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International Symposium on Strong Vrancea Earthquakes and Risk Mitigation Oct. 4-6, 2007, Bucharest, Romania
LIQUEFACTION PROBABILITY IN BUCHAREST AND INFLUENCING FACTORS
D. Hannich1, H. Hoetzl1, D. Ehret1, G. Huber2, A. Danchiv3, M. Bretotean4
ABSTRACT The questions if liquefaction within the shallow sandy layers can occur in Bucharest during strong earthquakes was until now not systematically analysed. Strong earthquakes can cause liquefaction and therewith ground failure in the form of sand boils, lateral spreading, or subsidence. In 2005 at 10 representative sites in Bucharest Seismic Cone Penetration Tests (SCPTu) within the frame of the CRC-461 project were executed. For the first time they have brought data for the evaluation of the liquefaction probability in Bucharest for the whole area of the city. The “factor of safety” (Fs) against liquefaction and the “probability of liquefaction” (PL) were computed from the obtained test-data. For the first time maps of the “liquefaction potential index” (Li) and of the “liquefaction severity index” (Ls) for Bucharest were outlined. These maps show how severe liquefaction phenomena during strong earthquakes in Bucharest could be. The main influencing factors of liquefaction under the local geologic and hydrogeologic conditions in Bucharest are also studied. Beside geotechnical factors, hydrogeological influences were analysed. The influence of seasonal groundwater level variations and variations due to extreme precipitation events upon the liquefaction potential index and liquefaction severity index are studied using a groundwater flow model of Bucharest elaborated for this purpose. Pore water pressure records, obtained by cells installed into liquefaction-prone layers at three sites in Bucharest study the triggering mechanism of liquefaction during earthquakes.
INTRODUCTION
The shallow geologic underground of Bucharest, represented by Quaternary sediments, is characterized by an alternation of soft cohesive and non-cohesive soil layers down to depths varying between 100 and 300 m. Within this sequence, three main porous aquifers bound to sandy and gravely layers exist, presenting specific seasonal variations and long-time trends of the groundwater level. These geological and hydrogeological conditions existing in Bucharest put into question if during strong earthquakes, like in other places with soft soils, liquefaction can take place and if liquefaction-induced ground failure can occur. The only observation of liquefaction occurrence in Bucharest during the 1977 Vrancea earthquake is limited to a small area along the old riverbed of the Dambovita (Ishihara and Perlea, 1984). In 1977 this area was sparsely populated and used mainly for farming. Today it is part of a park, the “Tineretului Park”. Here was described and reflected in a photo (Fig. 1) sand boils aligned along fissures of several meters developed in the covering cohesive soils. This site was then investigated by standard penetration tests (SPT) followed by laboratory tests of the ejected sand. Finally the factor of safety was computed.
1 University of Karlsruhe, Dept. of Applied Geology, Kaiserstr. 12, 76149 Karlsruhe, Germany
2 University of Karlsruhe, Inst. of Soil and Rock Mechanics, 12 Kaiserstr., 76149 Karlsruhe, Germany
3 University of Bucharest, Faculty of Geology and Geophysics, Dept. of Hydrogeology, 6 Traian-Vuia-Str.,
Bucharest, Romania 4 National Institute of Hydrology and Water Management, Dept. of Hydrogeology, 97 Sos. Bucuresti-Ploiesti,
Bucharest, Romania
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Within the CRC-461 project the decision was made to evaluate the liquefaction potential and the liquefaction risk over the whole surface of the city area of Bucharest. In 2005 Seismic Cone Penetration Tests (SCPTu) at 10 representative sites in Bucharest were executed (Hannich et al., 2006). By the use of an heavy equipment (29 tones) it was possible to reach depths up to 35m and to obtain a continuous registration of the cone resistance, the sleeve friction and the pore water pressure as well as a detailed distribution of the shear wave velocity. With the obtained data the factor of safety and the probability of liquefaction for certain shallow layers were determined. Afterward, to take the depth of the liquefied layer and the thickness of the covering cohesive layer in account, the liquefaction potential index and the liquefaction severity index were computed. Finally maps for the whole city area were outlined, showing the risk of ground failure at the surface during a specific earthquake. For the liquefaction potential of special importance is the time-variable groundwater level. A groundwater flow model of Bucharest elaborated for this purpose (Hannich and Danchiv, 2006) can forecast the most probable groundwater level at different sites in the city due to seasonal groundwater level variations, but also groundwater level variations due to extreme precipitation events can be predicted for a specific moment. The triggering mechanism of liquefaction due to strong Vrancea earthquakes is studied through pore water pressure records during earthquakes obtained by 7 measuring cells installed within liquefaction-prone layers at three sites in Bucharest (Tineretului Park, INCERC and NIEP) between 2003 and 2005.
