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Multichannel Analysis of Surface Waves for Assessing Soil Stiffness

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Multichannel Analysis of Surface Waves for Assessing Soil Stiffness Multichannel Analysis of Surface Waves for assessing soil stiffness Elín Ásta Ólafsdóttir FacultyFaculty of of Civil Civil and and Environmental Environmental Engineering Engineering UniversityUniversity of of Iceland Iceland 20162016 MULTICHANNEL ANALYSIS OF SURFACE WAVES FOR ASSESSING SOIL STIFFNESS Elín Ásta Ólafsdóttir 60 ECTS thesis submitted in partial fulfillment of a Magister Scientiarum degree in Civil Engineering Advisors Dr. Bjarni Bessason Dr. Sigurður Erlingsson Faculty Representative Dr. Benedikt Halldórsson Faculty of Civil and Environmental Engineering School of Engineering and Natural Sciences University of Iceland Reykjavik, January 2016 Multichannel Analysis of Surface Waves for assessing soil stiffness MASW for assessing soil stiffness 60 ECTS thesis submitted in partial fulfillment of a M.Sc. degree in Civil Engineering Copyright c 2016 Elín Ásta Ólafsdóttir All rights reserved Faculty of Civil and Environmental Engineering School of Engineering and Natural Sciences University of Iceland Dunhagi 5 107 Reykjavik Iceland Telephone: 525 4000 Bibliographic information: Elín Ásta Ólafsdóttir, 2016, Multichannel Analysis of Surface Waves for assessing soil stiffness, M.Sc. thesis, Faculty of Civil and Environmental Engineering, University of Iceland. Printing: Háskólaprent, Fálkagata 2, 107 Reykjavík Reykjavik, Iceland, January 2016 Abstract Knowledge of the geotechnical properties of subsoil sites is essential in various civil engineering projects. Multichannel Analysis of Surface Waves (MASW) is a relatively new method where Rayleigh waves are generated and used to infer the shear wave velocity profile of the test site. Based on this the stiffness of the soil stratum can be estimated. MASW is a low-cost method. It is further non-invasive and environmentally friendly since it neither requires heavy machinery nor leaves lasting marks on the surface of the test site. Moreover, the MASW method provides information down to greater depth than SASW, a comparable method that until now has been applied in Iceland. MASW is divided into three steps: field measurements, dispersion analysis and inversion analysis. The main objective of this project is to develop and test a new set of software tools for analysis of MASW field data and to apply the methodology to estimate site-specific shear wave velocity profiles at Icelandic test sites. Results of field measurements carried out at two locations in South Iceland are presented to demonstrate the performance of the MASW software tools. The field measurements were performed using diverse receiver setups. The results indicate that the measurement profile configuration has significant effect on the quality of the acquired data and that it is beneficial to combine dispersion curves obtained from several different records prior to the inversion analysis. Validation of the field observations was done by comparison with results obtained by SASW and profiles estimated based on empirical correlations. v Útdráttur Þekking á jarðtæknilegum eiginleikum setlaga og jarðvegsfyllinga, svo sem þykkt og stífni einstakra laga og heildarþykkt, er nauðsynleg í mannvirkjagerð. Fjölnema- greining á yfirborðsbylgjum (MASW) er nýleg aðferðafræði sem byggir á tvístrunar- eiginleikum yfirborðsbylgna í lagskiptum jarðvegi og tengslum á milli útbreiðsluhraða þeirra og fjaðureiginleika jarðvegs. Yfirborðsbylgjur eru framkallaðar með höggi á yfirborð jarðar, útbreiðsla þeirra mæld með röð hraðanema og gögnin, ásamt eðlisfræðilegu reiknilíkani, notuð til að ákvarða skúfbylgjuhraða og skúfstuðul sem fall af dýpi. Kostir MASW mælinga felast meðal annars í því að þær eru ódýrar og fljótlegar í framkvæmd. Einnig eru þær umhverfisvænar, þar sem þær valda hvorki skemmdum á yfirborði prófunarstaðar né krefjast þungs vélbúnaðar. Auk þess hefur með MASW aðferðinni tekist að ákvarða stífni jarðlaga á meira dýpi en með sambærilegri mæli- aðferð (SASW) sem til þessa hefur verið notuð hér á landi. Meginmarkmið verkefnisins er að innleiða og þróa MASW yfirborðsbylgjuaðferðina. Felur það í sér þróun úrvinnsluhugbúnaðar til að ákvarða tvístrunarferla út frá mæligögnum, auk líkanhugbúnaðar til að bakreikna tvístrunarferil og ákvarða skúf- bylgjuhraða/stífni jarðlaga sem fall af dýpi. Gerð er grein fyrir MASW mælingum sem framkvæmdar voru á tveimur stöðum á Suðurlandi. Niðurstöður gefa til kynna að hagstætt sé að byggja bakreikninga á meðaltalstvístrunarferli prófunarstaðar, sem ákvarðaður er út frá gögnum sem aflað er með mismunandi nemauppstillingum. Niðurstöður MASW mælinganna voru bornar saman við niðurstöður SASW mælinga og reynslulíkingar fyrir skúfstuðul jarðvegs. vii Contents List of Figures xiii List of Tables xxi Notation and symbols xxiii Acknowledgements xxxiii 1. Introduction 1 1.1. Background . .1 1.2. Objectives . .3 1.3. Overview . .4 2. Mechanical properties of soil 7 2.1. Empirical correlations for the small strain shear modulus . 