Top-down construction over wetlands

How bridges get built · Heavy lift, moving and accelerated construction · Top-down construction over wetlands

Also called: top-down viaduct, end-on construction, leapfrog construction

How it is done

Build the first spans from the bank or an access pad by conventional means, and design the deck and girders to carry the construction plant: crane tracks or outriggers, pile hammer, girder transporters.
Set up the end-on spread at the working front: a crawler crane or gantry on the completed span (or on a short advancing trestle), with a pile template cantilevered ahead.
Drive the piles for the next bent through the template (precast concrete or steel pipe), checking position and set; cut off and cap them with precast or cast-in-place caps.
Deliver girders along the completed deck and set them with the end crane or a launching gantry; place deck panels or cast the deck.
Advance the crane and template onto the new span once the girders or deck are rated for it, and repeat span by span.
Keep environmental controls in place throughout: no ground access below, turbidity barriers, and limits on piling noise in sensitive seasons.
Finish barriers, joints and surfacing from the deck behind the working front.

Key numbers

Spansabout 20–45 m precast girders or slab spans
Paceone span per 2–5 days per working front
Crane on the deckcommonly 100–300 t crawler, with track or outrigger loads checked against the girders
Scalesuited to kilometre-scale crossings (LA 1 Phase 1A, about 8.9 km and 435 spans)
Cost5–15 % above trestle-based construction per metre where a trestle is feasible (indicative); much more than conventional construction where fill or ground access would otherwise be used

Where it fits

Wetlands, mangroves, protected marshes, mudflats, sites where trestles are prohibited or too expensive; spans 25-45 m.

Where it does not

Spans that a launching girder cannot handle, foundations that need bored piles with slurry plants.

Choosing it

Choose it over a temporary work trestle when permits forbid access below, or the trestle would be several kilometres long and costly to remove.
Prefer a work trestle or barges when the foundations need bored piles with slurry plants or heavy drilling rigs that cannot work from the deck.
Prefer a span-by-span launching gantry for long viaducts where spans exceed what an end crane can set or the girders are full-span precast.
Accept the single-front pace, or open intermediate fronts only where access is permitted.

Plant, pace and money

PlantLaunching girder or end-crane, crane with hammer, deck transporters.

ProductivityOne span per two to five days.

Cost5-15 % above trestle-based construction per metre; cheaper where the trestle would be long.

Risks and controls

What goes wrong

Overloading the fresh deck, pile positioning accuracy from the deck, crane stability at the deck end.

Quality assurance

Deck load ratings posted, pile position survey, crane outrigger positions on the girders.

Origins

End-on construction from the completed deck became common in the US Gulf and Atlantic coastal states as environmental permits limited temporary fill and trestles in wetlands; Louisiana was building swamp viaducts this way by the early 1990s.

Examples

Louisiana's swamp viaducts and the LA 1 elevated highway to Port Fourchon, and other Gulf and Atlantic coast wetland crossings; the game's wetland sites.

Case studies

LA 1 elevated highway, Leeville to Fourchon (Phase 1A)USA · 2011About 8.9 km and 435 spans built end-on over sensitive wetlands, with gantry cranes on a short trestle that advanced as each span was completed.

Louisiana DOTD twin swamp viaducts (reported by ENR, 1991)USA · c. 1991About 3 km of twin elevated roadways built entirely from the decks, advancing one bay at a time with work platforms and piling templates; reported to cost over 50 % more than conventional wetland construction.

In the bridge game

What Bridge Master applies when you pick it

Deck types: PSC I-girder, PSC U-girder, Precast segmental box, Voided slab, Spliced PSC girder, Steel plate girder, Steel box girder, RC slab, RC T-beam. Spans 20-45 m, up to 55 m.

+35: Wetland rule: build from the deck.
+25: No temporary works allowed below: top-down.
-20: Unnecessary where access is free.
-25: Deck-mounted plant cannot place girders longer than about 50 m.

Try it in the game →