Page 80 - Buried Pipe Design

56 Chapter Two

In light of the many uncertainties associated with the initiating
earthquake, the soil response, and the pipe behavior, it is often neces-
sary to perform sensitivity analyses where key parameters are varied
around a best-estimate value to ensure that the design is safe,
accounting for uncertainties.

Permanent ground deformation
Ground deformation from earthquakes includes lateral spread of
sloped surfaces, liquefaction, and differential soil movement at fault
lines. Ideally, the routing of a buried pipe is selected to avoid these
seismic hazards. Where this is not possible, the effects of postulated
ground motions are considered in design. 15
The first step is to establish the seismic hazard, or design basis
earthquake, and predict the corresponding ground movement.
The second step is to establish the performance requirement for the
buried pipe. For example:

1. The pipe may need to remain serviceable and allow, e.g., the pas-
sage of pig inspection tools.
2. The pipe may need to remain operational, with valves opening on
demand to deliver flow or closing to isolate a hazardous material.
3. The pipe may only need to retain its contents, without being opera-
tional following the earthquake.

Based on the performance requirement, an allowable stress or strain
limit is established.
The third step is to analyze the pipe response to the postulated move-
ment, and the resulting tensile, bending, and compressive loads
applied to the buried pipe. This may be done very easily by hand cal-
culations to the extent that the deformations are small. For large defor-
mations, preferably the calculations should be done by finite element
analysis of the soil-pipe interaction. Finally, the computed stresses or
strains are compared to allowable limits established earlier based on
the required performance of the pipe following the earthquake.
The design for seismic loads and deformations associated with buried
piping depends on the accuracy of the predicted ground movement and
soil properties as well as the accuracy of the pipe-soil interaction model.
Parametric variations of the input and model are usually necessary to
bound the problem. Costs are therefore incurred in (1) developing the
range of soil and pipe properties used in analysis and (2) conducting the
parametric analyses, which may be linear for small displacements or non-
linear for large displacements. A cost-benefit decision must therefore be
made regarding the seismic design of buried pipe. To help in the process,
the designer may turn to the lessons learned from actual earthquakes.