Effect of Bedrock Depth on Site Classification

Transcription

1 Effect of Bedrock Depth on Site Claification Ritu Raj Nath & Ravi Sankar Jakka Indian Intitute of Technology, Roorkee, India (247667) SUMMARY: One of the mot important tep in eimic microzonation i ite characterization. Average Shear Wave Velocity up to metre [ V ] i a common parameter for claifying ite. But often problem are encountered in obtaining V from model of hallow hear wave velocity [where the oil column doen t extend up to m] and in that cae, extrapolation i done. But none of thoe extrapolation method conider the bed rock hear wave velocity. Thi article attempt to tudy the effect of bed-rock hear wave velocity, impedance contrat of bedrock and uperficial oil layer, and depth of bed-rock, on ite amplification and claification. Five ite in Delhi, India are choen for which the hear wave velocity profile and bed rock depth are known. Software SHAKE2000 i ued for etimating ite amplification. Thee ite are claified according to NEHRP ite claification cheme (BSSC, 2003) and Eurocode-8 (2003) adopting three method: extrapolation auming contant velocity; extrapolation uing the correlation between V & V (d) ; and the propoed method. Better ite cla i obtained adopting the propoed method. Keyword: Site cla, Average hear wave velocity up to metre, Bedrock hear wave velocity, Amplification ratio, Natural frequency. 1. INTRODUCTION From the early hour of civilization, mankind ha been challenged by variou natural calamitie. The greatet hazard it ha faced i probably the earthquake which can neither be predicted nor be prevented. However, the everity of the damage can be minimized by proper infratructure planning baed on microzonation tudie and by following appropriate contruction procedure adopting codal proviion. Seimic microzonation tudie are conidered a one of the mot important parametre in mitigating eimic hazard a it provide the expected level of haking in a region and aociated eimic rik uch a liquefaction, lateral preading, landlide, tunami, etc. India ha experienced mot diatrou earthquake i.e. Aam 1897 (M=8.7), Kangra 1905 (M=8.6), Bihar-Nepal 1934 (M=8.4), Aam-Tibet 1950 (M=8.7), Uttarkahi 1991 (M=6.5), Latur 1993 (M=6.4), Jabalpur 1997 (M=6.0), Chamoli 1999 (M = 6.8), Bhuj 2001 (M= 7.6) and Kahmir 2005 (M=7.4). The devatating Bhuj Earthquake ha created greater awarene among the engineering fraternity of India and variou cheme have been implemented to avoid loe from future earthquake. A a reult of thi, eimic microzonation of variou urban area, like Delhi, Chennai, Bangalore, Lucknow, Ahmedabad, Guwahati etc. are being carried out. Boominathan (2004) ha given the criteria for eimic ite characterization for nuclear power plant and tructure. However, problem lie with the fact that there are no tandard procedure and/or guideline available for eimic microzonation, and hence thee tudie are coniderably different from each other, particularly in ite characterization tudie. Alo the availability of different geotechnical and geophyical tudie for dynamic ite characterization often lead to confuion in adopting a particular tet or methodology.

2 In conventional method of ite claification, Average Shear Wave Velocity up to metre[ V ] i a widely ued parameter for claifying ite to predict their potential to amply eimic haking (Dobry et. al, 2000). However, in many cae model of hallow hear wave velocitie do not extend up to metre. For thi purpoe ome method of extrapolation are devied (Boore, 2004). Extrapolation i alo done for the ite with lack of information about the uburface geology or where hear wave velocity cannot be obtained up to metre due to variou practical problem. But in none of thee method, the effect of bed rock ha been conidered. The article aim to tudy the effect of bed-rock hear wave velocity, impedance contrat of bedrock and uperficial oil layer, and depth of bed-rock on ite claification and amplification of eimic wave. Two conventional method a commonly ued i.e. Extrapolation Auming Contant Velocity (Boore 2004) and Extrapolation uing the Correlation between V & V (d) (Boore 2004) are alo employed to perform a comparative tudy. Five ite in the Delhi region i.e. Janakpuri, Dilhad Garden, Rohini, Puhpa Vihar and J.N.U. Campu are choen for which the hear wave velocitie and bed rock depth are known (Rao and Neelima, 2004). Thee ite are claified according to National Earthquake Hazard Reduction Program- NEHRP (BSSC, 2003) ite claification cheme and Eurocode-8 (2003). To undertand the effect of local ite condition, theoretical one dimenional ground repone analyi i performed uing the oftware SHAKE2000. Several reearcher have already carried out ground repone analyi for many citie in India: e.g. Govindraju et. al. (2004) for Gujrat; Rajiv Ranjan (2005) for Dehradun; Boominathan et. al. (2007) for Chennai; Mohanty (2007) for Delhi; Raghukanth (2008) for Guwahati; & Choudhuri and Shukla (2011, a, b) again for Gujrat.Kowk and Stewart (2006) howed that 1-D ground repone analyi can be ueful in predicting the average effect of ediment nonlinearity.in thi articlebhuj earthquake (2001) i conidered a the input motion. The accelerationtime hitory at the urface, repone pectra for 2, 5, 10 & 20% damping and amplification ratio are obtained from thi analyi. 2. SITE CHARACTERIZATION METHODS Recently mot of code like Eurocode-8 (2003), NEHRP (BSSC, 2003), International Building Code (IBC, 2009) etc. pecify the ite claification baed on the average hear wave velocity value in the top m [ V ]. The denity and the hear wave velocity of the overlying oil layer play major role in the amplification of hear wave. However due to comparatively leer variation in the denity of oil, the amplification depend heavily on the hear wave velocity near the earth urface. V i calculated uing the equation V d N i i 1 vi Where d i - thickne of the i th oil layer in metre; v i - hear wave velocity for the i th layer in m/ and N no. of layer in the top m oil trata which will be conidered in evaluating V value. In many location the rock depth will be hallow (le than m) and hence the evaluation of V value will not be poible. In thoe cae, extrapolation of available V value ha to be done to evaluate V. The method propoed by Boore (2004) can be ued for thi purpoe. He ha uggeted different model to extrapolate the hear wave velocitie, for depth le than m, to get the V value. The firt method i extrapolation baed on contant velocity. In thi model it i aumed that the hear wave velocity remain contant from the deepet velocity meaurement to the m. (2.1)

