Plate Bearing Test – Standard plate size for modulus of subgrade reaction (k) What is the standard circular plate diameter commonly used to determine k in highway/airfield plate load tests?

Introduction / Context:
The plate bearing test is used to estimate the modulus of subgrade reaction k, an input for rigid pavement design and for evaluating foundation support characteristics. A standard plate size is specified so that test results are comparable and representative of slab behavior over a larger area than very small plates would sample.

Given Data / Assumptions:

• Load is applied through a rigid, circular steel plate.
• Deflections are measured under incremental loading.
• Subgrade is reasonably uniform over the test influence zone.

Concept / Approach:

For pavement applications, a 75 cm diameter plate (approximately 30 in) is widely adopted to derive the k-value. The subgrade reaction modulus relates contact pressure to surface deflection under the plate, approximating slab-on-grade behavior. Using the standard plate size helps align field measurements with analytical models and code-based design procedures (e.g., Westergaard-type formulations) that assume a certain load distribution footprint.

Step-by-Step Solution:

Verification / Alternative check:

Other diameters (e.g., 30 cm) are used for foundation tests, but for pavement k-value determination, standards and many specifications call for the 75 cm plate to represent slab action adequately.

Why Other Options Are Wrong:

25–50 cm plates under-sample the subgrade area and may overstate k; 100 cm is heavy and uncommon for routine fieldwork; 30 cm is typical for small footing studies, not pavement k.

Common Pitfalls:

Poor seating of the plate, measuring plastic settlement instead of elastic rebound, or not correcting for layering and moisture variability.

Final Answer:

75 cm diameter