Basics of Retaining Wall Design
6. EARTHQUAKE (SEISMIC) DESIGN Page 38
the energy of that of a magnitude 4 (5
10
/ 4
10
≈ 10). This measure is popular with the media but
does not have a direct correlation to ground acceleration that is used to determine the seismic
force for the design of structures.
Prior to using the Richter Scale, the Mercalli Intensity Scale was developed, which classified
earthquakes based upon their effect at the earth’s surface. It was developed by Guiseppe Mercalli
in 1902 and described in a USGS pamphlet as shown below – the higher the number the more
severe the damage:
(I) - Not felt except by a very few under especially favorable conditions.
(II) - Felt only by a few persons at rest, especially on upper floors of buildings. Delicately
suspended objects may swing.
(III) - Felt quite noticeably by persons indoors, especially on the upper floors of buildings. Many do
not recognize it as an earthquake. Standing motor cars may rock slightly. Vibration similar to
the passing of a truck. Duration estimated.
(IV) - Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes,
windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking
building. Standing motor cars rocked noticeably.
(V) - Felt by nearly everyone; many awakened. Some dishes and windows broken. Unstable objects
overturned. Clocks may stop.
(VI) - Felt by all; many frightened and run outdoors, walk unsteadily. Windows, dishes, glassware
broken... books off shelves... some heavy furniture moved or overturned; a few instances of
fallen plaster. Damage slight.
(VII) - Difficult to stand... furniture broken… damage negligible in buildings of good design and
construction; slight to moderate in well-built ordinary structures; considerable damage in
poorly built or badly designed structures; some chimneys broken. Noticed by persons driving
motor cars.
(VIII) - Damage slight in specially designed structures; considerable in ordinary substantial buildings
with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks,
columns, monuments, walls. Heavy furniture moved.
(IX) - General panic... damage considerable in specially designed structures, well designed frame
structures thrown out of plumb. Damage great in substantial buildings, with partial collapse.
Buildings shifted off foundations.
(X) - Some well built wooden structures destroyed; most masonry and framed structures destroyed
with foundation. Rails bent.
(XI) - Few, if any masonry structures remain standing. Bridges destroyed. Rails bent greatly.
(XII) - Damage total. Lines of sight and level distorted. Objects thrown into the air.
The Richter magnitude scale, used mostly by the media and for general intensity comparisons, is
now replaced by site-specific ground accelerations as explained in following sections. An
excellent source of information on earthquakes, including hazard maps, is http://www.usgs.gov.
When is Seismic Design Required for Retaining Walls?
It depends upon what guides you. The evidence of earthquake damage to properly designed
retaining walls is nearly non-existent, excluding waterfront walls where liquefaction occurred,
and walls poorly designed for static loads. Based on the senior author’s observations and reviews
of inspection reports from both the Northridge and Loma Prieta earthquakes, incidents of damage
were not noted for walls properly designed for static loads. Building code changes are usually
prompted by failures observed, such as that of wall-to-roof diaphragm connections on tilt-up
buildings during the San Fernando earthquake of 1971 which prompted corrective code changes.
However, this does not seem to be the sequence for retaining walls because of the lack of failure
evidence. It can, however, be argued that we have not yet experienced “the big one”, and more