-
n a
ical Engineering, 06800 Beytepe, Ankara, Turkey
settlement. In addition, the comparison between fundamental site
periods and fundamental building periods, which were measured
It is well known and widely accepted that the effects of
particularly important because most of urban settlementshave
occurred along river valleys over such young andsoft surface
deposits. Ground-shaking site effect caused
Engineering Geology 87 (20surface geology on seismic motion
exist and can be large.in a few buildings and estimated from an
empirical expression, indicate that prime attention should be paid
to resonancephenomena, particularly for the northern part of the
settlement where high-rise buildings are still in construction.
2006 Elsevier B.V. All rights reserved.
Keywords: Microzonation; Microtremor measurement; Numerical
modeling; Predominant site period; Shear wave velocity; Site
amplification;Yenisehir (Bursa)
1. Introduction The earthquake damage is generally larger over
softsediments than on firm bedrock outcrops. This isReceived 14
March 2006; received in revised form 23 May 2006; accepted 24 May
2006Available online 11 July 2006
Abstract
Seismic micro hazard zonation for urban areas is the first step
towards a seismic risk analysis and mitigation strategy.
Essentialhere is to obtain a proper understanding of the local
subsurface conditions and to evaluate ground shaking effects. In
this study,present and future settlement areas of Yenisehir, which
is located in the earthquake-prone Marmara Region of Turkey,
wereevaluated with respect to site amplification and site period.
Borings in conjunction with in-situ penetration tests, seismic
velocitymeasurements, resistivity surveys and microtremor studies
were performed, and available data from previous investigations
werecomplied to determine the variation of the soil profile as well
as the characteristics of the soil layers within the study site.
Inaddition, new empirical correlations between shear wave velocity
(Vs) and number of blows from standard penetration test (SPT-N)were
also developed to be used for the estimation of amplification
factors. Site amplification was assessed using empirical
methodsbased on estimated values of Vs, 1-D site response numerical
modeling program and microtremor measurements. Among the
threemethods employed, the numerical technique and microtremor
method yielded considerably higher amplification factors
whencompared to those obtained from the empirical method. This
situation is considered as a limitation of the empirical methods.
Thesurvey of site response suggests ground amplification. The
microzonation map based on soil site amplification
suggestsamplification factors between 1.6 and 5 in the present
settlement, while the areas at the north and south of the
settlement generallyamplify the motion 5 to 9 times. The site
periods obtained from microtremor studies vary from 0.51 to 0.8 s
throughout theHacettepe University, Department of Geologmethods for
an earthquake-prone settlement in Western Turkey
Nilsun Hasancebi, Resat Ulusay Evaluation of site amplificatio
Corresponding author. Fax: +90 312 299 20 34.E-mail address:
[email protected] (R. Ulusay).
0013-7952/$ - see front matter 2006 Elsevier B.V. All rights
reserved.doi:10.1016/j.enggeo.2006.05.004nd site period using
different
06) 85104www.elsevier.com/locate/enggeoby an earthquake can vary
significantly within a smalldistance. This is because at sites
having soft soil and/or
-
ineeritopographic and basement undulations, seismic energygets
trapped, leading to amplification of vibration to man-made
structures. Man-made structures with resonancefrequency matching
that of the site have the maximumlikelihood of getting damaged.
Therefore, informationabout the site response is an integral part
of theconstruction of seismically-safe structures and
urbanplanning. One of the well known examples of such effectsis
Mexico City. In Mexico City, there exist very soft claydeposits
underneath the downtown area of the city. Theseled to very large
amplifications which caused loss of lifeand structural damages
during the distant GurreroMichoacan earthquake of 1985 (Kramer,
1996).
Turkey is one of the most seismically active countriesin the
World. In particular, the August 17, 1999 Kocaeliearthquake, which
resulted in more than 20,000 fatalitiesand extensive structural
damage, was a major disaster forthe most industrial and urbanized
region of Turkey calledMarmara Region. Therefore, this earthquake
focused theattention on densely urbanized and industrialized
settle-ments. In addition to extensive liquefaction and
associatedground failures, and submarine landslides at
differentparts of the earthquake region, site amplification
andrelated damages were also reported. The most
typicalamplification during this earthquake was experience
atAvcilar district of Istanbul (Tezcan et al., 2002; Ergin etal.,
2004).
