What Is Geotechnical Instrumentation? 10 Types Used in Practice

What Is Geotechnical Instrumentation and Why Install It?

Geotechnical instrumentation is a system for monitoring how soil and structures actually behave in the field. Foundation design, deep excavations and earth fills all rest on 3 assumptions—soil strength, rate of settlement and pore water pressure—calculated from soil investigation results. But soil is not homogeneous and often responds differently from what was calculated. Instrumentation therefore provides numerical evidence of whether what is actually happening remains within the limits the designer can accept.

There are 3 main benefits:

• Early warning. An abnormal change in trend usually shows up several days before visible damage.
• Verifying design assumptions. For example, checking that settlement has stopped on schedule before the slab is cast.
• Evidence in a dispute. When a neighboring building cracks during construction, continuously recorded data shows when the movement occurred and how large it was.

How Many Types of Instruments Are There, and What Does Each Measure?

Geotechnical work in Thailand uses 10 main types of instruments, arranged in 4 groups by the quantity to be measured: 3 types for settlement, 3 for groundwater and pore pressure, 2 for movement and tilt, and 2 for structural forces and data logging. The table below summarizes what each type measures and the work it suits.

The cross-section below places all 10 instruments where they are actually installed. The left side is an earth fill on soft clay; the right side is a deep excavation right next to an existing building. Note 3 points. First, the inclinometer casing must reach the stiff layer so its bottom end serves as the reference point. Second, the water level in the standpipe piezometer under the embankment rises above the groundwater level because the weight of the fill creates excess pore pressure. Third, the observation well sits outside the embankment and is open to water over a long interval, so the water level in the pipe equals the groundwater level.

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Group 1: Settlement

A settlement plate is a steel base plate placed on the original ground surface before filling, with a measuring rod extended above the fill level. As the fill loads the ground, the original soil compresses and pulls the rod head down with it, so the reading is the cumulative settlement of the original ground at that location. In fills on Bangkok soft clay, the settlement plate is the instrument used to decide whether settlement is close enough to its final value to start structural work. For detailed installation, reading frequency and interpretation of settlement curves, see Settlement Plate: Monitoring Settlement of Earth Fills and Embankments

A multi-level settlement point differs from a settlement plate in that measuring points are installed at several depths in a single hole, so it can separate which layer contributes most to the total settlement seen at the surface. This information is important for ground improvement with PVD and preloading, which needs to know how much the drained layers have compressed. A surface settlement marker, by contrast, is a leveling point on the ground or road surface, used to monitor the area around an excavation or fill through periodic level surveys.

Group 2: Groundwater and Pore Pressure

A standpipe piezometer is a pipe sealed with bentonite so that it takes in water only over the depth interval of interest. The measured value is therefore the pore water pressure of that specific layer. This differs from an observation well, which is open to water over a long interval and gives the overall groundwater level. Choosing the wrong one of the two is a common mistake. Work that must track pore pressure dissipation during filling needs a piezometer, not an observation well, because the weight of the fill creates excess pore pressure in the clay layer. The water level in the piezometer pipe therefore rises above the normal groundwater level and then gradually falls as the soil consolidates and expels water.

A vibrating wire piezometer works by measuring the vibration frequency of a wire tensioned inside the sensor. When the water pressure changes, a diaphragm deflects and changes the wire tension, and the change in frequency is converted back into pressure. Its advantages are a much faster response than the open-pipe type and the ability to connect to a datalogger for automatic reading. ASTM D7764 covers the acceptance check of this type of sensor before installation—a step that should not be skipped, because a sensor that has been embedded cannot be retrieved for recalibration. For a detailed comparison of the two types, see What Is a Piezometer? Measuring Pore Water Pressure (Standpipe vs Vibrating Wire)

Group 3: Movement and Tilt

An inclinometer measures lateral soil movement with depth. A casing with orientation grooves is installed in a borehole, and a probe is lowered to read the tilt angle interval by interval. Accumulating the angles of all intervals from the bottom up gives a displacement profile over the full depth, showing the level of any slip surface. This probe method follows ASTM D6230. A key condition is that the bottom of the casing must reach a layer considered stable; otherwise the readings are movements relative to a base that is itself moving. For an in-depth look at the operating principle, installation steps and how to read the result plots, see What Is an Inclinometer?

The inclinometer is therefore the main instrument for deep excavations using temporary earth retaining systems and slopes at risk of sliding. A tiltmeter, on the other hand, measures the tilt of whatever the sensor is fixed to, such as the wall of a neighboring building or the back of a retaining wall. Its strengths are high resolution and installation without drilling, so it is often used together with crack assessment and monitoring of neighboring buildings. For detailed point layout, reading frequency and alert criteria for neighboring-building monitoring, see Settlement Points and Tiltmeters: Measuring Settlement and Tilt of Neighboring Buildings

Group 4: Structural Forces and Data Logging

A strain gauge measures strain in a structural member, which is then converted into the force the member actually carries using material properties and cross-sectional area. Common applications are struts in deep excavations, retaining wall anchors and instrumented test piles, where strain gauges separate end bearing from shaft friction. For an in-depth look at measuring principles, gauge types and how strain is converted to force, see Strain Gauges in Geotechnical Engineering

A datalogger is a recording unit connected to vibrating wire and other electrical sensors. It takes readings at preset intervals and stores them in memory. With an added communication module, it can transmit data for remote viewing and trigger alerts when measured values exceed the specified criteria.

