Soil freezing force, internationally known as adfreeze strength or frozen bond strength, is the shear and adhesion resistance that develops when soil freezes and bonds to a foundation surface, and its accurate determination is essential for safe design in permafrost and seasonally frozen ground. This article explains the principle of soil freezing force measurement, laboratory and in-situ testing methods, applicable standards such as GB/T 50123, GB 50324, JGJ 118 and ASTM frozen-soil test methods, sample preparation, and typical engineering applications. It is intended for geotechnical engineers, cold-regions designers, and project owners who need reliable frozen soil–structure interface parameters.
Technical Overview
Soil freezing force refers to the tangential (shear) and normal (adhesive) resistance developed at the interface between frozen soil and a structural material such as concrete, steel, or timber. When pore water in soil freezes against a foundation surface, ice cementation creates a bond whose unit resistance, expressed in kilopascals (kPa), is called the adfreeze strength or frozen bond strength. This parameter governs two critical design scenarios: the load-carrying capacity of piles and anchors embedded in permafrost, where adfreeze along the frozen interface transfers structural loads to the ground, and the frost-jacking uplift of foundations in seasonally frozen ground, where adfreeze in the active layer mobilizes upward forces as the soil freezes.
Adfreeze strength is not a fixed material property; it is a system response that depends strongly on environmental and interface conditions. Key influencing factors include:
- Ground temperature: adfreeze strength increases substantially as temperature drops below 0 °C, often approximately linearly in the range of about -1 °C to -10 °C before gradually leveling off.
- Soil type and gradation: fine-grained silts and clays and ice-rich soils behave differently from sands and gravels, and ice lensing at the interface strongly affects the bond.
- Moisture and total water (ice) content: strength generally peaks near full saturation and decreases when an excess ice film separates soil particles from the structural surface.
- Salinity: dissolved salts depress the freezing point and can drastically reduce adfreeze strength, a decisive issue in coastal, saline lacustrine, and treated soils.
- Interface material and roughness: rougher concrete surfaces and certain surface conditions increase mechanical interlock, while coatings may be used intentionally to reduce bond for anti-frost-heave measures.
- Loading rate and duration: because ice at the interface creeps, short-term (rapid) adfreeze strength is markedly higher than long-term strength under sustained load, and design of permanent structures in frozen ground is usually controlled by the long-term value.
Consequently, a professional soil freezing force determination program must control or document temperature, soil state, interface preparation, and loading regime, and must distinguish clearly between short-term peak values and long-term allowable design values.
Testing Samples
Undisturbed Frozen Soil Cores
For permafrost sites, undisturbed frozen cores are recovered by core drilling with refrigerated or low-heat bits and are transported in insulated containers with temperature loggers to ensure the samples never thaw or experience unintended freeze-thaw cycles. Cores are stored in cold rooms at in-situ or lower temperatures (commonly at or below -10 °C) and are trimmed in a frozen state to standard cylindrical or ring specimens for testing.
Remolded (Reconstituted) Specimens
When project-specific fill, backfill, or parametric studies are required, remolded specimens are prepared at controlled dry density and water content, then frozen directionally under a temperature gradient to reproduce the ice lens structure and interface conditions that form in nature. This approach allows systematic investigation of the influence of moisture content, density, salinity, and temperature on adfreeze strength.
Soil-Structure Interface Specimens
Because freezing force is an interface property, representative structural material blocks or plates are required. Concrete blocks are cast with the project mix design and finished to the specified surface roughness (for example, formwork or roughened surfaces), cured and air-dried to field moisture conditions; steel, timber, or coated plates may also be evaluated. The soil specimen is frozen in intimate contact with the material surface so that a continuous ice bond forms before loading.
Saline and Special Soils
Saline soils, soils with high organic content, and artificially frozen or chemically treated soils require dedicated sampling and testing protocols, including measurement of salt content and unfrozen water characteristics, since salinity and unfrozen water fundamentally alter the freezing bond.
