Precast panels, made ground and a live 11kV line
You are managing a 12-week extension at a primary school. Twelve precast wall panels, 6.5t each, are to be landed on a new slab. Ground conditions are made ground over backfilled cellars, and the crane will stand on the only hardstanding, with its outriggers at a 22m radius. A 100t mobile crane is booked. The hire company's lift plan is a single-page standard form; it lists 'outrigger pads 2.5m x 2.5m, ground bearing 100 kN/m² assumed'. The slew will come within 7m of an 11kV overhead line along the boundary, and the only pedestrian route to the school hall passes beneath the planned load path. The head teacher asks whether you can lift on Saturday 'so the children aren't around'. No lift supervisor has been named other than the crane operator.
1. Is this a basic or a complex lift, and what planning outputs follow from your answer?
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Complex. Not because of the load weight - a 100t crane handling 6.5t at 22m is well inside its capacity - but because of the configuration and the environment:
- the slewing load will pass close to a live 11kV overhead line;
- third parties (children, parents, staff) use a route under the load path;
- the crane stands on made ground over cellars, not on ground designed for it;
- there is a single access route, so any failure blocks the site and possibly the school.
Outputs: an appointed person to plan the lift, a written lift plan specific to this crane, this configuration and this ground, a named lift supervisor with the authority to stop the operation, a permit to work, a verified ground assessment under each outrigger, a defined exclusion zone, a rescue and emergency plan, and a briefing for everyone involved including the school. The hire company form is a starting point for a planner, not a lift plan - nothing on it was written for this site.
2. How do you deal with the ground bearing question, and what do you do if the answer comes back that the ground is not strong enough?
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The word that should stop you is 'assumed'. Outrigger reactions are point loads that can far exceed a simple share of the load, so the pressure under a 2.5m pad can be many times higher than the assumed bearing value - and under backfilled cellars there may be voids and loose fill that a pad will simply punch into.
So the assumption is replaced with information: ground investigation or plate bearing tests at each outrigger position, carried out by a competent person, with the results fed back to the crane planner to size the pads or spreader mats. That is the sequence - measurement, then design, then lift.
If it comes back insufficient, the options in order of preference: engineered spreader mats or a crane mat designed for the load, relocating the crane to sound ground or reducing the radius so a smaller crane is closer in, or constructing a designed hardstanding. Which one you choose is a commercial and programme decision, but it is made after the ground information, not instead of it. What you must not do is accept 100 kN/m² because it is printed on the hire company's form, and what you must not do is allow a trial lift to 'see how it goes'.
3. How do you control the overhead line and the footpath, and why doesn't moving the lift to Saturday solve the problem?
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Overhead line - work the hierarchy from the top:
- Avoid - ask the network operator to divert or isolate the line, arranged well in advance, in writing, with confirmation of isolation before the lift. Nothing on site substitutes for this.
- If isolation is not possible, treat the line as live at all times: goalposts and barriers at the safe clearance distance advised by the network operator, marker boards, a banksman watching the slew, and a lift plan that assumes the line cannot be seen and will not be respected by a distracted operator.
Seven metres sounds generous until you add boom deflection, wind and a slewing load. Do not plan to the optimistic case.
Footpath: the load must not pass over people. Either design the lift so the load path never crosses the route, or close and divert the route properly - a signed diversion, and a traffic regulation order or the highway authority's agreement if the diverted route puts people on the road. 'Nobody will be there' is an assumption, not a control.
Why Saturday fails: a school site on a Saturday still has community users, hirers, dog walkers, deliveries and contractors. The children's absence reduces one population and leaves the rest. Moving the lift to a quiet time is a risk-reduction measure at best; it is not a substitute for an exclusion zone and a planned load path. And the exclusion zone under the load is absolute whatever day of the week it is.