Seismic design and analysis of underground structures - ITA-AITES

47 Pages·2001·1.8 MB·English

TunnellingandUndergroundSpaceTechnology16Ž2001.247(cid:1)293
ITA(cid:1)AITESAccreditedMaterial
Seismic design and analysis of underground structures
Youssef M.A. Hashasha,(cid:1), Jeffrey J. Hooka, Birger Schmidtb,
John I-Chiang Yaoa
aDepartmentofCi(cid:2)ilandEn(cid:2)ironmentalEngineering,Uni(cid:2)ersityofIllinoisatUrbana-Champaign,205N.MathewsA(cid:2)enue,MC-250,
Urbana,IL61801,USA
bParsonsBrinckerhoff,SanFrancisco,CA,USA
Abstract
Underground facilities are an integral part of the infrastructure of modern society and are used for a wide range of
applications,includingsubwaysandrailways,highways,material storage,andsewageandwatertransport.Undergroundfacilities
built in areas subject to earthquake activity must withstand both seismic and static loading. Historically, underground facilities
have experienced a lower rate of damage than surface structures. Nevertheless, some underground structures have experienced
significant damage in recent large earthquakes, including the 1995 Kobe, Japan earthquake, the 1999 Chi-Chi, Taiwan
earthquakeandthe1999Kocaeli,Turkeyearthquake.Thisreportpresentsasummaryofthecurrentstateofseismicanalysisand
design for underground structures. This report describes approaches used by engineers in quantifying the seismic effect on an
underground structure. Deterministic and probabilistic seismic hazard analysis approaches are reviewed. The development of
appropriate ground motion parameters, including peak accelerations and velocities, target response spectra, and ground motion
time histories, is briefly described. In general, seismic design loadsfor undergroundstructures are characterized in terms of the
deformations and strains imposedon the structure by the surrounding ground,often due to the interaction between the two.In
contrast,surfacestructuresaredesignedfortheinertialforcescausedbygroundaccelerations.Thesimplestapproachistoignore
the interaction of the underground structure with the surrounding ground.The free-field ground deformations due to a seismic
eventareestimated,andtheundergroundstructureisdesignedtoaccommodatethesedeformations.Thisapproachissatisfactory
when low levels of shaking are anticipated or the underground facility is in a stiff mediumsuch as rock. Other approaches that
account forthe interaction between the structural supportsandthe surroundinggroundare then described.In the pseudo-static
analysis approach, the ground deformations are imposed as a static load and the soil-structure interaction does not include
dynamic or wave propagation effects. In the dynamic analysis approach, a dynamic soil structure interaction is conducted using
numerical analysis toolssuch as finite element orfinite difference methods.Thereportdiscusses special designissues, including
the design of tunnel segment joints and joints between tunnels and portal structures. Examples of seismic design used for
underground structures are included in an appendix at the end of the report. (cid:1) 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Seismicdesign;Seismicanalysis;Undergroundstructures; Tunnels;Subways;Earthquakedesign
(cid:1)Correspondingauthor.Tel.:(cid:3)1-217-333-6986;fax:(cid:3)1-217-265-8041.
E-mailaddress:[email protected]ŽY.M.A.Hashash..
0886-7798(cid:1)01(cid:1)$-seefrontmatter(cid:1)2001ElsevierScienceLtd.Allrightsreserved.
PII: S0886-7798Ž01.00051-7
248 Y.M.A.Hashashetal.(cid:1)TunnellingandUndergroundSpaceTechnology16(2001)247(cid:1)293
Preface
This paper was developed as part of the activities of
the International Tunnelling Association (ITA) Working
Group No 2: Research. The paper provides a state-of-
the-art review of the design and analysis of tunnels
subject to earthquake shaking with particular focus on
practice in the United States of America. The Authors
wish to acknowledge the important contribution of
WorkingGroup2membersincludingMr.YannLeblais,
Animateur, Yoshihiro Hiro Takano, Vice-Animateur,
Barry New,Member,HenkJ.C.OudandAndresAssis,
Tutor and Former Tutor, respectively, as well as the
ITAExecutive Councilfortheir reviewandapprovalof
this document.
