Subsurface investigation — when is the auger boring method unsuitable? Auger boring (hand or portable power augers) is commonly used in cohesive, self-supporting soils above the water table. In which of the following ground conditions is auger boring NOT suitable for safe and reliable boring without special casing or support?

Introduction / Context:
Auger boring is a simple and economical exploration method for shallow depths. However, it relies on the surrounding soil remaining stable (self-supporting) and on the auger cutting efficiently. Recognizing when auger boring becomes unsafe or ineffective helps you select better alternatives (cased borings, wash boring, rotary, percussion).

Given Data / Assumptions:

• Hand or portable power helical augers.
• No casing or drilling mud for wall support unless noted.
• Objective is to advance the hole safely while obtaining representative information.

Concept / Approach:

Augers work best in cohesive or otherwise self-supporting soils above the water table. In very hard/cemented layers the bit cannot cut effectively; in fully saturated cohesionless soils the walls slough and the hole collapses without casing; in very soft soils there is insufficient stand-up time and excessive squeezing into the auger flights, leading to unsafe and poor-quality holes.

Step-by-Step Solution:

Verification / Alternative check:

Most geotechnical manuals recommend casing or switching to wash/rotary or percussion in saturated sands and to percussion/rotary coring for very hard or cemented layers; hand augers are explicitly limited to shallow, self-supporting soils above the water table.

Why Other Options Are Wrong:

• (a), (b), (c) individually identify true limitations, but each alone is incomplete.
• (e) “None” is incorrect because several unsuitable cases exist.

Common Pitfalls:

• Attempting to auger below the water table in sand without casing and expecting stable holes.
• Assuming the presence of silt gives cohesion; saturated silty sand may still collapse.

Final Answer:

All of the above.