Required Documentation. Investigations of geologic
Differential Settlement. Loosely compacted soils either
hazards shall be documented. As noted in the paragraph
above or below the water table can consolidate during
entitled "Required Investigation" above, a preliminary
earthquake shaking, producing surface settlement. The
geologic hazard investigation shall be conducted and a
potential for total and differential settlements beneath a
report issued during the siting phase for a facility.
structure shall be assessed. If liquefaction is not expected
However, unless the geologic hazard investigations have
to occur, then in most cases, differential settlement would
been documented in a stand-alone report, they shall be
not pose a significant problem to construction.
addressed in a section of the geotechnical engineering
Flooding. Earthquake-inducing flooding can
report prepared during the design phase of a project. The
by tsunamis, seiches, and dam and levee failures. The
geologic hazard report, whether it is a separate report or a
possibility of flooding shall be addressed for new
section of the geotechnical engineering report, shall as a
construction located near bodies of water.
minimum contain the following:
List of hazards investigated, which must include the five
Duration of Strong Ground Shaking. Estimates of the
described earlier in this section.
duration of strong ground shaking at a site are defined by
Description of the methods used to evaluate the site for
earthquake magnitude and shall be used to assess geologic
hazards such as liquefaction and slope failure. Strong
motion duration is strongly dependent on earthquake
Results of any investigations, borings, etc.
Estimates of the duration of strong ground shaking
Recommendations for hazard mitigation, if required.
shall be based on the assumption of the occurrence of a
In some cases, estimates of site ground motions may be
maximum considered earthquake generally accepted by
needed for assessment of geologic hazards such as
the engineering and geologic community as appropriate
liquefaction and slope failure.
to the region and to the subsurface conditions at the site.
Mitigative Measures. A site found to have one or more
geologic hazards may be used, provided the hazards are
removed, abated, or otherwise mitigated in the design,
or if the risk is judged to be acceptable. Examples of
mitigative measures include: removal and recompaction
of poorly compacted soils; use of special foundations;
stabilizing slopes; and draining, compaction, or chemical
treatment of liquefiable soils. The geological hazard report
shall identify feasible mitigative measures.
352 F A C I L I T I E S S T A N D A R D S
A.5 Surveys and Geotechnical Reports Geologic Hazard Report
Revised March 2005 PBS-P100