INDIAN GEOTECHNICAL CONFERENCE (DECEMBER 18-20, 2003)



MAPPING AVERAGE SHEAR WAVE VELOCITY FOR AHMEDABAD

SOIL SITES: A CASE STUDY

S.S. TRIVEDI

Research Scholar, Deptt. of Civil Engineering, IIT Delhi, New Delhi–110016, India, and Associate Professor, Civil Engineering Department, Institute of Technology, Nirma University, Ahmedabad–382 481, India.

E-mail: sandip.trivedi@nirmauni.ac.in

K.S. Rao

Professor, Deptt. of Civil Engineering, Indian Institute of Technology Delhi, New Delhi–110016, India.

E-mail: raoks@civil.iitd.ac.in

K.K. Gupta

Associate Professor, Deptt. of Civil Engineering, Indian Institute of Technology Delhi, New Delhi–110016, India.

G.W. Rathod

Research Scholar, Deptt. of Civil Engineering, IIT Delhi, New Delhi–110016, India.

ABSTRACT: Mapping the shear wave velocity profile is very important part in seismic hazard and microzonation studies. The shear wave velocity of soil in the Ahmedabad region was measured and mapped using the Multichannel Analysis of Surface Wave (MASW) technique. The test locations covered almost entire Ahmedabad region. A total of 54 MASW tests were performed and the 1-D and 2-D velocity profiles are determined. The shear wave velocity of Ahmedabad soils was evaluated for 2 m depth interval up to 30 m depth. The average shear wave velocity for all locations have been worked and are used to generate map on GIS platform.

1. INTRODUCTION

GEOPHYSICAL TESTS, BASED ON THE GENERATION AND PROPAGATION OF SEISMIC WAVES, ARE WIDELY USED IN EARTHQUAKE GEOTECHNICAL ENGINEERING. SHALLOW SHEAR-WAVE VELOCITY HAS LONG BEEN RECOGNIZED AS A KEY FACTOR IN STUDY OF VARIABLE GROUND MOTION AMPLIFICATION AND SITE RESPONSE IN SEDIMENTARY BASINS. IT IS AN IMPORTANT PARAMETER IN BUILDING CODES AND THE EARTHQUAKE ENGINEERING COMMUNITY WIDELY USES VS IN DESIGN APPLICATIONS (KRAMER, 1996).

Borehole logging is generally considered the standard for obtaining Vs data, but drilling and logging to the depths generally required for earthquake ground motion investigations is very expensive, and it is becoming increasingly problematic in heavily urbanized settings. This, in part, has led to the development of numerous surface acquisition techniques to obtain shallow shear wave velocity. The conventional active-source seismic reflection/refraction has been used extensively for the shallow shear wave velocity characterization up to 50 m.

The SASW method has been widely used for shallow shear wave velocity characterization. Multichannel Analysis of Surface Waves (MASW) overcomes the drawbacks associated with SASW method. In the recent years, MASW method has become very popular amongst professionals and researchers all over the world.

The multichannel analysis of surface waves method (MASW) analyzes dispersion properties of certain types of seismic surface waves (fundamental-mode Rayleigh waves) propagating horizontally along the surface of measurement directly from impact point to receivers. It gives the shear-wave velocity (or stiffness) information in either 1-D (depth) or 2-D (depth and surface location) format in a cost-effective and time-efficient manner. The fundamental framework of the MASW method is based on the multichannel recording and analysis approach long used in seismic exploration surveys.

This paper presents the mapping of average shear-wave velocity through the assessment of s-wave velocity profiles for 54 locations in Ahmedabad region. These results are preliminary outcome of the seismic ground response project undertaken for Ahmedabad region.

2. EXPERIMENTAL WORK

In the present experimental work, the MASW tests were performed at 54 locations in Ahmedabad region with the McSEIS-SX 48 digital seismograph of 48 channels. This equipment consists of engineering seismograph, geophones, connectors, connecting cables, trigger geophones, hammer with plate and batteries. 24 geophones of 4.5 Hz were inserted in ground placed in a straight array, spacing between the geophones was kept as 3 meters. Geophones 1–12 and 13–24, are connected with seismograph, through connecting cables. Trigger source location was kept between each receiver, and also at both ends of the straight line, beyond

1.5 meters. Trigger geophone was also connected with seismograph. A wooden sledge hammer of 11 kg weight was used as trigger source to create wave energy.

For each of the source, the data file was created and stored in the hard disk. The acquired data was then transferred from the seismograph for the analysis using SeisImager/SW software. The two dimensional Vs velocity profiles are developed for the test locations and the average wave velocity up to 30 meters, is calculated. The Vs values so obtained for different depths can be used to calculate the average shear wave velocity.

The setting parameters for the MASW tests are shown in Table 1.