1. Introduction
Underground structures have features that make
their seismic behavior distinct from most surface struc-
tures, most notably Ž1. their complete enclosure in soil
or rock, and Ž2. their significant length Ži.e. tunnels..
The design of underground facilities to withstand
seismic loadingthus, has aspects that are very different
Fig.1. CrosssectionsoftunnelsŽafterPoweretal.,1996..
from the seismic design of surface structures.
This report focuses on relatively large underground
facilities commonly used in urban areas. This includes Cut-and-cover structures are those in which an open
large-diameter tunnels, cut-and-cover structures and excavationismade,the structure isconstructed,andfill
portal structures ŽFig. 1.. This report does not discuss is placed over the finished structure. This method is
pipelines or sewer lines, nor does it specifically discuss typically used for tunnels with rectangular cross-sec-
issues related to deep chambers such as hydropower tions and only for relatively shallow tunnels Ž(cid:2)15 mof
plants, nuclear waste repositories, mine chambers, and overburden.. Examples of these structures include sub-
protective structures, though many of the design meth- way stations, portal structures and highway tunnels.
odsand analyses described are applicable to the design Immersed tube tunnels are sometimes employed to
of these deep chambers. traverse a body of water. This method involves con-
Large-diameter tunnels are linear underground structing sections of the structure in a dry dock, then
structures in which the length is much larger than the moving these sections, sinking them into position and
cross-sectional dimension. These structures can be ballasting or anchoring the tubes in place.
grouped into three broad categories, each having dis- This report is a synthesis of the current state of
tinct design features and construction methods: Ž1. knowledge in the area of seismic design and analysis
bored or mined tunnels; Ž2. cut-and-cover tunnels; and for underground structures. The report updates the
Ž3. immersed tube tunnels ŽPower et al., 1996.. These work prepared by St. John and Zahrah Ž1987., which
tunnels are commonly used for metro structures, high- appearedin TunnelingUndergroundSpaceTechnol.The
waytunnels, andlarge water andsewagetransportation report focuses on methods of analysis of underground
ducts. structures subjected to seismic motion due to
Bored or mined tunnels are unique because they are earthquake activity, and provides examples of perfor-
constructed without significantly affecting the soil or mance and damage to underground structures during
rock above the excavation. Tunnels excavated using recent major earthquakes. The report describes the
tunnel-boring machines ŽTBMs. are usually circular; overall philosophy used in the design of underground
othertunnels mayberectangular orhorseshoeinshape. structures, and introduces basic concepts of seismic
Situations whereboringorminingmaybepreferable to hazard analysis and methods used in developingdesign
cut-and-cover excavation include Ž1. significant excava- earthquake motion parameters.
tion depths, and Ž2. the existence of overlying struc- The report describes how ground deformations are
tures. estimated and how they are transmitted to an under-
Y.M.A.Hashashetal.(cid:1)TunnellingandUndergroundSpaceTechnology16(2001)247(cid:1)293 249
ground structure, presenting methods used in the com- by improving the contact between the lining and
putationofstrains, forces andmomentinthe structure. the surrounding ground through grouting.
The report provides examples of the application of 5. Tunnels are more stable under a symmetric load,
these methodsforundergroundstructures inLosAnge- whichimprovesground-lininginteraction. Improv-
les, Boston, and the San Francisco Bay Area. ing the tunnel lining by placing thicker and stiffer
This report does not cover issues related to static sections without stabilizing surrounding poor
design, although static design provisions for under- ground may result in excess seismic forces in the
groundstructures often providesufficient seismic resis- lining.Backfillingwithnon-cyclically mobilemate-
tance under low levels of ground shaking. The report rial and rock-stabilizing measures may improve
doesnotdiscuss structural designdetails andreinforce- the safety and stability of shallow tunnels.
