What Does a Strain Gauge Measure in Geotechnical Work?

A strain gauge is a sensor that measures the strain of a material — the ratio of elongation or shortening to the original length. The measured values are very small, so they are reported in microstrain (με), where 1 με is a change in length of one part per million. Structural steel under normal service loads typically reads in the range of tens to several hundred microstrain.

Other instruments in a monitoring system measure the "movement" of soil and structures, whereas a strain gauge measures the "force" a structure is currently carrying. For example, an inclinometer tells you how much a retaining wall has deflected, but not how much force each strut level is carrying. A strain gauge answers that second question directly, so the two instruments are often installed together in one system. For an overview of the full instrument set, see the Geotechnical Instrumentation Guide

An SPN technician bolting a strain transducer to the side face of a square prestressed concrete pile, with the signal cable leading from the sensor
Fixing a strain transducer to the pile surface before the test — the measuring principle is the same as an embedded strain gauge; only the fixing method and the period of use differ (photo: SPN Soil Engineering)

How Is Strain (με) Converted to Force?

Strain is converted to force using Hooke's Law in two steps. First, multiply the strain by the modulus of elasticity to get stress (σ = ε × E). Second, multiply the stress by the cross-sectional area to get force (F = σ × A). Combined into one formula:

F = ε × E × A
where ε = measured strain (dimensionless) · E = modulus of elasticity of the material · A = cross-sectional area carrying the force

Example calculation for an H-beam steel strut with a cross-section of 21,870 mm² and a steel modulus of elasticity of about 200,000 N/mm², with a reading of 150 με:

  • Stress = 150 × 10-6 × 200,000 = 30 N/mm²
  • Force = 30 × 21,870 = 656,100 N, or about 656 kN (approx. 67 t)

The engineer then compares this figure with the design force for that strut level.

A word of caution: the E value of steel is fairly constant and reliable, but the E value of concrete varies with compressive strength, age and mix. Using a table value for concrete piles without verification can therefore make the calculated force significantly wrong. Where accuracy matters, E is evaluated from the test's own data.

What Types of Strain Gauge Are There, and How Do They Differ?

There are three main types of strain gauge used in civil and geotechnical engineering: Vibrating Wire, Electrical Resistance and Strain Transducer. Each suits a different kind of work. The table below summarizes the differences that drive the choice in practice.

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TypeSignal principleKey strengthBest for
Vibrating Wire (VW)Vibration frequency of a tensioned wire changes with strainTolerates long cables and moisture, no drift over time, can be read automatically by a dataloggerPermanently installed in struts, anchors and pile reinforcement for long-term readings
Electrical Resistance (foil)Electrical resistance of the foil grid changes with strainMillisecond-level response, thin and bonds flush to the surface, low cost per pointRapidly changing forces; short-term measurements in a controlled environment
Strain Transducer (temporary mounting)Small steel frame with an internal strain gauge, bolted to the material surfaceFast to install and remove, reusable, factory-calibratedTemporary tests that must be installed within a day

Monitoring that requires continuous readings over months or years mainly uses the Vibrating Wire type. Because the signal is a frequency, it is not distorted by cable length or changes in resistance, and a built-in temperature sensor allows temperature effects to be compensated. When connected to a datalogger, it can be read automatically at a set interval (see the pros and cons of this approach in Manual vs Automatic Monitoring). The foil type suits work where forces change so fast that VW gauges cannot keep up, such as measuring impact forces in dynamic pile load testing (Dynamic Load Test, PDA)

A set of Vibrating Wire strain gauges with coiled signal cables laid out on a tarpaulin on site before being tied to the pile reinforcement cage, with reinforcement cages and a crawler crane in the background
Strain gauge sets and signal cables prepared before installation — the cables for each depth level are color-coded from the start so they are not mixed up when connected to the datalogger after concreting (photo: SPN Soil Engineering)

How Are Strain Gauges Installed on Excavation Struts?

Strain gauges on a strut are installed as a set around the cross-section at mid-length, and their readings are averaged to obtain the axial force. In deep excavations, the struts are the members that carry the earth pressure from the retaining walls on both sides, so the actual force in each strut level is direct safety information.

Typically, a set of 2-4 gauges around the cross-section of a steel strut is used. The reason is that a strut in the field almost never carries pure axial force — some bending is always present, from self-weight, misalignment of the strut ends or asymmetric load transfer. One side of the section is therefore compressed more than the other. A single gauge would read higher or lower than the true value; averaging the readings from gauges on opposite sides cancels out the bending effect, leaving only the axial force that is needed.

