Geothermal Rock-Soil Thermal Response Test: Principles, Methods, and Applications

The geothermal rock-soil thermal response test (TRT) is an essential in-situ investigation method used to determine the thermal conductivity of underground formations and the thermal resistance of borehole heat exchangers for ground-source heat pump projects. By circulating a heat carrier fluid at a controlled heating power through a test borehole, engineers can obtain reliable ground thermal parameters that directly govern the design depth, quantity, and spacing of ground heat exchangers. This article explains the technical principles, test samples, measured parameters, testing methods, engineering applications, and frequently asked questions related to geothermal rock-soil thermal response testing.

Technical Overview

The geothermal rock-soil thermal response test is an in-situ geothermal testing technique designed to evaluate the thermal properties of subsurface rock and soil formations surrounding a borehole heat exchanger. During the test, a fluid is circulated through a vertical ground heat exchanger while a constant heat injection or extraction power is applied, and the inlet and outlet fluid temperatures, flow rate, and power are continuously recorded over a sustained period. From the measured temperature evolution, key parameters such as the initial ground temperature, the effective thermal conductivity of the formation, and the borehole thermal resistance can be derived through analytical or numerical models.

These parameters are fundamental to the rational design of ground-coupled heat pump (GCHP) systems, because the required total borehole length and the long-term operational performance of the system are highly sensitive to ground thermal properties. National and international standards, including GB 50366 (the Chinese technical code for ground-source heat pump systems) and ISO 17628 (geotechnical investigation and testing — geothermal testing), specify the equipment requirements, test procedures, duration, and data evaluation methods for thermal response tests. Laboratory methods such as ASTM D5334 provide complementary thermal conductivity measurements on recovered soil and rock samples, and guidelines published by ASHRAE are also widely referenced for test planning and interpretation.

Testing Samples

Test Borehole with Vertical Heat Exchanger

The primary test object is a dedicated test borehole, typically drilled to a depth of 100 to 150 meters (or to the designed depth of the actual project), fitted with a high-density polyethylene (HDPE) U-loop heat exchanger, either single-U or double-U configuration. The borehole is backfilled with grout or bentonite-based material and allowed to reach thermal equilibrium, generally for at least 24 to 48 hours after drilling and grouting, before testing begins.

Soil and Rock Core Samples

Core or cutting samples recovered during drilling of the test borehole can be subjected to laboratory thermal conductivity testing. These samples represent different geological strata and allow the stratified thermal properties of the formation to be assessed in combination with the in-situ test results.

Backfill and Grouting Materials

The thermal conductivity of the grouting or backfill material placed in the annular space between the U-loop pipes and the borehole wall may also be evaluated, because it directly influences the overall borehole thermal resistance.

Heat Carrier Fluid

The circulating fluid, normally water or a water-antifreeze mixture, is characterized for its volumetric heat capacity and flow characteristics, since these quantities are required inputs for the analysis of the test data.

Testing Items

A complete geothermal rock-soil thermal response test program typically determines the following parameters:

• Initial undisturbed ground temperature (°C), representing the equilibrium temperature of the formation before heat injection.
• Effective thermal conductivity of the rock-soil formation (W/(m·K)), averaged over the tested borehole depth.
• Comprehensive borehole thermal resistance (m·K/W), including the resistance of the pipes, circulating fluid, and grout.
• Thermal diffusivity and volumetric heat capacity of the formation, where the analysis method permits.
• Heat injection or extraction rate per unit borehole length (W/m) and its stability during the test.
• Inlet and outlet fluid temperatures (°C) and circulating flow rate (m³/h) as continuous time-series data.
• Stratified laboratory thermal conductivity of soil and rock samples (W/(m·K)).
• Influence of groundwater seepage on the apparent thermal properties, where hydrogeological conditions indicate significant flow.

Testing Methods

Initial Ground Temperature Measurement

The initial ground temperature is measured before any heat is injected, either by circulating the fluid through the loop without heating and recording the stabilized outlet temperature, or by lowering a temperature probe or distributed optical fiber sensor into the borehole to obtain a vertical temperature profile.

