When the soil near the surface is too weak to carry a building's load — soft clay, fill, expansive soils, or a high water table — engineers stop trying to spread the load (shallow footings) and start trying to reach past it. That's a pile foundation: a slender column driven, drilled, or cast deep into the ground to transfer load to competent material below.

Piles carry load through two fundamentally different mechanisms, and most piles use both:

• End-bearing piles push their tip into a hard stratum — rock, dense gravel, or stiff till. The load travels down the shaft and dumps into the bearing layer like a column on a footing. Think of a piling that punches through 40 feet of soft river silt and sits on bedrock.
• Friction piles (shaft-friction or "floating" piles) transfer load along the entire embedded length through skin friction with the surrounding soil. Used where no firm stratum exists within reach — common in deep alluvial deposits like Mexico City or Boston's clay.

Common pile types:

• Driven piles — steel H-piles, pipe piles, or precast concrete hammered in. Fast, displaces soil, generates ground vibration and noise.
• Drilled shafts (caissons) — a hole is augered, rebar cage dropped in, concrete poured. Quiet, large diameter (2–10 ft), great for high single-pile loads.
• Micropiles — small-diameter (under 12") grouted piles drilled into tight access spots, used for underpinning existing buildings.

Rule of thumb — ultimate capacity:

Q_ultimate = Q_tip + Q_shaft = (q_p × A_tip) + (f_s × A_shaft)

Where q_p is the tip bearing pressure and f_s is the average skin friction. Apply a factor of safety of 2–3 for allowable load. For an HP12×74 H-pile (~1.0 ft² tip area) driven to refusal on dense sand with q_p ≈ 200 ksf, the end-bearing alone gives ~200 kips — before counting any side friction along the shaft.

Why this matters in practice: the Leaning Tower of Pisa is a friction-pile failure analog — its shallow foundation found no firm stratum and slowly punched into soft clay on one side. Modern code requires a geotechnical investigation (borings + Standard Penetration Tests) before any pile design, because pile capacity depends entirely on what the engineer can't see.

Driving records matter too: contractors log blow counts per foot during installation. If a pile drives easier than predicted, it didn't reach the design stratum and capacity is suspect. "Refusal" — typically defined as 5+ blows per inch — is the field signal that end-bearing has been achieved.