In a plate bearing test, the modulus of subgrade reaction (k) is taken from the load–deflection curve at a standard settlement. Which settlement value (in centimetres) should be used for computing k?

Introduction / Context:
The modulus of subgrade reaction, k, is a key input for Westergaard plate theory and rigid pavement design. In practice, k is derived from the load–deflection curve of a plate load test at a standard, small deflection so that the response approximates the linear range and represents near-surface stiffness relevant to slabs-on-grade.

Given Data / Assumptions:

• Field plate load test provides pressure vs. settlement (load–deflection curve).
• We must select a standard settlement to compute k = pressure / settlement.
• Rigid pavement design practices typically reference a small elastic deflection.

Concept / Approach:
To allow comparison across sites and tests, standards adopt a reference settlement of 1.25 mm, which equals 0.125 cm. Using this point, the secant stiffness to the origin approximates the subgrade spring constant per unit area. Larger settlements would capture nonlinear or bearing failure effects, not the desired near-elastic response for slab design.

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