The artificial frozen soil shear test is a specialized geotechnical laboratory procedure used to determine the shear strength parameters—cohesion and internal friction angle—of soil that has been artificially frozen for ground freezing construction. By evaluating shear behavior at controlled negative temperatures, this test provides the essential strength data required for designing frozen walls in mine shaft sinking, metro tunnels, and deep excavations. This article presents a comprehensive technical overview of artificial frozen soil shear testing, including specimen preparation, test methods, applicable standards, and typical engineering applications.
Technical Overview
Artificial ground freezing (AGF) is a temporary ground improvement technique in which refrigerant—typically chilled calcium chloride brine or liquid nitrogen—is circulated through buried freeze pipes to convert pore water into ice, creating a strong, impermeable frozen soil structure. Artificially frozen soil behaves as a four-phase composite material consisting of mineral grains, ice, unfrozen water, and gas. The ice acts as a cementing agent, giving frozen soil substantially higher strength and stiffness than the same soil in an unfrozen state.
Because frozen walls must safely resist lateral earth pressure, water pressure, and construction loads, their thickness and stability calculations depend directly on the shear strength parameters of the frozen soil, namely cohesion (c) and internal friction angle (φ). The artificial frozen soil shear test is therefore a critical step in the design and safety verification of freezing projects. Since frozen soil strength is strongly temperature-dependent, tests are performed at the negative temperatures expected in the frozen wall during construction and service.
In China, shear testing of artificial frozen soil is generally performed in accordance with the MT/T 593 series of coal industry standards for physical and mechanical testing of artificial frozen soil, together with the national geotechnical testing standard GB/T 50123. These standards specify requirements for specimen preparation, temperature conditioning, loading rates, and data interpretation.
Testing Samples
Undisturbed Samples
Undisturbed samples are recovered from exploration boreholes at the project site, targeting the soil strata that will be frozen. Sampling techniques include thin-walled tube sampling and block sampling from test pits. To preserve the natural structure, moisture content, and density of the soil, samples are immediately sealed, insulated, and transported under frozen or temperature-controlled conditions, then stored in a low-temperature chamber until specimen preparation.
Remolded Samples
When undisturbed sampling is impractical—for example, in deep alluvium or highly disturbed strata—remolded specimens are prepared in the laboratory. Soil is compacted in layers into molds at the target dry density and moisture content matching the field conditions. After compaction, the density and water content of each specimen are verified before freezing to ensure consistency across the test program.
Specimen Dimensions and Freezing Conditioning
Triaxial shear specimens are typically cylindrical, commonly 61.8 mm in diameter with a height-to-diameter ratio of approximately 2.0 to 2.5, while direct shear specimens are prepared to match the dimensions of the shear box. Specimens are placed in an environmental chamber at the design test temperature, such as −5 °C, −10 °C, −15 °C, or −20 °C, and conditioned for at least 24 hours. Temperature sensors embedded at the specimen center confirm that the internal temperature has reached the target value, typically within a tolerance of ±0.5 °C, before loading begins.
Testing Items
Typical parameters and properties determined in an artificial frozen soil shear test program include:
- Shear strength of the frozen soil at specified negative temperatures
- Cohesion (c) and internal friction angle (φ) derived from the Mohr–Coulomb failure envelope
- Stress–strain curves, peak deviator stress, and failure strain
- Shear strength as a function of normal stress or confining pressure
- Relationship between shear strength and freezing temperature
- Optional studies on strain-rate effects, soil salinity, and long-term (creep) shear behavior
Testing Methods
Direct Shear Test
In the direct shear test, a frozen cylindrical or square specimen is placed in a temperature-controlled shear box inside a climatic chamber. A constant normal stress is applied vertically, and a horizontal shear load is then applied at a controlled displacement rate until failure. A series of specimens is tested under different normal stresses, and the peak shear stresses are plotted against normal stress to define a linear failure envelope, from which cohesion and friction angle are obtained. Quick shear conditions are commonly used to evaluate the short-term strength of frozen ground.
