Ground Pressure and Proper Matting

There are technical formulas that you can use to determine if the ground can support your crane and machinery. To make it simple most manufacturers and third party lift plan software can help you calculate the pressure your crane is exerting on the supporting soil. But that is only half of the equation. You will still need to know more about the soil and ground to make a safe lift. The soil information that you are looking for is: soil/surface type, soil/surface properties, topographical properties, soil make-up and buried utilities or other underground obstructions. This information, depending on the site requirements, can be as detailed as getting a geotechnical engineer to perform soil tests to using known bearing capacities of the type supporting soil. In either situation you need to be very aware of the amount of pressure you are exerting onto the supporting soil and what your supporting soil can handle. Now I am using the term supporting soil, but don’t think of that as just compact gravel, clay, etc. Asphalt and concrete also have their own allowable bearing pressure, too. An unloaded crane, meaning no load on the hook, can be exerting hundreds of thousands of pounds of force on the soil through the outrigger system or the tire contact patch. So the question becomes, can the soil support the weight? Knowing what we know now in 2015 vs say 1920 and the current regulations that exist, the answer to this question is a big factor. But there are ways to mitigate and spread out the force over a wider area, thus limiting or reducing the chance of soil/surface failure.

In the beginning soil and ground pressure may have not have been an issue like it is today. Think of the old Lorain 15 ton lattice boom truck crane vs the Grove 550 ton crane. The loads that cranes lift have increased in weight over time and that means that the cranes have had to increase in capacity to stay relevant. This increase in size and load weights translate to an increase in pressures exerted on the supporting soil. So much so that no matter what surface you are on, you need to have proper crane mats and matting under your cranes. There are a few industry standard types of outrigger pads and crane mats and each mat has its own advantages: timber mats, synthetic or thermoplastic pads, fiberglass mats and steel mats. We won’t get into the advantages of each, but we will take a more high level view about the mats and general practice rules. Each type of outrigger pad or mat has its own properties, as well. You will need to pick the mat that has the proper stiffness and strength to support your crane and the load it is subsequently hoisting. Proper stiffness of the mat you chose is important. If you chose a mat that isn’t stiff enough for the forces the outriggers are exerting, then the mat isn’t helping you. Because if the mat isn’t rigid enough, the forces will be concentrated in the middle of the mat in a smaller area. Which could lead to failure. This can be visually seen on synthetic mats or plastic mats when they start to get a bowl shape while being under the outrigger. An offset outrigger placement on a sufficiently strong and stiff mat could also lead to this same effect, where you will see lifting of a mat on the corner. This is due to the offset pressure that the outrigger is placing on the mat. Without a properly strong mat the forces exerted on the mat could actually fracture or make the mat fail and/or the supporting soil to fail, as well. Not having the proper mat could also lead to the mat actually “punching through” the supporting soil. This is common on lifts with asphalt/concrete as the supporting soil, but can also happen on soil, too. Supporting soil can be thought of like a pie. You can tell we are in the holiday mood here. The top of the pie crust is hard and usually takes a bit of force to get the fork through. But once it breaks the fork cuts through the cherry filling and finds the bottom with little force. Much like soil, once you have punctured the crust, you may not find a solid soil beneath you.

All of the aforementioned can be mitigated by simply preparing for the job and knowing; the type of supporting soil that you will be working with, the pressures you will be placing on that supporting soil, having a mat that is sufficiently stiff and properly placing the outrigger on the mat.