Soil Thermal Test pit at Point 2 in the factory area, with the team preparing the temperature data logger and tool box beside the pit and a sign showing the test point and date
Soil Thermal Resistivity and Ambient Temperature Test at Point 2 for a 115 kV transmission line extension inside a factory in Prachin Buri Province. The pit was excavated to the cable-laying depth of 3.0 m and the probes were installed at the pit bottom (Photo: SPN Soil Engineering)

What Is a Soil Thermal Resistivity Test and What Does It Measure?

Soil thermal resistivity (ρth) is the property that describes how strongly soil resists the flow of heat. Its unit is meter-kelvin per watt (m·K/W). Its reciprocal is thermal conductivity (k), in watts per meter-kelvin (W/m·K). The lower the resistivity, the faster the soil carries heat away from a power cable.

The value is controlled by moisture content, density, soil type and grain size, and mineral composition. Water conducts heat about 25 times better than air, so a saturated soil dissipates heat several times better than the same soil when dry. Soils rich in quartz also conduct heat better than clays or organic soils. Guessing the value from a visual soil description can therefore be badly off.

Where Soil Thermal Resistivity Test results are used

Current rating design for underground transmission and distribution cables, 22 kV / 115 kV and above · Underground cables at substations, power plants and solar farms · Outgoing feeders from data centers and industrial estates · Selecting and accepting cable trench backfill (Thermal Backfill) · Heat pipelines and ground heat exchanger systems · Assessing the risk of soil drying around cables (Moisture Migration / Dry-out)

Why Do Underground Power Cables Need Soil Thermal Data?

Every cable generates heat while carrying current, following the I²R law. A cable run in air dissipates heat easily, but a buried cable relies on the surrounding soil as its only heat path. If the soil dissipates heat slowly, the conductor temperature rises until it exceeds the insulation limit (typically 90°C for XLPE insulation). The designer must then reduce the permissible current or switch to a larger cable.

Cable current ratings are calculated to IEC 60287, which takes two parameters directly from the soil: the soil thermal resistivity and the natural ground temperature at cable-laying depth (Ambient Ground Temperature). Even a small error in these inputs carries through to the current rating. For example, using a typical reference value of about 1.0 m·K/W when the actual soil is 1.7 m·K/W means the cable can carry less current than calculated and will overheat at full load.

Soil drying around cables (Thermal Runaway)

Heat from the cable drives moisture out of the surrounding soil. As the soil dries, its thermal resistivity rises, so the cable gets hotter and drives out even more moisture in a vicious cycle. This is one cause of failure in high-voltage underground cables. IEEE Std 442 therefore recommends evaluating thermal resistivity at both the natural moisture content and in the dry condition, so the designer can allow for the critical case.

The 3D view below shows how heat from the cable must pass through the backfill and the native soil before reaching the ground surface, and where the values from the tests in this article fit into the current rating calculation.

3D cutaway model of an underground cable trench, showing heat from the cables dissipating through the backfill and native soil up to the ground surface Conceptual 3D cutaway of a block of native soil showing an underground cable trench in section. Three single-core cables are laid in trefoil within a controlled thermal backfill zone at the trench bottom, with general backfill and warning tape above. The cable ends show the copper conductor, XLPE insulation and outer sheath, with conductor temperature not exceeding 90 degrees Celsius. Isotherm rings around the cables shade from orange near the cable to pale yellow farther out. The dashed ring is the dry-out zone around the cables. Red arrows show heat flowing through the soil to the ground surface. Numbers one to four are the IEC 60287 inputs: the native soil thermal resistivity measured with a Needle Probe, the backfill value, the ground temperature at cable depth from the ATT, and the burial depth and cable spacing set by the designer. The right-hand box summarizes the inputs for the current rating (ampacity), with a Thermal Runaway warning: dry soil raises ρth, the cable gets hotter and drives out more moisture; water conducts heat about 25 times better than air; and the acceptance value is set by the cable designer. 3D model: soil around a buried cable is its heat path Not to scale 4 Depth/ spacing (designer) Isotherms Hottest at cable Fading Heat flow path Conductor ≤ 90 °C (XLPE) 2 ρth backfill (Thermal Backfill) 1 ρth native (measured, Needle Probe) Dry-out zone Heat drives moisture out 3 Ground temp. at cable depth (ATT) Heat flows up to surface Conceptual · not for construction IEC 60287 inputs → Ampacity 1 ρth native Needle Probe · ASTM D5334 IEEE Std 442-2017 2 ρth backfill Thermal Backfill replaces soil if measured value too high 3 Ground temp. at cable depth ATT ≥ 24 h · IEC 60287-3-1 4 Depth / cable spacing set by designer Dry soil → ρth rises → hotter cable → more drying (Thermal Runaway) Water conducts ~25× better than air Acceptance value set by the cable designer
Underground cable trench model: heat from the cables must flow through the backfill and native soil to the ground surface. The ρth of the native soil, the ρth of the backfill, the ground temperature at cable depth (ATT) and the burial depth/cable spacing are therefore inputs to the current rating calculation to IEC 60287 (Diagram: SPN Soil Engineering)

What Is an Ambient Temperature Test (ATT) and Why Measure for 24 Hours?

