What is a piezometer, and what does it measure?

A piezometer is an instrument that measures pore water pressure in soil at a specific depth. The tip is installed in a borehole, and the measuring zone is sealed off from other water-bearing layers with bentonite, so the reading is the water pressure of that soil layer alone, not mixed with water from layers above or below. The piezometer is one of the 10 main geotechnical instruments; read the overview of the whole system in the article What is geotechnical instrumentation?

Water pressure matters because soil strength does not depend directly on the overburden load, but on the effective stress, which equals the total stress minus the water pressure, written as σ′ = σ − u. When the water pressure (u) rises, the effective stress (σ′) falls, and the shear resistance of the soil falls with it.

Many deep excavations, embankments and slopes fail because the water pressure differs from what was assumed in design. Measuring the actual water pressure in the field therefore provides data used directly to judge the safety of the works.

How does a piezometer differ from an observation well?

A piezometer differs from an observation well in the width of its "measuring zone." An observation well has a screen along almost its full length, so water from every layer the pipe passes through flows in and mixes, and the reading is an averaged combined water level. A piezometer seals off a narrow measuring zone, so the reading is the water pressure at the selected depth only.

Work that needs layer-by-layer values therefore requires piezometers, for example in areas with several stacked aquifers, or when monitoring pore water pressure in clay layers during filling. For a detailed comparison with the internal construction of an observation well, read the article Observation wells for groundwater level monitoring. For an example of a project that installed both types side by side, see Case study: 8 m piezometer and monitoring well, Samut Prakan

How does a Standpipe (Casagrande) piezometer work?

A Standpipe piezometer (Casagrande type) is a small-diameter standpipe with a porous tip at its lower end, lowered into a borehole to the depth to be measured. The tip is surrounded by filter sand, a bentonite seal is placed above it to close off the measuring zone, and the rest of the hole is backfilled up to the surface. Water from the soil layer seeps through the tip into the pipe until the water level in the pipe is in equilibrium with the water pressure of that layer.

The water level in the pipe therefore represents the water pressure at the tip. The water pressure at the tip equals the unit weight of water multiplied by the height of water above the tip (u = γw × h). A 1 m column of water is equivalent to a pressure of about 9.81 kPa, or roughly 1 t/m².

Readings are taken by lowering a dip meter into the pipe. When the probe touches the water surface, the instrument beeps; the depth is read from the measuring tape and converted back to water pressure. The advantages are a simple, robust design that lasts for decades and makes faults easy to check.

The main limitation is the response delay (time lag). In low-permeability soils such as clay, water flows into the pipe slowly, so the reading lags behind the actual pressure. In addition, someone has to go to each point to take readings every round.

Standpipe piezometer installation drawing in a borehole, showing a 50 mm protective casing and a concrete pad at the collar, a 17 mm PVC pipe, cement-bentonite backfill, a bentonite pellet seal over the measuring zone at 14.00–14.50 m, and filter sand around the tip from 14.50–15.50 m
Standpipe piezometer installation drawing — the filter sand around the porous tip is the sensing zone, while the bentonite pellets above seal the measuring zone off from other water-bearing layers (drawing: SPN Soil Engineering)
SPN team drilling to install a piezometer at a pond-side site, with a project sign identifying the piezometer installation work
Drilling to install a piezometer in the field — the measuring zone must be packed with filter sand around the tip and then sealed with bentonite to isolate it from other water-bearing layers (photo: SPN Soil Engineering)

How does a Vibrating Wire piezometer work?

A Vibrating Wire piezometer is a pore pressure sensor based on the vibrating wire principle. Inside the sensor, a wire is tensioned between a diaphragm and the body. When water pressure pushes on the diaphragm, the wire tension changes and so does its natural frequency. The readout unit sends a signal to excite the wire, measures the frequency, and then converts it back to water pressure.

The frequency signal is highly resistant to interference over long cables, so the cable can be run from a deeply buried sensor up to the borehole collar for reading, or connected to a datalogger for automatic remote reading.

