Question

The thickness of gravel pack in tubewells varies from

• A. 5 to 8 cm
• B. 8 to 15 cm
• C. 15 to 20 cm
• D. 20 to 25 cm

Hint:

An ideal gravel pack thickness is 3 inches (approx. 7.5 cm) to 6 inches (approx. 15 cm).
This provides a stable, porous filter without restricting the well development process.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
In tubewell construction, a gravel pack (an envelope of clean, graded sand and gravel) is placed in the annular space between the borehole wall and the well screen.
Its primary function is to prevent fine formation sand from entering the well while allowing water to flow freely.
Detailed Explanation:
- The thickness of the gravel pack must be designed carefully:
- If the pack is too thin ($< 8 \text{ cm}$), it is difficult to place uniformly without leaving gaps, which could lead to sand pumping.
- If the pack is too thick ($> 15 \text{ cm}$), it increases head losses and makes well development (cleaning of fine mud/clays from the formation) extremely difficult.
- Based on standard well-drilling specifications (such as those by the Central Ground Water Board, CGWB), the optimal thickness of a gravel pack ranges from ______8 to 15 cm______ (or approximately 3 to 6 inches).

Step 2: Final Answer:
The standard thickness of a gravel pack in tubewells varies from 8 to 15 cm, corresponding to option (B).