Figure 1: Sand ejection (sand boils) observed after the 1977 Vrancea earthquake in the Dambovita River meadow (Photo in Ishihara and Perlea, 1984)
NEAR-SURFACE GEOLOGY AND HYDROGEOLOGY OF BUCHAREST For an earthquake endangered region, the city area of Bucharest presents quite special geological conditions: the absence of hard bedrock till 5000 m depth and near the surface an alternation of up to 300 m thick Quaternary sand and clay layers and among them three porous aquifer systems. Vertical thickness variations of these soft soil deposits complicate the geological structure. Based on the geological and lithological description of the Quaternary deposits in the area of Bucharest (Liteanu,1951) a classification of 7 main layers (beginning from the surface to depth) is accepted having following names and general characteristics: (1) Anthropogenic backfill and soil, (2) the “Upper clayey-sandy complex” with Holocene deposits of Loess, sandy clays and sands, (3) the “Colentina gravel complex”, including the “Colentina-aquifer”, (4) the “Intermediate clay layer”, (5) the “Mostistea sandbank”, including the “Mostistea-aquifer”, with sands of medium to fine grain size, (6) the “Lacustrine complex”, with a
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variable thickness from about 60m in the South to about 130m in the North and (7) the “Fratesti complex” or “Lower gravel complex”, bearing the “Fratesti aquifer”, lying discordant on Pliocene Levantine clay layers. Due to importance of them for near-surface geology and hydrogeology problems only the 5 upper layers were taken into consideration. These are shown in a W-E cross section in figure 2. The dotted line in this figure shows approximately the maximum depth (20m from the terrain surface) until which liquefaction can produce ground failure at the surface (Seed and Idriss, 1971). Complex hydrogeological conditions are characterizing the shallow underground of Bucharest. The upper-most aquifer, the “Colentina-aquifer” is a unconfined aquifer in direct hydraulic connection with the alluvial deposits of the two rivers, Colentina and Dambovita.
Within this aquifer the average permeability (k) lies between 1,2x10-4 and 9,7x10-5 m/s (Ciugudean and Martinof, 2000).
The second, deeper aquifer, the “Mostistea-aquifer” presents large variations of thickness in the area of Bucharest and is a mainly confined aquifer. The average permeability (k) lies at
8,3x10-5 m/s (Ciugudean and Martinof, 2000). The deepest (depths of 100-300 m) and thickest (thickness of 100-150 m) Quaternary aquifer, the “Fratesti-aquifer” represents the base of the Quaternary layers. Due to its relative great depth, it is considered outside of the near-surface geology range and without importance for liquefaction. The permeability of its uppermost layer A lies at about 1,3 –
5,4x10-5 m/s (Ciugudean and Martinof, 2000).
Figure 2: Cross-section showing the near-surface geology and hydrogeology in Bucharest (processed after Lungu et al., 1999)
DO LIQUEFIABLE SOIL CONDITIONS EXIST IN BUCHAREST? The term liquefaction involves soil deformations caused by transient cyclic loading of saturated cohesionless soils under undrained conditions. The generation of excess pore water pressure is the key of the triggering mechanism to the liquefaction initiation. Thus, the knowledge of the local hydrogeological conditions plays also an important role in analysing the saturation of the soil and the pore water pressure increasing and implicit liquefaction occurrence.
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In the evaluation of the liquefaction hazard of a region three questions have to be answered: - Is the soil susceptible to liquefaction? - If the soil is susceptible, will liquefaction be triggered? - If liquefaction is triggered, will damage occur?
Liquefaction susceptibility Not all soils are susceptible to liquefaction. There are several criteria by which liquefaction susceptibility can be judged. These include historical, geologic, hydrogeologic, compositional and state criteria. Historical criteria refer to information on liquefaction behaviour coming from post-earthquake investigations. Liquefaction case histories can be used to identify specific sites or site conditions, that may be susceptible to liquefaction in future earthquakes. For Bucharest the description of liquefaction phenomena as sand-boils after the 1977 Vrancea earthquake in the actual Tineretului Park (Ishihara and Perlea, 1984) represents historical criteria, showing that mainly the young alluvial sediments (Holocene age), which fill the old riverbed in the river meadows of Dambovita and Colentina, are susceptible for liquefaction. Geologic criteria deal with the age of a soil deposit, the depositional environment – fluvial, colluvial or aeolian deposits, the hydrogeological environment, uniform or non-uniform grain size distribution, the loose or compact state of the deposits. The liquefaction susceptibility is higher for younger soils – Holocene deposits are more susceptible than Pleistocene ones. Liquefaction of pre-Pleistocene soils is rare (Kramer, 1996). The experience and numerical evaluations has shown, that the maximum depth for liquefiable conditions is about 20 m (Seed and Idriss, 1971). In Bucharest the geological conditions influencing the liquefaction susceptibility consist mainly of the presence of loose Pleistocene and Holocene sand, silt and clay deposits along the two rivers, the medium-dense to dense Colentina layer (layer 3 in Fig.2) of sand with gravel and the fine to medium sand of the Mostistea layer (layer 5 in Fig. 2). Thick aeolian loess deposits on the flat planes outside the inner city, reduce generally there the liquefaction susceptibility of the Colentina sandy-gravely layer. The hydrogeologic environment plays an important role, because only in saturated soils liquefaction can occur. The depth of groundwater influences liquefaction susceptibility (Hannich et al., 2005; Hannich et al., 2006). It decreases with increasing depth of groundwater level. For a region it is necessary to know exactly the hydrogeological conditions, that means the number of aquifers, the type of aquifers (confined or unconfined), the piezometric level for each aquifer, the groundwater flow direction and the groundwater level variations in time (seasonal and long-time variations). It must be understood that the groundwater level is not a stationary state, so its range of fluctuation and time-variability must be known, because at sites where groundwater levels fluctuate significantly, liquefaction hazards may also fluctuate (Kramer, 1996). For Bucharest the two near-surface aquifers, Colentina and Mostistea, are of interest for the evaluation of their liquefaction susceptibility. The depth of the groundwater level, its seasonal and long-time variation and the confined and unconfined character within these aquifers is well known (Bretotean, 2003). The variation of the groundwater level depth due to seasonal or extreme precipitation event influences as well as long-time trends can influence significantly the liquefaction susceptibility of these aquifers. Compositional criteria refer to grain size distribution, gradation and particle shape. For many years, liquefaction-related phenomena were thought to be limited to sands. Finer-grained soils were considered incapable of generating the high pore water pressure associated with liquefaction, and coarser-grained soils were considered too permeable to sustain any generated pore pressure long enough for liquefaction to develop. More recently (Perlea and Perlea, 1984; Studer and Koller, 1997; Yan and Lum, 2003), the bounds on gradation criteria
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for liquefaction susceptibility have broadened (Fig. 3). It was observed in laboratory and in the field that even non-plastic silts can liquefy (Kramer, 1996). Just as well it was observed that gravely soils are susceptible for liquefaction if undrained conditions are assured by the presence of impermeable layers above and beneath the gravely layer (Kramer, 1996). Only plastic clays remain non-susceptible for liquefaction. For Bucharest these insights allow the acceptance that from this point of view the sandy-gravely Colentina layer is liquefaction susceptible if also other influencing criteria are fulfilled (Fig. 3). The medium-to-fine sands of the Mostistea layer and the alluvial sands in the Dambovita meadow, fit to the easy-liquefiable area in figure 3.
