Wear on the liquid limit device is considered excessive when the contact point exceeds what diameter?

The correct answer indicates that wear on the liquid limit device is considered excessive when the contact point exceeds a diameter of 13 mm. This specification is important because the liquid limit device plays a critical role in determining the plasticity of materials, particularly soils. When conducting tests, the precision of the equipment is vital to ensure accurate readings and results.

When the contact point wears down to a diameter greater than 13 mm, it can affect the consistency of the results since the wear impacts how the device interacts with the soil sample. A larger contact point may not be able to effectively replicate conditions required for the test, leading to discrepancies in the measured liquid limit. Therefore, maintaining the integrity of the device through regular checks on wear is crucial for reliable testing outcomes.

In the context of the other options, the specified wear thresholds of 10 mm, 11 mm, and 15 mm do not align with the defined standard for excessive wear. A diameter of 15 mm would imply even greater wear, at which point the results could be significantly compromised. Therefore, the 13 mm threshold serves as a practical limit for ensuring testing accuracy and reliability.