The strength that makes the Vibrating Wire type the main choice for monitoring work is its response speed. The diaphragm needs only a very small volume of water to move, so pressure changes are read almost instantly, even in clay where a standpipe would need several days.

Before installation, each sensor must pass a pre-installation acceptance test following ASTM D7764-20 to confirm that it works correctly before it is buried, because once buried it cannot be retrieved for inspection or repair.

Installation drawing of multi-level Vibrating Wire piezometers in a single borehole; each sensor is surrounded by filter sand and sealed with bentonite above and below, with cement-bentonite at a 1:5:16 ratio by weight between zones, plus a detail of the 113 mm sensor connected to PVC cable up to the collar
Installation drawing of multi-level Vibrating Wire piezometers in a single borehole — each measuring zone is sealed with bentonite above and below and separated by cement-bentonite, so that water pressures from different layers do not leak into one another (drawing: SPN Soil Engineering)

Sensor depths are not chosen as round numbers but according to the soil layers whose pressure needs to be known. The drawing below places the borehole soil profile next to the installation drawing, showing that the sensor is set in the lower clayey sand layer, surrounded by sand, capped with bentonite balls, and then the remaining length is grouted with bentonite-cement grout.

Vibrating Wire piezometer installation drawing compared with the borehole soil profile, from top to bottom: crushed rock, yellowish-brown silty sand, yellowish-brown clayey sand, light gray clayey sand, light brown sandy clay and grayish-brown sandy clay; the sensor is embedded in the lowest layer, surrounded by sand, capped with bentonite balls, and the full length of the hole is grouted with bentonite-cement
Sensor position is chosen by soil layer, not by round-number depth — the borehole soil profile (left) determines which layer the sensor goes in, while the bentonite-cement grout filling the full length (right) prevents water from upper layers flowing down and disturbing the readings (drawing: SPN Soil Engineering)
SPN staff reading a piezometer with a field readout, with the signal cable and a dip meter on site
Reading a piezometer in the field with a portable readout — in practice it is often read together with a dip meter water level check for cross-verification (photo: SPN Soil Engineering)

Standpipe or Vibrating Wire: which should you choose?

Choose a standpipe for robust long-term monitoring points on a limited budget. Choose Vibrating Wire when values change quickly, the sensor is in clay, or readings must be automated. Both types measure the same water pressure, but they suit different kinds of work.

The 2D diagram below shows the boreholes of both types side by side. Note 3 points. First, the backfill materials are the same for both types: filter sand, bentonite seal and cement-bentonite. Second, how the value is obtained: the standpipe is read from the water level in the pipe, while the Vibrating Wire sends its reading through a signal cable. Third, the small chart at the bottom compares the response speed when the pore water pressure in the soil changes.

