mounted a field effort to observe and map the damageof the magnitude 8.3 event as a long series of Profes-sional Papers, the content of which has served as the ongoing model for the appl
Trang 1mounted a field effort to observe and map the damage
of the magnitude 8.3 event as a long series of
Profes-sional Papers, the content of which has served as the
ongoing model for the application of engineering
geology to earthquake mitigation learning
Subsequent American earthquakes have been given
similar, yet lesser overall, attention, for instance the San
Fernando (California) earthquake of 9 February 1971
(magnitude 6.6) and the Loma Prieta event of 17
Octo-ber 1989 (magnitude 7.1) located just south of San
Francisco (California) on a San Andreas splinter fault
Purpose
Strictly speaking, the purpose of engineering geological
mapping in this context is to record the physical nature
of ground-rupture earthquakes and to elucidate the
related types of motion-induced damage (Table 1)
Once defined, these can be applied on a worldwide
basis, wherever similar conditions of tectonics and
near-field geology and topography occur
Engineering geological mapping for earthquake
miti-gation (Table 2) determines how engineered works and
human safety can be protected by judicious design
con-siderations, most of which are governed by geological
conditions related to the site characterization
Geological Profile (or Ground Profile)
This term geological profile has taken over in
engin-eering geology from ‘geological section’ and considers
only the depth of influence of engineering works
(generally less than 15 m;Table 3)
Exploration Trenches and Trench Logging
During the peak of nuclear power-plant siting and
construction in the 1970s considerable effort was
made to confirm the absence of active faults within the footprint of the power block (i.e the location of the reactor and the critical cooling linkage) The con-cept of avoiding active fault traces was promoted so that the risk of damage to the reactor and its contain-ment as a result of rupture-type earthquake ground motion could be minimized As a result, the technique
of exploratory trenching advanced from the original method of observation from hand-dug pits (Table 4) Site Characterization
As an integral part of site characterization in seismo-genic regions, attention should be given to identify-ing geological conditions that may make the site susceptible to physical damage from strong motion (Table 5)
Post-Event Surveys Certain elements of the earth media are particu-larly susceptible to being lifted, shifted, toppled, or cracked by earthquake strong motion The patterns
of damage reveal much about the frequency charac-teristics of the incident ground motion and the rela-tive duration of the strong motion Particularly affected are fine soils, boulders on slopes, blocks of rock defined by joints, overly steep stream and shore banks and cliffs, and hillside masses saturated with groundwater
Geologists have but hours to locate, photograph, and map these features before they are destroyed, first
by human visitors and soon after by rainfall and other natural erosive agents (Table 6)
Cultural features within the built environment (in-cluding engineered works) offer additional potential
Table 1 Engineering geological mapping of earthquake effects
Stratigraphy Identify and describe the geological
formational units to be expected in design and construction
Individual engineering geological units
Groundwater regime Define character of groundwater, as it is
affected by ground motion and diminishes the shear strength ofearth media to resist dynamic deformation
Perched water Vadose zone and fluctuations Peizometric surface
Potentiometric surface Rock mass characterization Delimit observable or likely subsurface
bounds ofeach detectable hard rock unit
Identify bodies of discontinuity bounded rock masses that may become unstable from shaking
Presence ofweak rock Basis ofdefinition, including why the
rock is determined to be weak
Recommendations as to how and why such weak rock may pose problems to design and/or construction, operation, and maintenance
Potential problems related to
sedimentological, structural,
or geomorphological
conditions
Portions of surface or subsurface that appear to be related to mapped patterns ofearthquake damage
Use special map symbols to portray these features; the Geological Society of London Engineering Geomorphological map symbols are ideal for this purpose
460 ENGINEERING GEOLOGY/Aspects of Earthquakes