On a continuous system the sequence is largely a productivity question. On a unit-placed system it is an engineering one, because a partially complete installation is a real condition that has to survive whatever arrives while it is partial.
Where to start
The bank asks directly whether placement should begin at the downstream end, the upstream end, or the lowest elevation.
The toe, or the lowest elevation, first. Three reasons:
- It is the mechanism. The toe is what is being attacked, and armoring it arrests the progression above. See toe protection
- It is the hardest to reach if conditions deteriorate. Water rises; a toe you could reach on Monday may be out of reach on Thursday
- Everything above bears on it. Building down onto an unfinished toe means disturbing completed work
Then build upward and outward from that established base.
Establish the edges early
Flow works at discontinuities, and a partially complete installation has more of them than a finished one.
A perimeter that is defined — even if the area inside it is not yet full — is more stable than a ragged advancing front. Where the sequence allows, running the boundary first and filling inward gives every intermediate state a defensible edge.
This matters most where the work is happening during or shortly after an event. See placing armor at flood stage.
How closely units should sit
The bank covers how closely adjacent units should touch after they settle, and whether they need to be tied together.
The principles:
Contact matters. Units bearing on each other behave differently from units sitting independently. Interlock and mutual support are part of the stability of the layer, in the same way packing matters for riprap.
Gaps are paths. A gap in the armor is a place for flow to reach what is underneath.
Settlement will close some gaps and open others. Which is why the bank asks how a layout accommodates expected settlement without opening large gaps — a layout that is exactly right on day one and has no allowance for settling is a layout that will develop gaps.
The specific spacing guidance for a given size and arrangement comes from the engineering team rather than a web page.
Coverage, and proving it
Placing units is one job. Demonstrating that the protected area is actually protected is another.
Above water it is visible and photographable.
Below water it is not, and coverage confirmation is a survey exercise. See underwater placement.
Either way, the record wanted at the end is: what was placed, how many, where, at what elevations, with the extents of the coverage. That is what makes the installation assessable later, and it is what the first inspection compares against.
Extend past the problem
The single most common defect in a hurried installation, and worth repeating here because sequence is where it happens.
Under pressure, the instinct is to cover the visible damage and stop. The affected footprint is larger than the visible damage, and armor terminating at the edge of an actively eroding zone puts its most vulnerable part in the worst place.
Terminations belong in ground that is not under attack. See revetment anatomy and why scour protection fails.
Leave every stop defensible
Work halts: for weather, for tide, for darkness, for a shift end, for a work window closing.
The question at each of those points is whether what is in place is stable on its own. On a tidal site with a few workable hours a day, this is not an occasional consideration — it is the state the installation spends most of its time in.
Planning the sequence so that each day’s work is a coherent, terminated increment rather than an advancing ragged front is the difference between a partial installation that holds and one that unravels overnight.
Settlement after placement
Expected, and normal within limits. The bank covers what amount of settlement is considered normal after installation, and it is a question worth asking for your size and arrangement.
Two consequences for sequence:
Do not assume the day-one geometry is final. Units bed in, the bed adjusts beneath them, and the layer finds its position.
Set the datum at completion. The first post-installation inspection establishes the baseline that all later inspections compare against. See post-installation inspection.
A working sequence
- Confirm layout, extents, and elevations against the survey
- Establish setting-out references that will survive the work
- Place the toe, or the lowest elevation, along the full length
- Establish the flank terminations
- Build upward and inward from the toe and edges
- Complete the crest and its termination
- Confirm coverage and elevations
- Record the as-built
- Initial inspection, which becomes the datum
Where to go next
- Layouts, single, stacked and keyed
- Transitions and terminations
- Underwater placement
- Post-installation inspection
- The rock bag FAQ for the full filling and placement section
- The installation hub for the rest