87° Congresso della Società Geologica Italiana e90° 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)

GROUND INVESTIGATION IN URBAN AREAS: A NEW FIELD FOR THE NEAR SURFACE GEOPHYSICSUrban 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

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

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 appliedfor 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

Among geophysical methods, the seismic survey is the most commonly appliedfor 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

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 RESOLUTION

HIDDEN

HIDDEN

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

Winch (powered by a 100 Ah/12 VBattery)

Traction springs

Propelled mass

Uncoupling devices

Source of energy(ACCELERATED

WEIGHT)

LiftThe prototype has been designed and produced for Techgea by TREFFE Sas

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

Distance [m]

Küçükbakkalköy station projection

COLOUR SCALEShear 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

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 AREASUBSIDENCE AREA

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

87° Congresso della Società Geologica Italiana e90° 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

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