Figure 3: Optimum grain size distribution for liquefaction (Perlea and Perlea, 1984), with the mean grain size distribution of the Colentina and the Mostistea layers as well as the alluvial sands in the Dambovita River meadow. Well-graded soils are generally less susceptible to liquefaction than poorly graded soils. Particle shape can also influence liquefaction susceptibility. Soils with rounded particle shapes can densify more easily than soils with angular grains. Fluvial and alluvial environments present frequently rounded particles. From this point of view the alluvial soils in the river meadows in Bucharest are favourable for liquefaction susceptibility. State criteria refer to the initial state of the soil, i.e. its stress and density characteristics at the time of the earthquake. Parameters like critical void ratio, steady state of deformation and relative density play an important role in assessing the liquefaction susceptibility of a soil. These parameters are generally difficult to evaluate and need sophisticated laboratory tests. Initiation (triggering) of liquefaction The fact that a soil deposit is susceptible to liquefaction does not mean that the liquefaction will necessarily occur during a given earthquake. The earthquake must be strong enough (as magnitude, as frequency content and duration) to initiate or to trigger the liquefaction. The generation of excess pore water pressure is the key to the initiation of liquefaction. Without changes in pore pressure, hence changes in effective stress, nothing will happen (Kramer,1996). The level of excess pore water pressure required to initiate liquefaction is related to the amplitude and duration of earthquake-induced cyclic loading. Pore water pressure changes during earthquakes can be observed through in-situ records in liquefaction-susceptible soil layers (Youd and Holzer, 1994). Such records were started in Bucharest in 2003, through the installation of 7 devices at three sites, in the Tineretului Park, at the test-site INCERC and at NIEP.
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During the magnitude-6-Vrancea-earthquake from 27. October 2004, first pore water pressure increasing was observed in the records in the Tineretului Park (Fig. 4). The amplitude and duration of the earthquake were obviously not sufficient to maintain the increased pore pressure more than 6 seconds. The fact that the pore water pressure has begun to increase even during this moderate earthquake, let assume that at stronger earthquakes the pore water pressure increase will be high and long enough to trigger liquefaction at this site.
Figure 4: Pore water pressure and Earth pressure increase recorded in Tineretului Park during the Vrancea earthquake of magnitude 6 from 27.0ct.2004, 20:34 UTC Effects of liquefaction The most frequently occurring liquefaction phenomena are small to moderate displacements like sand boils and ground surface settlements amongst others. As result of large liquefaction induced displacements, lateral spreading can occur. Between the liquefaction-induced damages the most often occurring are tilting of heavy structures (high buildings), subsidence of buildings and failure of retaining structures. The evaluation of the possible ground failure can be done through the calculated liquefaction potential index (Sonmez, 2003).
EVALUATION OF LIQUEFACTION SUSCEPTIBILITY IN BUCHAREST, BASED ON CONE PENETRATION TESTS (CPT)
Cone Penetration Testing (CPT) is the most versatile device for soil investigation. Without disturbing the ground, it provides information about soil type, geotechnical parameters like shear strength, density, elastic modulus, rates of consolidation, etc. The Seismic Cone Penetration Test (SCPTu) is a reliable technique to determine in addition in-situ seismic wave velocities and pore water pressure.
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The analysis of the liquefaction probability, the potential of liquefaction and of ground failure at a site can be evaluated using Cone Penetration Test (CPT) data. The evaluation procedure can be outlined using empirical equations deduced by different authors (Seed and Idriss, 1971; Olsen, 1997;Robertson and Wride, 1998; Chen and Juang, 2000; Lee et al., 2003; Yuan et al., 2003). All these empirical methods follow the general stress-based approach pioneered by Seed and Idriss (1971) and require the determination of two variables, namely, the cyclic stress ratio (CSR) and the cyclic resistance ratio (CRR). Since 1971, the determination of CSR, as proposed by Seed and Idriss, representing the cyclic load in simplified methods, remained unmodified. For the determination of CRR, different simplified methods have been proposed till today. In this paper the method proposed by Olsen (1997) is applied. The calculation of CSR after Seed and Idriss (1971) is carried out as follows:
( ) MSFrg
aCSR d
v
v
= max
5.7'
65.0σ
σ (2)
where
σv = vertical total stress of the soil at the depth studied
σ’v= vertical effective stress amax= maximum horizontal ground surface acceleration g = acceleration of gravity rd = shear stress reduction factor MSF = magnitude scaling factor
56.2
5.7
−
= wM
MSF (3)
where: Mw = moment magnitude
rd= 1.0 - 0.00765z for z ≤ 9.15m
rd= 1.174 – 0.0267z for 9.15 < z ≤ 23m; z is the depth in [m]. For Bucharest, in the presented determination of CSR, the characteristics of the Vrancea-earthquake of 1977 were used: for Mw = 7.4 and Peak Ground Acceleration, i.e. PGA-values at different sites in Bucharest deduced by a method proposed by Sokolov and Bonjer (2006). CRR, after Olsen (1997) has the following equation:
( )[ ] 327.00016.0028.017.0025.0'00128.0 fffvc RRRqCRR +−+−= σ (4)
where: qc = the cone penetration resistance in atm, from CPT Rf = the friction ratio in percent, defined as the sleeve friction f s divided by qc from CPT
σ’v= vertical effective stress Then, the factor of safety Fs is determined after Lee et al., 2003 as: Fs = CRR / CSR7.5 (5)
A soil is predicted to liquefy if Fs ≤ 1.2 (Sonmez, 2003). Starting from the factor of safety, the probability of liquefaction can be evaluated.
