Plate load test for ultimate bearing capacity: For determining the ultimate bearing capacity of soil, a square steel bearing plate of size 30 cm to 75 cm is recommended. What should be the minimum plate thickness to ensure rigidity and avoid flexure during loading?

Introduction / Context:
In a field plate load test, a thick, rigid steel plate is used to simulate the foundation contact area and to obtain a realistic load–settlement curve. If the plate is too thin, it can bend under load, corrupting settlement readings and leading to an unsafe estimate of bearing capacity. This question checks knowledge of the standard minimum plate thickness used in geotechnical practice.

Given Data / Assumptions:

• Square plate size typically between 30 cm and 75 cm on a side.
• Objective: measure load–settlement behavior to infer ultimate bearing capacity.
• Plate must behave as a rigid body (no flexural distortion).
• Soil reaction assumed uniform beneath a sufficiently rigid plate.

Concept / Approach:

The plate must be thick enough that its flexural stiffness greatly exceeds the soil reaction induced curvature. Field guidance and codal practices widely adopt a nominal minimum thickness so that the plate response remains essentially rigid under commonly applied jack loads in granular or cohesive soils. Among practical choices, 25 mm thickness has become a conventional baseline that prevents noteworthy bending for the plate sizes mentioned.

Step-by-Step Solution:

Verification / Alternative check:

Empirical practice and many test setups use 25 mm thick plates successfully without measurable flexure, validating settlement readings. Heavier loading or larger plates sometimes use even thicker plates, but 25 mm is the accepted minimum for the stated size range.

Why Other Options Are Wrong:

5–20 mm thicknesses are insufficient for rigidity over 30–75 cm spans under high jack pressures; they can bend and distort settlements.

Common Pitfalls:

Using thin plates due to convenience; not accounting for bending that masks true soil deformation; ignoring plate roughness and seating corrections.

Final Answer:

25 mm