10.5.1 Standpipe Piezometer
These piezometers are used to monitor piezometric water levels using observational well. A standpipe piezometer consists of a filter tip joined to a riser pipe (Figure 9). The filter tip is made of polyethylene or porous stone of 60 micron pores size. Riser pipe are installed downhole after the filter tip. A sand filter zone is tremied into place around the filter tip. The top of the filter zone is sealed with bentonite to isolate the porewater at the tip. The annular space between the riser pipe and the borehole is backfilled to the surface with a bentonite grout to prevent vertical migration of water. The riser pipe is terminated above ground level with a vented cap. Water levels in either the standpipe piezometer or the observation well are measured with a water level indicator.
A 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. Contact is signaled by a light and buzzer built into the cable reel. The water level reading is taken from the cable or tape. Typical applications of standpipe piezometer include monitoring of pore-water pressure to determine the stability of slopes and embankments and assessment of ground improvement techniques such as vertical drains, sand drains, and dynamic compaction.
Figure 9: Schematic of a standpipe Piezometer.