Artificial islands for marine foundations (sand, rock-bund and cellular islands)

How bridges get built · Caissons and marine foundations · Artificial islands for marine foundations (sand, rock-bund and cellular islands)

Also called: construction island, sand island, sheet-pile island, cellular island, steel cylinder island, bridge-tunnel island

How it is done

Define the island's purpose, plan and crest level: working area around the caisson or pile group, access, crest above the highest construction water level plus wave run-up, and the effect on flow, scour and navigation.
Investigate the seabed for bearing and settlement under the fill, and plan ground improvement (vertical drains with surcharge, sand compaction piles or replacement of soft clay) where the island must not sink or slide.
Build the perimeter: dumped rock bunds with filter layers, sheet-pile cells or rings, or large steel cylinders vibrated into the seabed (Hong Kong–Zhuhai–Macao Bridge).
Fill with sand pumped from a dredger or placed by bottom-dump barges, and compact the fill (vibro-compaction) where foundations or heavy plant will stand on it.
Armour slopes and toe with rock or concrete units against waves and current, and place scour aprons around the perimeter.
Monitor settlement, pore pressure, perimeter deflection and slope stability during filling and use, and control plant loads on the island.
Build the foundation from the island (sink the caisson, drill the piles or build the tunnel portal); dredge temporary islands away to the agreed bed level afterwards and keep permanent ones under inspection.

Key numbers

Water depthtemporary sand islands with plain slopes up to about 3–5 m; deeper with sheet-pile, cellular or cylinder perimeters
Crestabout 0.5–1.0 m above the highest construction water level, plus a wave allowance (Chinese bridge practice)
Working bermat least about 2 m clear around a caisson, more where cranes travel
Cellular perimeterscells about 15–25 m in diameter; the HZMB steel cylinders were 22 m
Underwater sand slopesabout 1:2 to 1:3 without a retaining perimeter

Where it fits

Shallow rivers and estuaries for caisson sinking and pile drilling with land plant; bridge-tunnel transitions in sea crossings; protective islands around piers near shipping channels; sites with dredged sand available nearby.

Where it does not

Deep or fast water where fill is washed out or the island obstructs flood flow and navigation.
Very soft seabeds without ground improvement (slips and large settlement).
Environmentally sensitive beds where fill and later dredging are not permitted.

Choosing it

Choose a temporary island over a floating or piled platform for caisson sinking or pile drilling in shallow, slow water, where fill is cheap and land plant can be used.
Choose a cellular or steel-cylinder perimeter over rock bunds when time, soft seabed or a tight footprint matter; the HZMB cylinders formed the island walls without long dredging and rock placement.
Prefer a floated caisson, jack-up or trestle in deep or fast water, or where an island would obstruct flood flow or navigation.
Use a permanent island where a bridge must change to a tunnel under a shipping channel, sizing it for the portal, ventilation and ship-impact protection.

Plant, pace and money

PlantCutter-suction or trailing suction hopper dredgers, bottom-dump barges, rock-placing vessels, vibratory hammers for sheet piles or cylinders, vibro-compaction rigs, survey vessels, and crawler cranes on the island.

ProductivityA temporary pier island in shallow water takes days to a few weeks; a permanent sea island takes one to three years including ground improvement and settlement.

CostIndicative: USD 0.1–1 million for a temporary river pier island; permanent sea islands cost tens to hundreds of millions of USD, driven by fill volume, perimeter structure and ground improvement.

Risks and controls

What goes wrong

Erosion of fill and scour at the toe in floods or storms.
Slope or perimeter failure on soft seabed during filling.
Island settlement tilting caissons or plant during work.
Flow constriction raising flood levels upstream, and environmental objections.

Quality assurance

Fill quantity and grading records, compaction checks by CPT, settlement plates and piezometers, perimeter deflection monitoring, bathymetric surveys of slopes and scour after floods and storms, and a load plan for plant on the island.

Origins

Sand islands were used to start caissons in shallow rivers from the 19th century; permanent islands for bridge-tunnel transitions followed with the Hampton Roads (1957) and Chesapeake Bay (1964) crossings, and the Hong Kong–Zhuhai–Macao Bridge (2011–2018) formed its island walls with giant steel cylinders instead of conventional seawalls.

Examples

Temporary sand islands for caissons and bored piles in shallow rivers worldwide; permanent islands on the Chesapeake Bay Bridge–Tunnel, Tokyo Bay Aqualine (Umihotaru), Øresund (Peberholm), King Fahd Causeway and Hong Kong–Zhuhai–Macao Bridge.

Case studies

Chesapeake Bay Bridge–TunnelUSA · 1964Four artificial islands, each about 3 ha, carry the transitions between the trestles and the two tunnels under the shipping channels.

Tokyo Bay AqualineJapan · 1997The Umihotaru island marks the change from the 4.4 km bridge to the 9.6 km tunnel; a second artificial island off Kawasaki serves the tunnel's ventilation.

Øresund Link (Peberholm)Denmark · 2000An artificial island about 4 km long, built largely from material dredged for the crossing, links the cable-stayed bridge to the immersed tunnel.

Hong Kong–Zhuhai–Macao BridgeChina · 2018Two artificial islands join the bridge to the 6.7 km tunnel; their walls were formed in 2011 by 120 steel cylinders about 22 m in diameter, vibrated into soft seabed with a synchronised group of vibratory hammers.