The most common plant problem on this work is not a machine that is too small. It is a machine that is big enough close in and not at the reach the job actually requires.
Work it from the furthest unit
Placement reach is not uniform. The units nearest the machine are easy; the ones at the far edge of the protection, at the toe, out in the channel, are the governing case.
So the sequence is:
- Filled weight of the unit, which depends on size and on aggregate density. See fill stone selection
- The maximum working radius you will need, measured to the furthest placement position from where the machine can actually stand
- The load chart for that machine in that configuration at that radius
- Rigging weight added to the load
- A margin, because site conditions are not chart conditions
The bank covers how required capacity is calculated from bag weight and placement reach, and what working radius contractors should use when checking capacity.
Where the machine can stand
This constrains the radius, and it is frequently the real limiting factor.
- Is there a standing position with a line to the work?
- Is the ground sound? The bank covers ground-bearing checks beneath filling and lifting equipment. On a failing bank this matters twice over, because the ground the machine stands on may be the ground that is failing. See safety during emergency placement
- Is the standing position stable through the shift? On an active site it may not be
- Can the machine reposition as the work progresses, or does one position have to cover the whole run?
That last question often decides the machine. A job that can be worked from several positions needs less reach than one worked from a single hardstanding.
Overhead and clearance
The bank addresses planning placement near overhead utilities, bridges, and limited-clearance structures.
Under a bridge deck this is usually the binding constraint rather than capacity: boom height is limited by the soffit, which limits reach, which limits how far out you can place. It is the reason abutment work is awkward. See stabilizing a scoured abutment fast.
Overhead power lines are a separate and non-negotiable constraint with statutory clearances.
Land or water
The bank covers when placement from a barge is more practical than from the bank.
From the bank where reach and standing allow, which is cheaper and simpler.
From a barge where the work is beyond reach from land, where there is no standing position, on a berth face, or where the site is only accessible by water. It brings its own considerations, covered in barge and vessel-based placement.
From a vessel with a crane and ROV or diver for subsea work. See underwater placement.
The filling operation needs plant too
Easy to plan the placement and forget the production.
- A loader or excavator feeding the filling frame or hopper
- A machine to move filled units to the placement position, if that is a separate operation
- The lifting machine itself
Whether one machine can do more than one of those depends on the cycle and the layout. The bank covers staging space requirements for empty units, aggregate, filling, and lifting. See site access and staging.
Attachments and accessories
The bank addresses which lifting accessories are supplied and which the contractor provides. Establish this in writing before mobilisation.
The rigging arrangement is a single attached ring, and the connection to the machine has to suit it. See lifting and rigging.
What decides the bag size, in reverse
Worth noting because it runs the other way sometimes.
Larger units place faster and offer more mass per placement. They also need more capacity, more reach, and heavier plant.
On a site where access limits you to a small machine, the practical size is the one that machine can place at the required radius, and the layout is worked from there. That is a legitimate way to arrive at a size, and it should be a stated assumption rather than a discovery on site.
The bank covers how bag size affects the equipment required, and the calculator will size from conditions. Where access is the binding constraint, say so when you send the site details.
The check that avoids a wasted day
Before mobilisation, confirm in one place:
- Machine, configuration, and load chart
- Standing positions and ground bearing
- Maximum radius required, to the furthest unit
- Capacity at that radius, with rigging
- Overhead clearance
- Connection compatibility with the rigging
- Who supplies what
Where to go next
- Lifting and rigging
- Site access and staging
- Barge and vessel-based placement
- Underwater placement
- The rock bag FAQ for the full equipment and access section
- The installation hub for the rest