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Vibrating Wire Piezometer

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priyainuk2009

priyainuk2009

Geotechnical
Joined
Jan 20, 2010
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Location
IN
We have installed Vibrating Wire Piezometer in Underground Metro site construction at 25m depth from ground.
If the pore pressure as per Vibrating Wire Piezometer reading is 0.16 MPa presently where is the GWT presently?
Though as per bore logs observations GWT is 3-4m below ground surface we have assumed GWT at ground surface for design.
When we installed the piezometer in borehole we found that the Pore pressure recorded is 0.25MPa and soon after the borehole is filled with sand and sealed with bentonite the pressure dropped to 0.177MPa on the next day.The pressure is gradually decreasing now.
Can we interpret any Consolidation phenomenon during construction period due to drop in pore pressure from base readings?
Regards
Debasis
 
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In case you haven't caclulated the head of water from the pressure, it's simply 0.16*10e6Pa/9.81m/s2/1000kg/m3, about 16.3 m. If you assume this is a measurement of the static phreatic surface (GWT), then it would be 8.7m below the surface.

Not sure how to explain the increasing/decreasing pressure since you apparently encapsulated the VWP in sand with a bentonite seal. This behavior would be very typical for VWPs installed in fully grouted drill holes, which I personally prefer to sand/bentonite. I doubt there would be much consolidation of the sand pack, especially over the span of a few days.

Over what time frame have you observed the readings you mentioned?
 
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