ment requirements in concrete or steel linings for 6. Damage may be related to peak ground accelera-
underground structures. The report briefly describes tion and velocity based on the magnitude and
issues related to seismic design associated with ground epicentral distance of the affected earthquake.
failure such as liquefaction, slope stability and fault 7. Duration of strong-motion shaking during
crossings,but doesnotprovideathoroughtreatment of earthquakes is of utmost importance because it
these subjects. The reader is encouraged to review may cause fatigue failure and therefore, large
other literature on these topics to ensure that relevant deformations.
design issues are adequately addressed. 8. High frequency motions may explain the local
spalling of rock or concrete along planes of weak-
ness. These frequencies, which rapidly attenuate
with distance, may be expected mainly at small
2.Performanceofundergroundfacilitiesduringseismic
distances from the causative fault.
events
9. Ground motion may be amplified upon incidence
with a tunnel if wavelengths are between one and
Several studies have documented earthquake da- four times the tunnel diameter.
mage to underground facilities. ASCE Ž1974. describes 10. Damage at and near tunnel portals may be sig-
the damage in the Los Angeles area as a result of the nificant due to slope instability.
1971 San Fernando Earthquake. JSCEŽ1988. describes
the performance of several underground structures, The following is a brief discussion of recent case
including an immersed tube tunnel during shaking in histories of seismic performance of underground struc-
Japan. Duke and Leeds Ž1959., Stevens Ž1977., Dowd- tures.
ing and RozenŽ1978.,Owen and Scholl Ž1981.,Sharma
and Judd Ž1991., Power et al. Ž1998. and Kaneshiro et
2.1. Undergroundstructures in the UnitedStates
al. Ž2000., all present summaries of case histories of
damage to underground facilities. Owen and Scholl
Ž1981. have updated Dowding and Rozen’s work with 2.1.1.BayArearapidtransit(BART)system,San
127 case histories. Sharma and Judd Ž1991. generated Francisco, CA, USA
an extensive database of seismic damage to under- The BART system was one of the first underground
ground structures using 192 case histories. Power et al. facilities to be designed with considerations for seismic
Ž1998. providea further update with 217 case histories. loading ŽKuesel, 1969.. On the San Francisco side, the
The following general observations can be made re- system consists of underground stations and tunnels in
garding the seismic performance of underground struc- fill and soft Bay Mud deposits, and it is connected to
tures: Oakland via the transbay-immersed tube tunnel.
Duringthe1989LomaPrietaEarthquake,theBART
1. Underground structures suffer appreciably less facilities sustained no damageand,in fact, operatedon
damage than surface structures. a 24-h basis after the earthquake. This is primarily
2. Reported damage decreases with increasing over- because the system was designed under stringent
burden depth. Deep tunnels seem to be safer and seismic design considerations. Special seismic joints
less vulnerable to earthquake shaking than are ŽBickel and Tanner, 1982. were designed to accommo-
shallow tunnels. date differential movements at ventilation buildings.
3. Underground facilities constructed in soils can be The system had been designed to support earth and
expected to suffer more damage compared to water loads while maintaining watertight connections
openings constructed in competent rock. and not exceeding allowable differential movements.
4. Lined and grouted tunnels are safer than unlined Nodamagewasobservedattheseflexiblejoints,though
tunnels in rock. Shaking damage can be reduced it is not exactly known howfar the joints movedduring
by stabilizing the ground around the tunnel and the earthquake ŽPB, 1991..
250 Y.M.A.Hashashetal.(cid:1)TunnellingandUndergroundSpaceTechnology16(2001)247(cid:1)293
Fig.2. SectionsketchofdamagetoDaikaisubwaystationŽIidaetal.,1996..
