Subsurface exploration — limitations of rotary drilling Which of the following statements correctly describe limitations or drawbacks of rotary drilling for geotechnical site investigation?

Introduction / Context:
Rotary drilling is a mainstay of subsurface exploration, enabling rapid penetration and continuous sampling with appropriate coring systems. However, like any method, it has limitations related to formation type, borehole conditions, and test compatibility. Recognizing these constraints leads to better investigation planning and data quality.

Given Data / Assumptions:

• Standard rotary methods using drilling fluid or air for cuttings removal.
• Focus on geotechnical site investigation for foundations, not deep oil/gas wells.
• Small to moderate hole diameters are typical.

Concept / Approach:

In very coarse gravel or boulder beds, rotary methods can be inefficient or impractical because tools skate on large particles and suffer damage; percussion methods or casing advance may be preferable. Drilling fluids can mask natural groundwater levels and disturb hydraulic conductivity around the hole, complicating observation wells and falling/rising head tests. For very small boreholes, mobilisation and tooling costs make rotary drilling uneconomical compared to hand augers or lightweight systems.

Step-by-Step Solution:

Verification / Alternative check:

Geotechnical investigation manuals recommend alternate methods (e.g., shell-and-auger, down-the-hole hammer, sonic) in coarse formations and prescribe standing periods before groundwater readings after drilling with fluid.

Why Other Options Are Wrong:

• Any single option alone is incomplete; the method has multiple limitations as listed.

Common Pitfalls:

• Reading groundwater immediately after drilling with bentonite slurry, leading to erroneous levels.
• Attempting standard permeability tests without allowing bore stabilization.

Final Answer:

All of the above.