In the field of geotechnical investigation and soil testing, the Vane Shear Test is an in-situ testing method widely used to determine the undrained shear strength of soft clay. According to differences in measurement and transmission systems, it is generally classified into two types: the electronic vane shear test and the mechanical vane shear test. Although both methods share the same testing principle, they differ significantly in equipment structure, operational procedures, and data acquisition methods.

This article, prepared and published by our company, provides a systematic comparison of the operating procedures of these two testing methods to help industry users better understand their differences and select the most suitable solution.

I. Procedure for Electrically Tested Vane Shear Test

1. Install ground anchors on both sides of the test location by screwing them into the soil, and set up and secure the penetration testing main frame. Level the system using a spirit level with a 1 mm graduation, and install the torque application device.

2. Connect the vane head to the torque sensor and tighten it. Insert the drill rod with the cable threaded through it into the drill rod clamp hole of the torque device. Connect the sensor cable plug to the socket passing through the drill rod, and perform waterproof sealing treatment. Switch on the measuring instrument, then tighten the drill rods. The drill rods shall be kept straight, and all joints must be tightly connected. It is recommended to install an enlarging reamer at the drill rod joint approximately 1 m above the vane shear head.

3. After pressing the vane shear head into the soil to the predetermined test depth, adjust the frame so that the drill rod is aligned with the center of the guide hole on the frame panel. If a relatively hard interlayer is encountered at the test depth, the test shall be conducted after penetrating through the layer.

4. After the vane shear head is pressed to the test depth, it shall be left undisturbed for 2–3 minutes before starting the test.

5. Tighten the drill rod clamp on the torque device, and set the measuring instrument to zero or record the initial reading.

6. Rotate the hand crank of the torque device clockwise. When the reading on the measuring instrument begins to increase, start the stopwatch. The drill rod should be rotated at a rate of 1°/10 s to 2°/10 s. Record the reading for every 1° of rotation. After a peak or stable value is reached, continue rotating and recording for an additional 1 minute. The peak or stable value shall be taken as the reading corresponding to the shear failure of the undisturbed soil.

7. After completing the peak or stable value test, rotate the vane head clockwise for six turns to fully remold the surrounding soil.Then determine the undrained shear strength of the remolded soil in accordance with Clause 6 above. The shear strength test of remolded soil shall be carried out according to project requirements.

8. If further testing is required, loosen the drill rod clamp and press the vane shear head to the next test depth, then repeat the procedure described above.

9. After completing the full borehole test, remove the drill rods and vane shear head section by section. Clean all components thoroughly and inspect them to ensure they are in good condition.

10. For vane shear tests conducted in boreholes, the borehole shall be drilled and casing installed to a depth of 3–5 times the casing diameter or 0.5 m above the target test depth. The hole shall then be cleaned using a perforated auger. The vane head, shaft rod, and drill rods shall be assembled section by section and tightened with a pipe wrench. The assembly shall be lowered into the borehole until the vane shear head contacts the borehole bottom. The penetration depth of the vane shear head into the borehole bottom shall not be less than 3–5 times the borehole or casing diameter, or 0.5 m.

11. During the test, avoid exposing the vane head to direct sunlight or freezing. For open-ring vane shear testers, the influence of frictional resistance between the shaft and the soil should be corrected.

12. Before and after engineering tests, the torque sensor of the vane shear head shall be calibrated. The validity period of each calibration is generally 1 to 3 months. If any abnormality occurs during use, recalibration shall be performed. The sensor, cables, and measuring instruments used during calibration shall be the same as those used during the test.

13. When conducting vane tests on water, if the soil at the bottom of the hole is soft, a casing controller should be used to prevent the casing from sinking during the test.

II. Mechanical vane shear test procedure

1. At the test site, lower the casing to a depth 3 to 5 times the casing diameter or 0.5 m above the desired test depth, according to the drilling depth.

2. Secure the casing with wooden casing clamps or chain clamps to prevent it from sinking or rotating in the opposite direction due to excessive torque.

3. Remove residual soil from the borehole. To avoid disturbing the test soil layer, a perforated auger drill is generally used for cleaning.

4. Assemble the vane shear head, shaft rod, and drill rods section by section and tighten them with a pipe wrench. Then lower the assembly into the borehole until the vane shear head contacts the borehole bottom.

