Outrigger Setup Beside an Unshored Excavation
A 100-tonne all-terrain crane is set up to lift a 9,500 kg chiller at an 18 m radius with 40 m of main boom. The load chart gives a maximum right-rear outrigger reaction of 520 kN for this configuration. The ground is 1.5 m of compacted fill over soft clay; the geotechnical report gives an allowable bearing pressure of 120 kPa. Each outrigger float is 600 mm square. The right-rear float sits 2.6 m from the crest of an unshored 3.0 m deep excavation that runs parallel to the crane, and a 200 mm water service is marked 2.0 m from the float. The crew has four 1.2 m × 2.4 m hardwood mats, 150 mm thick. The site superintendent wants the lift completed within 45 minutes and says, “The last crane set up in the same spot.”
1. Calculate the ground bearing pressure under the right-rear float and determine whether the available mats are adequate. Show your units and assumptions.
Read the worked answer
Ground pressure = 520 kN ÷ 0.36 m² = 1,444 kPa, about 12 times the allowable 120 kPa. Required bearing area = 520 ÷ 120 = 4.33 m². One 1.2 m × 2.4 m mat has 2.88 m² of plan area, but load spreads only at about 45° through the mat thickness. For one 150 mm mat, effective area ≈ (0.6 + 0.3)² = 0.81 m², giving 642 kPa. Even four stacked mats (600 mm) give ≈ (0.6 + 1.2)² = 3.24 m² and 160 kPa, still over 120 kPa. The mats are not adequate as an engineered load-spreading solution; a designed crane mat, steel plate, or relocation is required.
2. Evaluate the excavation setback and the water service. What minimum actions are required before any lift?
Read the worked answer
Required setback from an unshored excavation is commonly 1.5 × depth = 1.5 × 3.0 = 4.5 m from the crest. The float is at 2.6 m, so it is inside the influence zone. The excavation is unshored and the soil is fill over clay, so a geotechnical engineer must approve the setback or the crane must be moved. The 200 mm water service at 2.0 m must be located by hand, protected, and confirmed by the utility owner; outrigger load can crush a buried pipe. Also confirm the outrigger beam is fully extended and the float is pinned.
3. State your decision and the sequence you would follow.
Read the worked answer
Stop the lift and do not set up. Inform the superintendent and lift director that the ground, excavation setback, and utility clearance do not meet requirements. Document the chart reaction, soil allowable, measurements, and photos. Options: relocate to a designed pad outside the 4.5 m zone; obtain a shoring design and engineered mat from a qualified engineer; use a larger crane or different configuration to reduce the outrigger reaction; or pick from the opposite side if geometry permits. Do not accept “everyone else does it” as engineering evidence.