On the other hand, a large earthquake, which isexpected to occur
in the Marmara Sea within the next30 years (Parsons et al., 2000),
also pose a threatparticularly to the settlements located in the
MarmaraRegion. In addition to Istanbul and Kocaeli provinces,Bursa
is also one of the three most industrialized andpopulated cities of
the Marmara Region. There are 17towns officially belonging to
Bursa. One of them isYenisehir which is found 50 km east of Bursa
(Fig. 1).Increase in its population resulted in urbanization
andconstruction of new buildings. The 1999 Kocaeliearthquake was
also felt in Yenisehir, but did not causeserious structural damage
in this settlement. After thisdevastating earthquake, prime
consideration began to bepaid to geological and geotechnical
investigations bymunicipalities particularly by those in the
affected regionincluding the municipality of Yenisehir.
The first geotechnical study in Yenisehir wasperformed by
Doyuran et al. (2000) for the evaluationof the foundation
conditions of the present and futuresettlement areas of the town.
The study involved drillingat 17 locations, standard penetration
testing (SPT), trialpitting and laboratory testing. Doyuran et al.
(2000)established a microzonation map of the town based on
86 N. Hasancebi, R. Ulusay / Engthe earthquake risks and
geotechnical characteristics ofthe foundation material and
identified two zones interms of suitability of settlement. However,
ground-shaking site effects such as site amplification
andfundamental site periods were not included in thisprevious
study.
In this most recent study, site amplifications andfundamental
site periods in the settlement area ofYenisehir and its close
vicinity were investigated usingGrade-2 and Grade-3 methods
recommended by theTechnical Committee for Earthquake Geotechnical
Engi-neering (TCEGE, 1999). For the purpose, availablegeotechnical
data from Yenisehir were compiled, geo-technical studies involving
borings with SPT tests andgroundwater level measurements, and
laboratory testing.In addition, the data of seismic and resistivity
surveys andmicrotremor measurements (MTM) collected by theGeneral
Directorate of Disaster Affairs (GDAA) (Dikmenet al., 2004) for
this study were also evaluated. The soilamplification was assessed
using three methods, such asshear wave velocity-based empirical
relationships, 1-Dsite response program SHAKE andmicrotremor data.
Siteperiods obtained by SHAKE modeling were presentedand compared
with those obtained from microtremorstudy. Finally, an attempt was
made to establish micro-zonation maps derived from amplification
factors and siteperiods for Yenisehir town.
2. Description of the site
The town of Yenisehir is situated within an ellipticalbasin
called the Yenisehir Plain (Fig. 2). This basin isseparated from
Iznik Plain and Inegol Plain by ridges atthe north and south,
respectively. The Kocasu stream,flowing from southwest to northeast
(Fig. 2), is the mainstream of the basin. Yenisehir is founded on a
flat areamainly consisting of alluvial deposits (Fig. 3a).
Howev-er, towards the south and the north, where urbanizationhas
not extended yet, elevations gradually increase. Theaverage slope
of the site is generally less than 5. Itreaches to 10 in the north,
while is between 10 and 30along the ridges formed by metamorphic
rocks in thesouth.
The population of Yenisehir is 26,000. Increase in itspopulation
resulted in urbanization. Therefore, newbuildings particularly
high-rise buildings are underconstruction in the northern part of
the settlement(Fig. 3b). The present and future settlement areas
ofYenisehir cover about 18 km2. An organized industrialdistrict on
gentle slopes at the southern part of thesettlement is being
planned. In addition, a civil airportlocated 4 km west of the town
was opened to domestic
ng Geology 87 (2006) 85104flights one year ago (Fig. 2).
-
Fig. 1. Location map of the study area.
87N. Hasancebi, R. Ulusay / Engineering Geology 87 (2006)
85104
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3. General settings
3.1. Geology
The geology of the Yenisehir basin was studied byGenc (1986) in
detail. In this study, therefore, formationnames assigned by this
investigator were used. Yenise-hir settlement and its vicinity are
comprised of Pre-Neogene basement rocks, Neogene deposits and
Quaternary deposits (Fig. 4). The basement rocks,belonging to
Dereyoruk formation, are mainly repre-sented by the foliated
metamorphic rocks such asmicaschists, talcschists and fillates with
occasionalmarble bands. These rocks crop out only on the slopesat
the south of the site (Fig. 4). The schistosity planesstriking in
NESW direction generally dip towards thenorth. These units are
unconformably overlain by theNeogene deposits.
88 N. Hasancebi, R. Ulusay / Engineering Geology 87 (2006)
85104Fig. 2. Digital elevation model of the Yenisehir Plain.
-
The Kopruhisar formation consists of the Neogenedeposits (Genc,
1986). These deposits are mainlycomposed of loosely cemented
conglomerates andsandstones, and claystonesiltstone alternations
croppingout along the ridges in the north and marls in the
south(Fig. 4). The rounded and semi-rounded particles formingthe
conglomerates are of sedimentary and metamorphicorigin. Bedding
planes in this sequence dip towards southand southeast with
inclinations of 34. The yellowish-greenmarls observed in the south
areweak rocks and havetransformed into clay at shallow depths.