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Probability of liquefaction
The probability of liquefaction, PL can be estimated after Juang et al., 2003 as
( ) 5.496.01
1
s
LF
P+
= (6)
where Fs is the factor of safety defined by Eq.5. After Chen and Juang (2000) the likelihood of liquefaction can be interpreted using the calculated PL values in Table 1.
Table 1. The classification of probability of liquefaction
Probability Description (likelihood of liquefaction)
0.85 ≤ PL < 1.00 Almost certain that it will liquefy
0.65 ≤ PL < 0.85 Very likely
0.35 ≤ PL < 0.65 Liquefaction/non-liquefaction is equally likely
0.15 ≤ PL < 0.35 Unlikely
0.00 ≤ PL < 0.15 Almost certain that it will not liquefy
It can be seen in Table 1, that liquefaction will occur only if the probability of liquefaction is greater than 35%. The calculated probability permits to observe if a layer is susceptible to liquefy during a specific earthquake. Starting also from the CPT-data the soil type index, Ic, defined by Robertson and Wride (1998), permits to obtain a detailed lithological depth profile. The index is calculated by Eq. 7 and depends on the normalized stress-adjusted cone tip resistance (Eq. 8) and the normalized friction ratio (Eq. 9):
( )( ) ( )[ ] 5.022
1 22.1loglog47.3 ++−= FqI Ncc (7)
where qc1N = normalized (stress-adjusted) cone penetration resistance, defined as
( )[ ]5.0
1 '10 vcNc qq σ= (all terms in kPa) (8)
F = normalized friction ratio, defined as
( ) %100×−= vcs qfF σ (9)
The boundaries of the soil behaviour type are indicated in Table 2.
Table 2. Soil types defined by the soil type index (Eq. 7)
Soil type index Ic Soil behaviour type
Ic < 1.31 Gravelly sand to dense sand 1.31 < Ic < 2.05 Sands: clean sand to silty sand 2.05 < Ic < 2.60 Sand mixtures: silty sand to sandy silt 2.60 < Ic < 2.95 Silt mixtures: clayey silt to silty clay 2.95 < Ic < 3.60 Clays: silty clay to clay
Ic < 3.60 Organic soils: peats
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The soil type index Ic helps among others to restrict the probability of liquefaction to soils with Ic < 2.8 (Yuan et al., 2003):
Liquefaction potential index, Li
The liquefaction potential index Li is used to evaluate the ground failure risk. Its severity categories were proposed originally by Iwasaki et al. (1982) and modified by Sonmez in 2003. The calculations are using the factor of safety Fs (Eq. 5) and a depth weighting function W(z) (Eq.10). In this way the contribution of soil liquefaction at different depths to the failure of the ground is estimated. Sonmez (2003) proposed the change of the threshold value of Fs between non-liquefiable and liquefiable layers from 1.0 (Iwasaki et al., 1982) to 1.2 and suggested following equations:
∫ ⋅⋅=
20
0
)()( dzzWzFLi where Fs = CRR/CSR (Factor of safety) (10)
sFzF −= 1)( for Fs < 0.
F(z) = 2*106*EXP(-18.427*Fs) for 0.95<Fs<1.2
F(z) = 0 for Fs≥1.2
zzW ⋅−= 5.010)( , for mz 200 ≤≤
mzforzW 200)( ≥= KKKK
z = depth
To interpret the obtained values of Li (Eq. 10), a classification proposed by Iwasaki (1982) and modified by Sonmez (2003) is used. In Table 3 liquefaction potential categories are presented.
Table 3. Liquefaction potential classification proposed by Sonmez (2003)
Liquefaction potential index Li Liquefaction potential category
0 Non-liquefiable (based on Fs ≤ 1.2)
0 < Li ≤ 2 Low
2 < Li ≤ 5* Moderate
5* < Li ≤ 15** High
15** > Li Very high * for CRR calculated after Olsen (1997) this value is 8 (Lee et all., 2003) **for CRR calculated after Olsen (1997) this value will be 16 (Lee et all., 2003) The liquefaction potential index proposed by Sonmez (2003) is used to evaluate the severity of the liquefaction induced ground failure. In Table 4 are presented the thresholds of Li and the expected corresponding ground failure.
Table 4. Liquefaction potential index and ground failure (Sonmez, 2003)
Liquefaction potential index, Li
Ground failure
Li < 11.5 No ground failure appears 11.5 < Li < 32
Small to moderate liquefaction induced displacements appear: sand boils and ground settlements
32 < Li Large liquefaction induced displacements: lateral spreading
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Liquefaction severity index, Ls An improved method to evaluate the severity or risk of liquefaction-induced ground failure, was proposed by Sonmez & Gokceoglu (2005), based on previous methods proposed by Ishihara (1985), Lee et al. (2003) a.o. The calculation of the liquefaction severity index, Ls (Eq. 11), is using also the factor of safety Fs (Eq. 5) and the same weighting function W(z) like in Eq. 10.
