasement above and to the sides of the pipe. If the pipeline is unable to produce local soil failure in response to fault displacements, pipeline structural integrity may be compromised for even relatively small displacements. It is critical to limit the depth of soil freezing within the fault crossing sections.
In summary, get an expert. I've worked quite a few of these and have some general knowledge, but wouldn't even think about tackling it.
Greg Lamberson
Consultant - Upstream Energy
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