The Quaternary deposits cover the middle of the basin.Yenisehir
town is located within these deposits which arecharacterized by
alluvium and detritic materials. Based onthe information from the
previous boreholes drilled in theYenisehir Plain by the State
HydraulicWorks (DSI), at thenorthern part of the plain by the State
Harbors andAirports Directority (DLH, 2002) and in the
settlementarea by Doyuran et al. (2000), and from those
drilledduring this study, thick deposits of sand and gravel
withclay interlayers dominate at the south, particularly alongthe
Kocasu stream. While the sequence is mainlyrepresented by thick
clay and silt deposits with sand andgravel interbeds and/or lenses
at the northern part of the
basin. Hydrogeological boreholes opened byDSI indicatethat the
thickness of these deposits reaches up to 115 m.
3.2. Seismotectonics
The Yenisehir Basin is a pull-apart basin bounded bythe
Yenisehir fault extending in NESW direction andsmall faults (Barka
and Kadinsky-Cade, 1988) as seen inFig. 5. The field studies
performed by Doyuran et al.(2000) indicated that there is no field
evidencesuggesting that these faults are still active. In
addition,a fault, which was described as a probable fault
betweenthe pre-Neogene and Neogene units by Doyuran et al.(2000)
(Fig. 5) was clarified by electrical soundingstudies during this
study. This fault dips towards northand is a normal fault.
The main active faults controlling the seismicity ofthe
Yenisehir Plain are GeyveIznik fault zone (GIFZ),which is the
southwestern strand of the North AnatolianFault Zone (NAFZ) and
Bursa Fault (BF) (Doyuranet al., 2000) (Fig. 5). The GIFZ includes
right lateralstrike-slip faults and its distance to Yenisehir is 25
km. Ithas a potential to generate an earthquake with amagnitude of
7.5 (Gulkan et al., 1993). The BF is a
89N. Hasancebi, R. Ulusay / Engineering Geology 87 (2006)
85104Fig. 3. Views from (a) Yenisehir settlement and (b) high-rise
buildings at the northern part of the town.
-
right-lateral strike-slip fault with some normal compo-nent. The
most recent earthquake on this fault, which iscalled BursaMustafa
Kemal Pasa earthquake, occurredin 1855 with maximum modified
Mercalli intensity IX(Coburn and Kuran, 1985).
The recent destructive 1999 Kocaeli earthquake wasalso felt in
Yenisehir, but it did not caused any loss of lifeand structural
damage in this settlement. Based on theevaluations by Doyuran et
al. (2000) on previous earth-quakes occurred in the region, these
investigators indicatethat theGIFZ andBFmay cause destructive
earthquakes in
the region due to the accumulation of strain energy alongthese
fault zones since 145 and 500 (?) years, respectively.
4. Geotechnical investigations and subsurfaceconditions
In the present and future settlement areas of
Yenisehir,geotechnical studies for the assessment of
foundationconditions (Topal et al., 2003; Doyuran et al., 2000)
andrailway route conditions by DLH (2002) were con-ducted. These
studies included a total of 37 boreholes
90 N. Hasancebi, R. Ulusay / Engineering Geology 87 (2006)
85104Fig. 4. Geological map of the study area (modified from
Doyuran et al., 2000).
-
accompanied by SPT. The maximum depth of theboreholes drilled
during the study by Doyuran et al.(2000) was 20 m. By considering
the locations of thesepreviously drilled boreholes, 12 boreholes
were alsodrilled during this most recent study. It is well known
thataverage shear wave velocity of the uppermost 30 m of theground
is an important factor (Borcherdt, 1994; Dobryet al., 2000) for
ground characterization. Therefore, theboreholes were planned to
reach up to 30 m as possible as.Depth of 9 boreholes was 30 m and
others ranged between4.5 and 17 m. SPT tests were conducted at 1 m
intervalsand the samples from SPT were employed for
laboratorytesting. Depth of groundwater level in each borehole
wasalso measured. Locations of the boreholes and simplifiedlogs of
three selected boreholes are shown in Figs. 6 and
7,respectively.
