Responding is Kris Koberg, CEO, DICA, Guthrie Center, IA.
U.S. regulatory bodies and site owners continue to place emphasis on understanding ground conditions and using outrigger pads or crane mats to provide the necessary foundation support.
No. 1 on OSHA’s site inspection checklist (“Compliance Directive for Cranes and Derricks in Construction Standard,” issued Oct. 17, 2014) for its compliance officers is to determine the adequacy of ground conditions beneath the equipment set-up area, including the support/foundation, matting, cribbing, blocking, etc.
Know your equipment
What loads and pressures will your equipment exert onto the ground? Understanding the loads exerted through your outriggers and how the loads change based on the movement of the crane and the boom is critically important. If you know the actual loads exerted onto the outriggers, your next step is to understand the pressure being exerted through the outriggers and onto the ground or outrigger pads.
Know your ground bearing pressure
Ground bearing pressure is the pressure that is exerted onto the ground. The objective is to ensure the ground bearing pressure is always less than the ground bearing capacity, or lower than the maximum allowable ground bearing pressure that is dictated by the site owner. If the ground bearing pressure is greater than the ground bearing capacity, the ground will compress and compact to a point of support, or it will continue to move and fail under the load and pressure. Many site owners provide a maximum allowable ground bearing pressure. This is generally not the ultimate ground bearing capacity. Instead, it is a self-imposed maximum allowable ground bearing pressure that limits the user.
Know your ground
Ground bearing capacity is the strength of the ground, or its ability to support a pressure. To assist in ground assessment, many companies use roll or plate tests to test the surface, or crust of the ground. These tests are useful in the assessment of the top layer of the ground – but be careful. The surface is supported by the subgrade, the ground beneath the surface layers. To evaluate the subgrade, you will need to use geotechnical reports, soil borings, dynamic cone penetrometers, ground penetrating radar or other more advanced techniques to understand the strength of the subgrade and any unknown hazards.
Know your foundational support
The most common way to help prevent catastrophic ground failure is to use outrigger pads or crane pads. Accurately sized and engineered outrigger pads will increase the area of load contact onto the ground, thereby reducing the ground bearing pressure.
Outrigger pads and crane pads should be designed and selected based on two primary criteria. First, they need to be strong enough to not compress and shear under the known highly concentrated loads. Second, pads need to be rigid enough to distribute the concentrated loads over the intended area to reduce the pressures emitted onto the ground. The larger the load distribution area needed, the more rigid (generally thicker) the pad must be. If you are using materials that are not engineered, such as wood, be sure to account for degradation that is caused by moisture, ultraviolet light, insects, rotting and previous stress.
An additional measure to prevent failure of the ground is to improve the ground itself. The ground can be improved in many ways, such as compaction, adding rock or other dense inorganic materials, removing uncompacted surfaces, and allowing wet ground to dry.
Editor's note: This article represents the independent views of the author and should not be construed as a National Safety Council endorsement.
