Automatic Unsaturated Direct Shear Apparatus

Product Description

Fully Automatic Unsaturated Direct Shear Apparatus

Description

This equipment is a fully automatic unsaturated direct shear apparatus, models BTU-2U and BTU-2UM, used to determine the shear strength parameters of unsaturated soils under different suction conditions. It uses micro‑computer controlled automatic loading, is equipped with high‑precision sensors and an automatic bubble flushing device, and supports temperature control as an option.

Main Features:

• Specimen size: Ø61.8×H20 mm (other sizes optional).
• Normal loading: 2U – 0‑400 kPa automatic control; 2UM – 0‑3200 kPa automatic control.
• Normal displacement: 0‑12.7 mm, accuracy 0.003 mm, resolution 0.001 mm.
• Horizontal shear force: 2U – 5 kN load cell; 2UM – 10 kN load cell, accuracy ±0.1% FS.
• Shear rate: 0.0001‑6 mm/min, stepless adjustment.
• Pore water pressure: 0‑0.1 MPa, accuracy 0.1 kPa, resolution 0.1 kPa.
• Pore air pressure: 0‑0.9 MPa, accuracy 1 kPa.
• Volume change measurement: 0‑100 mL, accuracy ±0.01 mL, resolution 0.001 mL.
• Ceramic disk: 500 kPa high‑air‑entry, imported from the USA.
• Control method: 16‑key membrane keypad + 240×128 LCD.
• Acquisition and processing software: Includes data logger and Windows software.
• Special feature: Fully automatic diffused air flushing device for flushing bubbles under the ceramic disk and measuring the volume of diffused air.
• Optional: Temperature control system (5‑40 ℃).

Test Standards (International)

ASTM D3080(Direct shear test)

ASTM D5298(Suction measurement)

ASTM D6836(Unsaturated soil test methods, including axis‑translation technique)

ISO 17892‑10(Direct shear test)

BS 1377‑7(Direct shear test)

ASTM D7608(Residual strength – for reference)

The equipment controls matric suction using the axis‑translation technique and meets the basic requirements of the above standards for unsaturated soil testing.

Specification

Detail

• Axis‑translation technique– Applies pore air pressure (uₐ) while maintaining pore water pressure (u_w = 0 or controlled), enabling independent control of matric suction s = uₐ – u_w.
• High‑air‑entry ceramic disk– 500 kPa air‑entry value (imported), capable of controlling suction in the range 0‑500 kPa, covering most unsaturated soil testing needs.
• Diffused air flushing device– Fully automatic periodic flushing of air bubbles that accumulate under the ceramic disk, preventing blockage of water pathways and ensuring stable suction control; also measures the volume of diffused air for drainage correction.
• Normal loading difference–
• 2U: maximum normal stress 400 kPa, suitable for shallow unsaturated soils or low‑stress tests.
• 2UM: maximum normal stress 3200 kPa, can simulate deep unsaturated soils or high‑stress conditions.
• Horizontal shear force– 2U: 5 kN (max shear stress ~1.67 MPa); 2UM: 10 kN (~3.3 MPa), suitable for high‑strength or coarse‑grained soils.
• High‑precision volume measurement– Volume change resolution 0.001 mL, ideal for monitoring small drainage/absorption changes in unsaturated soils.
• Temperature control option– 5‑40 ℃ system for studying temperature effects on unsaturated soil strength.
• Software functions– Automatic data acquisition, real‑time curve display, calculation of shear strength parameters.

Application

• Unsaturated slope stability– Determine c′, φ′, φᵇ under different net normal stresses and suctions for rainfall‑induced landslide analysis.
• Expansive and collapsible soils– Study the effect of suction (water content) changes on shear strength.
• Landfill covers– Evaluate unsaturated soil mechanical behaviour.
• Subgrades and foundations– Obtain strength parameters for soils above the capillary zone.
• Deep unsaturated soils– The 2UM model can simulate normal stresses up to 3.2 MPa, suitable for deep soil layers or high fills.
• Temperature effect studies– Optional temperature control system for studying temperature‑suction‑strength coupling (e.g., geothermal, nuclear waste disposal).

