SLOPE INDICATOR - PNEUMATIC PIEZOMETERS

Application

Pneumatic piezometers are used to measure pore water pressure in saturated soils. Applications include:

• Monitoring pore pressures to determine safe rates of fill or excavation.
• Monitoring pore water pressures to determine slope stability.
• Monitoring the effects of dewatering systems used for excavations.
• Monitoring the effects of ground improvement systems such as vertical drains and sand drains.
• Monitoring pore water pressures to check the performance of earth fill dams and embankments.
• Monitoring pore water pressures to check containment systems at land fills and tailings dams.

Advantages

Operating Principle

In a typical installation, the piezometer is sealed in a borehole, embedded in fill, or suspended in a standpipe. Twin pneumatic tubes run from the piezometer to a terminal at the surface. Readings are obtained with a pneumatic indicator.

The piezometer contains a flexible diaphragm. Water pressure acts on one side of the diaphragm and gas pressure acts on the other. When a reading is required, a pneumatic indicator is connected to the terminal or directly to the tubing. Compressed nitrogen gas from the indicator flows down the input tube to increase gas pressure on the diaphragm.

When gas pressure exceeds water pressure, the diaphragm is forced away from the vent tube, allowing excess gas to escape via the vent tube. When the return flow of gas is detected at the surface, the gas supply is shut off.

Gas pressure in the piezometer decreases until water pressure forces the diaphragm to its original position, preventing further escape of gas through the vent tube. At this point, gas pressure equals water pressure, and the pneumatic indicator shows the reading on its pressure gauge.