Which type of soil typically has the highest bearing capacity for foundation design?
Correct Answer
B) Dense sand and gravel
Dense sand and gravel typically provide the highest bearing capacity, often 3000-6000 psf or more, due to their excellent drainage and load distribution characteristics.
Why This Is the Correct Answer
Dense sand and gravel have the highest bearing capacity because the particles are well-compacted and provide excellent interlocking between grains. The dense packing creates strong load distribution characteristics, and the coarse particles allow for excellent drainage, preventing water-induced settlement issues. These soils can typically support 3000-6000 psf or more, making them ideal foundation materials.
Why the Other Options Are Wrong
Option A: Clay with high plasticity
Clay with high plasticity has poor bearing capacity due to its tendency to expand and contract with moisture changes, poor drainage characteristics, and potential for significant settlement under load.
Option C: Organic silt
Organic silt has very poor bearing capacity because organic materials decompose over time, creating voids and instability, plus silts have poor drainage and are susceptible to liquefaction.
Option D: Loose fine sand
Loose fine sand has low bearing capacity because the particles are not well-compacted, creating potential for settlement, and fine particles can shift under load without proper confinement.
Memory Technique
Think 'Dense = Dollars' - dense sand and gravel can support the most 'weight' (money/load) because the particles are tightly packed like coins in a roll, while loose or organic soils are like loose change that shifts around.
Reference Hint
Florida Building Code, Chapter 18 - Soils and Foundations, or geotechnical engineering sections covering soil bearing capacities and allowable soil pressures
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