A crane operator needs to set up a 100-tonne mobile crane on a construction site with varying ground conditions. The planning team has identified a section of paved asphalt over compacted granular fill where the crane will operate. The maximum outrigger load for the planned lift is estimated at 600 kPa. The ground-bearing capacity of the asphalt/compacted aggregate is rated at 450 kPa. What is the minimum required size for cribbing pads, assuming a uniform load distribution and using four outriggers, if the outrigger floats are 0.6 m x 0.6 m?

Question: A crane operator needs to set up a 100-tonne mobile crane on a construction site with varying ground conditions. The planning team has identified a section of paved asphalt over compacted granular fill where the crane will operate. The maximum outrigger load for the planned lift is estimated at 600 kPa. The ground-bearing capacity of the asphalt/compacted aggregate is rated at 450 kPa. What is the minimum required size for cribbing pads, assuming a uniform load distribution and using four outriggers, if the outrigger floats are 0.6 m x 0.6 m?

Answer options:

• 0.6 m x 0.6 m ✅ 0.8 m x 0.8 m
• 1.0 m x 1.0 m
• 1.2 m x 1.2 m

Correct answer: 0.8 m x 0.8 m

Explanation: To determine the minimum required cribbing size, the pressure exerted by the outrigger must be less than or equal to the ground-bearing capacity. The outrigger load is 600 kPa, and the ground capacity is 450 kPa. Therefore, a larger area is needed. Pressure = Force / Area. Since Force is constant, Area Required = (Load Pressure / Ground Capacity) * Float Area. Area Required = (600 kPa / 450 kPa) * (0.6 m * 0.6 m) = 1.333 * 0.36 m² = 0.48 m². The square root of 0.48 m² is approximately 0.69 m. Therefore, 0.8 m x 0.8 m pads (0.64 m²) are the smallest option that provides sufficient area, as 0.6 m x 0.6 m (0.36 m²) is too small. 1.0 m x 1.0 m and 1.2 m x 1.2 m would also work but are not the minimum.