JJ M Brouwer

The vibrating wire piezometer (Figure 48) consists of a vibrating wire
pressure transducer and signal cable. It can be installed in a borehole,
embedded in fill or suspended in a standpipe. Readings are obtained
with a portable readout or a data logger. They are easy to read and very accurate. The response of the instrument to changes in pore pressure is good in all soils. As the measurement is obtained in terms of frequency it can be transmitted over long cable lengths and the
readings can be readily automated. The instruments must be protected
from electrical transients.
 
Applications
Typical applications are for monitoring:

• porewater pressures to determine safe rates of fill or excavation
   
• porewater pressures to determine slope stability
   
• effects of dewatering systems used for excavations
   
• effects of ground improvement systems (eg vertical and sand drains)
   
• pore pressures to check the performance of earth-fill dams and
  embankments
   
• pore pressures to check containment systems at landfills and tailings dams.

  
Installation
Grout-in method  The piezometer is lowered to the specified depth in
the borehole. The borehole is then filled with a bentonite–cement grout.
This method is particularly useful when multi-point measurements are
required in a single borehole.
 
Sand filter method  The borehole is flushed with water and a sand
filter is placed around the piezometer which is positioned at the specified depth. A bentonite plug is formed at the top of the sand filter. The remainder of the borehole is then filled with a bentonite–cement grout.
 
Push-in method  The push-in piezometer is generally pushed into soft,
cohesive soil using CPT equipment. The piezometer must be monitored
during installation to ensure that it is not over-pressured as it is pushed in.
 
Operation
The vibrating wire piezometer converts water pressure to a frequency
signal via a diaphragm, a tensioned steel wire and an electromagnetic
coil. The piezometer is designed so that a change in pressure on the
diaphragm causes a change in tension of the wire. When excited by the
electromagnetic coil, the wire vibrates at its natural frequency. The
vibration of the wire in the proximity of the coil generates a frequency
signal that is transmitted to the readout device. The readout device
processes the signal, applies calibration factors, and displays a reading
in the required engineering unit.