Health & safety bulletin
Hiab / crane lorry ground bearing requirements
25th May 2005
It has been reported that on occasion, mobile cranes fitted with outrigger / stabilizers have had bearing plates sink into the ground when lifting loads, dependent on the rate of settlement, it may result in a dangerous occurrence. Contractors should consider the potential bearing capacity problems with the Hiab crane lorry unit.
Xmo Strata MID validation surveyors already assess the position and general condition of the area in regard to the hiab unit position, however it is always best practice for the on site team to inspect the location themselves when the hiab unit is in place, as there may be something that has been overlooked
Many small lifts of a few hundred kilograms are undertaken on sites using the hiab crane lorry, these lifts normally rely on the judgment of the operator of the site ground conditions and bearing spread. The following describes some of the aspects which should be addressed on site if assessing the lift safety:
- The variability of the soil supporting the outrigger spread. Disturbed ground (excavated and backfilled) occurs commonly in site construction, and a degree of under compaction often results. Often localised voids can be present. Firm crusts of relatively thin aggregate are often used to form plant platforms on relatively weak and / or waterlogged geological strata. In such cases although the ground may be adequate for normal construction traffic, it needs to be regarded with concern when placing outrigger loads.
- Care must be taken when positioning the outrigger onto non carriageway class construction tarmac, as the stabilisers could settle down, into the potentially thin layer.
- Other areas of concern - the hiab driver or personnel on site must be aware of are as follows; soft verges, landscaped area, over or adjacent to manhole/access lids, footpaths, over known/suspected electrical cable or water main service lines, adjacent embankments, basements, pavement stone etc
- If in doubt follow the advice in CIRIA publication (for cranes) which states that 4 number 200x125x1500long sleepers should to be used to increase the bearing area capacity of the stabiliser.