2D model comparing the installation and reading of a Standpipe piezometer and a Vibrating Wire piezometer in boreholes Conceptual cross-section of two boreholes in a clay layer overlying a sand layer. The left borehole is a Casagrande-type Standpipe piezometer with a porous tip at the end of the standpipe, surrounded by filter sand that forms the measuring zone, capped with a bentonite seal, and backfilled with cement-bentonite up to the ground surface. The water in the standpipe rises to the pressure level of the soil layer at the tip; the height of water above the tip is h, and the water pressure equals the unit weight of water times h. Readings are taken with a dip meter lowered into the pipe. The right borehole is a Vibrating Wire piezometer with its sensor embedded in filter sand at the same depth, capped with the same bentonite seal and cement-bentonite. The signal cable runs up to the borehole collar to a readout box or datalogger. The bottom strip summarizes the differences: the standpipe responds slowly in clay and must be read manually, but is robust; the Vibrating Wire responds almost instantly and can be read automatically, but cannot be repaired once buried. The small chart in the middle shows that when the pore water pressure changes, the Vibrating Wire reading follows almost immediately, while the standpipe gradually catches up. 2D model: Standpipe vs Vibrating Wire Not to scale A Standpipe (Casagrande) Water level in pipe read by dip meter B Vibrating Wire Vibrating-wire sensor, cable output Dip Meter Lowered to water 1 Standpipe Water rises in pipe 2 Cement-bentonite Backfill to surface 3 Bentonite Seals the zone 4 Filter sand Sensing zone 5 Porous tip Porous Tip 6 Readout / Datalogger Manual or automatic 7 Signal cable Sensor to collar 8 VW sensor Diaphragm + wire Needs very little water 2 3 4 Clay (low perm.) Sand h Water level in pipe = pressure level of layer at the tip u = γw × h Water pressure at tip Water must seep in to balance A Standpipe Slow response in clay (time lag) Manual reading, point by point Robust, no electronics in ground B Vibrating Wire Near-instant response Datalogger for auto reading Sensor not repairable once buried When pore pressure changes B: Vibrating Wire A: Standpipe Time → Conceptual
2D model comparing Ⓐ Standpipe (Casagrande) and Ⓑ Vibrating Wire in a clay layer: ① standpipe with water rising to the pressure level of the soil layer at the tip ② cement-bentonite backfill to the ground surface ③ bentonite seal isolating the measuring zone ④ filter sand forming the sensing zone ⑤ porous tip ⑥ readout or datalogger ⑦ signal cable ⑧ Vibrating Wire sensor. The water height h above the tip gives the water pressure u = γw × h. The bottom strip summarizes advantages and limitations, and the conceptual chart shows that the standpipe responds more slowly in clay (image: SPN Soil Engineering)

The table below summarizes the key differences used in actual decisions.

← Scroll to see the full table →

TopicStandpipe (Casagrande)Vibrating Wire
Measuring principleWater level in the pipe balances with water pressure; read with a dip meterVibrating-wire sensor; frequency converted to pressure
Response speedSlow in clay; must wait for water to flow into the pipeAlmost instantaneous, even in low-permeability soil
ReadingManual reading at each point every roundDatalogger for automatic reading; remote alarms possible
DurabilityVery high; no electronic parts in the groundDepends on sensor life; cannot be repaired once buried
CostLowerHigher, for both sensors and readout system
Best forLong-term monitoring, easily accessible points, limited budgetMonitoring where values change quickly, clay, hard-to-access points

Many projects combine both types, placing Vibrating Wire piezometers in critical zones that need fast, frequent readings, and distributing standpipes as robust long-term monitoring points that are easy to cross-check. The type and number depend on each project's specification.

What are piezometers used for?

Piezometers are used in 4 main groups of work: deep excavations, fills on soft soil, embankments and dams, and slopes. What they all have in common is that water pressure governs stability.

Deep excavations — used to monitor water pressure behind retaining walls and beneath the excavation base, to confirm the effect of dewatering and to watch for water pressure beneath the clay layer at the base that could cause base heave failure. They are usually installed together with an inclinometer, which measures lateral movement of the soil and wall, to capture soil behavior in terms of both pressure and movement.

Fills on soft soil (PVD + preloading) — when fill is placed, soft clay develops excess pore pressure. The progress of consolidation can be seen from the rate at which this excess pore pressure dissipates, so the piezometer is the main instrument used to judge whether the soil is consolidating as planned. Read about the principles of this ground improvement method in the article How PVD and preloading improve soft soil

The 3D view below shows an embankment fill on soft clay, the application where the role of the piezometer is clearest. Sensors are installed at 3 depths beneath the embankment. The blue zone is where excess pore pressure (Δu) develops, and the chart in the right-hand panel shows Δu rising during filling and gradually falling during rest periods.