Constant Heat Injection Test

According to GB 50366 and ISO 17628, a constant heating power—commonly in the range of 30 to 80 W per meter of borehole, with a recommended fluctuation within ±1% to ±2% of the set value—is applied to the circulating fluid. The test is generally continued for at least 48 hours, and commonly 48 to 72 hours, until the temperature rise follows the theoretically expected logarithmic trend. Fluid temperatures at the inlet and outlet, flow rate, ambient conditions, and power consumption are logged continuously, typically at intervals of one minute or shorter.

Data Evaluation Models

The most widely used analysis is the infinite line-source model, in which the slope of the mean fluid temperature plotted against the logarithm of elapsed time yields the effective thermal conductivity, while the intercept provides the borehole thermal resistance. The cylindrical heat source model and full parameter-estimation techniques based on numerical simulation (finite difference or finite element models) are also applied, particularly when test durations are short or groundwater movement is suspected. Data from the initial stabilization period are excluded from the regression to reduce bias.

Laboratory Testing of Samples

Recovered soil and rock samples may be tested using the transient hot-wire or needle-probe method in accordance with ASTM D5334, providing stratified thermal conductivity values that support and cross-check the in-situ results.

Enhanced Thermal Response Testing

Enhanced or distributed thermal response tests employ optical fiber distributed temperature sensing along the borehole depth, enabling depth-resolved thermal conductivity profiles and the identification of groundwater flow zones that a conventional test cannot distinguish.

Applications

Geothermal rock-soil thermal response test results are applied throughout the lifecycle of shallow geothermal energy projects:

• Design of vertical borehole fields for ground-source heat pump systems in office buildings, hospitals, hotels, residential districts, and industrial facilities, including determination of borehole quantity, depth, and spacing.
• Feasibility studies and shallow geothermal resource assessment for district heating and cooling projects.
• Thermal design and optimization of energy piles and other thermo-active ground structures.
• Calibration of ground thermal storage models used to evaluate long-term heat accumulation or depletion beneath building clusters.
• Verification of construction quality of borehole heat exchangers, including the effectiveness of grouting, during commissioning of ground-source heat pump systems.
• Site characterization for closed-loop geothermal systems in regions with complex stratigraphy or active groundwater flow.

Accredited third-party laboratories and specialized geotechnical testing institutions commonly perform these tests and issue reports that serve as the technical basis for design review, system optimization, and acceptance of shallow geothermal engineering projects.

FAQ

Why is a thermal response test necessary for ground-source heat pump projects?

Design based only on literature values of ground thermal conductivity can carry errors of 20% or more, which may lead to significant over-sizing or under-sizing of the borehole field. A site-specific thermal response test reduces design uncertainty, controls initial drilling investment, and safeguards the long-term operating efficiency of the heat pump system. GB 50366 requires that a thermal response test be carried out for ground-source heat pump projects when the building cooling or heating load is large or the borehole quantity exceeds a specified threshold.

How long does a thermal response test take?

The standard test duration is typically 48 to 72 hours of continuous heat injection, following a stabilization period of 24 to 48 hours after borehole completion and an initial temperature measurement phase. The overall on-site program, including drilling, pipe installation, and grouting, usually requires one to two weeks.

What factors affect the accuracy of the test results?

Key factors include the stability of the heating power, the accuracy and calibration of temperature sensors, the test duration, the completeness of borehole backfilling, and natural groundwater seepage, which can increase the apparent thermal conductivity. Short tests, unstable power, and thermal disequilibrium before testing are the most common sources of error.

Can one test borehole represent the entire project site?

For sites with uniform geology, one test borehole is generally sufficient. For large projects exceeding certain area or borehole quantity limits, or where geological conditions vary significantly, GB 50366 and ISO 17628 recommend performing tests at multiple representative locations to capture spatial variability in ground thermal properties.

Is a heating test sufficient for systems that will extract heat from the ground?

In most formations, heat injection and heat extraction produce symmetrical results, so a heating test is the standard approach and is safer for the equipment. Where thermal asymmetry, groundwater flow, or freezing risk of the carrier fluid is a concern, a supplementary heat extraction test can be performed to verify performance under cooling-dominant operating conditions.