Which Projects Need Instrumentation?

Projects that should have a monitoring plan from the outset are those meeting at least 1 of 2 conditions. First, the consequences of a failure affect people or property outside the project. Second, the construction process relies on actual data to decide the next step. There are 5 common groups of such projects:

• Urban deep excavations with buildings or public roads right next to the excavation line. Inclinometers measure retaining wall movement, together with surface settlement markers and tiltmeters on neighboring buildings. See details in Deep Excavation Monitoring in Bangkok Soft Clay
• Earth fills and preloading on soft ground use settlement plates to track settlement, together with piezometers to see whether pore pressure is dissipating as expected before deciding to place the next lift.
• Embankments and dams that need monitoring of the water level within the embankment and slope movement throughout their service life.
• Slopes and hillside cuts with a history of sliding or located above traffic routes.
• Projects where the designer or owner specifies monitoring in the contract, which usually already defines instrument types, locations, reading frequency and alert criteria.

Pile driving near existing buildings is another case that requires monitoring, but it uses a different set of instruments from those above. Details are in the article Vibration Monitoring for Pile Driving

For point-by-point criteria to decide whether your project qualifies—including requirements from drawings, law and EIA, and when to start collecting baseline readings—read When Is Instrumentation Required? Which Projects Need It

How to Install and Read Instruments So the Data Supports Decisions

Monitoring data supports decisions when all 5 steps are completed: plan the monitoring, install and take baseline readings, read on schedule, process and compare against criteria, and report with alerts. If any step is missing, the numbers cannot be interpreted or reach the decision-maker too late.

1. Plan the monitoring. Define what to measure, where, with which instrument, how often and at what alert values. Alert criteria are project-specific values set by the design engineer. This step must be done by the design engineer together with the installer, because an instrument in the wrong place yields data that does not answer the project's questions.

2. Install and take baseline readings. After installation, take several initial readings until the values stabilize, then use them as the reference. A poorly taken baseline makes the entire set of later readings uninterpretable. Inclinometer baselines must be read before excavation begins, and settlement plates must be read before the first lift is placed.

3. Read on schedule. Always record the date, time, reader and construction activity at the time together with the numbers. Sudden changes can usually be explained by site activities, such as excavating to a new level or removing a strut.

4. Process and compare against criteria. Plot values against time to see the trend rather than single values. A change in the slope of the curve is often a more meaningful signal than an absolute value that has not yet reached the criterion.

5. Report and alert. When values approach the alert criterion, notify those concerned immediately through the reporting line agreed in advance—do not wait for the monthly report.

The 3D view below follows steps 2 to 5 as a data path: from instruments embedded in the ground, through readings by field staff or a datalogger, to a plot of readings versus time with criteria lines, ending with the engineer's decision. The right-hand panel summarizes the 5 conditions that make a data set truly usable for decisions. The criteria levels shown are conceptual examples only; the number of levels and their values depend on each project's specifications.

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Manual Reading vs Automatic Systems: What Is the Difference?

Manual reading means field staff visit the site and read each point on a schedule, while an automatic system has a datalogger read and transmit values continuously without anyone on site.

Manual reading on site suits projects where measuring points are easy to access and the required frequency is not high. Its advantage is that the reader sees actual site conditions and can notice cracks or seepage that instruments cannot measure. Its limitation is that data comes as discrete points on each site visit, so events between visits are missed.

An automatic system using a datalogger provides continuous data 24 hours a day, can trigger automatic alerts and suits areas that are hard to access or hazardous. The trade-offs are a higher initial installation cost, the need for power and communications on site, and sensors that output electrical signals, such as vibrating wire piezometers or strain gauges. Many projects therefore combine the two, with the automatic system covering critical points and field staff reading the rest on schedule. Read more in Manual vs Automatic (Real-time) Monitoring: What Is the Difference?

Which Standards Apply to Geotechnical Instrumentation?

Geotechnical instrumentation has no dedicated DPT standard of the kind that exists for pile testing. References therefore rely on ASTM standards covering each instrument type, together with project specifications and the sensor manufacturer's manuals.

SPN monitoring reports are certified by a licensed civil engineer (COE) with signature and registration number, and can be used in submissions to the relevant authorities. See the scope of our instrumentation and monitoring services on the service page. Fees depend on instrument type, number of points, installation depth and reading frequency, so they are assessed per project. Contact us for a free quotation within 24 hours. Prices exclude travel and equipment mobilization, which are charged according to the site distance.