Testing Items
Typical items determined in a soil freezing force test program include:
- Tangential (shear) adfreeze strength between frozen soil and concrete, steel, timber, or coated surfaces at specified temperatures.
- Normal freezing force (adhesion strength perpendicular to the interface).
- Short-term peak adfreeze strength under rapid loading at controlled displacement rates.
- Long-term (sustained-load) adfreeze strength and creep parameters of the frozen interface.
- Adfreeze strength versus temperature relationships over a temperature matrix (for example, -0.5, -1, -2, -4, -8, and -10 °C).
- Residual adfreeze strength after interface slip, relevant to post-peak behavior and seismic or cyclic considerations.
- Influence coefficients of water content, dry density, salinity, interface roughness, and loading rate.
- In-situ adfreeze capacity of prototype piles or anchors by pull-out testing.
Testing Methods
Laboratory Interface Shear Adfreeze Test
The most common laboratory method is a direct interface shear test conducted inside an environmental chamber or refrigerated shear apparatus. The soil specimen is frozen onto or against a plate or insert made of the foundation material and held at the target temperature for a sufficient conditioning period (typically 24 hours or more after thermal equilibration) to ensure complete ice bonding. A normal pressure representing the field confinement is applied, and a shear load is then imposed at a controlled displacement rate appropriate to the short-term or long-term objective. The peak shear stress divided by the contact area gives the adfreeze strength at that temperature and normal stress. The test is repeated at multiple temperatures and normal stresses to construct design charts.
Pull-Out (Extrusion) Adfreeze Test
In the pull-out configuration, a material core or block is frozen within a soil ring or mold and extracted axially, mobilizing the frozen bond over a defined cylindrical contact surface. This method is convenient for comparing foundation materials, surface finishes, and anti-adhesion coatings, and for quality control of anti-frost-heave measures, because it directly measures the force required to break the ice bond.
Long-Term Creep Adfreeze Test
Because ice-rich interfaces creep under sustained load, permanent structures in frozen ground must be designed against long-term adfreeze strength. In creep tests, sustained loads are applied in stages to interface specimens in cold rooms, and interface displacement is recorded continuously. The long-term strength is determined from creep curves, isochronous strength envelopes, or a limiting secondary creep rate criterion, and is typically substantially lower than the rapid peak value. Multi-stage and single-stage creep procedures at several temperatures are used to derive the time- and temperature-dependent design parameters.
In-Situ Pile Adfreeze (Pull-Out) Test
For important permafrost projects, adfreeze strength is verified in the field by instrumented pull-out tests on prototype piles or anchors. After installation, the pile is allowed to freeze back, with ground temperatures monitored by thermistor strings until the surrounding soil returns to the natural thermal regime. A reaction system applies axial uplift in maintained-load increments or at a controlled rate, and the measured ultimate resistance is converted to an average adfreeze stress along the frozen bonded length, taking into account the measured temperature distribution. Field results are used to calibrate laboratory parameters and confirm design assumptions.
Applicable Standards and References
Testing is performed in accordance with recognized geotechnical and cold-regions standards, including GB/T 50123 (Standard for Geotechnical Testing Method, which contains frozen soil test provisions), GB 50324 (Code for Engineering Geological Investigation of Frozen Ground), and JGJ 118 (Code for Design of Soil and Foundation of Buildings in Frozen Soil Regions), together with relevant ASTM frozen-soil practices such as ASTM D5918 for frost heave and thaw weakening susceptibility and ASTM D7099 terminology for frozen soil and rock. Where project specifications differ, accredited third-party laboratories establish detailed test procedures consistent with these frameworks and document all deviations.
Instrumentation and Quality Control
Temperature control and measurement are the backbone of reliable results: cold chambers and circulating baths typically maintain specimen temperatures within ±0.1 to ±0.2 °C, verified by calibrated thermistors or PT100 sensors embedded at and near the interface. Loads are measured with calibrated load cells and displacements with LVDT or dial gauges, all recorded through data acquisition systems. Water content, dry density, salinity, and interface surface condition are documented for every specimen, and duplicate tests are recommended to characterize scatter, which is inherently significant in frozen geomaterials.
