Geotechnical Earthquake Engineering

Transcription

1 Geotechnical Earthquake Engineering by Dr. Deepankar Choudhury Humboldt Fellow, JSPS Fellow, BOYSCAST Fellow Professor Department of Civil Engineering IIT Bombay, Powai, Mumbai , India. URL: Lecture 22

2 Module 6 Dynamic Soil Properties IIT Bombay, DC 2

3 Reference: NPTEL Video Course on Soil Dynamics Module - 4 IIT Bombay, DC 3

4 Important Soil Properties Density (Unit Weight) - Mass/Volume (Weight/Volume) Shear Modulus Damping characteristics IIT Bombay, DC 4

5 IIT Bombay, DC 5

6 IIT Bombay, DC 6

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8 IIT Bombay, DC 8

9 IIT Bombay, DC 9

10 Measurement of G max Usually accomplished by measuring shear wave velocity (V s ) (1) Direct field measurement Seismic reflection Seismic refraction Seismic cross-hole Seismic downhole, uphole SASW, MASW Suspension logger (2) Indirect field measurement Correlation to (N 1 ) 60, q c1, etc. (3) Laboratory measurement Resonant column Bender element Cyclic triaxial, shear, torsion tests G V max 10 2 s

11 From SPT N value IIT Bombay, DC 11

12 Resonant Column Test IIT Bombay, DC 12

13 Cyclic Triaxial Test Typical Triaxial Test Apparatus IIT Bombay, DC 13

14 Empirical Relationship between G max and In Situ Test Parameters IIT Bombay, CE 684, CE 402, DC 14

15 Effect of cyclic degradation on shear modulus (After Vucetic and Dobry, 1991). IIT Bombay, DC 15

16 IIT Bombay, DC 16

17 IIT Bombay, DC 17

18 Damping Ratio Variation of damping ratio of fine grained soils with cyclic shear strain amplitude and plasticity index (After Vucetic and Dobry, 1991). IIT Bombay, DC 18

19 Development of SPT N versus V s relationship IIT Bombay, DC 19

20 For Soil of Bangalore region in India (Sitharam et al. 2006) The regression equation developed between Vs and (N 1 ) 60cs is given by following equation:- Vs = 78 [(N 1 ) 60cs ] 0.40 Where Vs is the shear velocity in m/s and (N 1 ) 60cs is the corrected SPT N Value. The regression equations useful for residual soil such as silty sand and sand silt with small amount of clay content. Vs = 103 [(N 1 ) 60cs ] Upper bound (+ 47 to 17% variation) Vs = 53 [(N 1 ) 60cs ] 0.40 Lower bound (- 47 to 17% variation) IIT Bombay, DC 20

21 For Soil of Chennai region in India (Boominathan et al. 2006) The SPT N-values obtained in the field were corrected for various factors: overburden pressure, hammer energy, bore hole diameter, rod length and fines content. Shear wave velocity (Vs) was estimated from the corrected SPT-N values using the following empirical equations (JRA, 1980): Vs = 100 N 1/3 (For Clay) Vs = 80 N 1/3 (For Sand) IIT Bombay, DC 21

22 Application of Research for Dynamic Soil Characterization of Mumbai City Reference: Sumedh Y. Mhaske (2011), PhD Thesis, IIT Bombay, Mumbai, India. IIT Bombay, DC 22

23 Hazards for Mumbai City Maximum Population (density) above 16 million Generation of maximum solid waste materials in India 2.5 t/capita/day Limitations of space/land available for further development All Tallest buildings in India, presently about 60 storied building Composed of 7 Islands with Loose marshy soil/waste Amplification. Three fault planes meet near Thane Creek (North-Eastern Mumbai) One of the maximum rainfall area 250 cm yearly (mostly in 3 months) Prone to Flood, Coastline Arabian Sea. D. Choudhury, IIT Bombay, India

24 Seismic zonation map of India [as per IS 1893 Part 1 (2002)] with highlight on Mumbai city, which was originated from seven islands D. Choudhury, IIT Bombay, India

25 Earthquake history in and around Mumbai city [Mhaske and Choudhury, 2010, Jl. of Applied Geophysics, Elsevier] D. Choudhury, IIT Bombay, India

26 Reclaimed lands around original seven islands of Mumbai city showing marshy land along sea shore [Mhaske and Choudhury, 2010, Jl. of Applied Geophysics, Elsevier] D. Choudhury, IIT Bombay, India