An Ambient Temperature Test measures the natural ground temperature at the level where the cable will be laid. A temperature probe connected to a data logger records continuously for at least 24 hours at 10-minute intervals. The result is a temperature–time graph and four summary values: minimum (Tmin), maximum (Tmax), average (Tavg) and range (ΔT = Tmax − Tmin).

The reason to measure rather than use air temperature is that ground temperature at 1–3 m depth does not swing with the daily air temperature; it stays close to the local mean temperature and changes slowly with the seasons. IEC 60287-3-1 specifies the ground temperature at cable-laying depth as the reference condition, because it is the baseline temperature into which the cable must shed its heat. Ground that is 5°C warmer than assumed directly cuts the margin between ground temperature and the insulation limit by 5°C.

How Is the Field Test Carried Out?

Fieldwork at one point takes about 2 days, because the temperature record must run a full 24 hours. SPN's sequence is as follows.

1. Set out the location and excavate a pit to cable-laying depth

Test locations are set along the cable route on the drawings. Before excavation, existing underground utilities such as pipes, cables and power pole foundations must be checked. The pit is then dug with a mini excavator or by hand to the specified depth. The project used as an example in this article used 3.0 m below existing ground level, close to the 115 kV cable route level.

Soil Thermal Test pit at Point 1, dug with a mini excavator to 3 m beside a power pole foundation, with a sign identifying the 115 kV transmission line extension project in Prachin Buri Province
Test Point 1: pit excavated beside a power pole foundation along the cable route. The excavated soil is reddish-brown lateritic soil, consistent with test results classifying the layer as Laterite (GP) (Photo: SPN Soil Engineering)

2. Install the Thermal Needle Probe and measure STR

A Thermal Needle Probe is a metal needle containing a heater wire and a temperature sensor. The technician pushes the needle into the soil at the pit bottom so that it is in full contact with no air gaps, then applies a constant heat input to the soil while recording the temperature rise over time until a suitable data range for analysis is obtained. This is the Transient Line Heat Source method of ASTM D5334 and IEEE Std 442.

SPN technician in a hard hat inside the test pit installing the Thermal Needle Probe and temperature probe at the 3 m pit bottom in reddish-brown lateritic soil
A technician installs the probe at the bottom of the 3 m pit. The needle must be inserted into undisturbed native soil and be in full contact with the surrounding soil, because air gaps make the measured thermal resistivity falsely high (Photo: SPN Soil Engineering)

3. Calculate thermal conductivity from Line-Source Theory

When a constant heat input q (W/m) is applied to a line source, the needle temperature rises linearly when plotted against ln(time). The slope of this line gives the soil thermal conductivity directly.

Calculation formula per ASTM D5334

k = (q / 4π) × [ln(t2/t1) / (T2 − T1)]  and  ρth = 1 / k

where k = thermal conductivity (W/m·K) · q = heat input per unit needle length (W/m) · T1, T2 = temperatures at times t1, t2 · ρth = thermal resistivity (m·K/W)

Example from an actual job: Point 1 gave k = 0.725 W/m·K → ρth = 1 / 0.725 = 1.381 m·K/W

4. Install the temperature probe, backfill the pit and record for 24 hours

After the STR measurement, the technician installs a temperature probe with a data logger at the same level and backfills the pit so the probe sits in conditions close to the real ones, then leaves it recording every 10 minutes for at least 24 hours. Logger clocks must be checked for synchronization; if a logger clock has drifted, it must be corrected before the data are analyzed. In the example project, the Point 2 logger was found to be about 30 minutes off, which had to be recorded and corrected in the report.

Wide view of the Point 1 test pit showing the excavator, the pile of excavated laterite and the Soil Thermal Conductivity test equipment box with the yellow measuring cable
Overview of Test Point 1: the Soil Thermal Conductivity meter and 12 V battery sit in a box beside the pit, with the yellow measuring cable running down to the probe at the pit bottom (Photo: SPN Soil Engineering)

The figure below summarizes steps 2–4 in a single image, from the needle position at the pit bottom and the backfilling for the ATT to how the slope of the ΔT–ln(t) graph is read to calculate k and ρth.