Triaxial Shear Test
The triaxial shear test is the preferred method for deep freezing projects because it realistically simulates the confining stress conditions around a frozen wall. A frozen cylindrical specimen is sealed in a rubber membrane, placed in a low-temperature triaxial cell, and subjected to a hydraulic confining pressure. Axial load is then applied at a constant strain rate, typically in the range of 0.1 to 1.0 percent per minute, until the specimen reaches peak strength or a defined failure strain. At least three or four confining pressures covering the design stress range are tested. Because ice is essentially impermeable over short loading times, frozen soil triaxial tests are usually conducted under unconsolidated-undrained (UU) conditions.
Equipment, Procedure, and Data Interpretation
The test system consists of a triaxial or direct shear apparatus installed within a low-temperature environmental chamber, a refrigeration circulator for cell and chamber temperature control, and instrumented load, pressure, displacement, and temperature sensors connected to a data acquisition system. Temperature stability is maintained throughout loading, since even small temperature fluctuations can measurably alter frozen soil strength.
Test results are interpreted using the Mohr–Coulomb criterion. In direct shear testing, the failure envelope is expressed as τ = c + σ·tan φ. In triaxial testing, Mohr circles constructed from the major and minor principal stresses at failure are used to fit the envelope and determine c and φ. Frozen soils typically exhibit very high cohesion due to ice cementation, while the friction angle remains relatively moderate. Failure modes range from brittle, with a distinct peak stress, to ductile and plastic, depending on temperature, soil type, and confining pressure.
Applications
Artificial frozen soil shear test results directly support the design, construction, and safety assessment of ground freezing projects, including:
- Freezing method shaft sinking for coal mines and deep alluvium, where frozen wall thickness and strength govern shaft safety at depths of several hundred meters
- Metro tunnel cross passages and connecting tunnels constructed under rivers, railways, and urban infrastructure
- Frozen walls for shield tunnel launching and receiving shafts, work shafts, and deep excavations
- Emergency ground treatment and groundwater cutoff for tunnel collapses, quicksand conditions, and leakage remediation
- Temporary frozen sealing for pipe jacking and utility tunnel entry and exit operations
In each application, the measured cohesion and friction angle at the design freezing temperature are used in limit equilibrium, elasticity, or numerical analyses to verify frozen wall stability, determine required wall thickness, and confirm safety factors during excavation and permanent structure construction.
FAQ
What test temperatures are typically used for artificial frozen soil shear testing?
The test temperature is selected based on the average or design operating temperature of the frozen wall, and commonly ranges from −5 °C to −20 °C. Colder temperatures are used for deeper, more heavily loaded frozen structures. Testing at multiple temperature levels allows the strength–temperature relationship to be established for design use.
Which standards govern the artificial frozen soil shear test?
In China, testing generally follows the MT/T 593 series of coal industry standards for artificial frozen soil, which includes the triaxial shear strength test method, supplemented by the national geotechnical testing standard GB/T 50123. Project-specific technical specifications may impose additional requirements.
How long must specimens be frozen before testing?
Specimens are conditioned at the design temperature for at least 24 hours, and testing begins only after the temperature at the specimen center has stabilized within the specified tolerance. This ensures the ice phase and unfrozen water content are in equilibrium at the target temperature.
How does temperature affect the shear strength of artificial frozen soil?
As temperature decreases, unfrozen water content drops and ice cementation strengthens, causing shear strength—primarily cohesion—to increase significantly, often in an approximately linear or power-function relationship with negative temperature. The internal friction angle varies comparatively less with temperature.
Why are frozen soil shear tests necessary when unfrozen parameters are already known?
Frozen soil strength can be several times higher than unfrozen strength, but it is also highly sensitive to temperature, time, and stress conditions. Using unfrozen or assumed parameters would produce either unsafe designs or grossly uneconomical frozen wall dimensions, so laboratory shear testing at representative temperatures is essential for safe and economical ground freezing design.