2.1.2. AlamedaTubes, Oakland-Alameda,CA, USA of the station and at areas where the station changed
The Alameda Tubes are a pair of immersed-tube width acted as shear walls in resisting collapse of the
tunnels that connect AlamedaIsland toOakland in the structure ŽIida et al., 1996.. These walls suffered sig-
San Francisco Bay Area. These were some of the nificant cracking, but the interior columns in these
earliest immersed tube tunnels built in 1927 and 1963 regions did not suffer as much damage under the
withoutseismicdesignconsiderations.DuringtheLoma horizontal shaking. In regions with no transverse walls,
Prieta Earthquake, the ventilation buildings experi- collapse of the center columns caused the ceiling slab
enced some structural cracking. Limited water leakage to kink and cracks 150(cid:1)250-mm wide appeared in the
into the tunnels was also observed, as was liquefaction longitudinal direction. There was also significant sepa-
ofloosedepositsabovethe tube at the Alamedaportal. ration at some construction joints, and corresponding
Peak horizontal ground accelerations measured in the water leakage through cracks. Few cracks, if any, were
area ranged between 0.1 and 0.25 g ŽEERI, 1990.. The observed in the base slab.
tunnels, however,are prone to floatation due to poten- Center columns that were designed with very light
tial liquefaction of the backfill ŽSchmidt and Hashash, transverse Žshear. reinforcement relative to the main
1998.. Žbending. reinforcement suffered damageranging from
cracking to complete collapse. Center columns with
2.1.3. L.A. Metro, Los Angeles, CA, USA zigzagreinforcement inadditiontothehoopsteel,asin
The Los Angeles Metro is being constructed in sev- Fig. 3, did not buckle as much as those without this
eral phases, someofwhich were operational duringthe reinforcement.
1994 Northridge Earthquake. The concrete lining of According to Iida et al. Ž1996., it is likely that the
theboredtunnelsremainedintactaftertheearthquake. relative displacement between the base and ceiling
While there was damage to water pipelines, highway levels due to subsoil movement created the destructive
bridges and buildings, the earthquake caused no da-
mage to the Metro system. Peak horizontal ground
accelerations measured near the tunnels ranged
between 0.1 and 0.25 g, with vertical ground accelera-
tions typically two-thirds as large ŽEERI, 1995..
2.2. Undergroundstructures in Kobe,Japan
The 1995 Hyogoken-Nambu Earthquake caused a
major collapse of the Daikai subway station in Kobe,
Japan ŽNakamura et al., 1996.. The station design in
1962 did not include specific seismic provisions. It
represents the first modern underground structure to
fail during a seismic event. Fig. 2 shows the collapse
experiencedbythecenter columnsofthestation,which
wasaccompaniedbythe collapseofthe ceiling slab and
the settlement of the soil cover by more than 2.5 m.
Fig.3. Reinforcingsteel arrangement incenter columnsŽIidaetal.,
During the earthquake, transverse walls at the ends 1996..
Y.M.A.Hashashetal.(cid:1)TunnellingandUndergroundSpaceTechnology16(2001)247(cid:1)293 251
horizontal force. This type of movement may have
minor effect in a small structure, but in a large one
such as a subway station it can be significant. The
non-linear behavior of the subsoil profile may also be
significant. It is further hypothesized that the thickness
of the overburden soil affected the extent of damage
between sections of the station by adding inertial force
to the structure. Others attribute the failure to high
levels of vertical acceleration.
EQE Ž1995. made further observations about Daikai
Station: ‘Excessive deflection of the roof slab would
normally be resisted by: Ž1. diaphragm action of the
slab, supported by the end walls of the station; and Ž2.
passive earth pressure of the surrounding soils,
Fig. 5. Bolu Tunnel, re-mining of Bench Pilot Tunnels, showing
mobilizedas the tube racks. Diaphragmaction was less
typicalfloorheaveandbuckledsteelribandshotcreteshellŽMenkiti,
than anticipated, however, due to the length of the
2001..