Section through a Soil Thermal Resistivity test pit using a Thermal Needle Probe at the 3.0 m cable-laying depth, with a ΔT versus ln(t) analysis graph and a 24-hour ATT ground temperature graph The left panel is a section through a test pit in lateritic soil (Laterite, GP), 3.0 m deep to cable-laying level. The pit is split into two halves showing two stages. Stage one, open pit: the Thermal Needle Probe is pushed into undisturbed native soil at the pit bottom; disturbed soil must be trimmed off first, and the needle must be in full contact with no air gaps. A yellow measuring cable runs to the readout unit and 12 V battery in a box beside the pit. Stage two, backfilled (dashed line): the temperature probe sits at the same level, connected to a data logger recording every 10 minutes for at least 24 hours. Detail A enlarges the needle to show the heater wire q (W/m) and the temperature sensor at mid-needle. The upper-right panel is a conceptual graph of ΔT versus ln(t): it curves at first, then becomes a straight line between t1 and t2. The slope S is used to calculate k = q/(4π·S) and ρth = 1/k per ASTM D5334. The actual example at Point 1 gave k 0.725 W/m·K and ρth 1.381 m·K/W. The lower-right panel is a 24-hour ground temperature graph using the actual Point 1 values: Tmin 30.3, Tmax 34.8, Tavg 31.32 and ΔT 4.5 degrees Celsius; the curve shape is conceptual. 2D section: STR by Needle Probe and 24 h ATT Not to scale 3.0 m (cable depth) Laterite (Laterite, GP) 1 STR Open pit 2 ATT Backfilled A Data Logger (ATT) Log every 10 min ≥ 24 h Readout + 12 V battery in tool box by the pit Yellow cable Trim disturbed soil first Full contact, no air gaps A Detail: Thermal Needle Probe Heater wire q (W/m) Temp. sensor (mid-needle) Native soil around needle ΔT vs ln(t) (conceptual) Apply constant q, log needle temperature vs time ΔT (°C) ln(time) → ln t1 T1 ln t2 T2 S Linear range (used) k = q / (4π·S) · ρth = 1/k ASTM D5334 S = (T2 − T1) / ln(t2/t1) Point 1: k = 0.725 W/m·K → ρth = 1.381 m·K/W ATT: ground temp. at 3.0 m over 24 h Point 1 actual values · conceptual curve 0 12 24 h Tmax 34.8 °C Tavg 31.32 °C Tmin 30.3 °C ΔT 4.5 °C IEC 60287-3-1 ref.: ground temp. at cable depth
Section through a 3.0 m deep test pit in lateritic soil: STR is measured with a Thermal Needle Probe in native soil at the pit bottom, then the pit is backfilled to record ground temperature (ATT) every 10 minutes for at least 24 h. The right-hand graphs show k obtained from the slope of the linear range of ΔT–ln(t) per ASTM D5334, and the ATT results for Point 1 (Tmin 30.3 / Tmax 34.8 / Tavg 31.32 °C) (Diagram: SPN Soil Engineering)

What Is the Typical Thermal Resistivity Range for Each Soil Type?

This table gives reference ranges from IEEE Std 442-2017 and the work of Farouki (1986). Use it to check whether a measured value is reasonable for the observed soil conditions, not as a design value in place of actual test results.

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Soil type / conditionThermal conductivity k (W/m·K)Thermal resistivity ρth (m·K/W)
Sand, dry0.15–0.502.0–6.7
Sand, saturated1.50–3.000.33–0.67
Silty sand, dry0.20–0.402.5–5.0
Silty sand, saturated1.30–2.500.40–0.77
Clay, dry0.25–0.402.5–4.0
Clay, saturated1.00–1.500.67–1.0
Laterite, dry to natural moisture0.30–0.801.25–3.33
Laterite, wet to saturated0.80–1.800.56–1.25
Organic soil / peat, wet0.20–0.402.5–5.0
Gravel, saturated1.50–3.500.29–0.67
Rock (granite, limestone)2.5–4.00.25–0.40

What the table makes clear is that the same soil can vary by a factor of 5–10 with moisture content. Dry soil is always the critical case for underground cables, so designers often ask for values at both natural moisture and dry conditions, or specify that the cable trench be backfilled with a controlled thermal resistivity material (Thermal Backfill) instead of native soil when the measured value exceeds the project's acceptance limit. (Sources: IEEE Std 442-2017; Farouki, O.T. (1986) Thermal Properties of Soils; Clauser & Huenges (1995))

Actual Test Results: 115 kV Transmission Line Project, Prachin Buri Province

These data come from SPN's testing on a 115 kV transmission line extension project inside a factory in Kabin Buri District, Prachin Buri Province. Two points were tested at 3.0 m depth in September 2026. The site lies on Quaternary floodplain deposits (unit Qff on the Prachin Buri Province geological map, Department of Mineral Resources, 2008). The soil at test level is laterite (Laterite, USCS: GP) at natural moisture content.

