Geoscience Reference
In-Depth Information
Fig. 17.6. Geotechnical characteristics of theareas of concentrated pipeline damage
after the Northridge earthquake
conditionsareproneeithertogroundfailureoramplificationofstrongmotion.Eachzone
of concentrated damage is labeled in Figure 17.6 according to its principal geotechnical
characteristics.Ineffect,therefore,Figure17.5isaseismichazardmapfortheLosAnge-
les region, calibrated according topipeline damage during the Northridge earthquake.
Of special interest is the location of concentrated repair rate contours in the west central
part of San Fernando Valley (designated in Figure 17.6 as the area of soft clay deposits).
This area was investigated by USGS researchers, who found it to be underlain by local
deposits of soft, normally consolidated clay. Field vane shear tests disclosed clay with
uncorrected, vane shear undrained strength, S
uvst
=
20-25kPa, at a depth of 5m, just
below thewater table. USGS investigators concluded that the saturated sands underlying
this site were not subjected to liquefaction during the Northridge earthquake. Newmark
sliding block analyses reported by O'Rourke (1998) provide strong evidence that near
source pulses of high acceleration were responsible for sliding and lurching on the soft,
normally consolidated clay deposit. The results of GIS analysis and site investigations
have important ramifications because they show a clear relationship between PGD, con-
centratedpipelinedamage,andthepresenceofpreviouslyunknowndepositsofnormally
consolidated clay.
Therecordsfromapproximately240freefieldrockandsoilstationswereusedtoevaluate
the patterns of pipeline damage with the spatial distribution of various seismic parame-
ters.Figure17.7showstheCIpipelinerepairratecontourssuperimposedonpeakground
velocity (PGV) zones, which were developed by interpolating the maximum horizon-
tal velocities recorded at the strong motion stations. Using the GIS database, a pipeline
repair rate was calculated for each PGV zone, and correlations were made between the
repair rate and average PGV for each zone. As explained by O'Rourke (1998), similar
