Soil Standard Electric Strain Direct Shear Apparatus

1.Core principles and control methods
Q: What is 'Electric Strain' ?What is the difference between it and 'equal stress' ? 
Strain Controlled (most commonly used): The instrument advances the stroke at a constant speed of 0.02mm/min or 0.8mm/min through an electric motor. The magnitude of horizontal shear force depends on the soil sample's ability to resist that velocity. This method can fully record the strength changes (residual strength) of the soil sample before and after failure.
Stress Controlled: Apply horizontal shear force by adding weights in stages until the soil sample suddenly slides and cracks. It cannot accurately measure the softening behavior of soil samples after failure, and modern standard laboratories are rarely used.
Q: What are the "fast shear", "consolidation fast shear", and "slow shear" in direct shear tests? How should I choose?
These are the three core types of direct shear tests, which mainly depend on drainage conditions: unconsolidated undrained (UU): immediately start cutting after applying normal force (usually within 3-5 minutes), and do not allow water to drain out. Suitable for simulating situations where the construction speed is extremely fast and the soil cannot be consolidated and drained in time.
Consolidated Undrawed (CU): After applying normal force, the soil sample is fully consolidated and stabilized, but drainage is not allowed at the moment of shearing, and the speed is fast, such as 0.8mm/min.
Commonly used for stability analysis of highway and railway subgrade under load. Consolidated Drained (CD): After consolidation, shear is carried out at an extremely slow rate, such as 0.02 mm/min, to ensure that the excess pore water pressure generated during the shear process can be completely dissipated. It is closest to a long-term stable state, but the experiment takes an extremely long time.
2.On site operations and details
Q: When should the "Fixing Pins" of the upper and lower cutting boxes be removed during sample assembly?
Extremely important: During the process of loading soil samples, leveling surfaces, placing permeable stones, and applying vertical (normal) loads, the fixing pin must be inserted properly to ensure complete alignment of the upper and lower boxes.
The timing that must be unplugged: At the moment when preparing to click on the motor to start cutting, the fixing pin must be unplugged! If the fixing pin is forgotten to be removed, the thrust of the motor will directly act on the steel pin, causing severe overload of the instrument, bending of the transmission shaft, or damage to the force measuring ring (force gauge).
Q: Why do we need to leave small gaps between Shear Boxes? How to adjust?
Reason: If the steel edge walls of the upper and lower shear boxes are directly attached, the "shear force" measured during shearing will include the frictional force between the steels, resulting in a higher measured soil shear strength.Operation method: The instrument is usually equipped with gap adjustment screws. After pulling out the fixing pin, slightly rotate the adjusting screw to lift the upper cutting box slightly by a small gap of 0.1~0.2 mm, and then start cutting again.
3. Data anomalies and troubleshooting
Q: Why is the calculated Coulomb theory shear strength curve not a straight line, or does the cohesion $c $have a negative value?
Human operation error: In parallel experiments (usually four samples are tested and subjected to soil cutting at 50, 100, 200, and 300 kPa, respectively), the density of the soil samples is inconsistent, or the soil is disturbed during sample loading. Improper cutting speed: For example, slow cutting should have been done, but fast cutting speed was used, resulting in the failure to dissipate pore water pressure. Reaction stiffness issue: If the fixing screw of the force measuring ring (or pressure sensor) is loose, the transmission of shear force will lag, resulting in data distortion under low pressure.
Q: The motor is running normally, but the push rod (shear propulsion shaft) is suspended or not moving forward?
Check the limit switch: Check if the instrument has advanced to the maximum cutting stroke (usually 8-10 mm) and triggered the automatic limit protection. If so, the gearbox or software needs to be switched to "reverse/return" mode.Check clutch/transmission gear: Electric direct cutters usually have both manual and electric gears. Check if the manual/electric shift lever is engaged properly. If in neutral position, the motor's idle push rod will not move.