Geoscience Reference
In-Depth Information
In this paper, soil-structure interaction under extreme loading conditions is examined
with reference to earthquakes, which are used as an example of how extreme loading
influences behavior at local and geographically distributed facilities. The paper begins
with the effects of earthquake-induced ground deformation on underground facilities,
and then expands this treatment to consider the system-wide performance of the Los
Angeles water supply during the 1994 Northridge earthquake. Large-scale experiments
toevaluatesoil-structureinteractionunderextremeloadingconditionsaredescribedwith
reference to tests of abrupt ground rupture effects on steel and high density polyethylene
pipelines. Large-scale tests and the development of design curves are described for the
forces imposed on pipelines during ground failure. The paper covers performance from
the component to the system-wide level to provide guidance in developing an integrated
approach to the application of geotechnology over large, geographically distributed net-
works.
2. Geotechnical earthquake loading
Earthquakescausetransientgrounddeformation(TGD)andpermanentgrounddeforma-
tion (PGD), both of which affect underground pipelines. TGD is the dynamic response
of the ground, and PGD is the irrecoverable movement that persists after shaking has
stopped. PGD often involves large displacements, such as those associated with surface
fault rupture and landslides. TGD can cause soil cracks and fissures triggered by pulses
of strong motion that develop localized shear and tensile strains exceeding the strength
of surficial soils. In these cases, crack widths and offsets are primarily a reflection of
surficial ground distortion and gravity effects, such as local slumping. They should not
be mistaken as an expression of PGD generated by ground failure mechanisms of larger
scale.
The principal causes of PGD have been summarized and discussed by O'Rourke (1998).
They are faulting, tectonic uplift and subsidence, and liquefaction, landslides, and densi-
fication of loose granular deposits. Liquefaction is the transformation of saturated cohe-
sionless soil into a liquefied state or condition of substantially reduced shear strength
(Youd,1973).Liquefaction-inducedpipelinedeformationcanbecausedbylateralspread,
flowfailure,localsubsidence,post-liquefactionconsolidation,buoyancyeffects,andloss
of bearing (Youd, 1973; O'Rourke, 1998). It is widely accepted that the most serious
pipelinedamageduringearthquakesiscausedbyPGD.Furthermore,itiswellrecognized
that liquefaction-induced PGD, especially lateral spread, is one of the most pervasive
causes of earthquake-induced lifeline damage (Hamada and O'Rourke, 1992; O'Rourke
and Hamada, 1992).
Ground displacement patterns associated with earthquakes depend on PGD source, soil
type, depth of ground water, slope, earthquake intensity at a given site, and duration of
stronggroundshaking(O'Rourke,1998).Itisnotpossibletomodelwithaccuracythesoil
displacement patterns at all potentially vulnerable locations. Nevertheless, it is possible