3D model of piezometers at several depths beneath an embankment fill on soft clay, used to monitor excess pore pressure during filling and consolidation Conceptual 3D view of a soil block cut away to show a cross-section. The upper layer is soft clay and the lower layer is sand. On the ground surface is a trapezoidal embankment fill placed in layers. Beneath the embankment, within the soft clay, a blue zone shows the excess pore pressure generated by the weight of the fill. Blue arrows show water draining upward and down into the sand layer. Three piezometer sensors are embedded at different depths beneath the center of the embankment, with signal cables running along the embankment base to a datalogger box at the left toe. A settlement plate sits at the embankment base with its rod protruding above the crest, and an inclinometer casing is at the right toe. The right-hand panel has a conceptual chart showing the fill height increasing in stages and the excess pore pressure rising during filling and then gradually falling during rest periods, with a summary that effective stress equals total stress minus water pressure; if water pressure rises quickly and does not fall, filling is too fast; and readings should be interpreted together with the settlement plate and inclinometer. 3D model: Piezometers under fill on soft soil Not to scale Fill Soft clay Sand Δu Sensors at 3 depths In soft clay Drainage path (Δu falls) Excess pore pressure (Δu) 1 Piezometer Cable → Datalogger 2 Settlement Plate Settlement 3 Inclinometer Lateral movement What do readings show? Fill height Excess pressure Δu Fill Rest Fill Rest: Δu falls σ′ = σ − u u rises → σ′ falls Soil strength drops temporarily u rises fast, won't fall Sign of filling too fast Slow or pause filling per designer's criteria Read with ② and ③ Δu falls + settlement levels off = consolidation progressing Faster lateral movement = alert Conceptual view, not to scale
3D model of an embankment fill on soft clay: ① piezometers at 3 depths, with signal cables running along the embankment base to a datalogger ② settlement plate measuring settlement at the embankment base ③ inclinometer measuring lateral movement at the embankment toe. The blue zone is excess pore pressure (Δu), and the arrows show drainage directions. The right-hand panel summarizes how to read the results. Conceptual image, not actual monitoring data; fill control criteria are set by the design engineer (image: SPN Soil Engineering)

There are 3 rules for interpreting the results. First, during filling, Δu rises with the weight of the fill; effective stress has not yet increased, so soil strength has not yet improved. Second, when filling is paused, water drains out, Δu falls and the soil gains strength. Third, if Δu rises quickly and does not fall during the rest period, the fill is being placed faster than the soil can drain, and the engineer will slow or pause filling according to the criteria set by the designer. Piezometer readings should always be read together with settlement plates and inclinometers, to confirm soil behavior from more than one type of data.

Embankments and dams — used to monitor water pressure in the embankment body and foundation throughout the service life. Abnormal seepage paths often show up as unexpectedly high water pressure at the wrong location before damage becomes visible at the surface.

Slopes — rising water levels during heavy rain are a major cause of slope failure. Measuring water pressure together with movement enables early warning before failure occurs.

Which standards apply to piezometers, and how much does it cost?

There is no dedicated DPT standard for geotechnical instrumentation. Pre-installation acceptance testing of Vibrating Wire sensors refers to ASTM D7764-20 (Standard Practice for Pre-Installation Acceptance Testing of Vibrating Wire Piezometers), which specifies two tests: the zero test and the down-hole test. Installation methods and reading frequency follow the project-specific specification and the sensor manufacturer's manual.

SPN's service covers everything from drilling and installation, sensor acceptance testing, baseline readings and scheduled readings through to data processing and reporting. Reports are certified by a civil engineer licensed by the Council of Engineers Thailand (COE), with signature and registration number, and can be used for submissions to the relevant authorities. See the full service scope on the geotechnical instrumentation and monitoring page.

Fees depend on the sensor type, installation depth, number of points and reading frequency, so they are assessed per project. Contact us for a free quote within 24 hours. Prices do not include travel and equipment mobilization costs, which are charged extra according to the distance to the site.

⚠️ This article provides general information

This content explains the working principles and selection of piezometers for general understanding only. The sensor type, installation location and depth, and reading frequency for each project must be set by the responsible engineer according to the specific conditions of that project.