Reporting and Interpretation
A complete test report presents specimen provenance and preparation, soil index properties and ice structure, interface material and roughness, temperature and loading histories, individual and average adfreeze strengths with failure descriptions, strength versus temperature relationships, long-term strength or reduction coefficients, and recommended design values with stated reliability. Design use must integrate the measured values with the predicted thermal regime of the foundation over its service life.
Applications
Determination of soil freezing force supports design and assessment across a wide range of cold-regions and frozen-ground engineering scenarios:
- Bearing design of piles, caissons, and thermo-syphon foundations in permafrost, where adfreeze along the permanently frozen zone carries structural loads.
- Anti-frost-heave and anti-jacking design of foundations, buried utilities, and piers in seasonally frozen ground, where adfreeze in the active layer drives uplift forces.
- Anchor and guy foundations for transmission towers, communication masts, and cable systems in cold climates.
- Pipelines, storage tanks, and terminal facilities subject to differential frost heave and thaw settlement.
- Artificial ground freezing projects for mine shafts, tunnels, and deep excavations, where the frozen wall-structure interaction must be quantified.
- Railways, highways, bridges, and airports in seasonally frozen and permafrost regions, including thermal regime verification during operation.
- Cold storage facilities, ice rinks, and industrial freezer floors where slab and foundation adhesion and heave behavior matter.
- Research and specification development for saline permafrost, degrading permafrost under climate warming, and new anti-adhesion materials or coatings.
FAQ
What is the difference between freezing force and frost heave force?
Freezing force (adfreeze strength) is the bond resistance at the soil-structure interface produced by ice cementation, whereas frost heave force is the load imposed on a structure by the growth of ice lenses and volume expansion as soil freezes. Both must be considered: freezing force governs load transfer and uplift anchorage, while heave force represents the driving load that adfreeze in the active layer can transmit to foundations.
Why is long-term adfreeze strength lower than short-term strength?
The ice phase at the interface creeps under sustained stress, causing stress relaxation and slow interface slip over time. As a result, the long-term adfreeze strength that can be relied upon for permanent structures is substantially lower than the rapid peak measured in a short-term test, often by a factor of two or more for ice-rich contacts, which is why creep testing or established reduction coefficients are required for design.
What temperature range should be tested?
The temperature matrix should bracket the coldest and warmest ground temperatures expected at the interface during the structure's service life. A typical matrix spans about -0.5 °C to -10 °C for most permafrost foundations, while deep permafrost or artificial freezing projects may require tests down to -15 °C or colder.
How are frozen samples protected during transport and storage?
Undisturbed frozen cores are sealed, packed in insulated containers with cold accumulators or portable freezers, and monitored with temperature loggers so that any thermal excursion is detected. In the laboratory they are stored in cold rooms at or below their in-situ temperature, and freeze-thaw cycling is strictly avoided because it destroys the natural ice structure and invalidates results.
Can freezing force be tested for saline or coated surfaces?
Yes. Saline soils are tested at their natural salinity, with salt content and unfrozen water reported, because salinity can reduce adfreeze strength dramatically. Coated, painted, oiled, or specially finished surfaces are tested in the pull-out or shear configuration precisely to quantify how much the bond is reduced for anti-frost-heave design, or increased where higher anchorage is desired.
How long does a freezing force test program take?
Short-term interface shear or pull-out testing at several temperatures, including freezing and thermal conditioning, typically requires one to two weeks. Long-term creep programs, which may run multiple load stages at multiple temperatures, can extend from several weeks to a few months, so projects with tight schedules should schedule creep testing early.
What deliverables should a client expect?
Clients should receive a formal report containing specimen and interface documentation, calibrated temperature and load data, adfreeze strength values with statistical treatment, strength-temperature charts, long-term design values or reduction coefficients, and clear statements of test conditions and limitations, enabling direct input into foundation design calculations.