station. The method of construction Žcut-and-cover,
involving a sheet pile wall supported excavation with
fact that the structure was underground instead of
narrow clearance between the sheet pile wall and the
beingasurface structure mayhavereducedtheamount
tube wall. made compaction of backfill difficult to
of related damage.
impossible, resulting in the tube’s inability to mobilize
A number of large diameter Ž2.0(cid:1)2.4 m. concrete
passive earth pressures. In effect, the tube behaved
sewer pipes suffered longitudinal cracking during the
almostasafreestanding structure withlittle ornoextra
Kobe Earthquake, indicating racking and(cid:1)or compres-
supportfrompassive earth pressure.’ However,it is not
sive failures in the cross-sections ŽTohda, 1996.. These
certain thatgoodcompactionwouldhavepreventedthe
cracks were observed in pipelines constructed by both
structural failure of the column. Shear failure of sup-
the jacking method and open-excavation Žcut-and-
portingcolumnscaused similar damagetothe Shinkan-
cover. methods. Once cracked, the pipes behaved as
sen Tunnel through Rokko Mountain ŽNCEER, 1995..
four-hinged arches and allowed significant water leak-
Severalkeyelements mayhavehelpedinlimitingthe
age.
damage to the station structure and possibly prevented
complete collapse. Transverse walls at the ends of the
2.3. Undergroundstructures in Taiwan
station and at areas where the station changed width
providedresistance to dynamic forces in the horizontal
direction. Center columns with relatively heavy trans- Severalhighwaytunnels werelocatedwithinthezone
verse Žshear. reinforcement suffered less damage and heavily affected by the September 21, 1999 Chi Chi
helped to maintain the integrity of the structure. The earthquake ŽML 7.3. in central Taiwan. These are
large horseshoe shaped tunnels in rock.All the tunnels
inspected by the first author were intact without any
visible signs of damage. The main damage occurred at
tunnel portals because of slope instability as illustrated
in Fig. 4. Minor cracking and spalling was observed in
some tunnel lining. One tunnel passing through the
Chelungpu fault was shut down because of a 4-m fault
movementŽUengetal.,2001..Nodamagewasreported
in the Taipei subway, which is located over 100 km
from the ruptured fault zone.
2.4. Bolu Tunnel, Turkey
The twin tunnels are part of a 1.5 billion dollar
project that aims at improving transportation in the
mountainousterrain tothe westofBolubetween Istan-
bul and Ankara Žhttp:(cid:1)(cid:1)geoinfo.usc.edu(cid:1)gees.. Each
tunnel wasconstructedusingtheNewAustrianTunnel-
ing Method ŽNATM. where continuous monitoring of
Fig.4. SlopeFailureatTunnelPortal,Chi-ChiEarthquake,Central
Taiwan. primary liner convergence is performed and support
252 Y.M.A.Hashashetal.(cid:1)TunnellingandUndergroundSpaceTechnology16(2001)247(cid:1)293
elements are addeduntil a stable system is established. 3.Engineeringapproachtoseismicanalysisanddesign
The tunnel has an excavated arch section 15 m tall by
16 m wide. Construction has been unusually challeng-
ing because the alignment crosses several minor faults Earthquake effects on underground structures can
parallel to the North Anatolian Fault. The August 17, be grouped into two categories: Ž1. ground shaking;
1999 Koceali earthquake was reported to have had and Ž2. ground failure such as liquefaction, fault dis-
minimalimpactonthe Bolutunnel. Theclosure rate of placement,andslopeinstability.Groundshaking,which
one monitoring station was reported to have temporar- is the primary focus of this report, refers to the defor-
ilyincreased foraperiodofapproximately1week,then mationofthegroundproducedbyseismicwavespropa-
became stable again. Additionally, several hairline gating through the earth’s crust. The major factors
cracks, which had previously been observed in the final influencing shaking damage include: Ž1. the shape,
lining, were being continuously monitored for additio- dimensions and depth of the structure; Ž2. the proper-
nal movement and showed no movement due to the ties of the surrounding soil or rock; Ž3. the properties
earthquake.TheNovember12,1999earthquake caused of the structure; and Ž4. the severity of the ground
the collapse of both tunnels 300 m from their eastern shaking ŽDowding and Rozen, 1978; St. John and
portal. At the time of the earthquake, a 800-m section Zahrah, 1987..