An accurate evaluation of seismic site dependingparameters needs
a proper shear wave velocity profile.Seismic refraction is one of
the techniques which arelargely used in determining dynamic
properties of theunderlying layers. Shear wave velocities were
measured at
the locations of 9 boreholes drilled during this study. Dueto
some restrictions encountered at the locations ofboreholes H6 and
H7, and shallow depth of boreholeH12, seismic refraction
measurements could not beperformed at these locations. In order to
obtain someinformation about stratigraphical knowledge of the
basin,three electrical sounding profiles (resistivity surveys)
werealso taken at a total of 11 points (Fig. 6). In addition,
asseen from Fig. 6, microtremor measurements were alsotaken at
different points in the study area for microzona-tion. Assessments
on microtremor records are discussed inthe next section. All these
geoseismic investigations wereperformed by the team of the General
Directorate ofDisaster Affairs (Dikmen et al., 2004) for this
study.Besides, during the seismic refraction studies,
somecontributions were also provided from the
GeophysicalEngineering Department of Ankara University.
Both previous and recent geotechnical borehole logssuggest that
the Quaternary deposits generally start withlight brown silty clay
with high SPT blow-countsindicating a stiff soil. Below this, there
exists medium-
91N. Hasancebi, R. Ulusay / Engineering Geology 87 (2006)
85104Fig. 5. Seismotectonic map of the eastern Marmara Region
(after Doyuran et al., 2000).
-
dense-to-loose silty sand (Fig. 8a and b). At the
south,particularly in the vicinity of boreholes H7, H8 and
H11,which are located in the old flood plain of the Kocasustream,
sand layers form the top layer of the sequence.However, at some
places the silty clay may also appearbelow the sandy zone.
Occasional gravel layers ofvariable thickness are also observed in
the Quaternarysequence at shallow depths (Fig. 8a and b). During
theelectrical soundings, marls were penetrated at a depth ofabout
10 m below the Quaternary deposits at the southernpart of the site.
The higher resistivity values (Fig. 8c)
obtained in the north when compared to those in the southsuggest
that fine grained soils and saturated sandy layersdominate in the
north and south, respectively.
In laboratory, sieve and hydrometer analyses andAtterberg limit
determinations on 149 SPT samples werecarried out in accordance
with the standards of ASTM(1994). Then, based on the test results,
the samples wereclassified according toUnified Soil Classification
System.
The laboratory studies suggest that the Quaternarydeposits in
the southern part of the site are mainlyconsisted of poorly- and
well-graded sandy soils falling
92 N. Hasancebi, R. Ulusay / Engineering Geology 87 (2006)
85104Fig. 6. Location of geotechnical boreholes, geoseismic
investigations and microtremor recording points.
-
into SP and SW soil classes, and silty and clayey sandsof SC and
SM groups. While CL and CH groups fine-grained soils with low and
high plasticity dominatetowards the north. Range of grain size
distribution of thesoils and plasticity chart are given in Fig. 9a
and b,respectively. These findings show a good agreement tothose
obtained by Doyuran et al. (2000).
Depths of groundwater table measured in thepreviously drilled
boreholes (Doyuran et al., 2000) andin those drilled in this study
are shallow and rangebetween 2.2 and 10 m. However, Doyuran et al.
(2000)indicate that the groundwater levels are deeper (14 m)in the
north and shallower in the south. Sand and gravellenses and/or
interbeds in the alluvial sequence form thewater bearing zones.
5. Assessments on site amplification and site period,and
microzonation
5.1. Amplifications estimated from shear wave velocity
Prediction of ground shaking response at soil sitesrequires
knowledge of stiffness of the soil, expressed interms of shear wave
velocity (Vs). This property is usefulfor evaluating site
amplification (Borcherdt, 1994).While it is preferable to determine
Vs directly from fieldtests, it is not often economically feasible
to make Vsmeasurements at all locations. When the direct
measure-ments of Vs for soil layers are not available, the
existingcorrelations between SPT blow-counts (SPT-N) and Vscould be
used. For the purpose, a number of correlations
93N. Hasancebi, R. Ulusay / Engineering Geology 87 (2006)
85104Fig. 7. Some typical engineering logs illustrating the
subsurface ground conditions and depth of groundwater table in the
study area.
-
were developed (Table 1). The correlations given inTable 1 are
all based on uncorrected SPT-N values.
Since the aerial extend of the study site is large and
Vsmeasurements are limited only for the locations of nineboreholes
drilled during this study, derivation ofempirical correlations
between Vs and SPT-N values ofthe soils of the study site was
considered as a useful toolto be used in amplification
evaluations.
The correlation equations were developed using asimple
regression analysis for the existing database. Thedatabase consists
of 97 data pairs (Vs and SPT-N) obtainedboth from boreholes and
shear wave velocity profiles at
each borehole location. The relationships between Vs andSPT-N
were proposed in three categories considering soiltypes, such as
sandy soils, clayey soils and all soils.Because only a few data
from silty layers and no data fromgravelly layers were available,
these categories could notbe evaluated. These relationships are
shown in Fig. 10.Comparisons between measured and predicted values
ofVs using the equations given in Fig. 10 are presented inFig. 11.
