By end bearing at the toe, shaft friction along the length, or a combination. End bearing dominates where the pile reaches a dense stratum or rock capable of taking the load directly. Shaft friction dominates in deep uniform soils where no firm stratum is within reach. The ground profile determines which mechanism governs, and that in turn sets pile length and how installation is controlled.

For programme and ground conditions. Driven steel produces no spoil, needs no concrete cure, works immediately at full capacity, can be spliced to whatever depth the ground requires and drives through dense layers that would stop other types. Bored piles compete on very large individual capacities, in ground where driving is unacceptable for noise or vibration, and often on cost in straightforward conditions.

During driving. Blow counts and final set are recorded and compared against the capacity predicted by the driving formula or wave equation analysis, so each pile is checked as it is installed. Dynamic load testing on selected piles, and static load testing where required, confirm the relationship. This continuous verification is a significant advantage over pile types whose capacity can only be tested afterwards.

Yes, routinely, and it is one of the practical strengths of steel piling. Sections are joined by welded splices or by proprietary mechanical connectors as driving proceeds, so the final length is determined by the ground rather than fixed in advance. That matters where the depth to the bearing stratum is variable or uncertain across a site, since it avoids ordering to a guessed length.

The volume of ground displaced as a pile is driven. H-piles displace relatively little because their cross-sectional area is small, so ground heave and lateral movement around them are limited. That matters where piling is close to existing structures, services or slopes that could be damaged by displacement, and it is a reason H-piles are favoured in constrained urban sites.

It depends on the environment. Piles fully embedded in undisturbed natural soil corrode extremely slowly, because oxygen is effectively absent, and often need little or no allowance. Made ground, contaminated fill, water, and the tidal and splash zones on marine works corrode much faster and require sacrificial thickness, coatings, concrete encasement or cathodic protection as appropriate to the design life.

Boulders and obstructions, which deflect piles and can damage the toe; very dense layers, which may cause refusal above the design depth; and sloping rock surfaces, on which a pile toe can skid rather than seat. Pre-drilling, rock shoes on the pile toe, and adjusting the driving system are the usual responses, and all are decided from ground investigation rather than discovered on site.