Foundation Bearing Shortfall and an Open Excavation at the Crane Base
A 55 m jib tower crane will be erected on a fixed base on a new 6.0 m × 6.0 m × 1.4 m reinforced concrete foundation. The manual requires 200 kPa foundation bearing and the base level to 1:500. The geotechnical report gives 150 kPa allowable bearing for the silty clay at that depth, but the pad was poured to an earlier, preliminary value of 220 kPa. The pad sits 2.8 m from the top of a 4.5 m deep basement excavation that is open and unbraced on that side. After two days of rain the ground at the pad edge is ponded and a crack has opened across the adjacent apron slab. The crane will slew over a public footpath along the hoarding, and the site has no alternative crane position.
1. The site engineer argues the pad is poured and at full 28-day strength, so the 150 kPa bearing value is now 'academic.' How do you answer that, and what do you require before erection?
Read the worked answer
Concrete strength and soil bearing are two different checks. The 28-day strength proves the pad can carry its own internal stresses; it says nothing about the soil beneath it. The manufacturer's base reactions (vertical load, overturning moment, shear) were satisfied by an allowable bearing of 200 kPa. At a confirmed 150 kPa the pad is undersized, and the failure modes are bearing/shear failure of the clay and differential settlement that tilts the mast and loads the slew ring and mast bolts unevenly.
Required before erection:
- Written confirmation of allowable bearing from the geotechnical engineer at the actual founding level, not the preliminary value.
- A revised foundation design (larger pad, piles, or ground improvement) from the foundation designer who holds the crane's base reactions.
- Manufacturer's written acceptance if the foundation departs from the manual's standard detail.
- Re-survey of base level against 1:500 after any rework.
This is an engineering decision, not a site one, and the operator should not accept the crane until it is closed out.
2. What is your disposition on the 2.8 m setback from the open, unbraced excavation and the crack in the apron slab?
Read the worked answer
Treat it as a stop item. An allowable bearing value assumes a bearing stratum with lateral confinement and surcharge around the pad. A 4.5 m open cut 2.8 m away removes that confinement and creates a potential slip surface that can daylight beneath the pad; the crack in the apron is evidence the ground has already moved, and the ponding is softening the clay and adding load.
Actions:
- Suspend preparatory works and keep personnel and plant off the pad edge.
- Have the geotechnical engineer assess stability of the cut with the crane's bearing pressure applied, and design shoring, benching or backfill as required.
- Re-establish a setback that satisfies the manufacturer's minimum distance or the engineer's calculated requirement, whichever is greater.
- Install monitoring (survey points, tell-tales) and set action limits, then re-survey the pad level against 1:500 before erection.
'We will keep an eye on it' is not an acceptable control for a foundation that carries an eccentric overturning moment.
3. How do you control the risk of the crane slewing over the public footpath, given that no other position is available?
Read the worked answer
First establish what is actually permitted: check the manufacturer's manual and the local authority/regulator for oversail restrictions and any requirement for consent. The default safe position is that no load, and ideally no unloaded hook or jib, passes outside the site boundary over a place where people walk.
If the crane must stay where it is, the engineering controls are:
- A tested mechanical/electronic slew limit set so the jib stops before the footpath, combined with a trolley-in limit so the hook cannot reach the boundary. Test both and record it; confirm they cannot be casually overridden.
- If oversail cannot be eliminated, a designed protective structure over the footpath with the authority's approval, plus a written lift plan and exclusion of pedestrians during lifts.
- Brief and sign off every operator on the limits, include them in the daily check, and prohibit free-slew with the jib out.
Remember an empty hook or a dropped hand tool is as lethal as a load, so the limit must control the jib, not just the load.