Pile foundations — where are they generally preferred in civil engineering practice?

Introduction / Context:
Pile foundations transfer loads to deeper, stronger strata or mobilize skin friction where near-surface soils are weak, compressible, or scour-prone. Choosing piles is a function of load, settlement tolerance, groundwater, scour risk, and construction constraints.

Given Data / Assumptions:

• Bridges require foundations through water or deep alluvium with scour risk.
• Skyscrapers impose very high loads and settlement limits.
• Low-rise residences and runways may often be supported on shallow foundations where subgrades are competent.

Concept / Approach:
Piles are preferred when shallow foundations cannot safely carry loads or meet settlement criteria. They also provide uplift, lateral resistance, and scour resilience. For bridges, piles are common for piers and abutments founded in rivers or soft deposits. For very tall buildings, deep piles or barrettes distribute large axial and lateral loads to bearing layers or mobilize shaft resistance in deep deposits.

Step-by-Step Solution:

Verification / Alternative check:
Design codes and case histories show widespread use of piles for river bridges and high-rises, while many residential structures succeed with footings or rafts on competent soils.

Why Other Options Are Wrong:

• Low-rise residences (option C) often use shallow foundations when soils allow.
• Runways (option D) rely on ground improvement and thick granular bases rather than discrete deep piles in most cases.

Common Pitfalls:
Assuming piles are always superior; economy and constructability often favor shallow solutions when soils permit.

Final Answer:
Both bridge foundations and very tall buildings