3 V tt( d) ( d)/ V (Boore, 2004) (2.2) eff Where tt(d) i the travel time to depth d and V eff = V (d), V (d) i the timed average velocity to a depth of d. Even though thi method i imple, it give under etimated V value, ince the hear wave velocity increae with depth in mot of the oil. Another relation propoed by Boore (2004) wa baed on a power law relation, the V value can be etimated a: logv a blogv ( d) (Boore, 2004) (2.3) Where V S S d i the velocity at a depth of d m (10 < d < ). The value of the regreion coefficient a and bare given for depth ranging from 10m to m.to incorporate the effect of bed rock hear wave velocity on eimic ite characterization a method i propoed in thi article. It i calculated uing the equation: V N i1 d v i i ( d) v r Where d i the depth of oil layer (1 <d< ), d i and v i are depth and hear wave velocity of the i th layer repectively, N i the number of layer up to depth d and v r i the hear wave velocity of the bed rock. A comparative tudy i performed among the method decribed by the equation (2.2), (2.3) and (2.4). A ite claification cheme baed on V value wa propoed by Burckhardt (1994) and a imilar cheme wa adopted by the National Earthquake Hazard Reduction Program (NEHRP) alo. Eurocode-8 (2003) ha alo claified the ite baed on V, tandard penetration tet (SPT) and cone penetration tet (CPT) value. Here ite claification i done in accordance with both the code. (2.4) 3. GROUND RESPONSE ANALYSIS: The effect of bed rock depth on ite characterization i particularly of importance for ite like Delhi where the bedrock topography i undulating in nature with everal hump and depreion (Rao and Neelema, 2004). In their tudy, they have calculated V for 118 ite in Delhi uing MASW teting. Five ite in Delhi i.e. Janakpuri, Dilhad Garden, Rohini, Puhpa Bihar and J.N.U. campu are choen for which bed rock depth and hear wave velocity are known (Rao and Neelima, 2004). Table 3.1 how a comparion for the two extrapolation technique a decribed by equation (2.2) and (2.3) and the propoed method a given by equation (2.4).

4 Table 3.1.Comparion of Three Method Site Bed Rock Depth from urface (m) V (m/) Site cla NEHRP EUROCODE 8 Remark Janakpuri > 286 D C Since oil column extend up to m, no extrapolation i done. Dilhad Garden 24 Rohini 20 a. 277 D C b. 280 D C c. 3 D C a. 366 C B b. 403 C B c. 435 C B Higher value of V conidering bed rock hear wave velocity Puhpa Vihar 16 a. 338 b. 355 D D C C Better ite Cla i obtained conidering bed c. 425 C B rock hear wave velocity. J.N.U. Campu 8 a. 439 C B b. 541 C B c. 615 C B a. Extrapolation auming contant velocity (Boore, 2004). b. Extrapolation uing the correlation between V and V (d) (Boore, 2004). c. Propoed Method conidering the hear wave velocity of bed rock. To undertand the effect of bed rock depth a hypothetical cae tudy i preented. 4 cae are conidered with bed rock depth linearly reducing from 25 metre to 10 metre at 5 metre interval. Figure 3.1 how the comparion of the extrapolation technique and the propoed method for four ite where bed rock lie at 10m, 15m, 20m and 25m repectively.

