Ground improvement — definition and use of “sand piles” (civil engineering foundation practice) Pick the correct statement about sand piles used for densification/improvement of loose granular soils beneath foundations. Choose the description that best reflects how a sand pile is formed and its typical purpose.

Introduction / Context:
In foundation engineering, “sand piles” (also called sand compaction piles) are a classic ground-improvement technique for loose granular or soft cohesive soils. Rather than acting as conventional load-bearing piles, they densify the surrounding soil, reduce compressibility, and improve bearing capacity indirectly. This question checks whether you recognize the correct definition and typical role of sand piles in practice.

Given Data / Assumptions:

• We are considering improvement methods for weak ground under shallow or raft foundations.
• Sand piles are installed by driving a casing or augering, then backfilling with sand in stages and compacting.
• The primary intent is densification and drainage, not direct end-bearing like concrete or steel piles.

Concept / Approach:

Sand piles are constructed by forming a hole (with casing/auger/vibroflot), then filling with granular material (clean sand) in lifts with compaction. The sand column stiffens the composite soil mass and accelerates consolidation if soft fines are present. The system is analogous to stone columns but with sand as the column material. Load transfer is shared between improved soil and the sand columns; the improvement is often specified by replacement ratio and achieved relative density.

Step-by-Step Solution:

Verification / Alternative check:

Field practice (e.g., vibro-replacement with sand) increases SPT N-values, decreases settlement, and mitigates liquefaction susceptibility. Settlement records and plate load tests post-improvement substantiate the function as ground improvement rather than pure end-bearing action.

Why Other Options Are Wrong:

• (a) Mislabels any pile driven in sand as a “sand pile”, which is incorrect terminology.
• (c) Overstates the role; sand piles do not typically act as high-capacity structural piles.
• (d) Incorrect because (b) is correct.

Common Pitfalls:

• Confusing sand piles with cast-in-situ concrete piles or stone columns.
• Assuming capacity comes solely from the column; the composite action with surrounding soil is key.

Final Answer:

A drilled or driven hole that is then filled and compacted with clean sand is called a sand pile.