∫ ⋅⋅=
20
0
)()( dzzWzPL LS where 5.4)96.0/(1
1)(
s
LF
zP+
= for Fs<1.411 (11)
PL(z) = 0 for Fs>1.411
zzW ⋅−= 5.010)( , for mz 200 ≤≤
mzforzW 200)( ≥= KKKK
The liquefaction severity classification using Ls, suggested by Sonmez & Gokceoglu, (2005) is presented in table 5.
Table 5. Liquefaction severity classification (Sonmez & Gokceoglu, 2005)
Liquefaction severity index Ls
Severity class
85 ≤ Ls < 100 Very high
65 ≤ Ls < 85 High
35 ≤ Ls< 65 Moderate
15 ≤ Ls < 35 Low
0 < Ls < 15 Very low Ls = 0 Non-liquefied
SCPTu-measurements in Bucharest Seismic Cone Penetration Tests (SCPTu) were executed within the CRC-461 project in Bucharest at 10 representative sites (Hannich et al., 2006), located in the river meadow of the Dambovita-River, in the Inter-fluvial domain and in the Northern and Southern Plains (Fig. 5). In Bucharest it was possible to reach with the used equipment depths of 36 m. Through the continuously recorded cone tip resistance and the sleeve friction it was possible to get detailed lithological profiles, soil type index profiles and the factor of safety over the depth. Further these were used to calculate the liquefaction potential index and the liquefaction severity index, over a 20 m-depth interval. Based on these indices contour maps of the liquefaction potential and of the severity of liquefaction were outlined. Finally a ground failure analysis was executed. For the first time through the executed CPT measurements it was possible to evaluate the liquefaction risk analysis over the whole city area in Bucharest. In this paper the calculations were outlined for the 1977-Vrancea earthquake of magnitude 7.4
For all the ten investigated SCPTu-locations (Fig. 5) the above mentioned calculations based on the CPT-field data were performed and presented in special graphics. In this paper we present the obtained graphics for three sites, at the locations TINE (Fig. 6), BAZI (FIG. 7) and INCERC (Fig. 8).
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Figure 5: The location of the SCPTu-measurements in Bucharest In fig. 6 it can be observed, that at TINE for a PGA-value of 2.6 m/s² and for a depth interval of 12 m (between 6-18 m depth) the probability of liquefaction is over 35 %, partly even 60 %. For this site the calculations were performed also for a greater PGA-value of 4.0 m/s², showing an increase of the probability of liquefaction for the same depth interval over 80 %. This shows the scale of the influence of PGA upon the liquefaction evaluation. This site in the Tineretului Park presents the highest probability of liquefaction over a relative thick soil layer and the highest liquefaction severity and potential index from all the ten investigated sites. In fig. 7 the calculation results at the BAZI location are presented. The evaluation was performed for a PGA-value of 3.6 m/s² and presents a probability over 80 % for a depth interval of 6 m (between 7 m and 13 m). For this site, calculations for a 3 m higher groundwater level were performed also, showing an increase of the probability up to nearly 100 % and also an increase over 60 % for a deeper depth interval, where the probability was previous below 35 %. This shows the scale of the influence of groundwater level variations upon the liquefaction susceptibility. In fig. 8 the liquefaction evaluation for the INCERC location is presented. For a PGA-value of 2.5 m/s² estimated here from the ground motion records of the 1977 earthquake, the liquefaction evaluations using CPT-data, show a probability under 35 %, that means under that conditions no liquefaction will take place. Calculations with a higher PGA-value of 4.0 m/s² shows an increase of the probability over 35 %, even over 60 %, but only for small depth intervals. So, even for larger PGA-values, the liquefaction risk is here very reduced.
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0 20 40 60 80 100
-20
-16
-12
-8
-4
0
Gra
velly
san
d to
den
se
san
d
For PGA=3.3 m/s²
0 10 20 30 40
qc [MPa]Cone tip resistance
-28
-27
-26
-25
-24
-23
-22
-21
-20
-19
-18
-17
-16
-15
-14
-13
-12
-11
-10
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
De
pth
[m
]
0 10 20 30
Rf [%]Friction ratio
-28
-27
-26
-25
-24
-23
-22
-21
-20
-19
-18
-17
-16
-15
-14
-13
-12
-11
-10
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
0 1 2 3 4
Ic
Soil type index(Robertson & Wride, 1998)
-28
-27
-26
-25
-24
-23
-22
-21
-20
-19
-18
-17
-16
-15
-14
-13
-12
-11
-10
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
SCPTu-location TINER (Tineretului Park)Dambovita-River meadow
Liquefaction potential index, LI=13.23 (High)
Liquefaction severity index, Ls=38.89 (Moderate)
pga=2.6 m/s²
0 20 40 60 80 100
PL [%]
Probability of liquefaction(Juang et al., 2003)
-28
-27
-26
-25
-24
-23
-22
-21
-20
-19
-18
-17
-16
-15
-14
-13
-12
-11
-10
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
