Sheet piles interlock with one another to form a continuous wall that resists lateral pressure from soil and water, so they are primarily bending members. Bearing piles are individual sections driven to carry vertical load from a structure into competent ground, so they are primarily axial members. Some elements, notably king piles in combined walls, do both at once and are designed for the combination.

Because of what it permits on site. Steel is installed without excavation or spoil removal, reaches full capacity immediately with no curing, can be spliced to reach greater depth, works in water, and can be extracted and reused on temporary works. Its strength-to-weight ratio makes long sections handleable. Concrete competes on cost and on corrosion in aggressive ground, and the choice usually turns on programme and ground conditions.

The bending capacity a wall delivers for each metre of its length, rather than per pile. It is the correct basis for comparison because piles come in different widths: a wider pile covering more wall per unit needs proportionally more capacity to give the same performance. Comparing piles on their individual section modulus, or on weight per pile, leads to the wrong selection.

By impact driving with a hammer, by vibratory driving, or by silent hydraulic pressing that jacks piles in using the reaction of those already installed. Impact driving suits dense ground and gives the highest driving energy. Vibration is faster in granular soils. Pressing generates very little noise or ground vibration and is used near existing structures and in built-up areas where the other methods are unacceptable.

A sacrificial thickness added to the structural requirement to account for section loss over the design life. Rates differ sharply by exposure zone: buried soil, fresh water, the splash and tidal zones on marine structures, and the atmospheric zone all corrode at different rates, and the splash zone is usually the most aggressive. Design codes publish rates by zone and design life, and coatings or cathodic protection can be used where allowances become uneconomic.

Yes, and it is a significant part of their economics on temporary works such as cofferdams and excavation support. Extraction is by vibratory or hydraulic means and depends on the ground, the interlock condition and whether the piles have corroded or become locked by soil. Piles are commonly hired rather than bought for temporary works, with condition and expected damage agreed in advance.

Interlocks are not watertight by themselves; they resist flow but do not seal. Where the wall must control water, interlocks are treated with a swelling or bituminous sealant applied before driving, or welded after installation on the exposed face. Complete water tightness is rarely achievable, so designs usually combine sealed interlocks with a drainage or pumping strategy rather than relying on the wall alone.

Dense granular layers, cobbles, boulders and obstructions cause piles to deviate, declutch from their neighbours or refuse. Rock at shallow depth limits penetration and may require pre-drilling. Very soft ground gives little lateral support and can allow piles to lean. Declutching, where the interlock separates during driving, is the characteristic failure and is why interlock strength and driving control matter as much as section size.

A cantilever wall relies solely on the passive resistance of the soil below excavation level, which limits retained height. Beyond that, walls are anchored or propped: a waling beam spreads load along the wall and transfers it to tie rods and anchors behind, or to props across the excavation. A capping beam at the head distributes load and gives a finished level to build from.

Routinely, provided corrosion over the design life is accounted for. Quay walls, permanent basement walls, flood defences, bridge abutments and foundations are all built in steel piling with design lives measured in decades. The design combines an appropriate corrosion allowance with, where necessary, coatings, concrete encasement in the splash zone or cathodic protection on marine structures.