Table 1: Setting Parameters in Seismograph, for MASW Tests

|Recording system: |McSEIS-SX 48 |

|Sampling Interval: |500 (s (MASW) |

|Memory: |4kb |

|Recording Format: |SEG-2 |

|Pre trigger: |ON |

|Stack mode: |Summation |

|Geophones: |24 geophones of 4.5 Hz frequency, for |

| |MASW |

|Geophones array: |Linear with geophone spacing of 3 m |

|Source: |11 kg sledge hammer |

|Source array: |Source is shifted with 3 m interval |

|Offset distance |3 m |

3. AVERAGE SHEAR WAVE VELOCITY VS30

The shear wave velocity averaged over the top 30 m of soil is referred to as VS30 and is computed by dividing 30 m with the shear wave velocity from the surface to 30 m as given in following equation.

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where, hi and Vi denote the thickness and shear wave velocity of the N layers existing in the top 30 meters.

Modern seismic codes like NEHRP, UBC97, IBC200 and Eurocode8 use VS30 for doing the site characterization. These codes are developed after the recent strong earthquakes in America, Europe, Japan and other countries. In general the parameters describing the site effects in seismic codes are expressed through soil characterization and spectral amplification factor.

The VS30 for all the 54 test sites are calculated using the above equation, the Vs30 values for some of the sites are given in Table 2. The map based on all calculated Vs30 values is presented in Figure 1.

Table 2: Average Shear Wave Velocity Values (Vs30) for Some of the Sites

|Sr. |Location name |Latitude |Longitude |Vs30 at 30 |

|No. | | | |m. depth |

|1. |Bopal |23.03 |72.48 |306.40 |

|2. |Isanpur |22.98 |72.60 |261.09 |

|3. |Vatwa Railway Colony |22.95 |72.63 |321.86 |

|4. |Ghodasar |22.98 |72.64 |303.98 |

|5. |Sarkhej Cross roads |22.98 |72.52 |279.79 |

|6. |Odhav Cross road |23.08 |72.63 |442.75 |

|7. |Aman Party Plot, Vasana |23.01 |72.54 |329.58 |

|8. |Vasana Barrage, Vasana |22.99 |72.55 |223.21 |

|9. |Changodar |22.94 |72.46 |409.34 |

|10. |Kathwada |23.07 |72.69 |462.77 |

|11. |Lapkaman Village |23.14 |72.50 |324.41 |

|12. |VasantnagarGota |23.11 |72.53 |397.17 |

|13. |Maninagar-Sukhipura |22.99 |72.61 |389.05 |

|14. |Memnagar-Naranpura |23.05 |72.54 |435.57 |

|15. |NERF SCHOOL, S. G. |23.01 |72.38 |239.95 |

| |Highway | | | |

|16. |Ground near Adalaj Bus |23.17 |72.60 |283.46 |

| |stand | | | |

|17. |Ground near Vejalpur |22.99 |72.51 |386.43 |

|18. |Ground near RTO office, |23.06 |72.58 |275.08 |

| |near Subhash bridge | | | |

|19. |HomeGuards Ground, Nr. |23.03 |72.56 |304.78 |

| |Navrangpura BusStop | | | |

|20. |Shrinath Farm, Near |23.10 |72.62 |271.58 |

| |Torrent, Bhat | | | |

|21. |Near Ring Road, Od-Kamod|22.92 |72.54 |449.09 |

| |Chokdi, Aslali | | | |

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Fig. 1: Mapping of Average Shear Wave Velocity for Ahmedabad Region

4. CONCLUSIONS

Following conclusions are derived from the above study.

1. The Engineering Seismograph, and the MASW method prove to be useful for a quick assessment of site characterization and velocity profiling of the region.

2. The locations so tested, have soil sites with high sedimentary thickness, which have registered low shear wave velocities (120 to 290 m/s) in upper strata, increasing up to around 500 m/s at 30 m. The Vs30 values are varying between 210 m/s to 480 m/s.

3. The map on the basis of Average Shear Wave Velocity has been prepared and presented.

REFERENCES

Kramer, Steven (1996), Earthquake Geotechnical Engineering, Prentice Hall Publication.

Park, C.B., R.D. Miller and J. Xia, (1999). “Multi-channel Analysis of Surface Waves”, Geophysics, V.64, No.3, pp 800–808.

Rao, K.S. (2001). “Seismic Microzonation Studies of Delhi Region”, Indian Geotech. Conf. 2001, Keynote Lecture, Vol. 11, pp. 44–51.

Rao, K.S. (2003). “Seismic Microzonation of Delhi Region”, 12th ARC Soil Mechanics and Geotechnical Engineering, Singapore, GeoAsia, I, 327–330.

Rao, K.S. and Neelima Satyam D. (2005). “Seismic Microzonation Studies for Delhi Region”, Symposium on Seismic Hazard Analysis and Microzonation, September 23–24, Roorkee, pp. 213–234.

Xia, J., R.D. Miller and C.B. Park (1999). “Estimation of Near surface Shear Wave Velocity by Inversion of Rayleigh Waves”, Geophysics, V. 64, No. 3, pp. 691–700.

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IGC 2009, Guntur, INDIA

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