Advantages

• Fully automatic suction control– Axis‑translation combined with an automatic diffused air flushing device ensures long‑term stable suction control.
• High‑precision measurement– Displacement 0.001 mm, volume change 0.001 mL, pore water pressure 0.1 kPa – meeting research‑grade requirements.
• Two models cover wide stress range– 400 kPa for conventional unsaturated soils, 3200 kPa for deep high‑stress soils.
• High shear force option– 2UM provides 10 kN horizontal force for testing high‑strength or coarse soils.
• Automatic diffused air flushing– Eliminates manual intervention, ensures test continuity and improves data reliability.
• High resolution volume measurement– 0.001 mL accurately captures small volume changes during drainage/absorption.
• Temperature control option– Optional 5‑40 ℃ module for studying temperature‑suction‑strength coupling.
• Easy operation– Membrane keypad + LCD screen, with Windows software for fully automated test procedures.

What To Choose

Select the model based on testing requirements:

Note: If a different specimen size (other than Ø61.8 mm) is required, custom shear boxes can be ordered from the manufacturer.

Process Flow

Example:Unsaturated direct shear test (consolidated drained, controlling net normal stress and suction)using BTU-2U or 2UM.

• Ceramic disk preparation

Soak the 500 kPa ceramic disk in de‑aired water and apply vacuum to saturate (remove air bubbles).

Install the ceramic disk into the shear box base, ensuring a good seal. Connect the flushing lines.

• Specimen preparation

Cut undisturbed unsaturated soil or prepare remoulded specimen using a Ø61.8×20 mm cutting ring.

Measure initial water content and density.

• Specimen installation

Place the specimen on the ceramic disk, then install porous stone and loading plate.

Assemble the upper shear box and place the assembly into the direct shear apparatus.

Connect the pore air pressure line (to the top of the shear box) and the pore water pressure sensor (below the ceramic disk).

Connect the volume change measurement system (to the drainage line).

• Apply suction

Apply target air pressure (e.g., 100 kPa) via the pore air pressure controller.

Drainage line is open to atmosp, so matric suction s = uₐ = 100 kPa.

Start the automatic diffused air flushing device (periodic flushing), record volume change until drainage stabilises.

• Apply net normal stress

Apply target net normal stress σ_net (e.g., 200 kPa) via the automatic normal loading system.

Total normal stress σ = σ_net + uₐ = 300 kPa.

Record normal displacement until consolidation is stable.

• Set shear parameters

In the software, set shear rate (e.g., 0.02 mm/min for slow shear or 0.8 mm/min for fast shear).

Set shear displacement limit (e.g., 6 mm).

Select “standard shear module” or “fast shear module”.

• Start shearing

Start the horizontal stepper motor to move the shear box.

Software records horizontal shear force, horizontal displacement, normal displacement, volume change, pore water pressure, and time in real time.

Plot shear stress‑displacement curve.

• Stop condition

Automatically stops when the preset shear displacement is reached or after a clear peak shear stress drop.

• Disassembly and cleaning

Unload normal force, release pore air pressure.

Remove specimen, clean shear box and ceramic disk.

Run the flushing device to clean the lines under the ceramic disk.

• Data processing

Software calculates peak shear stress (or residual shear stress).

For constant suction and varying net normal stress, plot Mohr‑Coulomb envelope to obtain effective stress strength parameters c′ and φ′.

For constant net normal stress and varying suction, calculate φᵇ.

Generate test report.

Summary:The BTU-2U/2UM are fully automatic unsaturated direct shear apparatuses using axis‑translation for suction control, equipped with an automatic diffused air flushing device and high‑precision sensors. The 2U is suitable for conventional unsaturated soils (max. normal stress 400 kPa), while the 2UM is designed for high‑stress deep unsaturated soils (max. 3200 kPa). Both support an optional temperature control system. They are applicable to slope stability, expansive soils, landfill covers, deep fills, and other research fields. Selection should be based primarily on the required normal stress range and shear force capacity.