10 3. Seismic waves 15 3.1. Body waves . 15 3.2. Surface waves . 16 3.2.1. Rayleigh waves in homogeneous elastic half-space . 17 3.2.2. Rayleigh waves in vertically heterogeneous elastic half-space . 21 4. Surface wave analysis methods 23 4.1. Spectral Analysis of Surface Waves . 24 4.2. Multichannel Analysis of Surface Waves . 26 4.2.1. Advantages of the MASW method . 31 4.2.2. Two dimensional MASW surveys . 32 4.2.3. Passive MASW surveys . 32 5. Field measurements 35 5.1. Field procedures and measurement equipment . 35 5.2. Configuration of the measurement profile . 38 5.2.1. Length of receiver spread . 38 5.2.2. Receiver spacing . 39 5.2.3. Source offset . 39 5.2.4. Summary of recommended profile-setup parameters . 40 ix Contents 5.2.5. Topographical conditions . 40 6. Dispersion analysis 43 6.1. Swept-frequency approach . 43 6.1.1. Swept-frequency record . 45 6.1.2. Computation of a dispersion curve . 48 6.2. Phase shift method . 55 6.2.1. Fourier transformation and amplitude normalization . 57 6.2.2. Dispersion imaging . 59 6.2.3. Extraction of experimental dispersion curves . 64 6.3. Computation of average experimental dispersion curves . 69 6.4. Comparison of dispersion analysis algorithms . 70 7. Inversion analysis 73 7.1. Layered earth model and model parameters . 74 7.2. General inversion algorithms . 76 7.3. Computation of theoretical dispersion curves . 77 7.3.1. Thomson–Haskell method (Transfer matrix method) . 78 7.3.2. Stiffness matrix method . 82 7.4. Inversion analysis software tool . 86 7.4.1. Initial estimation of model parameters . 87 7.4.2. Computation of a theoretical dispersion curve . 90 7.4.3. Error estimation and misfit minimization . 91 7.5. Validation of dispersion curve computations . 93 8. Field tests 95 8.1. MASW measurement equipment . 95 8.2. MASW measurements at Arnarbæli . 97 8.2.1. Arnarbæli test site A1 . 99 8.2.2. Arnarbæli test site A2 . 104 8.3. MASW measurements at Bakkafjara . 108 8.3.1. Bakkafjara test site B1 . 110 8.3.2. Bakkafjara test site B2 . 118 9. Discussion 129 9.1. Observed effect of MASW measurement profile configuration . 129 9.1.1. Length of receiver spread (receiver spacing) . 129 9.1.2. Source offset . 132 9.1.3. Type of seismic source . 136 9.2. Comparison of MASW and SASW measurement results . 137 9.2.1. Measurements at Arnarbæli . 137 9.2.2. Measurements at Bakkafjara . 139 9.3. Evaluation of stiffness profiles and comparison to empirical models . 142 x Contents 10.Summary and future work 147 10.1. Software for analysis of MASW field data . 147 10.1.1. Dispersion analysis software tool . 147 10.1.2. Inversion analysis software tool . 148 10.2. Measurement profile configuration . 150 10.3. Comparison . 151 10.3.1. Comparison to results of SASW measurements . 151 10.3.2. Comparison to empirical correlations . 152 10.4. Future research topics . 152 References 157 A. Dispersion image resolution 163 A.1. Length of receiver spread . 163 A.2. Source offset . 165 A.3. Type of seismic source . 168 B. Empirical stiffness profiles 171 xi List of Figures 1.1. Natural sites in Iceland where MASW field data have been acquired.3 2.1. Variation of soil properties with strain, after Ishihara (1996). .7 2.2. Stress–strain curve. .8 3.1. Particle motion associated with compressional waves (Bolt, 1976). 15 3.2. Particle motion associated with shear waves (Bolt, 1976). 15 3.3. Particle motion associated with Rayleigh waves (Einarsson, 1991). 16 3.4. Particle motion associated with Love waves (Einarsson, 1991). 17 3.5. Distribution of compressional, shear and Rayleigh waves generated by a point load in a homogeneous, isotropic, elastic half-space (Woods, 1968). 18 3.6. Displacement amplitude of Rayleigh waves versus dimensionless depth (Richart et al., 1970). 18 3.7. Variation of compressional, shear and Rayleigh wave propagation velocities in a homogeneous medium with Poisson’s ratio. 20 4.1. Rayleigh wave components with different wavelengths propagating through a layered medium. Wave components with different frequen- cies reflect soil properties at diverse depths. 23 4.2. Example of a SASW measurement profile. Line-up of ten geophones with equal spacing. 24 xiii LIST OF FIGURES 4.3. Example of a SASW measurement profile. Symmetrical line-up of ten geophones with unequal spacing. 25 4.4. SASW data processing. Choice of receiver pairs for analysis. 25 4.5. Overview of the MASW method. 27 5.1. Example of a MASW measurement profile. 36 5.2. Effects of recording time in MASW surveys. Phase velocity spectra obtained by the phase shift method from surface wave records acquired with a recording time of (a) 1.2 s and (b) 2.2 s. 37 5.3. Effects of topography on the quality of the recorded multichannel surface wave data. 41 6.1. Overview of the swept-frequency approach. 44 6.2. Convolution of a trace from an impulsive multichannel surface wave record with a stretch function. (a) Stretch function. (b) Recorded data (single trace). (c) Resulting swept-frequency trace. 48 6.3. Pseudo swept-frequency record obtained by convolution. 50 6.4. Swept-frequency record. Local maxima of each swept-frequency trace identified. 51 6.5.
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