27 Some Necessary Information on Study Area Location of Mumbai City, India : Seismic Hazard Zone III as per IS 1893 : 2002 (Part I) Earthquake of 6.0 to 6.5 magnitude is possible to occur. Past disastrous earthquakes occurred in peninsular India Koyna (1967, M w = 6.3), Killari (1993, M w = 6.1), Jabalpur (1999, M w = 5.8) and Bhuj (2001, M w = 7.7) The Mumbai city population is more than 15 million. Active Fault zone :- Panvel flexture, Thane creek, Dharmatar Creek (Subramaniyam, 2001) 23 Active faults identified around Mumbai (Raghukanth and Iyengar, 2006) Mhaske and Choudhury (2010) in Journal of Applied Geophysics, Elsevier, Vol. 70(3),

28 Typical Soil Properties of Mumbai D. Choudhury, IIT Bombay, India

29 D. Choudhury, IIT Bombay, India Locations of boreholes in Mumbai city [Mhaske and Choudhury, 2011, GSP, ASCE]

30 D. Choudhury, IIT Bombay, India Typical soil profile for Mumbai city [Mhaske and Choudhury, 2011, GSP, ASCE]

31 Table 1. Typical Index properties of soil of Mumbai city Station Soil type Depth (m) Depth for SPT (m) SPT N value Ground water depth (m) Gravel (%) Sand (%) Silt (%) Clay (%) Liquid limit (%) Plas tic limit (%) Spe cific grav ity Tilaknagar Chembur Mulund Blackish soft marine clay Yellowish brown stiff clay with gravel Brownish medium stiff clay with boulders Wadala Sakinaka Bluish grey silty clay Brownish medium stiff clay Miraroad Blackish soft marine clay Bandra Yellow stiff clay

32 Worldwide used SPT N vs. Vs relationships [Mhaske and Choudhury, 2011, Natural Hazards, Springer]

33 Typical average shear wave velocity (Vs) of soil for Mumbai city at various borehole locations digitized in GIS map [Mhaske and Choudhury, 2011, Natural Hazards, Springer] D. Choudhury, IIT Bombay, India

34 Typical soil type with SPT N value and shear wave velocity (Vs) of soil for Mumbai city [Mhaske and Choudhury, 2011, Natural Hazards, Springer] D. Choudhury, IIT Bombay, India

35 Correlations between shear wave velocity (Vs) and Uncorrected SPT N value for typical soils of Mumbai city [Mhaske and Choudhury, 2011, Natural Hazards, Springer] D. Choudhury, IIT Bombay, India

36 Correlations between shear wave velocity (Vs) and Clean sand corrected SPT (N 1 ) 60,cs value for typical soils of Mumbai city [Mhaske 2011, PhD Thesis, IIT Bombay] D. Choudhury, IIT Bombay, India

37 Correlations between shear wave velocity (Vs) and Uncorrected SPT N value for various Indian soils [Mhaske and Choudhury, 2011, Natural Hazards, Springer] D. Choudhury, IIT Bombay, India

38 Thematic map of average soil shear wave velocity (Vs) more than 100 m/s soil for Mumbai city [Mhaske and Choudhury, 2011, Natural Hazards, Springer] D. Choudhury, IIT Bombay, India

39 Geospatial contour map of average soil shear wave velocity (Vs) with interval of 50 m/s for typical soil of Mumbai city [Mhaske and Choudhury, 2011, Natural Hazards, Springer] D. Choudhury, IIT Bombay, India

40 Thematic map of maximum shear modulus (G max ) more than 20 MPa for Mumbai city [Mhaske 2011, PhD Thesis, IIT Bombay] D. Choudhury, IIT Bombay, India

41 Soil site classification based on Vs 30 as per NEHRP (2000) Typical Mumbai soils come under soil site class D and E mostly in and around Mumbai city based on Vs 30 values [Mhaske 2011, PhD Thesis, IIT Bombay] D. Choudhury, IIT Bombay, India

42 Soil Liquefaction The transformation from a solid state to a liquefied state as a consequence of increased pore pressure and reduced effective stress (Kramer, 1996) If the shear resistance of the soil becomes less than the static, driving shear stress, the soil can undergo large deformations and is said to liquefy (Osinov 2003).