The plotted data are scattered between the lineswith 1:0.5 and 1:2
slopes confirming that the regressionequations generally show a
reasonable fit of the complieddata for the investigated soils.
94 N. Hasancebi, R. Ulusay / Engineering Geology 87 (2006)
85104Fig. 8. (a) and (b) typical cross-sections showing the
subsurface conditions oFig. 6 for section lines).f the study area
and (c) resistivity cross-section of profile A1A5 (see
-
In addition to comparison shown in Fig. 13a, in orderto compare
the performance of the relationships, a graphbetween the scaled
percent error given in Eq. (1) and
Table 1Some existing correlations between Vs and SPT-N for all
soils
Author (s) Vs (m/s)
Ohba and Toriumi (1970) Vs=84N0.31
Imai and Yoshimura (1970) Vs=76N0.33
Fujiwara (1972) Vs=92.1N0.337
Imai (1977) Vs=91N0.337
Ohta and Goto (1978) Vs=85.35N0.348
Imai and Tonouchi (1982) Vs=0.97N0.314
Jinan (1987) Vs=116.1(N+0.3185)0.202
Sisman (1995) Vs=32.8N0.51
Iyisan (1996) Vs=51.5N0.516
Kiku et al. (2001) Vs=68.3N0.292
95ineering Geology 87 (2006) 85104Fig. 9. (a) Range of grain
size distribution of 149 soil samples from SPT
N. Hasancebi, R. Ulusay / EngIn order to assess the effect of
soil type on thesecorrelations, the correlation curves for
different soilsshown in Fig. 10 are transferred onto the same
plot.Fig. 12 suggests that types of soil investigated in this
studyhas no influence on the values of Vs. This result shows agood
agreement with that obtained by Iyisan (1996), whostudied on the
soils collected from an earthquake-affectedarea in the eastern part
of Turkey. He indicated that expectthat for gravels, the
relationship equations developed forall soils, sands and clays
yield approximately similar Vsvalues. Therefore, in thismost recent
study by the authors,the relationship for all soils given in Fig.
10a wasemployed in further evaluations. This equation of thepresent
study is plotted in Fig. 13a together with otherexisting comparable
correlations given in Table 1. Fig.13a indicates that the equations
developed by Sisman(1995) and Kiku et al. (2001) underpredict the
Vs values.Except these equations, all the equations
comparedestimate Vs values close to each other in the case ofSPT-N
values less than 20 as seen from Fig. 13a. For SPT-N values greater
than 20, the equation developed in thisstudy compares well with the
regression equations byOhba and Toriumi (1970), and Imai and
Yoshimura(1970). The differences seen between the relationshipsmay
be due to the quantity of the processed data anddifferent methods
employed during Vs measurements.
tubes and (b) distribution of the fine-grained soils on
plasticity chart.Fig. 10. Correlations between Vs and SPT-N for (a)
all soils, (b) sandyand (c) clayey soils.
-
cumulative frequency was drawn (Fig. 13b) consideringthe data
employed in this study.
Scaled percent error VscVsm=Vsm100 1Where, Vsc and Vsm are the
predicted and measured
shear wave velocities, respectively. As seen in Fig. 13b,
A 68V0:61 V1 < 1100 m=s 2:1
A 1:0 V1 > 1100 m=s 2:2
AHSA 700=V1 for weak motion 3:1
AHSA 700=V1 for strong motion 3:2
Where; A is the relative amplification factor for peakground
velocity, AHSA is the average horizontalspectral amplification in
period range of 0.4 to 0.2 s,and V1 is the average shear wave
velocity over a depth of30 (in m/s). The Grade-2 methods
recommended by
96 N. Hasancebi, R. Ulusay / EngineeriFig. 11. Comparison of the
measured and predicted Vs for (a) all soils,(b) sand and (c) clayey
soils.85%of the values ofVs predicted from the equation of
thisstudy (Fig. 10a) fall into 20% of the scaled percent
errorindicating a better estimate for the studied soils
whencompared to those from the existing equations. Based
onadditional regression analyses performed by Hasancebi(Okan)
(2005) using corrected SPT-N and Vs values fromthe study site
revealed that the relationships with highestcorrelation
coefficients between Vs and SPT-N areobtained when uncorrected
SPT-N values are used forestimation of Vs. Thus, in this study, the
equation in Fig.10a was preferred to estimate the values of Vs
which areemployed to calculate soil amplification factors.
Shear wave velocity of surface layers is a usefulindex property
for evaluating site amplification. Shima(1978) found that the
analytically calculated amplifica-tion factor is linearly related
with the ratio of Vs of thesurface layer to that of bedrock.
