Site Assessment, Ground Conditions and Siting
Most self-erect crane incidents trace back to the ground or the siting decision. The exam expects a structured assessment rather than a single check.
Ground assessment
- Establish bearing capacity of the supporting material and compare with the maximum imposed pressure under each outrigger/base pad, including dynamic and wind effects.
- Allow for underground features: drains, basements, voids, services, recently filled ground, and excavations that reduce support.
- Provide suitable foundations: purpose-made pads or a designed raft distributing load so pressure stays within capacity. Timber packing alone is rarely a design.
- Consider drainage and frost: saturated or frozen ground changes capacity.
Siting
- Check clearances: to excavations, slopes, overhead power lines, adjacent structures, and the full slewing radius including counter-jib.
- Confirm access for the erection vehicle and for later dismantling.
- Consider wind exposure and the effect of tall buildings creating turbulence.
- Verify permit and licensing issues and any restrictions on overflying neighbouring land.
Worked rule of thumb
If the calculated pad pressure exceeds the assessed bearing capacity, you do not "add more timber" - you increase the pad area or obtain an engineered foundation. Doubling pad dimensions quarters the pressure.
Traps
Questions often describe a "firm, level surface" and expect you to reject it as insufficient without a documented assessment of capacity versus imposed pressure.