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S24 naldi

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Mobile system for seismic investigation in urban area

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S24 naldi

  1. 1. 87° Congresso della Società Geologica Italiana e 90° Congresso della Società Italiana di Mineralogia e Petrologia 10 - 12 Settembre 2014 - Milano A new prototype system for MASW2D roadside survey Mario Naldi, Emmanuele Duò – Techgea Srl (info@techgea.eu)
  2. 2. GROUND INVESTIGATION IN URBAN AREAS: A NEW FIELD FOR THE NEAR SURFACE GEOPHYSICS Urban areas represent some of the most challenging environments for civil works: •Metro lines •Underground utilities (sewage, pipe lines, heating system) •Underground parking In such a context, the geophysical surveys can play an important role for the assessment of the geology and soil mechanics properties
  3. 3. Some “obstacles” should be considered in the survey design : 1.Restricted space, 2.the necessity for traffic management, 3.high ambient noise (electrical and vibration), 4.underground utilities, … all THESE issues help to complicate the design and setting out of the geophysical profiles or grids. HOW TO SELECT THE RIGHT METHODOLOGY
  4. 4. The conventional seismic surveys (refraction or reflection) can have many limitations in urban areas: •do not work well in noisy areas •take long time for data acquisition (hard traffic management) •require high energy source Among geophysical methods, the seismic survey is the most commonly applied for civil engineering projects. MASW 2D profiling has many advantages for surveys in urban areas: •More energetic wavelets (less energy required) •Short acquisition time with just one shot and fast move of the geophone layout (no stop of the road traffic) •Favorable ratio between the length of the seismic line and the soil depth. MASW2D = 1:1  seismic refraction = 5:1 Seismic surveys in urban areas
  5. 5. Among geophysical methods, the seismic survey is the most commonly applied for civil engineering projects. Seismic sources generate a continuous spectrum of surface wave frequencies. Where there is vertical velocity variation, different frequency waves travel at different phase velocities. This is dispersion. By examining dispersion curves (phase velocity vs. frequency), we can determine material properties (e.g., layer thickness, depth, Vs) via Inversion BASIC THEORY OF MASW Lower frequency  deeper strata DISPERSION INVERSION
  6. 6. Low density shallow soil Gravel and sand (not cemented) Gravel and sand (partially cemented) Masw2D (Vs - m/s) Seismic refraction (Vp – m/s) Low density shallow soil MASW2D HAS A HIGHER VERTICAL AND LATERAL RESOLUTION COMPARED WITH THE REFRACTION METHOD Velocity inversion in the subsurface is one of the most serious limitations of the shallow seismic refraction method  GEOPHYSICAL IMAGING HAS A POOR HIDDEN HIDDEN
  7. 7. How to improve MASW 2D survey efficiency in urban areas? 1.Mobile system  continuous profiling (no traffic interruption) 2.High seismic energy  bypass of the urban noise Slow motion (low gear) STOP: One shot and recording (20 seconds) GO: Slow move to the next station (2 m) String of geophones on a land streamer Source of energy (ACCELERATED WEIGHT) Seismograph on board (DATA RECORDING) The streamer is towed at low speed with stop at every recording station
  8. 8. Winch (powered by a 100 Ah/12 V Battery) Traction springs Propelled mass Uncoupling devices Source of energy (ACCELERATED WEIGHT) Lift The prototype has been designed and produced for Techgea by TREFFE
  9. 9. Land streamer Sensors (4.5 hz geophones) mounted on a metal plate) String of geophones on a land streamer Source of energy (ACCELERATED WEIGHT)Seismograph on board (DATA RECORDING) The streamer is towed at low speed with stop at every recording station
  10. 10. Distance [m] Küçükbakkalköy station projection COLOUR SCALE Shear wave velocity [m/s] Made ground and/or soft soil [Vs: 0÷200 m/s] Dense soil and/or overburden [Vs: 200 – 370 m/s] Very dense soil/soft rock [Vs: 370÷800 m/s] Rock [Vs: 800÷1200 m/s] Hard rock [Vs > 1200 m/s] 800 m/s 370 m/s 200 m/s 1200 m/s Remark: soil classification from ASCE table (7-2005), (modified) Fault zone Solid rock (LIMESTONE) Fractured rock Fractured rock Weathered rock Top soil Line M5-13 - Shear Wave section Weathered rock Küçükbakkalköy station EXAMPLE 1 – ISTANBUL METRO LINE BOSTANCI – DUDULLU (TURKEY) Masw2d SURVEY FOR THE GEOLOGICAL ASSESSMENT AND THE SEISMIC SOIL CLASSIFICATION Fault zone THE MASW2D LINE IS POSITIONED ALONG A ROAD BETWEEN THE PROJECTION OF THE TWIN TUNNELS OVERNIGHT SURVEY
  11. 11. EXAMPLE 2 – SEWER PIPE SETTLEMENT (MILAN AREA, ITALY) ROAD SUBSIDENCE AFTER THE PIPELINE POSITIONING CEMENTED ZONE (GROUTING BELOW THE DEFORMED ROAD) DENSE GRAVEL VERY LOW DENSITY SAND- GRAVEL/VOIDS SUBSIDENCE AREA SUBSIDENCE AREA SUBSIDENCE AREA
  12. 12. EXAMPLE 3 – SLOPE FAILURES ASSOCIATED WITH ABANDONED UNDERGROUND QUARRIES (Alessandria, Italy) ROAD SUBSIDENCE AND DEFORMATION BA Siltstone (soft rock) Highly weathered siltstone (silt consistency) Landslide projection (IFFI) S1 Very low density zone (collapsed tunnels) Tunnel Subsidence area Subsidence area
  13. 13. 87° Congresso della Società Geologica Italiana e 90° Congresso della Società Italiana di Mineralogia e Petrologia 10 - 12 Settembre 2014 - Milano A new prototype system for MASW2D roadside survey Mario Naldi, Emmanuele Duò – Techgea Srl Thank you for the attention

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