Investigations based onthe observation and analyses of ground
motion haverevealed that the average Vs of surface soils to a
certaindepth shows strong correlation with the
relativeamplification (Midorikawa, 1987; Borcherdt et al.,1991).
The available correlation equations by Midor-ikawa (1987) and
Borcherdt et al. (1991) are given inEqs. (2) and (3.1) (3.2) ,
respectively.
Fig. 12. Effect of soil type on VsSPT(N) relationships.
ng Geology 87 (2006) 85104TCEGE (1999) include the use of
above-given empirical
-
equations for the estimation of amplification factor.
Byconsidering this, in this study, Eqs. (2.1) and (3.1)
wereemployed. The amplification factors computed from Eq.(2.1)
varied in a relatively narrow range of 2.3 to 2.8.But the
amplification factors obtained from Eq. (3.1)were slightly greater
and range between 2.4 and 3.4.
5.2. Site modeling
The Grade-3 approach recommended by TCEGE(1999) requires an
in-depth understanding of the necessaryanalytical models and
numerical procedures, when thegeotechnical characteristics of the
site are known; siteeffects can be, in principle, estimated through
numericalanalysis. These analyses may be performed considering
either linear or a non-linear behavior for the soil. The
non-linearity is very often approximated by a linear
equivalentmethod that uses an interactive procedure to adapt the
soilparameters, such as rigidity and damping to actual strain
itundergoes. The soil column is modeled as a series ofhorizontal
layers. These layers are subjected to basemotions that are
considered representative of those likelyoccur in the region of
interest. The SHAKE program is oneof the most widely used for such
calculations (Schnabelet al., 1972). In this study, preliminary
one-dimensionalshear wave prorogation analysis was conducted,
frombedrock to surface, for nine soil profiles at the locations
ofboreholes H1 toH5 andH8 toH11 using the SHAKE2000computer program
(Ordonez, 2004). The thickness andequivalent shear wave velocity
used for each layer were
97N. Hasancebi, R. Ulusay / Engineering Geology 87 (2006)
85104Fig. 13. (a) Comparison of VsSPT(N) regression equation (Fig.
10a) developof Vs predicted from different correlation
relationships.ed in this study with those given in Table 1, and (b)
scaled percent error
-
obtained from boreholes and seismic refraction surveys.The unit
weights of sands and clays used for modeling are18.6 kN/m3 and 18
kN/m3, which were taken from thetable of unit weights recommended
by Tatsuoka et al.(1980), and unit weight determinations in
laboratory,respectively. The time history motion assumed at
bedrocklevel is the EW-component of the 1999Kocaeli
earthquakerecorded at Sakarya strong ground motion station which
isfounded on rock ground and the closest station to the studysite
(see Fig. 5). The other parameters required for theanalysis, such
as G/Gmax ratio, damping ratio and shearmodulus were estimated from
the graphs recommended bySeed and Idriss (1970) and Sun et al.
(1988) for sands andclays, respectively. The amplification spectra
and responsespectra for 0.5% and 10% damping values obtained for
thesoil profiles representing the locations of boreholes H3 andH8
are depicted in Fig. 14 as typical examples, and siteperiods for
the locations investigated were also obtained(Table 2). Table 2
suggests that except borehole locationsH2 and H10, amplification
factors are between 3.5 and
9.03. Boreholes H2 and H10, where amplification factorsare
greater than 10 and generally range between 11 and 12were obtained,
are located near the basin margins in thenorth and south,
respectively, above an inclined bedrocktopography as seen in Fig.
8. Although the amplificationfactors at the locations of boreholes
H5 and H8 are nearly
Table 2Values of site amplification and natural site periods
estimated fromnumerical analysis
Borehole no. Amplification Period (s)
H-1 6.16 0.15H-2 12.06 0.19H-3 9.03 0.47H-4 7.08 0.57H-5 3.58
0.57H-8 3.59 0.80H-9 6.00 0.32H-10 11.05 0.17H-11 5.22 0.44
98 N. Hasancebi, R. Ulusay / Engineering Geology 87 (2006)
85104Fig. 14. Amplification spectra and response spectra for the
location of boreholes H3 and H8.
-
same, site period at location H8, where sand layersdominate, is
considerably higher. Site periods and amplifi-cation factors
obtained by SHAKEmodeling are comparedwith those obtained from MTM
in the followingparagraphs.
5.3. Microtremor measurements
The use ofMTM in estimation of site response has
beeninvestigated since it was proposed in the 1950s. Althoughthere
is ongoing discussion about the applicability of it invarious site
conditions and ground shaking levels, it hasbeen widely used to
estimate the fundamental period ofsoil deposits (Lermo and
Chavez-Garcia, 1994) andrecommended as one the approaches in
Grade-2 methodsin zoning for ground motions (TCEGE, 1999).
