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Crane Risk and Loss Root Causes: Ground Conditions

Crane sinking into dirt
Ground collapse often has a catastrophic outcome. A crane tips when the outrigger sinks, the load is dropped and the crane causes damage when it tips. When a crane’s outrigger punctures underground utilities, it damages underground water and sewage lines and can cause a gas line explosion. The site designated Lift Director should identify and mark known underground hazards and voids, address poor soil and backfill conditions and prepare the site adequately for crane operations.
 

Checking and knowing the soil type is paramount to avoiding ground collapse exposure.  In Florida, we have type C soil with an unconfined compressive strength of 0.5 tsf (48kPa) or less, which may flow and slump when compressed. Type C soil displays granular characteristics, and includes gravel, sand, loamy sand, submerged soil or soil from which water is freely seeping.  In a slope layered system, type C soil generally has a slope of 4:1 meaning 4 units distance horizontal to 1 unit distance vertical. This means when working near an excavation on type C soil, the crane must setup up 4 feet from the excavation for every foot of depth to avoid collapse! NOT “half the depth from the hole” which we commonly hear in the field.

Unconfined compressive strength measures the load per unit area at which soil will fail in compression and may be measured using laboratory testing, a field pocket penetrometer or thumb penetration test. Wet soil in technical terms refers to soil that contains a significant amount of moisture, and will slump of begin to flow when vibrated. Granular material that would normally exhibit cohesive properties when moist will lose those properties when wet.

Remember, the right equipment, proper site preparation and good communication between team members always makes for a safe lift.

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