Definition of a shallow foundation based on depth-to-width ratio A foundation is classified as shallow if its depth (measured from ground level to foundation base) is:

Introduction:
Foundation classification guides selection, analysis, and construction methods. The core distinction between shallow and deep foundations is the relative depth of the foundation compared with its breadth, which correlates with load transfer mechanisms and potential settlement behavior.

Given Data / Assumptions:

• Spread footings, strip footings, and rafts are typical shallow foundations.
• Depth is the embedment below ground surface; width is the least lateral dimension.
• Soil is reasonably uniform within the influence depth.

Concept / Approach:

Shallow foundations distribute loads near the surface such that depth D is equal to or less than width B (D/B ≤ 1). Deep foundations (e.g., piles, caissons) develop capacity significantly below the surface with D/B > 1 and differ in load-transfer mechanisms (skin friction and/or end bearing at depth).

Step-by-Step Solution:

Verification / Alternative check:

Textbook definitions and codes adopt this ratio-based criterion rather than absolute depths in metres, as soils and loads vary widely.

Why Other Options Are Wrong:

Absolute limits like 1 m or 3 m are arbitrary and not generally correct; “greater than its width” describes deep foundations.

Common Pitfalls:

Using depth-only criteria without considering width; confusing raft foundations (shallow) with caissons (deep).

Final Answer:

equal to or less than its width