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PROPOSAL 3-4 (2009)




SCOPE: Sec. 11.8



PROPOSAL FOR CHANGE:


Revise Provisions Section 11.8 as follows:

11.8 Geologic Hazards and Geotechnical Investigation


11.8.1 Geotechnical Investigation Report for Seismic Design Categories C through F. A
geotechnical investigation report shall be provided for a structure assigned to Seismic Design Category C,
D, E or F in accordance with this section. An investigation shall be conducted and a report shall be
submitted that shall include an evaluation of the following potential geologic hazards:

a.

slope instability;
b.

liquefaction;
c.

settlement;
d.

and surface displacement due to faulting or lateral spreading.

The report shall assess the potential consequences of geologic hazards, including but not limited to total
and differential settlement, lateral movement, and lateral loads on foundations. The report shall contain
recommendations for appropriate foundation designs, ground stabilization, or other measures to mitigate
the effects of the hazards. Where deemed appropriate by the authority having jurisdiction, a site-specific
geotechnical report is not required where prior evaluations of adjacent sites with similar soil conditions
provide sufficient direction relative to the proposed construction.

11.8.1.1 Earthquake Ground Motions for Geologic Hazards Investigations. Geologic hazards shall
be evaluated for site ground motions, seismic sources, and earthquake magnitudes consistent with Design
Earthquake ground motions. Effects of geologic hazards on structural performance shall also be checked
for Maximum Considered Earthquake (MCE) ground motions (without two-thirds factor). Ground
motions shall be determined based on Section 11.4 or based on a site-specific study following the
procedures in Chapter 21.

11.8.2 Additional Geotechnical Investigation Report Requirements for Seismic Design Categories
D through F. The geotechnical investigation report for a structure assigned to Seismic Design
Category D, E or F shall include the determination of lateral pressures on basement and retaining walls
due to earthquake motions.

REASON FOR PROPOSAL:


This proposal modifies the requirements for geologic hazards evaluations as follows: Proposal 3-4 (2009) Continued


Proposal 3-4 (2009) PUC Ballot Copy (September, 2007)

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Current Section 11.8.1 is deleted so as not to absolutely prohibit the siting of a structure
in Seismic Design Categories E and F on an active fault. The current requirement is
based on the assumption that the structure cannot be designed for fault displacement,
which may generally be the case but not always. Current Section 11.8.2 (proposed
renumbered Section 11.8.1) requires that a fault rupture hazard including consequences
of the displacement be evaluated and mitigated if needed. Mitigation would include not
siting the structure on the fault if other means of mitigation were not feasible.
Proposed Section 11.8.1.1 defines earthquake ground motions for geologic hazards
evaluations. A proposed requirement is that geologic hazards not only be evaluated for
Design Earthquake ground motions, but also that effects of such hazards be checked for
Maximum Considered Earthquake (MCE) ground motions (without two-thirds factor).
Geologic hazards potentially can result in much larger ground deformations at the MCE
level than at the design level. The effects of such larger deformations should be
considered in structural and foundation design with respect to structure performance
criteria. By limiting the geologic hazard evaluation to only the design earthquake, as
currently done, the level of risk for geologic hazards could be higher than that for
structural design for ground motions.
Proposed Section 11.8.1 refers to two parts of the Provisions for seismic ground motion
quantification. One of these parts -- Section 11.4 -- will be modified in another proposal
to be generated by E.V. Leyendecker that will add nationwide quantification of peak
ground accelerations (needed for many types of geologic hazards evaluations) to the
existing quantifications of spectral response accelerations. The other part is Chapter 21,
which contains the requirements for site-specific ground motion quantification.
In current Section 11.8.3 (proposed Section 11.8.2), the text relating to geologic hazards
evaluations for Seismic Design Categories (SDC) D-F is deleted because the
requirements for SDC D-F are the same as requirements for SDC C-F stated in proposed
Section 11.8.1 and 11.8.1.1. The text that remains in proposed Section 11.8.2 is the
requirement for SDC D-F to determine seismic lateral pressures on basement and
retaining walls.
TS 3 VOTE:

Yes = 7
Yes with Reservations = 1
No= 2
Not Voting = 3

Proposal 3-4 Comments from TS-3 members who voted No or YR.

M. Yegian YR

I generally agree with Proposal 3-4. The text under Section 11.8.1.1 is not very clear. Does one run
liquefaction analysis for both DE and MCE? Should the effect of liquefaction based on MCE be
included in the structural design when it is based on the DE?

I vote Yes, with a suggestion to further clarify the text of Section 11.8.1.

J. Stewart No

I support the change with respect to surface rupture. If this is a stand-alone proposal, put me down for a
yes vote.
Proposal 3-4 (2009) Continued


Proposal 3-4 (2009) PUC Ballot Copy (September, 2007)

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I do not support the proposal for ground motion levels to be considered for ground failure analysis. The
reason is that the wording is too vague and it implies a different standard for ground failure analysis
than for building analysis. With regard to the vagueness of wording, the statement in section 11.8.1.1
that the effects of geologic hazards should be checked for the MCE event in addition to the hazards
being evaluated for the DBE is the source of my concern. I can see remarks like this being in a
commentary (although even the terminology should be better defined), but I just dont see how this can
be in the provisions. If this is a stand along proposal, my vote is no. If the surface rupture and ground
motion issues are part of the same proposal, then my vote on the joint proposal would be no.

T. Hale No

I vote No on Proposal 3-4. My reasons are as follows:

The prohibition of siting a structure in Seismic Design Categories E and F on an active fault should
remain. There are no established methods of structural design or construction types to mitigate fault
displacements without at least a partial collapse. Current section 11.8.2 only mentions the effect on
foundation design, whereas the superstructure will also be effected due to flexible foundation systems.
Analytical methods and code provisions have not been established to determine appropriate fault
rupture displacements for design of structures located directly over the faults.

In addition, in California, removal of the original provision to prohibit construction of SDC E or F
buildings where there is a known potential for surface faulting under an active fault will result in
violations of the Alquist-Priolo Earthquake Fault Zoning Act.