had been excavated, and a 300-m section of unrein- Seismic design of underground structures is unique
forced concrete lining had been completed. The col-
in several ways. For most underground structures, the
lapse took place in clay gauge material in the unfin-
inertia of the surrounding soil is large relative to the
ished section of the tunnel. The section was covered
inertia of the structure. Measurements made by Oka-
withshotcrete Žsprayedconcrete.andhadboltanchors. moto et al. Ž1973. of the seismic response of an
Fig. 5 shows a section of the collapsed tunnel after it
immersedtube tunnel during several earthquakes show
has been re-excavated. Several mechanisms have been
that the response of a tunnel is dominated by the
proposed for explaining the collapse of the tunnel.
surrounding ground response and not the inertial
These mechanisms include strong ground motion, dis-
properties of the tunnel structure itself. The focus of
placement across the gauge material, and landslide.
underground seismic design, therefore, is on the free-
O’Rourkeet al.Ž2001.present adetaileddescriptionof
fielddeformationofthe groundandits interaction with
the tunnel performance.
the structure. The emphasis ondisplacement is in stark
contrast to the design of surface structures, which
2.5. Summary of seismic performance of underground focuses on inertial effects of the structure itself. This
structures led to the development of design methods such as the
Seismic Deformation Method that explicitly considers
the seismic deformation of the ground. For example,
The Daikai subway station collapse was the first
Kawashima, Ž1999. presents a review on the seismic
collapse of an urban underground structure due to
behavior and design of underground structures in soft
earthquake forces, rather than ground instability. Un-
ground with an emphasis on the development of the
derground structures in the US have experienced
Seismic Deformation Method.
limiteddamageduringthe LomaPrieta andNorthridge
The behavior of a tunnel is sometimes approximated
earthquakes, but the shaking levels have been much
to that of an elastic beam subject to deformations
lower than the maximum anticipated events. Greater
imposed by the surrounding ground. Three types of
levels of damage can be expected during these maxi-
deformations ŽOwen and Scholl, 1981. express the re-
mum events. Station collapse and anticipated strong
motions in major US urban areas raise great concerns sponse of underground structures to seismic motions:
regarding the performance of underground structures. Ž1. axial compression and extension ŽFig. 6a,b.; Ž2.
Itisthereforenecessarytoexplicitlyaccountforseismic longitudinal bending ŽFig. 6c,d.; and Ž3. ovaling(cid:1)rack-
loading in the design of underground structures. ing ŽFig.6e,f..Axial deformations in tunnels are gener-
The data show that in general, damage to tunnels is ated by the components of seismic waves that produce
greatly reduced with increased overburden, and da- motions parallel to the axis of the tunnel and cause
mage is greater in soils than in competent rock. Da- alternating compressionandtension.Bendingdeforma-
mage to pipelines Žbuckling, flotation. was greater than tions are caused by the components of seismic waves
torailorhighwaytunnels inbothKobeandNorthridge. producing particle motions perpendicular to the longi-
The major reason for this difference seems to have tudinal axis. Design considerations for axial and bend-
been the greater thickness of the lining of transporta- ing deformations are generally in the direction along
tion tunnels. Experience has further shown that cut- the tunnel axis ŽWang, 1993..
and-cover tunnels are more vulnerable to earthquake Ovaling or racking deformations in a tunnel struc-
damage than are circular bored tunnels. ture develop when shear waves propagate normal or
Y.M.A.Hashashetal.(cid:1)TunnellingandUndergroundSpaceTechnology16(2001)247(cid:1)293 253
Fig.6. DeformationmodesoftunnelsduetoseismicwavesŽafterOwenandScholl,1981..
