At DICA, we often discuss outrigger reaction forces, ground bearing pressure under outriggers and crane pads, crane mats, and ground bearing capacity. A recent article written by Geoffrey Goldberg, Ground Pressure Considerations and published in American Cranes & Transport, presents insightful information on the need to understand ground pressures within the soils under tire, outrigger, and crawler loads specifically when underground hazards are present. Goldberg states “The fact that OSHA requires that the operator be informed of the existence of the hazards implies that the operator has at least some capability to process the details of the hazards that are presented.”
First, Goldberg shares an easy way to calculate the ground bearing pressure imposed by a vehicle on tires by simply “measuring the tire’s inflation pressure with a tire pressure gauge. It is the pressure of the tire pushing against the ground that the ground is called upon to resist.”
Second, Goldberg explains how the load imposed at the surface of the ground dissipates at varying rates, which is important when considering structures that are below the surface at various depths, such as utilities and other underground hazards.
“The superincumbent load dissipates with the tiny particles of soil acting like little bricks, so the deeper into the soil one looks, the load is spread over a larger area. The rate that this spread takes place is dependent upon the soil properties, and rates anywhere from 1:1, which dissipates at a 45° angle, or 1:2, which spreads at 63.4° angle. “A ratio of 1:2 might be too conservative, so 60° (1:1.732) is often used and is generally a safe bet,” explains Goldberg.
In previous posts, we have shared information on understanding soil mechanics. A 2019 article in Pile Buck Magazine identified the different elements of soil that an engineer must know in order to identify soil type and their relative compactness. A 2020 article in the same publication provides definitions of the common types of soil and their characteristics.
In crane terms, the softer the ground or higher the loads, the larger the matting or pad area needs to be to spread the load and safely reduce ground bearing pressures to required levels. These requirements are based on both the strength of the ground (ground bearing capacity) and on the need to reduce pressure within the ground to prevent damage to underground hazards.
Two additional resources from DICA that are helpful are our Crane Pad and Outrigger Pad Sizing Graph, which you can use to identify the pad area needed to reduce X load to Y ground bearing pressure, and our 10 Tips for a Safe Setup for general safety protocols.
Previous Blogs referenced: