Anchored sheet-pile walls

How does Anchored sheet-pile walls work, step by step?

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   Step 1: Investigate the line and confirm drivability

   A ground investigation along the wall line establishes the strata, the strength and the presence of obstructions, and it is read as much for drivability as for design. Old quay structures, buried rubble, boulders and services are located, because a sheet that meets one of them will refuse. On most projects a drivability study and a trial drive follow, and the marine contractor uses them to choose the hammer, the section and the sequence. The designer sets the driven depth and the sheet section; nothing about those follows from the retained height alone.

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   Step 2: Set up the driving frame and pitch the first sheets

   Sheets are pitched in a guide frame - a substantial gate or template that holds the pans in line and vertical - because a sheet-pile wall goes where the first few sheets tell it to go and cannot be argued back afterwards. Work is either from a jack-up or a spudded barge, or from land where the reclamation is already in place. Pairs or panels are pitched, checked for verticality in both directions, and only then driven, so that lean is corrected while it is still small.

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   Step 3: Drive the sheets to design depth

   Driving is by impact hammer, vibratory hammer or hydraulic press, chosen for the ground and for the noise and vibration limits at the location. Panel driving - taking a set of sheets down in stages rather than one sheet to full depth at a time - keeps the wall in line and reduces the risk of a sheet dragging its neighbour or declutching. Driving records are kept for every sheet. Where a sheet refuses above design depth the marine contractor stops and reports it rather than pushing on, because a refused sheet is a design question, not a driving one.

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   Step 4: Cast the capping beam and install the walings

   A reinforced concrete capping beam is cast along the head of the wall to tie the sheets together and to give the true line and level for the berth. Walings - steel or concrete members running along the wall - spread the tie load across several sheets. The tie level is set by the designer and is generally near the top of the wall but below the working deck, so the beam and waling detail has to be built in the right sequence with the backfill.

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   Step 5: Install the anchor system

   Anchors take several forms: a buried concrete deadman block, a row of anchor piles or raking piles, a screw or plate anchor, or ground anchors drilled and grouted into the ground behind. Whichever is used, it has to sit far enough behind the wall to be effective, and how far is set by the designer from the geometry and the ground. Tie rods run from the waling back to the anchor, and they are the most vulnerable component in the whole structure - protected against corrosion, supported so they cannot sag or be bent by settling fill, and detailed so they can be tensioned and, where the design allows, inspected later.

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   Step 6: Backfill in the sequence the design requires

   The order of dredging in front and filling behind is not free. The designer specifies a sequence so that the wall is never asked to carry more out-of-balance load than it has support for at that moment, and the marine contractor follows it. Filter layers and drainage go in behind the wall to relieve water pressure and to stop fine material washing through the interlocks. Fill is placed in layers and the tie rods are protected as it rises around them. The compaction requirement for the fill is the designer's, not the plant operator's.

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   Step 7: Dredge the berth pocket to design level

   The ground in front is dredged away in stages to reach the berth level, with the wall monitored as the support in front is removed. Overdredging is the classic way of damaging a new sheet-pile quay, because it removes passive support the design assumed, so the dredge is surveyed as it proceeds and worked to a controlled level rather than to a plant operator's judgement. The finished pocket is surveyed by multibeam and reconciled against the design.

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   Step 8: Protect against corrosion and fit out

   The corrosion protection system is completed: coatings applied in the zones the designer nominates, cathodic protection anodes or an impressed current system installed and commissioned, and concrete encasement placed where it is specified. Fenders, bollards, ladders, edge protection, services and the apron surfacing follow. Baseline thickness readings on the steel and a baseline survey of the wall line are recorded at handover, because the port authority will be comparing against them for the rest of the berth's life.