Last reviewed 30 Sept 2026 · 7 min read
Special features of marine foundations
Compared with foundations on land, foundations in water have:
- Difficult access — construction from barges, jack-up platforms or temporary works;
- Large lateral loads — waves, currents, berthing, mooring and wind;
- Scour of the bed around the foundation;
- Aggressive environment — chlorides, sulphates and marine growth causing corrosion;
- Tidal variation — rise and fall of the water level affecting the construction and the design;
- Soft soils — clays and loose sands, often with a thick layer needing piles or dredging and replacement.
Piles for marine structures
Types
| Pile | Features |
|---|---|
| Steel tubular (pipe) piles | High strength and lateral stiffness, easily driven to a great depth, can be filled with concrete; need corrosion protection (coating, extra thickness, cathodic protection). Common for jetties and dolphins |
| Steel H or sheet piles | Lower cost; sheet piles for walls |
| Precast reinforced or prestressed concrete piles | Durable in the marine environment if of good quality; prestressed piles resist driving stresses and cracking; heavy to handle |
| Cast-in-situ (bored) piles | Formed inside a steel casing or permanent liner; large diameter for heavy loads, used where driving is not possible |
| Timber piles | Traditional; attacked by marine borers (teredo) above the mud-line; used for fenders and light works |
Design
- Axial capacity: from the soil (end bearing plus skin friction) in the same way as land piles, with an appropriate factor of safety (typically 2.5); negative skin friction where the fill settles; pile load tests (static, dynamic) to verify.
- Lateral capacity: the pile is analysed as a beam on an elastic foundation (p–y curves, subgrade reaction), for the berthing, wave and wind loads; raking (batter) piles carry the horizontal load by axial forces in the pile.
- Free length in water is large, so buckling and slenderness need attention; the point of fixity is at a depth below the mud-line.
- Group action: spacing normally at least 2.5–3 times the diameter.
- Marine durability: cover of at least 50–75 mm for concrete in the splash zone; corrosion allowance for steel (see the corrosion note).
Construction
Piles are driven from a barge or a jack-up platform with a pile-driving hammer (diesel, hydraulic, vibratory) guided by a template (frame) to keep the position and rake accurate; a follower may be used in deep water. The driving stresses are checked by the wave equation analysis and dynamic monitoring.
Well foundations (caissons) for bridges and river works
A well foundation is a hollow shaft of masonry or concrete (circular, D-shaped, rectangular or twin) that is sunk through soft soil to a firm stratum by excavating the soil inside (dredging with grabs) and letting it sink under its own weight. It is widely used in India for bridge piers and abutments in rivers.
- Parts: cutting edge (steel shod), curb (a tapered reinforced concrete ring above it), steining (the wall), bottom plug (concrete seal), sand filling or hearting, top plug and well cap which supports the pier.
- Sinking: the well is built in stages by adding lifts of steining; the soil is removed by grabs through the wells (dredge holes). Difficulties: tilt and shift (corrected by controlled dredging, eccentric loading, pulling with wire ropes, or water jets), hard obstructions (boulders and rock: blasting, chiselling), skin friction sticking (reduced by bentonite slurry or air/water jets, and kentledge loads).
- Grip length: the depth of embedment below the maximum scour level — under the IRC practice for well foundations it is taken as about one-third of the maximum scour depth (measured below the high flood level), subject to the stability checks.
- Design checks: allowable bearing pressure at the base, lateral stability (passive resistance of the soil at the sides), and the steining design (stresses under the loads). The design scour depth is = where is the mean scour depth = (Lacey's formula), with = the design discharge per metre width and = silt factor.