Site exploration — material where rotary drilling is generally the fastest For which type of soil/ground condition is rotary drilling typically the fastest and most efficient during geotechnical site investigation?

Introduction / Context:
Choosing the right drilling technique affects speed, cost, and data quality in subsurface investigations. Rotary drilling performance varies with formation type due to cuttings removal, bit interaction, and borehole stability considerations.

Given Data / Assumptions:

• Standard rotary rigs with appropriate bits and circulation (water/air/mud).
• Goal is production rate (penetration speed) with acceptable sample recovery.
• Comparing general soil categories, not specialized rock coring rigs.

Concept / Approach:

In clean to slightly silty sands, rotary augers or rotary wash methods typically achieve very high penetration rates due to easy cutting and efficient cuttings removal. In stiff clays, penetration is moderate and may ball up the bit; in rocky formations, rotary coring can be slow because progress depends on bit wear and fracture toughness of rock (unless using down-the-hole hammers in specific contexts). Hence, sandy soil is generally the fastest for rotary methods in routine geotechnical work.

Step-by-Step Solution:

Verification / Alternative check:

Production logs from site investigations often show peak ROP (rate of penetration) in sandy layers compared with stiff clays and intact rock.

Why Other Options Are Wrong:

• Rock drilling is slower due to coring requirements and bit wear.
• Clay can be problematic due to balling and stability issues.
• “All of these” is incorrect because speeds vary significantly.

Common Pitfalls:

• Assuming uniform behavior across sands; very loose saturated sands may need casing.

Final Answer:

Sandy soil.