Micro-tremormeasurements are relatively easy and
economicallyfeasible method to estimate site response under
earthquakeexcitations. For this study, microtremor measurementswere
conducted by the team of GDDA (Dikmen et al.,2004) at 131 points
within the settlement area of Yenisehirand its close vicinity to
estimate site amplification andpredominant soil periods. The
locations of measurement
sampled at 100 Hz was recorded. Nakamura's (1989)methods was
employed for the determination of the fun-damental site period and
estimation of seismic amplifica-tion at each point. The microtremor
measurementsindicated amplification factors ranging between 1.64
and8.5, and predominant site periods varying from 0.15 to 1 s.The
site amplifications and predominant site periodsobtained by this
method were compared to those obtainedfromothermethods employed in
the following paragraphs.
5.4. Comparison of the results and microzonation
The amplification factors obtained from Vs-basedempirical
equations, SHAKE modeling and MTM arecompared in Fig. 15a. Because
SHAKE analyses werecarried out only for soil columns at the
locations of nineboreholes, the comparisons among three methods
couldbe made only for these locations. It is clear from Fig.
15athat the amplifications obtained from the empiricalequations are
considerably lower than those obtainedfrom other two methods.
Although the coefficientcorrelations of the existing empirical
equations betweenVs and SPT-N are high, it should be kept in mind
that Vs
99N. Hasancebi, R. Ulusay / Engineering Geology 87 (2006)
85104points are shown in Fig. 6. A Datamark LS-8000wd typerecorder
and Akashi-Jep6a3 type three-component accel-erometer were used
through the measurements. At eachobservation point, a minimum of 3
min ambient noiseFig. 15. Comparison of the amplification factors
(and SPT-N values are affected from various factors, andtherefore,
values of Vs obtained from the empiricalequations should not be
evaluated as measured values.Fig. 15a also suggests that the
amplification ratios froma) and site period (b) for the Yenisehir
area.
-
SHAKE are generally larger than those obtained fromMTM, but at
few locations (H5, H8 and H11) thesefactors from both methods are
close to each others. Onthe other hand, the periods computed by MTM
are largerthan the SHAKE periods (Fig. 15b). As mentioned byVentura
et al. (2004), while the periods from SHAKEgenerally reflect the
stratigraphy, the periods determinedfrom microtremor measurements
may be affected bylocal variations in geology and may be also
reflective oftopographical (e.g. basin edge) effects and 3-D
wavereflection/refraction effects due to the geometry andrapid
changes in thickness of the layers.
Based on MTM, maps showing the distribution ofamplifications
(Fig. 16) and fundamental site periods(Fig. 17) over the
investigated area were established.Fig. 16 suggests that the
amplification ranges between1.6 and 5 in the present settlement
area of Yenisehir.While the northern and southern parts of the
basin gener-ally amplifies the motion 5 to 7 times, and locally 8
to 9times. Particularly greater amplifications found for
thesouthern part of the investigated area are probably asso-ciated
with the presence of thick and loose sand layersand very
shallow-seated groundwater table. As seen fromFig. 17 the
fundamental periods generally increase from
100 N. Hasancebi, R. Ulusay / Engineering Geology 87 (2006)
85104Fig. 16. Spatial distribution of amplification factors.
-
the south to north which is interpreted as a thickening ofthe
sediments of the alluvial deposits towards north.Similar trends
were also observed by Bour et al. (1998)and Ventura et al. (2004)
who studied on amplificationand natural period of soils on a plain
near Rhone Delta(France) and Fraser River Delta (Canada),
respectively.The periods in the current settlement area of
Yenisehirgenerally range between 0.51 and 0.8 s, while some
spotareas with periods between 0.91 and 1 s exist (Fig. 17).
Because the settlement area of Yenisehir extendsparticularly to
the north, where high-rise buildings arebeing constructed, it was
considered that a simple com-
parison between the fundamental periods of some buildingsand
site would be useful for the sake of providingpreliminary
information to future comprehensive micro-zonation of Yenisehir.
For the purpose, an additional MTMstudy was also performed by the
team of GDDA (Dikmenet al., 2004) at four buildings for residential
use (Fig. 17).Buildings numbered from 1 to 3 are four-storied
andbuilding 4 is six-storied. All buildings have reinforcedconcrete
framed structures and height of each floor isapproximately 3 m. At
the top and ground floors of threebuildings microtremor monitors
were observed. In thelatest Turkish building code (GDDA, 1999),
approximate
101N. Hasancebi, R. Ulusay / Engineering Geology 87 (2006)
85104Fig. 17. Fundamental site period distribution zoning map.
