Geoscience Reference
In-Depth Information
Fig. 17.4. Map of Los Angeles water supply systemaffected by Northridge earthquake
(O'Rourke and Toprak, 1997; O'Rourke, 1998)
Valley, Santa Monica Mountains, and Los Angeles Basin. The rectilinear system of
pipelines is equivalent to a giant strain gage. Seismic intensity in the form of pipeline
damage can be measured and visualized by plotting pipeline repair rates and identifying
theareaswherethelargestconcentrationsofdamagerateoccur.Theresultingareasreflect
the highest seismicintensities as expressed by the disruptiontounderground piping.
To develop a properly calibrated strain gage, it is necessary to select a measurement grid
with material having reasonably consistent properties and a damage threshold sensitive
to the externally imposed loads being measured. Figure 17.5 presents charts showing
the relative lengths of LADWPandMWDtrunkand distributionlines, according topipe
composition.Asshownbythepiechart,themostpervasivematerialintheLAdistribution
system is CI. The 7,800km of CI pipelines have the broadest geographic coverage with
sufficient density in all areas to qualify as an appropriate measurement grid. Moreover,
CI is a brittle material subject to increased rates of damage at tensile strains on the order
250 to 500
µε
. It is therefore sufficiently sensitive for monitoring variations in seismic
disturbance.
Figure 17.5 presents a map of distribution pipeline repair locations and repair rate con-
tours for cast iron (CI) pipeline damage. The repair rate contours were developed by
dividing the map into 2km
2km areas, determining the number of CI pipeline repairs
in each area, and dividing the repairs by the distance of CI mains in that area. Contours
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