0 0.5 1 1.5 2 2.5 3
Factor of safety, FS(Olsen, 1997)
-28
-27
-26
-25
-24
-23
-22
-21
-20
-19
-18
-17
-16
-15
-14
-13
-12
-11
-10
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
FS
<1.2
= liq
uefa
ctio
n
GW
PL>
35%
= liq
uefa
ctio
n
Gra
ve
lly s
an
d to
de
ns
e s
an
d
Cle
an
sa
nd
to s
ilty s
an
d
Silty
sa
nd
to s
an
dy
silt
Cla
ye
y s
ilt to s
ilty c
lay
Silty
cla
y to
cla
y
A -
All
uvia
l cla
yey
-sil
ty
m
ea
do
w d
ep
os
ite
s G
- A
llu
via
l sa
nd
y-g
rav
ely
m
ead
ow
dep
osit
es
4 -
In
term
ed
iate
c
lay
la
yer
pga=4.0 m/s²
LI=18.76 (Very High)
Ls=45.25 (Moderate)
pg
a=
4.0
m/s
²
pg
a=
2.6
m/s
²
Figure 6: Liquefaction evaluation at the TINE location
GW
0 10 20 30 40 50
qc [MPa]
Cone tip resistance
-27
-26
-25
-24
-23
-22
-21
-20
-19
-18
-17
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-3
-2
-1
0
De
pth
[m
]
0 2 4 6 8 10
Rf [%]
Friction ratio
-27
-26
-25
-24
-23
-22
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-18
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0
0 1 2 3 4
Ic
Soil type index(Robertson & Wride, 1998)
-27
-26
-25
-24
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0
0 20 40 60 80 100
PL [%]
Probability of liquefaction(Juang et al., 2003)
-27
-26
-25
-24
-23
-22
-21
-20
-19
-18
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-4
-3
-2
-1
0
PL >
35
% =
liqu
efa
ctio
n
0 1 2 3
FSFactor of safety
(Olsen, 1997)
-27
-26
-25
-24
-23
-22
-21
-20
-19
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-17
-16
-15
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-2
-1
0
FS
<1
.2 =
liqu
efa
ctio
n
SCPTu-location BAZI (Interfluvial domain)
Liquefaction potential index, LI=7.5(High) - GW1: LI=16.5 (Very high)
Liquefaction severity index, Ls=22 (Low) - GW1: LI=27.9 (Low) pga=3.6 m/s²
Gra
ve
lly s
an
d to
de
ns
e s
an
d
Cle
an
sa
nd
to s
ilty s
an
d
Silty
sa
nd
to s
an
dy
silt
Cla
ye
y s
ilt to s
ilty c
lay
Silty
cla
y to
cla
y
2 -
Up
pe
r cla
y
la
yer
4 -
In
term
ed
iate
cla
y la
yer
3 -
Co
len
tin
a a
qu
ife
r
5 - Mostistea
aquifer
6 -
La
custr
ian c
om
ple
x
GW1
Figure 7: Liquefaction evaluation at the BAZI location
International Symposium on Strong Vrancea Earthquakes and Risk Mitigation
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0 10 20 30 40 50
qc [MPa]
Cone tip resistance
-30
-29
-28
-27
-26
-25
-24
-23
-22
-21
-20
-19
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-17
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-8
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-1
0
GW
2-
Up
pe
r c
lay
lay
er
3 -
Co
len
tin
a a
qu
ife
r4
- In
term
ed
iate
cla
y la
yer
0 5 10 15 20 25
Rf [%]
Friction ratio
-30
-29
-28
-27
-26
-25
-24
-23
-22
-21
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-19
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0
0 1 2 3 4
IcSoil type index
(Robertson & Wride, 1998)
-30
-29
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0
0 20 40 60 80 100
PL [%]
Probability of liquefaction(Juang et al., 2003)
-30
-29
-28
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-25
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0
0 1 2 3 4
FS
Factor of safety(Olsen, 1997)
-30
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-28
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-1
0
Gra
ve
lly s
an
d to
de
ns
e s
an
d
Cle
an
sa
nd
to s
ilty s
an
d
Silty
san
d to
sa
nd
y s
ilt
Cla
ye
y s
ilt to s
ilty c
lay
Silty
cla
y to
cla
y
SCPTu-location INCERC (Interfluvial domain)Liquefaction potential index, LI= ~0 (Non-liqu.) - Li= 0.32 (Low)
Liquefaction severity index, Ls= ~0 (Non-liqu.) - Ls=7.06 (Very Low)
pga=2.5 m/s²
pga=4.0 m/s²
Figure 8: Liquefaction evaluation at the INCERC location
Summarized results of the liquefaction risk analysis for Bucharest Starting from the magnitude of the 1977 Vrancea-earthquake and the deduced PGA-values (Sokolov & Bonjer, 2006) at the ten SCPTu-sites in Bucharest, the factor of safety (Fs), the probability of liquefaction (Pl), the liquefaction potential index (Li) as well as the liquefaction severity index (Ls) were calculated. Using the categories and classes from Tables 4 and 5, the corresponding characterization was deduced. Table 6 contains the summarized results for Bucharest during the 1977 earthquake. It can be seen, that the highest liquefaction potential category (“High”) and the highest liquefaction severity class (“Moderate”) were obtained for the TINE site, in the Tineretului Park. Taking into account the corresponding expected ground failure using the thresholds from Table 4, this indicates: “small to moderate liquefaction-induced displacements: sand boils and ground settlements”. This result agrees with the observed liquefaction phenomena as sand boils in the Tineretului Park (Ishihara & Perlea, 1984).
Table 6. Liquefaction risk analysis for Bucharest
No.
Name
Coord. X
Coord.Y
PGA [m/s²]
