Plate load test – settlement criterion for modulus of subgrade reaction When deriving the modulus of subgrade reaction (k) from a plate bearing test for pavement design, the standard settlement used to compute k is:

Introduction:
The modulus of subgrade reaction k is the proportionality between bearing pressure and surface deflection used in slab-on-grade and rigid pavement analysis. Standardizing the deflection at which k is determined allows consistent comparison across sites and tests.

Given Data / Assumptions:

• Static plate load test performed on compacted subgrade.
• Pressure–settlement curve recorded for a standard plate.
• Rigid pavement design context (e.g., Westergaard).

Concept / Approach:

In pavement engineering practice, k is usually taken as pressure divided by settlement at a small reference deflection. A commonly adopted standard is 1.25 mm (0.05 in). This value provides a representative stiffness near the origin of the load–settlement curve and avoids nonlinearities at larger settlements.

Step-by-Step Solution:

Verification / Alternative check:

Design guides and standards for rigid pavements reference k at 1.25 mm as the benchmark; correlations to CBR or resilient modulus also rely on this convention.

Why Other Options Are Wrong:

Using larger settlements (2.50 or 3.75 mm) shifts k to a lower tangent/slope region and is not the standard reference; 1.75 or 5.00 mm are non-standard for the k-value definition.

Common Pitfalls:

Not correcting for plate size; using total (plastic + elastic) settlement without considering rebound; mixing units when converting mm to m in k calculations.

Final Answer:

1.25 mm