nearly normal to the tunnel axis, resulting in a distor- the response of the ground and the structure to such
tion of the cross-sectional shape of the tunnel lining. shaking. Table 1 summarizes a systematic approach for
Design considerations for this type of deformation are evaluating the seismic response of underground struc-
in the transverse direction.Thegeneral behavior ofthe tures. This approach consists of three major steps:
lining maybesimulated as aburiedstructure subject to
ground deformations under a two-dimensional plane- 1. Definitionofthe seismic environment anddevelop-
strain condition. ment of the seismic parameters for analysis.
Diagonally propagating waves subject different parts 2. Evaluation of ground response to shaking, which
of the structure to out-of-phase displacements ŽFig. includes ground failure and ground deformations.
6d., resulting in a longitudinal compression(cid:1)rarefac- 3. Assessment of structure behavior due to seismic
tion wave traveling along the structure. In general, shaking including Ža. development of seismic de-
larger displacement amplitudes are associated with sign loading criteria, Žb. underground structure re-
longer wavelengths, while maximum curvatures are sponse to ground deformations, and Žc. special
produced by shorter wavelengths with relatively small seismic design issues.
displacement amplitudes ŽKuesel, 1969..
The assessment of underground structure seismic Steps 1 and 2 are described in Sections 4 and 5,
response, therefore, requires an understanding of the respectively. Sections 6(cid:1)8 provide the details of Steps
anticipated ground shaking as well as an evaluation of 3a, 3b and 3c.
254 Y.M.A.Hashashetal.(cid:1)TunnellingandUndergroundSpaceTechnology16(2001)247(cid:1)293
Y.M.A.Hashashetal.(cid:1)TunnellingandUndergroundSpaceTechnology16(2001)247(cid:1)293 255
Fig.7. DeterministicseismichazardanalysisprocedureŽafterReiter,1990..
4. Definition of seismic environment marize the ground motion hazard at a site. This sce-
nario requires the ‘postulated occurrence’ of a particu-
The goal of earthquake-resistant design for under- lar size of earthquake at a particular location. Reiter
ground structures is to develop a facility that can Ž1990.outlinedthe followingfour-stepprocessŽseeFig.
withstand a given level of seismic motion with damage 7.:
not exceeding a pre-defined acceptable level. The de-
sign level of shaking is typically defined by a design 1. Identification and characterization of all earth-
ground motion, which is characterized by the ampli- quake sources capable of producing significant
tudes and characteristics of expected ground motions ground motion at the site, including definition of
and their expected return frequency ŽKramer, 1996.. A the geometry and earthquake potential of each.
seismic hazard analysis is used to define the level of The most obvious feature delineating a seismic
shaking and the design earthquakeŽs. for an under- zone is typically the presence of faulting. Reiter
ground facility. Ž1990. generated a comprehensive list of features
A seismic hazard analysis typically characterizes the that may suggest faulting in a given region. How-
potential for strong ground motions by examining the ever, the mere presence of a fault does not neces-
extent of active faulting in a region, the potential for sarily signify a potential earthquake hazard (cid:2) the
fault motion, and the frequency with which the faults fault must be active to present a risk. There has
release stored energy. This examination may be dif- been considerable disagreement over the criteria
ficult in some regions Že.g. Eastern USA. where fault- for declaring a fault active or inactive. Rather than
ing is not readily detectable. There are two methods of using the term ‘active’, the US Nuclear Regulatory
analysis: Ža. the deterministic seismic hazard analysis Commission ŽCode of Federal Regulations, 1978.
