Seismic risk assessment for oil and gas pipelines - Seismic Risk Analysis and Management of Civil Infrastructure Systems - page 685

movements can lead to potentially unacceptable strains in the pipelines.
Extensive research has been focused in the last few decades to investigate
the response of the pipes under permanent ground deformations. Common
causes of permanent ground displacement are related to surface fault dis-
placement, liquefaction-induced lateral spreading and fl ow slides, slope
instability and landslides, and ground subsidence.

Ground movements at fault crossings

Pipelines crossing faults can be subjected to displacements ranging from a
few centimeters to several meters. In addition to the lateral movements
signifi cant vertical movements can occur in the crossings involving reverse-
thrust faults. Large ground movements at the Trans-Alaska Pipeline cross-
ing of the Denali fault near Glennallen, Alaska, during the 2002 M7.9
Denali fault earthquake is a classic example of the type of ground move-
ments expected at fault crossings.

Liquefaction-induced ground movements

In saturated loose or soft granular soils, liquefaction occurs when the shear
strains induced due to seismic shaking cause transient pore water pressures
to increase in the soil mass. As a result, intergranular contact stresses will
reduce to negligible levels. In this transient state, the soil mass is subject to
signifi cant reduction in shear strength and behaves essentially as a viscous
fl uid that could deform or fl ow under gravitational or inertia forces. Areas
susceptible to liquefaction could undergo signifi cant vertical and lateral
movements even in gently sloping terrain. The extent of permanent ground
displacements is expected to increase with the increasing amplitude of
earthquake accelerations and with the duration of seismic shaking. The
extent of ground movements can be classifi ed as: fl owslides (more than
∼ 5 m), lateral spreading fl owslides ( ∼ 5 m to ∼ 0.3 m), and ground oscillation

(less than 0.3 m). In addition to liquefaction-induced ground movements,

fl otation and soil uplift could also be identifi ed as another potential concern
in relation to the reduction in soil strength associated with liquefaction. This
would be of particular concern if the pipe trench backfi ll materials are
poorly compacted and susceptible to liquefaction.
Liquefaction can produce overall volume changes in the liquefi ed soil
mass that take place due to the dissipation of earthquake-shear-induced
excess pore water pressures. The volume changes manifest in the fi eld as
post-liquefaction settlements, and they may occur both during and after
earthquake shaking. The adverse impacts of these settlements on the per-
formance of structural foundations and linear lifelines (such as buried pipe-
lines and bridges) have been well recognized (Tokimatsu and Seed, 1987;
Wijewickreme and Sanin, 2010).

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