5 V (m/) method 1 method2 propoed method Depth (m) Figure 3.1. Comparion of Three Method Table 3.2 give the difference of etimated V value uing the propoed method and method given by equation (2.3). Table 3.2. Percentage Difference Of Etimated V Value V (m/) Depth of Bed Rock Extrapolation uing the correlation from G.S (m) between V and V (d) (Boore, 2004). Propoed Method Difference (%) One dimenional ground repone analyi i performed for all thee ite uing SHAKE2000. The modulu reduction curve and damping ratio curve are choen from the databae. For a typical cae tudy the following parametre are choen: Depth of bed rock = 15 m from the ground urface. Type of oil = and. Shear wave velocity of and = 350 m/ Shear wave velocity of bed rock = 750 m/ Depth of W.T. = 12 m from the ground urface. The reult can be interpreted through the figure 3.2, 3.3 and 3.4 a hown below:

6 Spectral Acceleration (g) Acceleration (g) Time (ec) Figure 3.2. Acceleration Time Hitory at the Surface Soil Profile No. 1 - Analyi No. 1 - Profile No. 1 - Layer Sa for 2% damping - SHAKE 1.5 Sa for 5% damping - SHAKE Sa for 10% damping - SHAKE 1.0 Sa for 20% damping - SHAKE Period (ec) Figure 3.3. Repone Spectra at 2, 5, 10 & 20%damping

7 natural frequenct (Hz) Amplification Ratio 10 Layer 2 & Frequency (Hz) Figure 3.4. Amplification Spectrum The preence of oil modifie each and every component of the trong ground motion and it can be een from fig. 3.5in which natural frequency for all the cae are graphically repreented depth (m) Figure 3.5. Natural Frequency for Four Cae Table 3.3 give the amplification ratio for different cae at different frequencie. [up to 5Hz conidered]

8 Table 3.3. Amplification Ratio For Different Depth At Different Frequencie Depth of Bed Rock from G.S Amplification Ratio at Frequencie (m) 0-1 Hz 1-2 Hz 2-5 Hz DISCUSSION AND CONCLUSIONS: The main limitation of the conventional approache of claifying ite where the hear wave velocity model do not extend up to metre, i that it give underetimated ite cla. If actual hear wave velocity of the bed rock i conidered V value improve and conequently better ite cla i obtained. It i alo oberved that the difference of etimated value of V uing the extrapolation technique and the propoed method decreae a the depth of oil layer increae. For hallower depth extrapolation technique are relatively le reliable. Bed rock depth alo influence the amplification of the trong ground motion. Natural frequency hift toward right i.e. increae with decreae in the depth of oil column over bed rock. Amplification ratio increae a the depth of oil layer increae for lower frequencie. Wherea at higher frequencie, the oppoite pattern ha been followed. So from thee above obervation it can be concluded that depth of bed rock influence eimic amplification for different depth at different frequencie i.e. for hallow thickne of oil depoit above bed rock amplification occur at higher frequencie and for high depth of bed rock amplification occur at lower frequencie. Alo the decreae of amplification ratio (at lower frequencie) for the hallower depth of bed rock indicate the better repone of the ite toward eimic excitation (i.e. better ite cla). Hence it i recommended to ue the actual hear wave velocity of the bed rock in ite claification. REFERENCES Aki, K. (1988). Local ite effect on trong ground motion. Proc. Earthquake Engineering and Soil Dynamic II, Park City, Utah, June 27, Banal, B.K and Vandana, C. (2007). Microzonation Studie in India: DST initiative. Proc. of Workhop on Microzonation, Indian Intitute of Science Bangalore, 1-6. Bard, P.Y. (1995). Effect of Surface Geology on Ground Motion: Recent Reult and Remaining Iue. Proc. of the 10 th European Conference on Earthquake engineering, Vienna, Bonilla, L. F., J. H. Steidl, G. T. Lindley, A. G. Tumarkin, and R. J. Archuleta (1997). Site amplification in San Fernando Valley, California: variability of ite effect etimation uing the S- wave, coda and H/V method.bull. Seim. Soc. Am.87, Boore, D.M. (2004). Can ite repone be predicted. Journal of earthquake Engineering.Vol. 8, Sp. Iue 1, Boore, D.M. (2004). Etimating V (or NEHRP Site Clae) from hallow velocity model (Depth < m). Bull. Seim. Soc. Am. 94 (2), BSSC (2003). NEHRP recommended proviion for eimic regulation for new building and other tructure (FEMA 450), Part 1: Proviion Building Seimic Safety Council for the Federal Emergency Management Agency, Wahington, D.C., USA. Dobry, R., Borcherdt, R.D., Croue, C. B., Idri, I. M., Joyner, W. B., Martin, G. R., Power, M. S., Rinne, E. E. and Seed, R. B. (2000). New ite coefficient and ite claification ytem ued in recent building eimic code proviion. Earthquake Spectra, 16, Kramer, S.L. (1996). Geotechnical earthquake engineering. Prentice Hall Publiher, Englewood Cliff, New Jerey.

9 Lachet, C., Hatzfeld, D., Bard, P.Y., Theodulidi, N., Papaioannou C. and Savvaidi, A. (1996). Site effect and microzonation in the city of Thealoniki (Greece): comparion of different approache. Bull. Seim. Soc. Am., 86, Noack, T. and Fah, D. (2001). Earthquake Microzonation: ite effect and local geology. A cae tudy for the Kanton of Bael-Stadt. Sanchez- Sezma, F. J. (2002). Etimation of local ite effect during earthquake: an overview. ISET Journal of Earthquake Technology, Vol. 39, No. 3, Satyam, N. D. and Rao, K. S. (2008). Seimic ite characterization in Delhi region uing the multi channel analyi of hear wave velocity (MASW) teting. EJGE, 13,