non-destructive testing of ground sites using sasw

NON-DESTRUCTIVE TESTING OF GROUND SITES USING SPECTRAL ANALYSIS OF

SURFACE WAVE TECHNIQUE

by

Shubhrajit Maitra

Under the guidance of

Prof. Jyant Kumar

Department of Civil Engineering Indian Institute of Science

Bangalore

ORGANIZATION

• Introduction

• Literature Review

• Equipment details and description of sites

• Data acquisition

• Signal processing

• Results

• Concluding remarks

• References

2

INTRODUCTION

3

BACKGROUND

• In-situ Surface Wave Methods (SWM) : Identification of soil properties at

large scale, under undisturbed conditions, at very low strain.

• Advantages of SWM

Non-destructive and non-invasive method

Saves time and money

Can detect low-velocity layers

Can be used up to a considerable depth

• SWM Active Source Methods

Passive Source Methods

• Spectral Analysis of Surface Waves (SASW) : Active source surface

wave method that capitalize upon the dispersive nature of Rayleigh waves.

INTRODUCTION

4

OBJECTIVE

• Study the shear wave velocity (VS) profiles for site specific investigations

• Compare these profiles with available VS profiles obtained from other

tests, such as cross bore hole tests

• Study the variation of maximum depth of exploration with stiffness

characteristics of the site

• Study the effect of change in impact energy at the source on depth of

exploration

LITERATURE REVIEW

5

IN SITU NON-DESTRUCTIVE METHODS FOR OBTAINING THE

SUBSURFACE PROFILE OF THE GROUND

• Reflection survey

• Refraction survey

• Down-hole and up-hole seismic surveys

• Cross-hole seismic survey

• Steady state vibration technique

• Surface wave methods

SPECTRAL ANALYSIS OF SURFACE WAVES (SASW)

MULTI-CHANNEL ANALYSIS OF SURFACE WAVES (MASW)

LITERATURE REVIEW

6


• SURFACE WAVES

Propagates parallel to earth’s surface without spreading energy through

the earth’s interior

Most of the energy propagates in a shallow zone, roughly equal to one

wavelength (λ) (Richart et al., 1970)

More than two-thirds of total seismic energy generated is imparted into

Rayleigh waves (Richart et al.,1970)

• SPECTRAL ANALYSIS OF SURFACE WAVES

Non-intrusive method to determine the shear wave velocity profile

Based on the geometric dispersion of surface waves

LITERATURE REVIEW

7


• NON-HOMOGENEOUS MEDIUM

Fig. 1 Geometric dispersion of surface

waves in non-homogeneous medium

011422

22

1

2

22

2

2

S

R

P

R

S

R

V

V

V

V

V

V

• HOMOGENEOUS MEDIUM

[Lord Rayleigh (1885)]

Dispersion occurs

DISPERSION: Variation of phase velocity as a function of frequency or wavelength

LITERATURE REVIEW

8


• METHODOLOGY

Data Acquisition

Evaluation of dispersion curve by phase unwrapping method

Determination of shear wave velocity profile by inversion process

• INVERSION ANALYSIS

Can be achieved using various numerical techniques proposed by several

researchers (Thomson, 1950; Haskell, 1953; Lysmer, 1970; Kausel & Röesset,

1981; Nazarian, 1984; Nazarian et al., 1988; Hossain & Drnevich, 1989;

Tokimatsu et al., 1992a; Park et al., 1999; Xia et al., 2002; Kumar, 2011)

• SIMPLEST APPROACH TO INVERSION

According to Tokimatsu et al., 1997, (C=Phase Velocity)

D=λ/3 (Heisey et al., 1982) D=λ/2 (Heukelum et al., 1960)

CVS 1.1

EQUIPMENT DETAILS AND DESCRIPTION OF SITES

9

EQUIPMENT FOR GENERATING AND CAPTURING SEISMIC WAVES

Fig. 2 Cylindrical dropping mass along

with tripod and pulley arrangement

Fig. 3 Sledgehammer of

mass 20 lbs (9.07 kg)


10

EQUIPMENT FOR GENERATING AND CAPTURING SEISMIC WAVES

Fig. 4 Geophone fixed to

the ground

Fig. 5 Data acquisition

system

Fig. 6 Connecting cable of

geophones to DAqS

Fig. 7 Base Plate


11

DESCRIPTION OF SITES

• Testing was done on four sites (G-1, G-2, G-3, G-4)

• Location of sites: New BARC Campus, Visakhapatnam

• Sites G-1 and G-2:

Located near the foot of a hill

Separated by a distance of 300 m

• Sites G-3 and G-4:

Located in an open field close to sea

Separated by a distance of 120 m

DATA ACQUISITION

12

SOURCE DISTANCE (X)

• Plane-wave propagation of surface waves does not occur in most cases until

or,

max5.0 X X2max (Stokoe et al., 1994)

• D=λ/3 (Heisey et al., 1982)…………………………………………………… Eq. (2)

• D=λ/2 (Heukelum et al., 1960)

…Eq. (1)

Combining Eq. (1) and Eq. (2), we get

DX 5.1

Three values of S was considered (46 m, 56 m, 66 m).