Along the length, the mid-span of the strut is usually chosen, a reasonable distance from splices and end connections, because stress is distributed unevenly near the joints. A zero reading must always be taken before that strut level starts carrying load; otherwise the readings show only the change from the force already present, not the total force. The full monitoring system for deep excavations and how alarm levels are set are covered in Deep Excavation Monitoring in Bangkok's Soft Clay, and an example of an earth retaining system from a real project is in the Temporary Earth Retaining System Case Study

Installing strain gauges as a set around the cross-section of a steel strut in a deep excavation, averaging to remove bending, and converting strain to axial force The left image is a section of a deep excavation with retaining walls on both sides and two levels of steel struts. Earth pressure from both sides pushes the walls toward each other, so the struts carry compression. A strain gauge set is installed at mid-length of the upper strut, away from splices and strut ends, with the signal cable running to a data recorder at ground level. The right image is the cross-section of an H-shaped steel strut with four strain gauges on the top and bottom flanges. As the strut sags under its own weight, the top flange reads more than the bottom flange. Example assumed values: top flange 170 microstrain, bottom flange 130 microstrain, average 150 microstrain. The bending effects cancel out, leaving only the axial force, which is converted with F equals ε times E times A: 150 times 10 to the power of minus 6, times 200,000 newtons per square millimeter, times 21,870 square millimeters, giving about 656 kilonewtons or approx. 67 tonnes, which is then compared with the design force. The bottom strip summarizes four things that distort readings. Strain gauges on struts: install a set, average, convert to force Not to scale Gauge set at strut mid-length Strut compression Datalogger Deep excavation + steel struts Earth pressure from both sides into strut Away from splices/ends → uniform stress Excavation base Pressure H-beam strut section 4 gauges around section, averaged SG-1 · SG-2 SG-3 · SG-4 Top flange 170 με Bottom flange 130 με (assumed) Average of 4 = 150 με Bending cancels out, axial force remains F = ε × E × A 150×10⁻⁶ × 200,000 × 21,870 ≈ 656 kN (approx. 67 t) 1 Temp. compensation Sun/cool shifts readings 2 Zero at right time Read before loading 3 Symmetric mounting Opposite pairs, averaged 4 Concrete E value Piles only
Strain gauges are installed as a set around the strut cross-section and averaged, removing the bending effect and leaving only the axial force, which is compared with the design value

How Are Strain Gauges Used in Anchors and Piles?

In anchors, strain gauges track the remaining tension; in piles, they measure the force at each depth. Retaining walls that use ground anchors instead of struts have strain gauges installed on the anchor strands or bars to see whether the tension applied at stressing is maintained or decreases over time. Loss of tension can result from strand relaxation, creep of the soil around the fixed length, or movement of the anchor head — all of which directly affect wall stability.

In pile work, strain gauges are tied to the reinforcement cage in sets at different depths before the cage is lowered into the hole and concreted. Each level reports the pile force at that depth. The measured force decreases with depth because each soil layer takes up part of the load in friction, so it is possible to separate how much load each soil layer carries and how much reaches the pile toe. This type of test is called an Instrumented Static Load Test. Details of planning, reading the Load Transfer Curve and separating skin friction by soil layer are in the article Instrumented Static Load Test: Pile Testing with Strain Gauges, while the basic test principles are in Static Load Test: Testing Pile Load Capacity

The 3D view below combines all three applications on one site to show the installation locations. Orange dots are strain gauges and light-blue lines are signal cables running to the datalogger. Note the blue bars beside the pile that get shorter with depth, representing the decreasing pile force. The right-hand panel steps through the conversion using the strut example above and summarizes the causes of distorted readings.

