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surface of the earth in the crane industry it is slightly more complex. “When dealing with mobile cranes the ground can take different forms and applications from elevated decks to spider cranes and special equipment being brought into and onto buildings as well as marine applications. All of these will require specific approaches to determine stability.” Walsh says when it comes to Mother Earth there is no magic at play. “All objects on earth exert some amount of downward force on the earth’s surface due to gravity and this includes live and dead loads and also reactive forces, gravity opposed by tension forces of the ground surface,” he says. “Therefore it is important to not just focus on the big crane when considering what bears down on the ground.” He says having a clear understanding of this and Newton’s Third Law – for every action, there is an equal and opposite reaction – is critical in terms of creating a stable base for any equipment. It is also important to distinguish between the actual ground bearing capacity of soil to support applied loads on a sustained basis, without settlement or shear failure in the soil and the allowable ground bearing pressure. “This would be the calculated or assumed capacity of the soil,” explains Walsh. Methods used According to Walsh despite being the least reliable the most commonly used method to
determine bearing capacity is a visual assessment. “This really entails taking a look at the ground and then determining what class of soil it is so typically it is rock or shale, gravel or sand,” he says. “But unless there is a capital S on one’s chest, the reality is that you have no clue as to what is five feet down and what changes there are in the soil. There is no knowledge about any voids or unconsolidated soils all of which can have a significant impact on the actual capacity and bearing up the load.”
there is a uniform distribution of forces and uniform soil conditions. “This is just not true and can be catastrophic. It is important to take the time and determine what the ground bearing capacity is to ultimately ensure ground stability.”He says once the ground bearing capacity is developed decisions can be taken to deal with inadequate ground conditions.
It is with this in mind that Walsh advises always doing a geophysical assessment. This entails taking soil samples that are analysed in a laboratory. It is especially important on sites where there is no historical data available.
“This could mean lightening the load by breaking it down and lifting smaller loads at a time. Another option is to increase the distribution area for ground pressure, meaning one increases the footprint of the lift through increased square footage of mats and plates,” he says. “Alternatively one could modify the actual ground conditions to improve the stability.”
“The best way to get data is by geotechnical means. It is, however, also the most expensive way,” he says. Too often it is assumed that
Walsh emphasis the importance of ground preparation regardless of whether it is a rough terrain crane, a boom truck or a heavy crawler.
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