Combi-walls

How does Combi-walls work, step by step?

1. 1

   Step 1: Establish the ground and choose the wall type

   The investigation along the wall line has to be good enough to answer two questions: can a large tube be driven to the depth the design needs, and what fixity will it get there. Dense layers, boulders, obstructions and rockhead level all matter. The designer compares a combi-wall against a plain sheet-pile wall and against a piled relieving deck, and the answer usually turns on retained height and drivability. All the sizing - tube diameter, wall thickness, spacing, driven depth, infill section and any anchor - is the geotechnical and marine engineer's work for that berth.

2. 2

   Step 2: Fabricate the tubes with their interlocks

   Tubes are supplied or fabricated to length, with interlock sections welded along both sides to receive the infill sheets. That welding is a controlled fabrication activity with its own inspection regime, because an interlock that is out of line or out of tolerance will not thread a sheet on site, and correcting it over water is slow and expensive. Shop coatings are applied at the same stage where the corrosion protection design calls for them, since coating in a yard is far better work than coating on a barge.

3. 3

   Step 3: Set out and drive the king piles

   Tubes are pitched in a heavy guide frame and driven by impact or vibratory hammer, or drilled and driven where hard strata demand it. Position and plumb are surveyed on every tube, in both directions, because the infill sheets have to thread between them. Driving records - blow counts, energy, penetration - are kept for each tube and reviewed against the drivability assessment. Where a tube refuses high, the marine contractor stops and the designer decides, since a short king pile changes the wall's behaviour.

4. 4

   Step 4: Thread and drive the infill sheets

   Infill sheets are pitched into the interlocks on the tubes and driven to the depth the design requires, which is generally well short of the tube toe. Threading is the operation that punishes any error in the tubes, so it is done as soon after driving as practical while the survey is fresh and corrections are still possible. Interlock integrity is checked as the sheets go down, and a declutched sheet is dealt with rather than covered up.

5. 5

   Step 5: Complete the tubes and cast the capping beam

   Where the design calls for it, tubes are cleaned out, reinforced and filled with concrete, which stiffens the wall and adds corrosion life. A reinforced concrete capping beam is then cast along the head of the wall, connecting the tubes and the sheets into a single line and providing the level and alignment for the berth fittings. Where the wall is anchored, walings and tie rods are installed to the designer's arrangement at this stage.

6. 6

   Step 6: Backfill and dredge in the specified sequence

   The sequence of filling behind and dredging in front is set by the designer and followed exactly, with wall movement monitored as the balance changes. Filters and drainage go in behind the wall so that water pressure is relieved and fine material cannot wash through the interlocks. The berth pocket is dredged in controlled stages and surveyed, with overdredging treated as a defect because it removes support the wall was designed to have.

7. 7

   Step 7: Install the corrosion protection system

   Coatings are made good where driving has damaged them, encasement is placed at the levels specified, and the cathodic protection system - sacrificial anodes or impressed current - is installed, connected and commissioned with its readings recorded. Baseline thickness measurements are taken on the tubes and the sheets at the levels the port authority will re-measure in service. This is the part of the project most often squeezed by programme, and it is the part that decides how long the berth lasts.

8. 8

   Step 8: Fit out, prove and hand over

   Fenders, bollards and quick-release hooks are installed and proof loaded, crane rails are set where the berth carries rail-mounted cranes, and services and apron surfacing are completed. A multibeam survey of the pocket, a topographic survey of the apron, the as-built wall line and the full driving and corrosion protection records are issued together, along with the movement monitoring baseline the port authority will read against.