Featured in Lift Line Summer 2025
When ALL has a complex lift in need of ground condition assessment and ground stabilization engineering services, it turns to Dearborn Companies, an Illinois and Texas civil and structural engineering firm. They have been a strategic partner for decades, providing ground condition data collection including ground-penetrating radar, 3D laser scanning, and drone mapping.
"What we do is perceive lifts from the ground up,” said Mike Walsh, president of Dearborn. “Keep in mind that, in this context, ‘ground’ is a relative term – it’s not just “soil.” Because if there are vaults underneath … is the ground really the ground? This is why it’s so important to fully understand the surface that lies below any equipment, such as cranes, telehandlers, and aerial lifts - anything that will create a reaction force.”
Tools of the trade
Prior to a site visit, the Dearborn team assesses existing documentation including site logistics plans, utility plans, information on the cranes and lifts, as well as gauging the client’s perception of the bounds of the survey area. Walsh’s workers then use their hardware and tools to scan that area and a reasonable portion outside of it.
“In addition to ground-penetrating radar, we utilize 3D laser scanning to capture the smallest details of the above-ground crane deployment area,” said Walsh. “It will pick up pavement surface conditions, signage that might be there, light poles, manhole covers. We also look at the proposed area for the crane’s tail swing, boom swing, load swing, and load movement in and out of the area. It’s much more comprehensive than people might imagine, well beyond a simple ground condition survey.”
The importance of knowing ground bearing pressure
In recent years, ALL customers of every stripe have become more interested in understanding ground bearing pressure. It’s a change that Walsh has observed as well. “ There is recognition in the construction industry that ground condition is a critical issue to which attention must be paid,” said Walsh. “ The size and complexity of the lifts, enabled by bigger and bigger equipment, are what’s driving it, at least in part.”
However, Walsh cautions that it’s not just big cranes that require attention. Dearborn takes a holistic view of a jobsite. “We’re not only looking at where the crane will set up,” said Walsh. “We’re considering the entire construction village. Where are the work trailers, where is the assist crane, where are the telehandlers and other equipment? Where will cranes be assembled and disassembled? It helps us tell the full story of ground condition needs. We’re mapping the ground and accounting for total force to be exerted.”
Different pressures
Different types of cranes exert different kinds of force. All-terrain cranes, which rest on road-worthy tires and lift from static positions, typically have outriggers placed at their four corners for added stability. Crawlers run on tank-like tracks. Because they are designed to pick and carry with loads suspended from the boom.
The pressure derived from outriggers is easy to visualize. Think of someone jumping on a trampoline in high heels. They’re likely to poke through the trampoline because all their weight is concentrated on the tiny heel. It’s similar with a crane’s outriggers.
A crawler’s tracks might look like they offer more even load distribution, but Walsh cautions that it’s an illusion. “That’s a misconception about crawlers, that because you’ve got, say a 30-foot-long track that is four feet wide, it’ll have this great, uniform load distribution. It’s anything but that. You can get a lot of different load concentrations across those tracks. Some of the worst ground conditions are under the ‘toes’ of the crawler. If you’ve got a bunch of main boom and fair amount of jib, then go to pull the boom assembly off the ground, you get intense point loads on the front portion of crawlers.”
The solution to help mitigate these point loads, whether from outriggers or track toes, is usually crane matting.
Crane mats as equalizer
Every location has an “ultimate” bearing capacity – it’s the maximum load at which point the ground fails; a safety factor is applied to the ultimate bearing capacity value to calculate the “allowable” ground bearing pressure (GBP), as measured in pounds per square foot. The requirement is to keep ground bearing forces exerted during a lift below the allowable bearing capacity. The crane mats help spread forces over a larger area, thereby better distributing and reducing the forces applied to the ground.
In addition to matting, improvement of the actual ground’s capacity can also be undertaken. Compacting the ground, adding crushed stone, or mixing additives into the ground (such as lime) are potential ways to offset reaction forces generated during a lift. From there, the size of matting can be adjusted, and more mats can be stacked on top, if needed.
Mats are made of timber, steel, and synthetics, and can also be used in combination with one another. Larger cranes with outriggers use, as a rule of thumb, steel crane mats, with crawler cranes using bolted wood timber, or steel, mats. Smaller cranes typically use wood mats. The larger the area of the crane mat, the wider it broadcasts ground bearing pressures. Standard sizes for steel mats are 6' x 6', 8' x 10', and 8' x 12'. Bolted wood timber mats are typically 4' x 20' and can be placed together to create longer runs over which a crawler can pick and carry.
John Stolarczyk, P.E., S.E., is ALL’s heavy lift engineer. He is frequently called upon to recommend appropriate crane matting for a given lift. He also custom designs mats, sending his specs to ALL’s in-house fabrication shop, where they are custom-built in six to eight weeks.
“Most steel crane mats are done in a sandwich configuration,” said Stolarczyk. “ There are steel layers at the top and bottom, but it’s in-between where the real magic happens.” Many designs are considered trade secrets, but Stolarczyk says honeycombed, ribbed, and waffle configurations are what you might find there.
Dearborn’s Walsh says the idea with crane mats is to design for the worst-case scenario.
“That’s really the entire goal of assessing ground conditions. You’re establishing what’s the worst that can happen then taking steps to prevent it.”