What is a piezometer?
A piezometer is an instrument that measures pore water pressure, the pressure of the water in the pores of the soil, at a specific depth. Together with the total stress, pore water pressure determines how much stress the soil grains themselves carry (the effective stress), and therefore the strength and stability of the ground. Anyone checking whether an embankment, excavation or fill is stable needs to know the pore water pressure.
Types
- Standpipe (open) piezometer: a tube with a short filter tip at the required depth. The borehole is sealed above and below the tip, usually with bentonite, so only the pressure at that depth is measured. Readings are taken with a dip meter. Simple and reliable, but slow to respond in low-permeability soils.
- Closed (electric) piezometer: a pressure sensor buried in the ground, such as a vibrating wire or piezoresistive transducer. Responds quickly and can be read continuously with a data logger.
- Pore pressure during cone testing: in a piezocone test (CPTU) a pressure transducer is built into the cone, giving a continuous profile that also helps identify soil type.
Difference from a monitoring well
A groundwater monitoring well usually has a longer screen and shows the water level in an aquifer. A piezometer measures the pressure at one point. In layered ground with artesian pressure or excess pore water pressure, the two can differ considerably.
Uses
- Flood embankments and dikes: monitoring during high water, as rising pore pressure lowers stability.
- Excavations and dewatering: checking that dewatering works and whether there is a risk of base heave.
- Fills on soft ground: tracking how excess pore water pressure dissipates (consolidation) before the next layer is placed.
Related terms
- Excess pore water pressure
- Groundwater monitoring well
- Groundwater level
- Groundwater seepage
- Dewatering
- Pressure relief dewatering
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