Night Pour Beside a Retaining Wall
A 36 m five-section truck-mounted boom is set up at 21:30 for a night pour. The outrigger pad on the left rear sits on compacted fill that was placed eight weeks ago; the right rear pad is on a concrete apron. A 2.4 m high retaining wall runs 3.1 m behind the left rear outrigger, and the wall's backfill surface slopes down away from the boom. The supervisor says the wall is 'just a garden wall' and asks you to skip the outrigger mat to save time. Site lighting is two portable floodlights aimed at the pour area, leaving the outrigger zone in shadow. Wind is 12 km/h gusting to 22 km/h from the left. The pump operator has not yet seen the day's delivery docket, which shows 180 m³ of 40 MPa concrete with a 180 mm slump.
1. Identify the setup hazards you must resolve before extending any boom, and explain what evidence you need for each.
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The outrigger zone is the controlling hazard. Compacted fill eight weeks old is still settling; without a plate-load or dynamic cone test result, you cannot treat it as bearing capacity equivalent to the apron. The retaining wall matters because it retains soil: the wall plus its backfill creates a surcharge and a potential failure plane that can extend behind the wall for a distance related to the wall height. With a 2.4 m wall, the zone of influence reaches well past 3.1 m, so the left rear outrigger may be sitting on ground whose lateral support depends on the wall. You need the geotechnical or civil engineer's confirmation of allowable bearing pressure and whether the outrigger load falls inside the wall's zone of influence.
Second, you need the boom's actual outrigger reactions, not the machine's nominal weight. Those come from the manufacturer's load chart for the specific boom configuration and radius, including the counterweight and any stowed equipment.
Third, lighting: two floodlights aimed at the pour leave the outrigger zone unlit. You need task lighting that lets you see pad settlement, ground cracking and leg movement throughout the pour, because outrigger failure is progressive and visible before collapse.
Fourth, the delivery docket matters for planning, not for setup: 180 m³ at 180 mm slump means a long, continuous pour with high flow rates and possible line surge, which affects how often you must re-check stability.
2. The supervisor insists the wall is decorative and the mat is unnecessary. How do you respond, and what is your decision if the engineer is unavailable?
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I do not accept the supervisor's characterisation as a basis for a stability decision. A 2.4 m wall with backfill sloping away from the boom is a retaining structure by function regardless of what it was built for, and the relevant question is not its label but whether the outrigger load sits within its zone of influence and whether the ground beneath the pad has been verified.
My response is to state the specific information I need and why: allowable bearing pressure under the left rear pad, confirmation that the pad is outside the wall's failure zone or that the wall has been designed for the surcharge, and the load chart reactions for the planned radius. Until those are available, I will not set up on that side.
If the engineer is unavailable, the decision is to relocate. Options in order of preference: reposition the boom so the left rear outrigger lands on verified ground or the apron; use a load-spreading mat sized from the actual reaction and the verified bearing pressure, not a nominal mat; or reduce the working radius so the reaction on that leg decreases. I would not proceed on the supervisor's assurance alone, and I would record the refusal and the reason in the site diary and notify the PCBU.
3. Assuming setup proceeds lawfully, describe the checks you would run during the pour and the trigger points for stopping.
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Before the first concrete, I would mark the pad positions and photograph them, record the leg extensions on the outrigger beams, and confirm the boom is level using the machine's level indicator plus an independent check. I would set a monitoring routine: a nominated spotter checks each pad every 15 to 20 minutes and after every boom repositioning, looking for settlement, soil cracking radiating from the pad, leg piston movement, and mat deformation.
Trigger points for stopping the pour: any visible settlement of a pad; cracking in the fill or apron around a pad; a leg that loses contact or shows pressure loss; the level indicator going out of tolerance; wind sustained above the manufacturer's limit (the 22 km/h gusts are close to many limits and must be checked against the specific manual); or the pump operator reporting unusual boom response.
I would also manage the pour rate: 180 m³ at 180 mm slump produces high line pressure and surge, so I would keep the boom within the planned radius and avoid rapid swing-and-brake cycles that impose dynamic loads on the outriggers. If a trigger occurs, stop pumping, retract the boom to a safe position per the manufacturer's procedure, and do not resume until the cause is identified and corrected.