For D=30 m, mX 45

DATA ACQUISITION

13

• RECEIVER SPACING

Fig. 8 Conventional way of using just two receivers

Fig. 9 Source and receiver distance configurations at sites G-1, G-2, G-3 and G-4

PURPOSE OF USING MORE NUMBER OF GEOPHONES • Resolve the issue of phase unwrapping • Quick generation of input data for several simultaneous values of X

DATA ACQUISITION

14

• FIELD TESTING

Fig. 12 Lifting and dropping of

65 kg mass at site G-2

Fig. 11 Array of

geophones fixed to

ground

Fig. 10 Spike for

fixing geophones

• ACQUISITION OF DATA

Sampling rate : 1024 data points per second

Mass was dropped six-seven times for each value of X

SIGNAL PROCESSING

15

• SPECTRAL CALCULATIONS IN SASW TECHNIQUE

• y1(t) and y2(t) are time domain records of two geophones separated by distance X.

• Measured time records were transformed into the frequency domain, Y1(f) and Y2(f) with the help of Fast Fourier Transform(FFT).

• As per Nazarian & Desai, 1993:

)().( 2

*

121 fYfYG YY )().( 1

*

111 fYfYG YY )().( 2

*

222 fYfYG YY

)](/().([tan)( 2121

1

YYYYwrap GrealGimaginaryf

nwrapunwrap 2

unwrap

X

2 fC Coherence function

)().(

)(

2211

2

21

fGfG

fG

YYYY

YY

, ,

, ,

where, n= 0,1,2…

Here, GY1Y1,GY2Y2 are auto power spectra, GY1Y2 is cross power spectra.

ϕwrap and ϕunwrap are wrapped and unwrapped phase angle respectively.

λ=Wavelength, C= Phase Velocity, f=frequency.

SIGNAL PROCESSING

16

• PHASE UNWRAPPING PROCEDURE

• It deals with calculating ϕunwrap from ϕwrap.

• n needs to be found out.

• For correct value of n, (ϕunwrap /X) becomes constant for a given frequency.

• Thus, by making use of simultaneous records gathered by geophones (R1,

R2,R3 etc.), n can be found out by trial and error.

nwrapunwrap 2 where, n= 0,1,2…

SIGNAL PROCESSING

17

• ILLUSTRATIVE EXAMPLE ON PHASE UNWRAPPING

Fig. 13 Plot of signals from receiver R1 and R6 at site G-1 for 65 kg drop mass and

46 m source to R1 distance

SIGNAL PROCESSING

18


Fig. 14 Variation of wrapped phase angle with frequency

SIGNAL PROCESSING

19


Fig. 15 Variation of unwrapped and wrapped phase angle with frequency after filtering

out unwanted data

• For f < 22.3 Hz, n=0

• For 22.3 Hz < f < 50 Hz, n=1

SIGNAL PROCESSING

20

• CONSTRUCTION OF FIELD DISPERSION CURVE

• After obtaining unwrapped phase Fourier spectrum, field dispersion curve is

obtained using the procedure discussed below.

• Phase difference corresponding to one wavelength (λ) = 2π radians.

• Phase difference corresponding to receiver spacing X = ϕunwrap radians.

• Phase velocity (C) is then calculated using the expression:

• Calculations done for all combinations of frequency (f), source to sensor

distance (S), receiver spacing (X) and type of source used.

• Plot of C versus f and C versus λ is obtained.

2

unwrap

Xor,

unwrap

X

2

fC

SIGNAL PROCESSING

21

• CONSTRUCTION OF SHEAR WAVE VELOCITY PROFILE

As per Tokimatsu et al., 1997,

(C=Phase Velocity, VS=Shear Wave Velocity)

As per Heisey et al., 1982,

Equivalent depth (D) = λ/3

Thus, without doing any rigorous inversion analysis, rough estimate of shear

wave velocity profile can be determined.

CVS 1.1

RESULTS

22

• COMPARISON OF SIGNALS IN TIME DOMAIN

Comparison between various sites

Fig. 16 (a) Signals shown by geophone R1 for S = 46 m and 65 kg drop mass used as source for Site G-1 and Site G-2

RESULTS

23



Fig. 16 (b) Signals shown by geophone R1 for S = 46 m and 65 kg drop mass used as source for Site G-3 and Site G-4

RESULTS

24


Comparison of signals for various values of source to sensor distance (S)

Fig. 17 Signals shown by geophone R1 at site G-1 for (a) S = 46 m, (b) S = 56 m, (c) S =66 m for 65kg mass used as source

RESULTS

25


Comparison of signals for different types of sources

Fig. 18 Signals shown by geophone R1 at site G-1 for S = 46 m when (a) 65 kg mass dropped from a height of 4 m, (b) sledgehammer; was used as source

RESULTS

26

• COMPARISON OF SIGNALS IN FREQUENCY DOMAIN


Fig. 19 Fourier amplitude spectrum of measured time records at geophone R1 for S=46m and 65 kg drop mass for (a) Site G-1, (b) Site G-2, (c) Site G-3 and (d) Site G-4

