Outrigger Pad Sinks Mid-Pour on a Sloping Site
A 28 m three-section boom is set up on a site with a 1-in-12 fall. The operator has levelled the machine using the outriggers, with the front left leg extended 1.4 m and the rear left leg extended 0.6 m. The left side pads sit on a gravel hardstand over a buried stormwater line; the right side pads are on undisturbed clay. Halfway through a 90 m³ pour, the spotter notices the front left pad has sunk about 40 mm and the machine's level indicator shows a bubble off centre. The operator says the boom 'feels fine' and wants to finish the last 30 m³ before stopping. The pour is 45 minutes from completion and the concrete supplier has two more trucks en route.
1. Interpret the symptoms and decide whether to continue.
Read the worked answer
The symptoms indicate an active stability failure, not a nuisance. A 40 mm settlement of one pad with the level indicator off centre means the machine's centre of gravity has shifted toward the loaded side, and the load on the remaining legs has increased. The front left leg is the most extended at 1.4 m, so it is the most vulnerable to buckling and to further settlement, and it sits over a buried stormwater line where the gravel hardstand may be bridging a void.
The operator's 'feels fine' is not evidence: boom response feels normal until the margin is nearly gone, and the level indicator is the objective measure. Continuing for 45 minutes adds load cycles, boom movements and dynamic surge from 30 m³ of concrete, each of which can trigger sudden failure.
The decision is to stop. Stop pumping, stop boom movement, and keep personnel clear of the machine's tipping side.
2. Describe the safe shutdown and recovery sequence.
Read the worked answer
First, stop the pump and stop all boom functions. Do not retract or swing the boom while the machine is unstable, because movement shifts the centre of gravity and can precipitate collapse. Clear the area beneath the boom and around the outriggers, and establish an exclusion zone on the settling side.
Second, assess the ground: expose or probe the front left pad area to determine whether the stormwater line has been crushed or the hardstand has voided. Do not send anyone under or beside the machine to do this until the machine is secured.
Third, plan the recovery with the manufacturer's procedure: the usual sequence is to reduce the boom radius to lower the overturning moment, then carefully take load off the affected leg only under controlled conditions, then re-support on a properly sized mat or relocate. Recovery is a planned lift, not an improvised adjustment, and should be supervised by a competent person.
Fourth, dispose of or return the remaining concrete per the supplier's instructions; do not rush the recovery to save 30 m³.
3. What should have been done differently at setup, and what does this case tell you about the site's buried services?
Read the worked answer
At setup, the buried stormwater line should have been identified from service plans and located on the ground before any pad was placed. A pad over a buried line is a pad over a potential void, and the gravel hardstand does not change that. The left side should have been either relocated or bridged with a load-spreading mat sized from the actual outrigger reaction and the verified bearing capacity of the ground around the line.
The 1-in-12 fall also required more than levelling with the legs. Levelling by extending one leg to 1.4 m while the opposite leg is at 0.6 m creates an asymmetric support geometry and increases the load on the extended leg. The better approach is to cut or build a level pad, or to use mats to reduce the leg extension needed, so the machine sits closer to its design attitude.
The case shows that service plans are a setup document, not a desk reference, and that the spotter's routine pad checks are what caught the problem. That check should have been scheduled from the first load, not left to chance.