1.38–1.73
Thermal resistivity (m·K/W)
0.58–0.73
Thermal conductivity (W/m·K)
30.8–31.3°C
Average ground temp. at 3.0 m

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ItemTest Point 1Test Point 2
Test depth3.0 m3.0 m
Soil type (USCS)Laterite (GP)Laterite (GP)
Thermal conductivity k (W/m·K)0.7250.578
Thermal resistivity ρth (m·K/W)1.3811.733
ATT duration / logging interval24 h / every 10 min (145 readings)24 h / every 10 min (145 readings)
Minimum temperature Tmin30.3°C29.8°C
Maximum temperature Tmax34.8°C33.8°C
Average temperature Tavg31.32°C30.75°C
Range ΔT4.5°C4.0°C

How to read the results Thermal resistivities of 1.38–1.73 m·K/W fall within the range for laterite at natural moisture (1.25–3.33 m·K/W), showing that the soil is not saturated, dissipates heat moderately, and is higher than the reference value of about 1.0 m·K/W that designers often use when no test data are available. The roughly 25% difference between the two points comes from differences in soil density and moisture. The average ground temperatures at the two points differ by only 0.57°C, with similar ranges of 4.0–4.5°C, so they can be used as representative values for the site during the measurement period.

Acceptance values must come from the project's cable designer

This article explains how to test and how to read the values; it does not define which values pass or fail. The measured thermal resistivity and ground temperature must be used in the current rating calculation together with cable size, cable spacing, burial depth and backfill material by the project's electrical design engineer, following IEC 60287. In addition, a 24-hour ATT reflects only the period measured; if the critical value over the year is needed, seasonal data should also be considered.

Which Standards Does This Test Follow?

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StandardCovers
ASTM D5334-22ae1Test method for thermal conductivity of soil and rock by Thermal Needle Probe (Transient Line Heat Source). The main standard for the measurement method and calculation formula; applicable in both the field and the laboratory.
IEEE Std 442-2017Guide for Thermal Resistivity Measurements of Soils and Backfill Materials: guidance on measuring thermal resistivity of soils and backfill for underground power cables, covering field and laboratory measurement and assessment of dry soil conditions.
IEC 60287-1-1Formulas for calculating the current rating of electric cables, using soil thermal resistivity and ground temperature as inputs.
IEC 60287-3-1:2017Site Reference Conditions, including the ground temperature and soil thermal resistivity used in current rating calculations. Used as the reference for the Ambient Temperature Test.
ASTM D2487Soil classification by the USCS system, used to identify the soil type at test level in the report, e.g. Laterite (GP).

What Mistakes Make Test Results Unusable?

Measuring at the surface instead of cable depth Surface soil dries quickly and heats up with the air, so the value is not that of the soil the cable will actually be in. Excavate to cable depth, or use a borehole and measure at that level.

Poor needle contact with the soil Air gaps around the needle act as insulation, making the thermal resistivity falsely high. In stiff soil or gravelly laterite, pre-drill with a rod of the same diameter as the needle before inserting it.

Testing on disturbed soil Soil at the pit bottom that has been crushed by the excavator or loosened by digging has a different density from the native soil. Trim off the disturbed soil and measure in the native soil below.

Not recording moisture and weather The same value means different things at the end of the dry season and in the middle of the rainy season. The report must state the date and weather, and where possible soil samples should be taken for moisture content and density so the designer can allow for dry conditions.

Temperature record under 24 hours or logger clock drift Without a full daily cycle, the true Tmax and Tmin cannot be found, and loggers with unsynchronized clocks make it impossible to compare data from two points. Check logger time before installation and state any corrections in the report.

How Does It Work Together with Other Tests?

Soil Resistivity Test measures the electrical resistivity of soil (Ω·m) for designing earthing systems at substations and solar farms. It tests a different property from thermal resistivity, but underground transmission projects often do both on the same trip to save on travel costs.

Soil investigation (Soil Boring) provides the soil profile, groundwater level and soil samples for laboratory moisture content and density, the variables that directly control thermal properties. The groundwater level also indicates whether the soil at cable depth is likely to dry out in the dry season. See how groundwater level affects foundations and laboratory soil testing.

Field Density Test is used to accept the compaction of cable trench backfill, because Thermal Backfill compacted below specification has more air voids and higher thermal resistivity than designed. Foundations for power poles and substations use the Plate Bearing Test and pile testing as usual.

How Much Does It Cost and How Long Does It Take?

Costs are estimated per project. The biggest factors are the number of test points, the depth and pit method (excavator or borehole), the number of conditions to be measured (natural moisture only or also dry), and travel distance. In terms of time, fieldwork takes about 2 days per point including the 24-hour temperature record, and the report is ready within about 1 week after fieldwork is complete. For a price estimate, please prepare the cable route drawing, cable burial depth and the number of points set by the designer. Prices exclude travel and equipment mobilization, which are charged extra according to site distance. — Contact us for a free quotation within 24 hours