Non-invasive Geophysical Investigation for Subsurface Profiling in Urbanized Megacities

Transcription

1 Non-invasive Geophysical Investigation for Subsurface Profiling in Urbanized Megacities Arindam Dey Assistant Professor Civil Engineering Department Indian Institute of Technology Guwahati nd KGS-IGS Workshop, Seoul, Introduction Invasive geotechnical investigations are unaffordable in urbanized areas damage to existing structures Non-invasive geophysical investigations Seismic exploration methods Evaluate the ground stiffness profiles Utilizes the wave propagation characteristics through soil Commonly applied techniques Seismic refraction survey Seismic reflection survey Nuclear/Proton magnetic resonance survey Ground penetrating radar SASW and MASW 1

2 nd KGS-IGS Workshop, Seoul, Introduction Advantage over conventional boring and drilling Drilling and boring technique is excessively costly Spacing between boreholes have every possibility of missing the subsurface profile variation nd KGS-IGS Workshop, Seoul, Target of NDT Geophysical Surveys Identification of the subsurface shear wave velocity (small-strain V s ) in 1-D, 2-D or 3-D formats 2-D 2

3 nd KGS-IGS Workshop, Seoul, Surface Wave Techniques Waves generated from various sources Sledge Hammer Automatic Generator Traffic noise Ambient vibration Collected by geophone receivers Field layout configurations Analysis of the obtained records Various pre-processing characteristics Different analysis techniques Analyzed for propagation velocities Shear wave velocity pattern Affected by several data acquisition and processing parameters nd KGS-IGS Workshop, Seoul, Overall Procedure Signal records 3

4 nd KGS-IGS Workshop, Seoul, SASW Approach Spectral Analysis of Surface Waves (SASW) Two-receiver approach Hammer or weight-drop as impact energy source Operates on the cross-power spectrum of the signals recorded by the two receivers Only the fundamental (M0) dispersion curve is considered nd KGS-IGS Workshop, Seoul, Applicability SASW Approach Good resolution information is obtained for very shallow strata thicknesses (less than 4-5 m) Pavements and extremely shallow foundations Uncertainties Noise at low frequencies Poor data quality Heterogeneity of soil layer and presence of obstacles Scatter in dispersion curve Prominent far-field and near-field effects Attenuation of surface wave and Intrusion of body waves, respectively 4

5 nd KGS-IGS Workshop, Seoul, SASW Approach Generation of good quality data Coherence function based quality check Frequency filtering or masking based on threshold power Cumulative weight stacking for enhancing data quality nd KGS-IGS Workshop, Seoul, SASW Approach Final outcome Dispersion curve based on various geometrical layouts, satisfying Heisey s filtering criterion Inversion of combined dispersion curves using Occam s algorithm Mukherjeeand Prashant(2013) 5

6 nd KGS-IGS Workshop, Seoul, Schematic and Instrumentations Multi-receivers placed in a linear array 10 kg Sledgehammer 40 kg PEG nd KGS-IGS Workshop, Seoul, Generation of unimodal or multimodal dispersion image space (frequency-phase velocity-energy space) Dispersion characteristics of wave propagation 6

7 nd KGS-IGS Workshop, Seoul, Ascertaining the collected data quality Governed by sampling parameters (sampling frequency and time of acquisition) nd KGS-IGS Workshop, Seoul, Ascertaining the collected data quality Governed by receiver layout (near-field and far-field effect) 7

8 nd KGS-IGS Workshop, Seoul, Enhancing the collected data quality Filter type (Band-pass, Band-stop, high-cut, Low-cut) nd KGS-IGS Workshop, Seoul, Enhancing the collected data quality Combined application of band-pass filtering and muting 8

9 nd KGS-IGS Workshop, Seoul, Enhancing the collected data quality Input energy and dispersion image stacking nd KGS-IGS Workshop, Seoul, Final outcome Good resolution shear wave velocity up to m Cater the needs of the founding depths of pile foundations Portability of impact hammer is an issue Shear wave velocity (m/s) Depth (m) V s from 1 m reciever spacing V s from 2 m reciever spacing V s from 3 m reciever spacing 9

10 nd KGS-IGS Workshop, Seoul, Passive Roadside MASW Approach Utilizes the roadside traffic movement for estimating the subsurface characteristics Modified form of Active MASW survey Source is generally offline Azimuth determination Fundamental and higher modes are considered nd KGS-IGS Workshop, Seoul, Passive Roadside MASW Approach Enhancing the data quality Removal of noise and contaminated traces by noise analysis B Higher Modes A 10

11 nd KGS-IGS Workshop, Seoul, Passive Roadside MASW Approach Final outcome Governed by undulations in the road, weight of the traffic vehicles, speed and traction forces The depth of investigation can vary from 10 m 30 m nd KGS-IGS Workshop, Seoul, Passive Remote Survey Approach Long wavelength ambient vibrations are used as the stimulating source (Long duration survey in serenity) Uses varied patterns of geophone layouts (Space may be an issue) 11

12 nd KGS-IGS Workshop, Seoul, Passive Remote Survey Approach Enhancing the data quality Ambient vibrations are of very low energy More stacking required nd KGS-IGS Workshop, Seoul, Passive Remote Survey Approach Enhancing the data quality Dependent upon shape of the array Circular proves to be best using simple testing configurations Circular Triangular Square 12

13 nd KGS-IGS Workshop, Seoul, Roll-along survey 2-D V s Profiling Changing the position of the array relative to the source or the original position of the array nd KGS-IGS Workshop, Seoul, Conclusions Geophysical NDT tests (SASW and MASW) can be effective in determining the shear wave profiling of the subsurface Such information can be extremely helpful in various geotechnical assessments in a highly urbanized area without damaging the existing structures Tests are relatively quick and the depth of investigation can be controlled based on the guidelines set for the field investigation and numerical analysis Choice of the method should be as per requirement 13

14 nd KGS-IGS Workshop, Seoul, Thank You 14