Under-Reamed Piles – Minimum centre-to-centre spacing of bulbs For multi under-reamed piles having bulb diameter du, what is the minimum recommended centre-to-centre vertical spacing between successive bulbs?

Introduction / Context:
Under-reamed piles are commonly used in expansive soils. Enlarged bulbs provide additional bearing and uplift resistance. Proper spacing of multiple bulbs on a single pile ensures that their stress zones do not overlap excessively and that concrete can be placed and compacted effectively.

Given Data / Assumptions:

• Bulb diameter = du.
• Multiple bulbs formed along the same pile shaft.
• Standard practice for vertical spacing to avoid interference.

Concept / Approach:
Guidelines specify a minimum vertical spacing between bulbs as a multiple of bulb diameter. A widely adopted minimum is 1.5 du, balancing structural performance and constructability. Closer spacing risks overlapping stress bulbs and inadequate formation of the enlarged cavities.

Step-by-Step Solution:
Define spacing requirement: centre-to-centre distance along the pile.Apply recommended minimum: 1.5 du.Confirm that options larger than this are acceptable but not “minimum”.

Verification / Alternative check:
Typical standards and textbooks for under-reamed piles in expansive clays cite 1.5 du to 1.75 du; the smallest acceptable is 1.5 du.

Why Other Options Are Wrong:

• du and 1.25 du: too close; risk stress overlap and poor bulb formation.
• 1.75 du and 2 du: safe but not the minimum recommended; thus not the correct answer to “minimum”.

Common Pitfalls:
Ignoring construction tolerances; ensuring adequate neck diameter and reinforcement anchorage between bulbs is essential for performance under uplift and cyclic loading.

Final Answer:
1.5 du