43 Soil Liquefaction due to Earthquakes Modern Liquefaction Engineering Niigata (1964) & Great Alaskan (1964) earthquakes Every new earthquake creates new research areas necessary Key Elements of Soil Liquefaction (Seed et al., 2003)

44 Susceptibility of Soils to Liquefaction Earthquake intensity and duration Soil type Soil relative density Particle size distribution Presence or absence of plastic fines Ground water table location (saturation) Hydraulic conductivity Placement conditions or depositional environment Aging and Cementation Overburden pressure Historical liquefaction

45 Liquefaction Susceptibility Criteria - Fine Grained Soils Chinese Criteria and Modified Chinese Criteria Andrews and Martin (2000) Youd et al. (2001) Seed et al. (2003) Bray et al. (2004) Bray and Sancio (2006) Boulanger and Idriss (2006) Other Studies

46 Liquefaction Susceptibility Criteria - Chinese Criteria and Modified Chinese Criteria Chinese Criteria Wang (1979) Haichen (1975) and Tangshan (1976) Criteria: 1. Percent finer than mm < = 15% 2. Liquid Limit (LL) < = 35% 3. Water Content (w c ) > 0.9(LL) Modified Chinese Criteria Modified Chinese Criteria (Seed et al. 2003) Fine (cohesive) soils that plot above the A-line as shown in the figure are considered to be susceptible to liquefaction if, 1. Percent clay fines (clay size less than 0.005mm) < 15% 2. Liquid Limit (LL) < = 35% 3. In-situ Water Content (w c ) > = 0.9(LL)

47 Liquefaction Susceptibility Criteria - Youd et al. (2001) Seed et al. (1985) developed CRR / CSR curves for granular soils with fines content of 5% or less, 15% and 35%, based on available empirical data. Revised approximate corrections to better fit the empirical database as shown in Fig.3 for Fines Content (FC): i) FC < = 5%, ii) 5%< FC<35% and iii) FC>=35% The CRR / CSR curves developed are valid only for magnitude 7.5 earthquakes and are to be adjusted to other magnitudes using scaling factors. Cyclic Resistance ratio (CRR) curves for Fines Content (FC) (Youd et al. 2001)

48 Liquefaction Susceptibility Criteria - Seed et al. (2003) Adapazari (1999) and Chi-Chi (1999) earthquakes Field and laboratory performance represented recommendations regarding liquefiability of soils with significant fines contents as shown in the Fig. 4. Do not address, how the criteria was developed Recommendations Regarding Assessment of Liquefiable Soil Types (Seed at al. 2003)

49 Liquefaction Susceptibility Criteria - Bray et al. (2004) After the Kocaeli (1999) earthquake Seven different sites throughout the Adapazari City Laboratory tests, over 100 cyclic triaxial tests, 19 static strength tests, 24 consolidation tests with incremental loading and constant strain rate, numerous index tests undisturbed soil specimens. on Liquefaction Susceptibility Criteria (Bray et al. 2004) Liquid Limit is not considered, as the authors observed that a number of specimens with LL > 35% were found to be moderately susceptible to liquefaction.

50 Liquefaction Susceptibility Criteria - Bray & Sancio (2006) Criteria based on: Ten Cyclic Simple Shear (CSS) tests performed for the same soil specimens in addition to all the tests carried out by Bray et al. (2004b),. Revaluation of data for soils liquefied during Northridge (1994) earthquake by Bennett et al. (1998), data in China from Wang (1979) and Some observations of Chi-Chi (1999) earthquake in Taiwan by Chu et al. (2004). As shown in Figure, only the upper limit of PI given by Bray et al. (2004) is modified for moderately liquefiable soils from 20 to 18. Graphical Representation of Proposed Liquefaction Susceptibility Criteria (Bray & Sancio, 2006) Authors suggest that the proposed criteria should be applied with engineering judgments as there may be cases where sensitive soils with PI > 18 may undergo sever strength loss as a result of earthquake induced straining.