Ls
Liquefaction severity class
Li
Liquefaction potential category
1 INMH 426800 4929086 3.0 1.19 Very low 0.09 Non-liqu.
2 MOGO 421947 4929594 3.5 14.55 Very low-Low 2 Low-Moderate
3 BAZI 423080 4926502 3.6 22 Low 7.5 Moderate-High
4 VICT 426908 4922577 3.0 16.2 Low 1.62 Low
5 EROI 426463 4920702 4.0 33.38 Low-Moderate 9.83 High
6 AGRO 425899 4924823 3.0 3.9 Very low 0.64 Low
7 IMGB 431752 4913306 2.7 2.82 Very low 0.15 Low
8 INCERC 433298 4921207 2.5 0 Non-liqu. 0 Non-liqu.
9 METRO 437184 4917155 2.5 0.99 Very low 0.18 Low
10 TINE 429791 4917315 2.6 38.89 Moderate 13.23 High
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Analysis of the influence of groundwater level variations for liquefaction in Bucharest Seasonal groundwater level variations and groundwater level variations due to extreme precipitation events must be taken into account as a main time-variable influencing factor for liquefaction in Bucharest. The extend of the groundwater level variations within the upper-most aquifer, the Colentina-aquifer was studied by continuous level-records during the research period of the CRC-461 project. In addition, a groundwater flow model was elaborated (Hannich and Danchiv, 2007) to forecast levels at different sites at different future moments. To emphasize the scale of the influence of groundwater level variations, the liquefaction probability and the other liquefaction indices were calculated for different level heights. In Figure 7 these differences are presented for the case of the BAZI-location. It can be seen, that for a 3 m higher groundwater level, the probability of liquefaction increase for the whole depth interval of 12 m, but especially for the depth interval between 13-16 m, where the probability was below 35 %, it is now greater than 50 %. The liquefaction potential became from “high” to “very high”, but the liquefaction severity remains “moderate”, mainly due to the great thickness (about 5 m) of the covering clay layer at this site. The influence of the local PGA-value The local PGA-value of a site, influences directly the factor of safety Fs through the CSR-value (Eq. 2). To observe the scale of this influence, for the TINE site and the INCERC site the liquefaction probability was calculated for PGA-values of 2.5 m/s² and of 4.0 m/s² (Figures 6 and 8). At the location TINE (Fig. 6) the liquefaction probability increase very strong for a higher PGA-value, the liquefaction potential became “very high”, but the liquefaction severity remains “moderate”. In Figure 8 it can be seen, that for 2.5 m/s² the probability is lower than 35 %, but for 4.0 m/s² the probability became greater than 60% and even 80 %, but only for thin soil layers. The liquefaction potential became from “non-liquefiable” to “low” and the liquefaction severity became from “non-liquefiable” to “very low”. The liquefaction severity increases only a little, also due to the great thickness (about 6 m) of the covering clay layer. It can be said, that for a stronger earthquake than the 1977 one, at greater PGA-values, liquefaction effects can appear also at sites, that until now were considered as “non-liquefiable”. The intention of the authors is to execute the liquefaction risk analysis for mean PGA-values obtained for return periods of 100 and 475 years. In the same way threshold-PGAs will be outlined, for which liquefaction and ground failure will occur at different sites.
LIQUEFACTION POTENTIAL AND LIQUEFACTION RISK MAPS FOR BUCHAREST The evaluation of the liquefaction potential of a liquefaction-prone area is one of the important tasks in geotechnical earthquake engineering. Based on the SCPTu-data and the liquefaction potential index Li as well as on the liquefaction severity index Ls maps for Bucharest were prepared, showing the areas with higher risk to liquefaction during strong earthquakes like the 1977 one. Figure 9 shows the map for the liquefaction potential index Li for Bucharest, by interpolation of the Li-values at the ten SCPTu-locations and additional 4 phantom locations with adopted values for Li. It can be observed that a larger area with “low“ potential extends SE-NW, comprising the Dambovita-River meadow and the eastern part of the Colentina-River meadow. Within this larger area, two local areas –one in the Tineretului Park and the other in the Eroilor Park – around the locations TINE and EROI, areas with “high” potential were contoured. Nearly the same aspect can be observed in Figure 10, which contains the map for the liquefaction severity index Ls for Bucharest.
International Symposium on Strong Vrancea Earthquakes and Risk Mitigation
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These two maps will be improved, by applying a rectangular grid over the whole area and by according similar values for Li and Ls function of appropriate geologic, hydrogeologic and geotechnical conditions.
Figure 9: Contour map of the liquefaction potential for Bucharest for the 1977 earthquake
Figure 10: Contour map of the liquefaction severity for Bucharest for the 1977 earthquake
D. Hannich et al.
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CONCLUSIONS For the first time a liquefaction risk analysis for the whole area of Bucharest was performed. The CPT-field data executed in 2005 were of high quality. The investigations reached optimal depths of 25-35 m, permitting to evaluate accurately the liquefaction probability and the real risk at representative sites in Bucharest. The analysis was performed for the 1977 earthquake, taking into account the magnitude of this earthquake and a PGA distribution deduced by Sokolov and Bonjer (2006). The calculated factor of safety (Fs) using the Olsen (1997) method for the Cyclic Resistance Ratio (CRR) has led to reliable results after calculating the liquefaction probability as well as the liquefaction potential index (Li) and the liquefaction severity index (Ls). The contour maps outlined using these indices show a reliable distribution of the liquefaction risk over the city area. An improvement of these maps through additional procedures is intended. The analysis of ground failure based on the calculated indices shows moderate effects at the TINE location, which corresponds with the sand boils observed there after the 1977 earthquake. The influence of groundwater level variations is studied, showing with the example of the BAZI location the possible scale of this influence. An elaborated groundwater flow model make possible to forecast the probably groundwater level at different sites in Bucharest during future earthquakes. Local PGA-values have a direct influence upon the liquefaction probability, the potential and the severity. The possible scale of this influence is demonstrated at the locations TINE and INCERC. Installed instruments at 3 sites in Bucharest (Tineretului Park, INCERC and NIEP) are recording the pore water pressure in liquefaction prone layers, to study the triggering mechanism during future earthquakes. An increase of pore water pressure was registered at the Tineretului Park during the 27. October 2006 earthquake of magnitude 6.
ACKNOWLEDGEMENTS This study was made possible by the German Research Foundation (Deutsche Forschungsgemeinschaft-DFG), which funded this project as part of the CRC-461 “Strong Earthquakes – a Challenge for Geosciences and Civil Engineering”.