5. Connect the guide rod, pass the base through the guide rod, and fix it onto the casing. Tighten it with locking screws, then slowly press the vane shear head to the test depth. If a relatively hard interlayer is encountered at the test depth, the test shall be conducted after penetrating through the layer.

6. After the vane shear head is pressed to the test depth, it shall remain undisturbed for 2–3 minutes before the test is started.

7. Install the transmission components and rotate the base plate to align the guide rod keyway with the fixed clamp keyway of the steel ring. Lock the fixed sleeve to the base with locking screws. Then rotate the hand crank so that the special key falls freely into the keyway. Align the pointer with any integer scale, install the dial gauge, and set it to zero.

8. Start the test by rotating the hand crank at a speed of 1°/10 s to 2°/10 s, while simultaneously starting the stopwatch. Record the dial gauge reading once for every 1° of rotation. When a peak or stable value is reached, continue rotating and recording for an additional 1 minute. The peak or stable reading shall be taken as the maximum dial gauge value Ry corresponding to the shear failure of the undisturbed soil.

9. Remove the special key, install the rotating handle at the top of the guide rod, and rotate it clockwise for six turns to fully disturb the soil around the vane shear head. Then remove the rotating handle and reinsert the special key. In accordance with Clause 8, measure and record the maximum dial gauge value Ry at shear failure of the remolded soil. The shear strength test of remolded soil shall be conducted according to engineering requirements.

10. For clutch-type crossheads, remove the special key, lift the guide rod 2cm~3cm to disengage the clutch teeth, then insert the special key again, rotate the hand crank at a constant speed, and record the stable reading Ry of the gauge for the friction between the shaft and the soil;

11. For the toothed crosshead, rapidly rotate the handle counterclockwise more than ten times to disengage the shaft from the crosshead, then rotate the handle clockwise at a constant speed and record the gauge reading Ry when the shaft rubs against the soil.

12. After the test, remove the rotating parts and base. Insert a hook into the guide rod hole and remove the drill rod and crosshead section by section. Clean the crosshead and check for loose screws and bent shaft.

13. When conducting crosshead tests on water, if the soil at the bottom of the hole is soft, a casing controller should be used to prevent the casing from sinking during the test.

14. For open steel-ring vane shear instruments, the influence of friction between the shaft rod and the soil shall be corrected.

15. Before and after engineering tests, the torque sensor of the vane shear head shall be calibrated. If any abnormality occurs during use, recalibration shall be performed. The sensor, cables, and measuring instruments used for calibration shall be the same as those used during the test.

III.Core Comparison of the Two Test Methods

1. Measurement Principle

Electronic method: Uses a torque sensor to directly convert shear resistance into electrical signals.

Mechanical method: Relies on mechanical transmission and dial gauge (or pointer) readings.

2. Data Acquisition Mode

Electronic method: Automatic digital data collection and recording.

Mechanical method: Manual reading and recording by operators.

3. Testing Accuracy

Electronic method: Higher accuracy with reduced human reading error.

Mechanical method: Relatively lower accuracy, influenced by manual operation.

4. Operation Complexity

Electronic method: Requires electrical connection, calibration, and system setup.

Mechanical method: More mechanical steps, but simpler system structure.

5. Data Stability

Electronic method: Strong anti-interference capability and stable output.

Mechanical method: Data may fluctuate due to operator reading differences.

6. Equipment Structure

Electronic method: Integrated system including sensor, cables, and digital instrument.

Mechanical method: Mainly mechanical components such as rods, crank, and dial gauge.

7. Maintenance Requirement

Electronic method: Requires periodic sensor calibration and electronic maintenance.

Mechanical method: Mainly mechanical inspection and lubrication.

8. Cost and Application Scope

Electronic method: Higher cost, suitable for high-standard engineering and research projects.

Mechanical method: Lower cost, widely used in routine geotechnical investigations.

9. Field Adaptability

Electronic method: Suitable for advanced and continuous monitoring conditions.

Mechanical method: More adaptable in basic field environments with limited power supply.

10. Overall Development Trend

Electronic method: Represents modern, digital, and automated development direction.

Mechanical method: Traditional but still widely used due to simplicity and reliability.

IV. Conclusion

Both electrical and mechanical vane shear tests are essentially used to determine the shear strength of soft clay, but they differ significantly in data acquisition methods and engineering applicability.

With increasing demands for data accuracy and automation in geotechnical engineering, electrical testing equipment is gradually becoming the mainstream development direction; while mechanical equipment continues to maintain important application value in basic engineering and traditional surveying.