-
b2 0.660.67 0.45 0.510.60b3 0.71b 0.45 0.610.70
ineerifundamental period, T1, of moment-resisting framed
con-crete buildingsmaybe estimated by the following
empiricalexpression.
T1 CtH3=4N 4Where; HN is the total height of building and Ct is
a
constant taken as 0.07 for buildings with HN25 m inthe first-
and second-degree earthquake zones. Table 3tabulates the observed
and approximately computed(Eq. (4)) fundamental periods of the
buildings with theranges of fundamental soil periods at the
location ofthese buildings obtained from the microzonation mapgiven
in Fig. 17. In building 1, the observed andcomputed fundamental
periods are close to each others,while in buildings 2 to 4 the
observed fundamentalperiods are larger than the computed ones.
However,except that of building 2, the ranges of
measuredfundamental periods of three buildings and the
enclosedsites show good agreement. In addition, Eq. (4) suggestsa
fundamental period of 0.76 s for 8-storey buildingsunder
construction at the northern part of the settlementarea. At the
location of these apartment blocks, thefundamental site period
ranges between 0.71 and 0.8 s(Fig. 17). Although this preliminary
evaluation is basedon a very limited observations and estimations
from anempirical equation, it emphasizes that the coincidence
of
b4 0.750.76 0.60 0.510.60a Measured at ground floor and top
floor.b Periods both in ground and top floors are same.Table
3Comparison of the selected building periods and site periods
BuildingNo.
Building period (s) Siteperiods (s)
Measureda Estimated
b1 0.500.51 0.45 0.510.60
102 N. Hasancebi, R. Ulusay / Engthe fundamental periods of the
buildings particularlythose of high rise buildings, and site seems
to be highlypossible. Therefore, resonance phenomena should betaken
into consideration to select appropriate and safestructural
configurations.
6. Conclusions
In this study, ground amplification was evaluated usingempirical
relationships, 1-D numerical modeling andMTM, and an empirical
relationship between VsSPT(N) values was established. In addition,
natural site periodswere also determined and compared to those of
someselected buildings. The following conclusions are drawnfrom
this study.
Among the three methods employed in the study, thenumerical
modeling and microtremor measurementsyielded higher soil
amplifications. This situation isprobably due to different methods
of shear wave velocitymeasurement, the quantity of processed data
and procedureof SPT which are considered in derivation of
empiricalequations for VsSPT(N) and amplification factor. Thesurvey
of site response, using both numerical method andMTM, has shown
that ground amplification exists. Map ofamplification obtained from
MTM indicates amplificationfactors ranging between 1.6 and 5 in the
present settlementarea.While northern and southern parts of the
basin, wherethe settlement extends, generally amplifies the motion
5 to9 times. The presence of loose sand layers and shallow-seated
groundwater table at the southern part of the site areprobably the
main factors contributing high amplifications.
The fundamental site periods computed from MTMwere larger than
those computed from the numericalmethod. The site periods obtained
from MTM vary from0.15 to 1 s, and 0.51 to 0.8 s throughout the
whole studysite and in the current settlement area, respectively.
Inaddition, preliminary evaluations based on the compar-isons
between fundamental site and building periodsindicate that prime
attention should be paid to resonancephenomena particularly for
high-rise buildings in the town.
This study, which is related to a particular townlocated in the
first-degree earthquake zone of Turkey,clearly shows the importance
of microzonation mapsshowing various degrees of risk zones
associated withdynamic soil behavior such as soil amplification
andliquefaction. With the availability of such maps, theengineers
and architects will be able to select appropri-ate and safe
structural configurations.
Acknowledgements
This study was supported by the Research ProjectGrants Division
of Hacettepe University (Project No.0302602008). The authors
sincerely acknowledge theGeneral Directorate of Disaster Affairs of
Turkey andthe geophysical team of this organization for the
effortsexhibited during the geoseismic experiments and
dataprocessing. The authors also wish to express theirgratitude to
B. Hamdi Cingil, the mayor of Yenisehir,and architecture Sinan
Suzgun and the personnel ofmunicipality for their kind interest and
the logisticsupport they provided throughout the site
investigations.Prof. Dr. Ahmet T. Basokur and research assistant
IrfanAkca of Ankara University, and Dr. Nihat Sinan Isik of
ng Geology 87 (2006) 85104Gazi University are also acknowledged
for their kind
-
103N. Hasancebi, R. Ulusay / Engineering Geology 87 (2006)
85104contributions during the interpretations of the seismicrecords
and numerical analysis, respectively.
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