Which soil type typically has the highest bearing capacity for foundation design?
Correct Answer
D) Rock
Rock has the highest bearing capacity, often exceeding 10,000 psf, while clay, silt, and sand have much lower capacities. Rock provides the most stable foundation support due to its density and structural integrity.
Why This Is the Correct Answer
Rock has the highest bearing capacity among all soil types because it is a solid, consolidated material with exceptional structural integrity and density. Rock formations can typically support bearing loads exceeding 10,000 pounds per square foot (psf), and in many cases can handle much higher loads. The crystalline or cemented structure of rock provides superior resistance to compression and settlement compared to any unconsolidated soil material. This makes rock the most desirable foundation material when available at reasonable depths.
Why the Other Options Are Wrong
Option A: Clay
Clay typically has low to moderate bearing capacity, usually ranging from 1,000 to 4,000 psf depending on its consistency and moisture content. Clay soils are problematic for foundations because they expand and contract significantly with moisture changes, leading to potential settlement and heaving issues.
Option B: Silt
Silt has relatively low bearing capacity, typically ranging from 1,000 to 2,000 psf. Silt particles are smaller than sand but larger than clay, and when saturated, silt can become unstable and prone to liquefaction, making it unsuitable for heavy foundation loads.
Option C: Sand
While sand has good bearing capacity compared to clay and silt (typically 2,000 to 6,000 psf for dense sand), it still falls significantly short of rock's capacity. Sand's bearing capacity depends heavily on its density and confinement, and it can experience settlement under load.
Memory Technique
Think 'ROCK SOLID' - rock is literally the most solid foundation material you can build on, while soils get progressively weaker as particle size decreases and cohesion becomes the primary binding force.
Reference Hint
Florida Building Code, Chapter 18 - Soils and Foundations, or geotechnical engineering sections covering allowable bearing pressures
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