JJ M Brouwer

 
 
The standpipe piezometer is used to monitor piezometric water levels.
It consists of a filter tip joined to a riser pipe. Readings are obtained
with a water level indicator.
 
Standpipe piezometers are simple, reliable and relatively inexpensive.
The accuracy depends on the skill of an operator. They can be very slow
to respond to changes in porewater pressure since large fluid volumes
are required to change the level in the standpipe. It is not usually
possible to monitor these instruments remotely.
 
Applications
Typical applications are for monitoring:

• porewater pressures to evaluate slope stability
   
• ground improvement techniques (eg vertical drains, sand drains, and
  dynamic compaction)
   
• dewatering schemes for excavations and underground openings
   
• pore pressures to check the performance of earth-fill dams and
  embankments
   
• pore pressures to check containment systems at landfills and tailings
  dams
   
• water drawdown during pumping tests.

Installation
The standpipe piezometer, which is either installed in a borehole or
pushed into position using CPT equipment, consists of a filter tip joined
to a riser pipe. In a borehole the filter tip is placed in a sand zone and a
bentonite seal is placed above the sand to isolate the porewater
pressure at the tip. The annular space between the riser pipe and the
borehole is backfilled to the surface with a bentonite grout. This is to
prevent unwanted vertical migration of water.
 
Where the push-in method is used there is usually no need to backfill
around the riser pipe with bentonite grout. The riser pipe is terminated
above ground level with a vented cap.
 
Operation
Water levels in the standpipe piezometer are measured with a water
level indicator. The water level indicator consists of a probe, a
graduated cable or tape, and a cable reel with built-in electronics. The
probe is lowered down the standpipe until it makes contact with water.
This is signalled by a light and a buzzer built into the cable reel. The
depth-to-water reading is taken from the cable or tape. The water level
indicator features a sensitivity adjustment which helps the user obtain
consistent measurements and eliminates false triggering.