3D model of strain gauges measuring forces in struts, anchors and piles, with the steps for converting strain to force A conceptual 3D view of a soil block with the front corner cut away to show a deep excavation pit, whose walls are retaining walls on two sides. One: two H-shaped steel struts brace the walls at the same level; the front strut has a strain gauge set around its cross-section at mid-length, and the signal cable runs along the strut and up the pit edge to a datalogger box at ground level. Two: an anchor slopes down into the soil behind the wall, with a strain gauge on the steel bar in the free length and a fixed length bonded in the soil at the end. Three: a bored pile on the front section, with a test load arrow pressing on the pile head and strain gauges on the reinforcement cage at four depths. The orange bars beside the pile are longest at the top and get shorter with depth, showing that the pile force decreases as each soil layer takes up friction. Four: the datalogger box collects cables from the strut and the pile. The right-hand panel shows the three-step conversion: a strain reading of 150 microstrain times the steel modulus of elasticity of 200,000 newtons per square millimeter gives a stress of 30 newtons per square millimeter; times the cross-sectional area of 21,870 square millimeters gives a force of about 656 kilonewtons, or approx. 67 tonnes. It also summarizes five causes of distorted readings: no temperature compensation, zero reading at the wrong time, asymmetric installation, a concrete modulus that does not match reality, and damaged signal cables. 3D model: strain gauges measuring force in 3 applications Not to scale Pit base Pile force drops with depth Each layer takes up friction (conceptual view) Orange = strain gauge · blue line = cable 3 applications in one view to explain the principle 3 Bored pile Multi-depth gauges 4 Datalogger Cables in, auto reading 1 Strut Set around section 2 Anchor Gauge on bar Convert readings to force A Strain ε Reading 150 με B Stress σ = ε × E 150 με × E = 30 N/mm² C Force F = σ × A 30 × 21,870 mm² ≈ 656 kN H-beam strut example (above) E = 200,000 N/mm² · A = 21,870 mm² 656 kN ≈ 67 t vs. design force Why readings go wrong No temp. compensation Zero at wrong time Asymmetric, bending mixed in Concrete E not actual Damaged cables Conceptual view, not to scale
3D model: ① steel struts in the excavation, with a strain gauge set around the cross-section at mid-length ② an anchor behind the retaining wall, with strain gauges on the steel bar to track tension ③ a bored pile with strain gauges at 4 levels; the blue bars show the pile force decreasing with depth ④ a datalogger collecting the signal cables. The right-hand panel (a)–(c) shows the steps for converting strain to force. Conceptual view; the actual number and locations are as specified by the design engineer (image: SPN Soil Engineering)
Bored pile reinforcement cages laid on timber sleepers in the site yard, with strain sensors installed along the cage, and a crawler crane and temporary borehole casing in the background
A bored pile reinforcement cage with strain gauges installed at different depths before being lowered into the hole — the position of each level must be recorded to match the drawings, because it determines which depth each reading represents (photo: SPN Soil Engineering)
A technician routing sensor signal cables at a bored pile head in an excavation pit beside a sheet pile wall, while preparing for a test on an actual site
Installing sensors at a bored pile head in an excavation pit — signal cables must be routed and protected from the preparation stage, because they cannot be repaired after concreting (photo: SPN Soil Engineering)

What Makes Strain Gauge Readings Go Wrong? 4 Common Causes

Strain gauge readings go wrong for 4 main reasons: no temperature compensation, a zero reading taken at the wrong time, asymmetric installation, and using the wrong E value for concrete. Strain gauges always give numbers that look credible, even when they are wrong, so all four points must be checked every time.

1. No temperature compensation Steel expands and contracts with temperature. A strut in the afternoon sun and the same strut in the morning give different readings even when the actual force is unchanged. Most Vibrating Wire strain gauges therefore have a built-in thermistor that records temperature alongside strain for later compensation.

2. Zero reading at the wrong time If the zero reading is taken after the member has started carrying load, the pre-existing force disappears from the report entirely. The zero reading must therefore be taken several times until it stabilizes, before the member actually starts working.

3. Asymmetric installation Installing a single gauge or an off-center gauge mixes bending effects into the axial force so that they cannot be separated. This is solved by installing gauges in opposite pairs and averaging, as explained above.

4. Using the wrong E value for concrete This only applies to installations in concrete piles. The modulus of elasticity used to convert strain to force directly affects the final figure. Taking a standard value without checking it against actual data can make the reported force wrong by several tens of percent.

Another problem that cannot be fixed once work has started is damaged signal cables. Strain gauges embedded in concrete or buried underground cannot be repaired, so in practice more gauges are installed than strictly needed, and cables are thoroughly protected from the installation stage.

Crew members in hard hats steadying a reinforcement cage with sensor signal cables running up along the longitudinal bars as the crane lowers it into a borehole by a river
Lowering a reinforcement cage fitted with strain gauges into the borehole — the signal cables must be bundled and secured tightly along the longitudinal bars, because a cable scraped or kinked at this stage will leave that measuring point permanently dead after concreting (photo: SPN Soil Engineering)

Which Standards Do SPN's Strain Gauge Services Reference, and How Much Do They Cost?

There is no DPT standard specifically for strain gauges in geotechnical instrumentation, so the reference depends on the application. For installation in piles for static load testing, the references are ASTM D1143/D1143M-20e1 together with DPT 1251. Strain transducers used in dynamic testing reference DPT 1252-51 and ASTM D4945-26. For installation on struts and anchors, the installation positions, number of points and reading frequency follow the project-specific monitoring requirements set by the design engineer.

SPN's services cover selecting the strain gauge type, installation, cable routing and protection, datalogger connection for automatic readings, zero readings, periodic readings, and processing into force-versus-design-value graphs, through to a report signed and certified by a licensed civil engineer (Council of Engineers Thailand) with signature and registration number, suitable for submission to the relevant authorities. See the full scope of services on the Geotechnical Instrumentation and Monitoring page and pile testing on the Pile Testing page

Fees depend on the strain gauge type, the number of installation points, the reading period and the datalogger connection, so they are assessed per project. Contact us for a free quotation within 24 hours. Prices exclude travel and equipment mobilization, which are charged extra according to the distance to the site.

⚠️ This article provides general information

This content explains how strain gauges work for general understanding. The sensor type, the number and location of installation points, the modulus of elasticity used for conversion and the force comparison criteria for each project must be set by the design engineer and the responsible engineer according to the specific conditions of that project.