ŽDSHA.;andŽb.the probabilistic seismic hazardanaly- coined the term capable fault to indicate a fault
sis ŽPSHA.. A deterministic seismic hazard analysis that has shown activity within the past
develops one or more earthquake motions for a site, 35000(cid:1)500000 years. For non-nuclear civil infras-
for which the designers then design and evaluate the tructure, shorter timeframes would be used.
underground structure. The more recent probabilistic 2. Selection ofa source-to-site distance parameter for
seismic hazard analysis, which explicitly quantifies the each source,typicallythe shortest epicentral(cid:1)hypo-
uncertainties in the analysis, develops a range of ex- central distance or the distance to the closest rup-
pectedgroundmotionsandtheir probabilities ofoccur- tured portion of the fault. Closest distance to rup-
rence. These probabilities can then be used to de- tured fault is more meaningful than epicentral dis-
termine the level of seismic protection in a design. tance especially for large earthquakes where the
ruptured fault extends over distances exceeding 50
4.1. Deterministicseismic hazardanalysis (DSHA) km.
3. Selection of a controlling earthquake Ži.e. that
A deterministic seismic hazard analysis involves the which produces the strongest shaking level at the
development of a particular seismic scenario to sum- site., generally expressed in terms of a ground
256 Y.M.A.Hashashetal.(cid:1)TunnellingandUndergroundSpaceTechnology16(2001)247(cid:1)293
motionparameter at the site. Attenuation relation- displacement, response spectrum ordinates, and
ships are typically used to determine these site- ground motion time history of the maximum credi-
specific parameters from data recorded at nearby ble earthquake. Design fault displacements should
locations. Several studies have attempted to corre- also be defined, if applicable.
late earthquake magnitudes, most commonly mo-
ment magnitudes, with observed fault deformation A DSHA provides a straightforward framework for
characteristics, such as rupture length and area, the evaluation of worst-case scenarios at a site. How-
and have found a strong correlation. However, the ever,it providesnoinformation about the likelihoodor
unavailability of fault displacement measurements frequency of occurrence of the controlling earthquake.
over the entire rupture surface severely limits our If such information is required, a probabilistic ap-
ability to measure these characteristics. Instead, proach must be undertaken to better quantify the
researchers have tried to correlate the maximum seismic hazard.
surface displacement with magnitude (cid:2) to varying
results. Empirically based relationships, such as
4.2. Probabilisticseismic hazardanalysis (PSHA)
those developed by Wells and Coppersmith Ž1994.,
can be utilized to estimate these correlations. An-
other, more basic way to evaluate the potential for A probabilistic seismic hazard analysis provides a
seismic activity in a region is through examination framework in which uncertainties in the size, location,
ofhistorical records.These recordsallowengineers and recurrence rate of earthquakes can be identified,
to outline and track active faults and their release quantified, and combined in a rational manner. Such
of seismic potential energy. The evaluation of fore- an analysis provides designers with a more complete
and aftershocks can also help delineate seismic description of the seismic hazard at a site, where varia-
zones ŽKramer, 1996.. In addition to the examina- tions in ground motion characteristics can be explicitly
tionofhistorical records,astudyofgeologicrecord considered.Forthis type of analysis, future earthquake
of past seismic activities called paleo-seismology events are assumed spatially and temporally indepen-
can be used to evaluate the occurrence and size of dent. Reiter Ž1990. outlined the four major steps in-
earthquakes in the region. Geomorphic Žsurface volved in PSHA Žsee Fig. 8.:
landform. and trench studies may reveal the num-
ber of past seismic events, slip per event, and 1. Identification and characterization of earthquake
timing of the events at a specific fault. In some sources, including the probability distribution of
cases, radiocarbonŽ14C.datingofcarbonizedroots, potential rupture locations within the source zone.
animal bone fossils or soil horizons near the fea- These distributions are then combined with the
tures of paleoseismic evidence can be utilized to source geometry to obtain the probability distribu-
approximate ages of the events. tion of source-to-site distances. In many regions
4. Formal definition of the seismic hazard at the site throughout the world, including the USA, specific
in terms of the peak acceleration, velocity and active fault zones often cannot be identified. In
Fig.8. ProbabilisticseismichazardanalysisprocedureŽafterReiter,1990..