RESULTS

27


Comparison of signals for different values of source to sensor distance (S)

Fig. 20 Fourier amplitude spectrum of measured time records at geophone R1 for 65kg drop mass at Site G-2 for (a) S =46 m, (b) S = 56 m and (c) S = 66 m

RESULTS

28


Comparison of signals for different types of sources

Fig. 21 Fourier amplitude spectrum of measured time records at geophone R1 for S=46m at Site G-2 for (a) 65 kg drop mass and (b) 20 lbs sledgehammer

RESULTS

29

• DISPERSION CURVES

Fig. 22 Phase velocity (C) versus frequency (f) plots for sites G-1, G-2, G-3 and G-4

RESULTS

30


Fig. 23 Phase velocity (C) versus wavelength (λ) plots for sites G-1, G-2, G-3 and G-4

RESULTS

31


Fig. 24 Comparison of dispersion curves for sites G-1, G-2, G-3 and G-4

Comparison between four sites

RESULTS

32


Fig. 24 Comparison of phase velocity versus wavelength plots for X= 46 m at site G-2 for (a) 65 kg drop mass and (b) 20 lbs sledgehammer used as input energy at the source

Comparison for different types of sources

RESULTS

33

• SHEAR WAVE VELOCITY PROFILE

Fig. 24 Comparison of approximate Shear Wave Velocity profile for sites G-1, G-2, G-3 and G-4

Comparison between four sites

CONCLUDING REMARKS

34

• CONCLUDING REMARKS

• Using SASW method, exploration can be carried up to a greater depth on hard soil

stratum as compared to soft stratum for the same input source energy.

• Exploration up to considerable depth can be achieved by

proper configuration of equipment in fields, proper choice of parameters during testing, In order to explore up to greater depth, higher values of source to sensor

distances needs to be considered. • An increase in input impact energy at the source results in increase of value of λmax.

• Higher sampling rate is not necessary for exploration up to a greater depth in case

of ground sites.

CONCLUDING REMARKS

35

• SCOPE FOR FURTHER WORK

• Determination of the stiffness profiles of various layers needs to be done using

an inversion analysis. This would allow obtaining shear wave velocity profile

more accurately. Sharp changes in values of shear wave along depth can be

detected.

• Obtained shear wave velocity profile can be then compared with available

cross bore hole data to validate the results obtained.

• More field tests needs to be done on various kinds of ground conditions to

validate the results obtained. The primary aim is to explore more number of

sites more effectively.

REFERENCES

• Banab, K.K., & Motazedian, D., 2010. On the efficiency of the multi-channel analysis of surface wave method for shallow and semideep loose soil layers. International Journal of Geophysics, Volume 2010, Article ID 403016, doi:10.1155/2010/403016.

• Ceballos, M.A., & Prato, C.A., 2011. Experimental estimation of soil profiles through spatial phases analysis of surface waves. Soil Dynamics and Earthquake Engineering, 31, 91–103.

• Chen, L., Zhu, J., Yan, X., & Song, C., 2004. On arrangement of source and receivers in SASW testing. Soil Dynamics and Earthquake Engineering, 24, 389–396.

• Haskell, N.A., 1953. The dispersion of surface waves on multilayered media. Bull. Seismol Soc Am, 43(1), 17–34.

• Heisey, J.S., Stokoe II, K.H., Hudson, W.R., & Meyer, A.H., 1982b. Determination of in situ shear wave velocities from spectral analysis of surface waves. Summary report 256- 2(S), Project 3-8-80-256, Center for Transportation Research, Bereau of Engineering Research, The University of Texas at Austin, November 1982.

• Heukelom, W., & Foster, C.R., 1960. Dynamic testing of pavements. Journal of the Soil Mechanics and Foundations, ASCE, Vol. 86, No. SM1, Part 1, 2368-2372. 36

REFERENCES

• Nazarian S., 1984. In situ determination of elastic moduli of soil deposits and pavement systems by spectral-analysis-of-surface-waves method. Ph.D. Dissertation, The University of Texas at Austin.

• Park, C.B., Miller, R.D., & Xia, J., 1999. Multichannel analysis of surface waves, Geophysics, 64(3), 800–808.

• Rakaraddi, P.G., 2012. Non-destructive testing of ground and pavement sites using surface wave technique. PhD thesis, Department of Civil Engineering, Indian Institute of Science.

• Tokimatsu, K., Kuwayama, S., Tamura, S., & Miyadera, Y., 1991. Vs determination from steady state Rayleigh wave method. Soils and Foundations, 31 (2), 153–163.

• Tokimatsu, K., 1997. Geotechnical site characterization using surface waves. Earthquake Geotechnical Engineering, Ishihara (ed.), Balkema, Rotterdam, 1333 – 1368.

• Jones, R., 1962. Surface wave technique for measuring the elastic properties and thickness of roads: Theoretical development. British Journal of Applied Physics, 13, 21-29.

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THANK YOU

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non-destructive testing of ground sites using sasw

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