Raft (mat) foundations — typical uses and advantages Select the most appropriate combined statement describing when raft foundations are adopted in building and civil works.

Introduction / Context:Raft or mat foundations consist of a continuous slab (sometimes with beams or ribs) supporting multiple columns or walls. They are used where soil bearing capacity is low or variable, or where uplift and settlement control motivate a large-area foundation system.

Given Data / Assumptions:

• Subsoil has relatively poor bearing capacity or inclusions of soft pockets.
• Structure may be subject to uplift or flotation effects (e.g., high groundwater).
• Multiple columns or walls need a unified foundation.

Concept / Approach:By spreading loads over a larger area, rafts reduce net contact pressure. They can bridge over localized weak zones (soft pockets) and, with adequate thickness and reinforcement, resist uplift or hydrostatic pressures by mobilizing self-weight plus structural action. They also help control differential settlement across a building footprint.

Step-by-Step Solution:

Verification / Alternative check:Settlement analyses and flotation checks (buoyancy vs dead load) often confirm the suitability of a raft where isolated footings would be uneconomical or risky.

Why Other Options Are Wrong:

• Selecting only one reason underestimates the versatility of rafts; all are valid.
• none of these: incorrect because each listed application is standard practice.

Common Pitfalls:Neglecting punching shear at column locations; ignoring groundwater drawdown or uplift changes; insufficient joint and waterproofing detailing.

Final Answer:all the above