Geotechnical site exploration guideline: As a rule of thumb, the minimum depth of exploration recommended for each storey of a building is

Introduction / Context:
Determining an adequate depth of geotechnical exploration helps identify soil layering, groundwater conditions, and potential weak strata influencing settlement. While rigorous criteria use stress bulbs and percentage stress increase, rules of thumb are commonly referenced at the planning stage.

Given Data / Assumptions:

• Low- to mid-rise building with conventional spread footings or raft.
• Preliminary planning stage (before detailed foundation modeling).
• Uniform soil profile is not guaranteed; exploration should capture critical layers.

Concept / Approach:
A practical guideline is to explore to at least about 2 metres per storey, subject to a minimum depth sufficient to intercept any compressible layer that could influence total or differential settlement. In detailed design, depth should extend where the net foundation pressure causes less than about 10% stress increase, or until a firm stratum is confirmed, whichever is deeper. The rule of 2 m per storey provides an initial planning benchmark for borehole depth scheduling.

Step-by-Step Solution:
Adopt the planning rule: ~2 m per storey.Cross-check with structural loads and expected foundation type to adjust as necessary.Ensure the exploration captures soft layers or perched water tables affecting settlement.Select “2 metres” as the thumb-rule answer.

Verification / Alternative check:
Many practice guides suggest 1.5–3.0 m per storey, with 2 m a balanced nominal figure; final depths are refined using stress distribution analysis (e.g., influence charts) and field observations.

Why Other Options Are Wrong:
1 m: Often insufficient to capture influence depth for storey loads.3–4 m: May be conservative or uneconomical at planning stage without specific triggers.0.5 m: Clearly inadequate for foundation decisions.

Common Pitfalls:
Using the rule blindly without checking stress bulbs; stopping before encountering a compressible layer that later governs settlements.

Final Answer:
2 metres.