51 Criterion: Liquefaction Susceptibility Criteria - Boulanger & Idriss (2006) fine-grained soils having PI < 3 are named as sand-like and they can exhibit cyclic liquefaction type response; for fine-grained soils having PI > 7 are named as claylike and they are expected to exhibit cyclic mobility type response. In between these PI ranges, i.e. 3 to 7, a transition is expected from sand-like behavior to clay-like behavior. Figure provides a schematic illustration of the transition behavior of fine grained soils with increasing PI. For sand-like soils, initial liquefaction is achieved when excess pore pressure ratio (r u ) becomes equal to 1.0; whereas clay-like soils undergo cyclic mobility when r u > = 0.8. Schematic Illustration of Liquefaction Susceptibility Boundaries on Cyclic Resistance Ratio (CRR) Vs. Plasticity Index (PI) Domain (Boulanger and Idriss, 2006) Criteria in the CRR vs. PI domain without a scale. Distinction of sand-like and clay-like fine grained soils is based on solely PI of the specimens. r u based liquefaction susceptibility definition requires the determination of CSR levels and duration of the excitation.

52 Seismic Liquefaction Hazard Map of Mumbai city using GIS-GPS Mhaske, S.Y. and Choudhury, D. (2010) in Journal of Applied Geophysics, Elsevier, Vol. 70(3), D. Choudhury, IIT Bombay, India

53 Evaluation of Soil Liquefaction Simplified Procedure for evaluation of liquefaction potential (Seed and Idriss -1971, Youd and Idriss -1997, Youd et al. 2001) Stepwise Procedure:- Step:-I The surface data used to access liquefaction should include, location of ground water table, SPT (N-value), V s value etc. unit weight of soil, fines content of soil moisture content Step:-II Evaluate total vertical stress and effective vertical stress (i.e. σ v, σ v ) for all potential liquefiable layer with in deposit D. Choudhury, IIT Bombay, India

54 Calculate Cyclic Stress ratio (CSR) induced by design earthquake CSR a 0.65 r max v d g v Ref; Seed and Idriss a max = peak horizontal acceleration at ground surface (in g) σ v =Total overburden pressure ( kn/m 2 ); σ v = effective overburden pressure ( kn/m 2 ); r d = Stress reduction factor g = Acceleration due to gravity These parameter are determine at particular depth z D. Choudhury, IIT Bombay, India

55 Various relationships for stress reduction coefficient (rd) with shear wave velocity Vs = 120 m/s for (a) Mw = 6.5 and (b) M = 7.5 (Adapted from Idriss and Boulanger, 2010) D. Choudhury, IIT Bombay, India

56 Cyclic Resistance Ratio (CRR) at M w = 7.5, calculated using (SPT) data (N 1 ) 60 CRR 2 3 a cx ex gx bx dx fx hx 4 Ref; Blake s equation (Youd and Idriss, 1997) Where; x = (N 1 ) 60, a = , b = , c = , d = , e = , f = , g = x 10-5 Factor of safety against Liquefaction: FS L CRR CSR 7.5 D. Choudhury, IIT Bombay, India

57 Factor of safety against liquefaction of soil:- Sr.No Factor of safety against Remark liquefaction of soil FS l 1 FS l < 1.0 Critically liquefiable soil < FS l 1.3 Moderately liquefiable soil 3 FS l > 1.3 Not liquefiable soil Mhaske and Choudhury (2010) in Journal of Applied Geophysics, Elsevier, Vol. 70(3),

58 Map of the critically liquefiable areas in Mumbai at M w = 6.0 [Mhaske and Choudhury, 2010, Jl. of Applied Geophysics, Elsevier] D. Choudhury, IIT Bombay, India

59 Liquefaction Hazard Map of Mumbai City for M w = 7.5 Mhaske and Choudhury (2010) in Journal of Applied Geophysics, Elsevier, Vol. 70(3),

60 Factor of Safety against Liquefaction for Mumbai City Mhaske and Choudhury (2010) in Journal of Applied Geophysics, Elsevier, Vol. 70(3),

61 Concluding Remarks Typical shear wave velocity (Vs) of soil varies from 140 m/s to 350 m/s from depth 3m to 10 m in Mumbai. Typical areas like Kandivali, Borivali, Goregaon, Malad of Mumbai city can be prone to critically liquefiable at M w =7.5. The soil amplification factors for acceleration in Mumbai is about 2.5 to 3.5 typically for earthquake similar to 2001 Bhuj motion. Significant effect of depth of liquefied layer for design of pile and other founadtions is necessary to incorporate in design. D. Choudhury, IIT Bombay, India

62 End of Module 6 IIT Bombay, DC 62