REFERENCES Bretotean, M., 2003. Phreatic level variations in Bucharest between 1973-2003, Internal
Report, National Institute for Meteorology and Hydrology, Bucharest, 2001 (in Romanian)
Chen, C.J., Juang, C.H., 2000. Calibration of SPT- and CPT-based liquefaction evaluation methods. In: Mayne, P.W., Hryciw, R. (Eds.) Innovations and Applications in Geotechnical Site Characterization. Geotechnical Special Publication, vol. 97. ASCE, Reston, VA, pp. 49-64
Ciugudean, V. and Martinof, D., 2000. Geological, geomorphological and hydrogeological conditions in the city area of Bucharest, Internal Report S.C. Metroul S.A., Bucharest
Hannich, D. and Danchiv, A. 2007. A hydrogeologic groundwater flow model for Bucharest and its connection with site effects at earthquakes, Scientific Communications at the University of Karlsruhe, AGK, June 2007
International Symposium on Strong Vrancea Earthquakes and Risk Mitigation
221
Hannich, D., Hötzl, H., and Cudmani, R., 2006. The influence of groundwater on damage caused by earthquakes – an overview [in German]. Grundwasser, Vol. 11, No. 4, pp. 286-294
Hannich, D., Hötzl, H., Ehret, D., Bretotean, M., Danchiv, A., and Ciugudean, V., 2005. The impact of hydrogeology on earthquake ground motion in soft soils. Proceedings,
Intern. Conference 250th
Anniversary of the 1755 Lisbon Earthquake, pp. 358-361 Hannich, D., Huber, G., Ehret, D., Hötzl, H., Balan, S., Bala, A., Bretotean, M., and
Ciugudean, V., 2006. SCPTu-Techniques used for shallow geologic/hydrogeologic site characterization in Bucharest, Romania. ESG 2006 – Third Inter. Symposium on the Effects of Surface Geology on Seismic Motion, Vol. 1, pp. 981-992
Ishihara, K., Perlea, V., 1984. Liquefaction-associated ground damage during the Vrancea earthquake of March 4, 1977, Soils and Foundations, vol.24, No. 1, pp. 90-112
Iwasaki, T., Arakawa, T., Tokida, K., 1982. Simplified procedures for assessing soil liquefaction during earthquakes. Proceedings of the Conference on Soil Dynamics and Earthquake Engineering, Southampton, UK, pp. 925-939
Juang, C.H., Yuan, H., Lee, D.H., Lin, P.S., 2003. Simplified CPT-based method for evaluating liquefaction potential of soils. Journal of Geotechnical and Geoenvironmental Engineering, ASCE 129 (I), pp. 66-80
Kramer, S.L., 1996. Geotechnical Earthquake Engineering. Prentice Hall, Upper Saddle River, NJ
Lee, D.-H., Ku, C.-S, Yuan, H., 2003. A study of the liquefaction risk potential at Yuanlin, Taiwan, Engineering Geology, 71,pp. 97-117
Liteanu, G., 1951. Geology of the city of Bucharest. Technical Studies, Series E, Hydrogeology, Bucuresti, No.1, (in Romanian)
Lungu, D., Aldea, A., Moldoveanu, T., Ciugudean, V., Stefanica, M., 1999. Near-surface geology and dynamic properties of soil layers in Bucharest. In: Wenzel, F., Lungu, D., Novak, O. (Eds.): Vrancea Earthquakes: Tectonics, Hazard and Risk Mitigation. Kluwer Academic Publishers, Dordrecht,137-148
Olsen, R.S. (1997): Cyclic liquefaction based on the cone penetration test. In: Youd, T.L. and Idriss, I.M. (Eds.), Proceedings of the NCEER Workshop of Evaluation of Liquefaction Resistance of Soils, State University of New York at Buffalo, Buffalo, NY, 225-276
Perlea, V. and Perlea M., 1984. Dynamic stability of sandy layers (in Romanian Language). Editura Tehnica, Bucuresti, Romania
Robertson, P. and Wride, C., 1998. Evaluating cyclic liquefaction potential using the cone penetration test, Canadian Geotechnical Journal, Vol. 35, No. 3, pp. 442-459
Seed, H.B. and Idriss, I.M., 1971. Simplified procedure for evaluating soil liquefaction potential, Journal of Geotechn. Engin. Division, ASCE, Vol. 97, No. 9, pp. 1249-1273
Sokolov, V. and Bonjer, K.-P., 2006. Modeling of distribution of ground motion parameters during strong Vrancea (Romania) earthquakes, Proceedings of First Europ. Conf. on Earthquake Engineering and Seismology, Geneva, Switzerland, paper 363
Sonmez, H., 2003. Modification to the liquefaction potential index and liquefaction susceptibility mapping for a liquefaction-prone area (Inegol-Turkey), Environmental Geology, 44 (7), pp. 862-871
Sonmez, H. and Gokceoglu, C., 2005. A liquefaction severity index suggested for engineering practice, Environmental Geology, 48, pp. 81-91
Studer, J.A. and Koller, M.G., 1997. Bodendynamik, Springer Verlag Berlin, Heidelberg Yan, L. and Lum, K., 2003. Liquefaction assessment of gravely soils for dam safety
evaluation, ICOLD Congress, Montreal Yuan, H., Yang, S.H., Andrus, R.D., Juang, C.H., 2003. Liquefaction-induced ground failure:
a study of the Chi-Chi earthquake cases, Engineering Geology 17, pp. 141-155 Youd, T.L., Holzer, Th.L., 1994. Piezometer performance at Wildlife liquefaction site,
California. Journal of Geotechnical Enginerring, 120(6), p. 975-995
