Tread Softly16 September 2020
Ground can give way and the results can be catastrophic. It is not only soft ground that needs mats or outrigger pads before it can support the weight of a crane lifting a load. Julian Champkin investigates.
There is soft and hard ground; there is clay and rock; sandy, wet and dry soil; tarmac and smooth concrete; compacted and uncompacted hardcore; and they all behave differently when you put a weight on them—the weight, for the purposes of this article, of a crane and the load it is lifting.
Once upon a time crane operators carried a few railway sleepers along on a job and put them down under their crawler tracks or mobile outriggers if the ground looked soft or boggy. Times have, fortunately, changed. Timber still has its place; but other, more highly-engineered alternatives, and more scientific techniques, are available.
Mat & Timber Services supplies crane mats to customers all over the UK. Alan Farmer, operations manager of Inverness-based Stoddart Crane Hire, is one of them. “We have a complete mixture,” he says. “We have hardwood bog mats, of 5m x 1m timbers; we have about 50 of those. We have some smaller ones as well. Each crane crew has composite nylon roundmats, stored in boxes on the truck or the crane; the bigger cranes have ramps to roll the mats up onto the boxes, because they are heavy. They go out with the crane as standard.
And we have steel mats of various sizes from 2m2 up to 5m2. We have seven sets of those, and they are stored in our yard and go out when the standard mats are inadequate and particular jobs require them.”
The softer the ground, of course, the larger the area of pad that is needed to spread the load. But crane operators are not, on the whole, geologists; how does an operator know how great an area of matting he has to lay safely to support his crane and load?
“Each job depends on outrigger calculations,” says Farmer. “A client can hire in two ways. There is a standard hire, in which the client decides the size of crane that he needs and we supply the crane and our standard mats, unless the client has asked for bigger.
Responsibility for establishing that the ground is safe is with the client. We won’t have visited the site, so the hirer takes responsibility and any damage that results, to the crane or the surroundings, is on his insurance not ours.” Since a crane can cost a million pounds, that is not small responsibility.
“Or there is contract hire, where we choose the crane and we do the lift plan. In those cases, we say to the client: ‘The ground loading will be 40t per square metre on the crane mat; can your ground stand that pressure?’ So in that case too, ultimate responsibility for establishing that the ground is strong enough lies with the client.”
This might seem a trifle hard on the client who, like the crane operator is unlikely to have ground geology among his specialist subjects of knowledge. The way round it is to get a ground pressure test done by a civil engineering company.
The principle is simple: “They come along the week before the lift and drive something heavy—say the size and weight of a digger—on to the site; then they jack it up and measure how far and how quickly it sinks into the ground,” Farmer explains. “On almost any soil the rate of sinking varies over time.
There is an initial compression, a sinking of typically a millimetre or so, just due to compression of the ground; then as you increase the pressure you see more sinking. 5mm is about all you want to see. And you want the graph to level off over say half an hour to no further sinking at all.”
There is a rule of thumb: “Cranes impose a pressure of around 50t/m2 just driving onto the site. So if your unladen crane sinks into the ground just driving onto the site, then the ground is not strong enough. But it is a bit late to discover it then because the job is already costing the client money.
Those are the times when you get the panic call to the yard: ‘Please can you send more mats!’ Which of course we do, but it takes time, and added transport and added costs.”
Tarmac and concrete surfaces seem strong and smooth but they are, counterintuitively, among the more problematic. “It is because you don’t know what is underneath them,” says Farmer, “And you cannot carry out testing without cutting a hole in them. Highways Specification tarmac, for example on the A9 which is our local trunk road, is designed to take 10 to 15t/m2, and the highways authority guarantee that. It is professionally built and maintained and monitored so you would be comfortable accepting that value on any main road.
“But on a school playground, which looks just the same, nobody knows what is under the tarmac.
It could be a wafer-thin coating laid on three inches of hardcore on top of a peat bog. The danger is that it looks like a nice hard crust when you drive on it but the outrigger can punch through it with no warning at all and the crane will go straight over. Experience is a guide, and of course you go for safety every time and put in larger mats. Look at the terrain around you. If it is boggy, then that is a warning—your playground will be resting on that!”
Southern Cranes and Access is another Mat & Timber Services customer. Kelvin Prince is a director. “We have 50 machines in our fleet, up to 300t capacity,” he says. “Our smallest cranes, up to 90t capacity, carry composite mats. Over 90t we use steel; we prefer to have something more substantial there but many would go up to 120t on composite. We do have some wood also, a selection of 3m x 1m, 4m x 1.2m, 5m x 1m and 6m x 1m timbers just to give bigger mat areas for high-capacity lifts.”
Their timbers are not recycled railway sleepers. “There is an issue with sleepers—you don’t see them now. They can wear, they can fray or splinter, which makes them not fit for purpose. Now clients want steel, or they want to inspect the timbers themselves for soundness. Ekki is the best quality of wood as far as the species is concerned— Ekki timbers last practically forever. Don’t buy cheaper.” Ekki is a tropical hardwood from West Africa. It is available FSC-certified for sustainability; uncertified timber of course should not be purchased.
Southern Cranes do a lot of their work in central London.
“We are getting stipulations now from local councils about ground loading on their roads,” he says.
“Westminster has a limit of 10t/m2. That is an anomaly. Neighbouring Kensington and Chelsea allow 15t/ m2, which is more general.”
“Best practice is to assess the ground conditions before driving on,” says Charles Grizzle, business development manager of Shropshire, UK-based Outriggerpads who, as the name suggests, make outrigger pads.
“Even if you are only driving the crane in a stowed position the machine could easily get stuck in unexpected soft or boggy ground. Delays on site while you are waiting to be rescued cost money—and you may have to end up paying recovery costs as well.”
“Uncompacted fill can be dangerous; that is the term for soil or other fill material placed into a backfilled trench without being compacted. One telltale sign is visible cracks along the line of the trench. Nearby excavations are another hazard. Industry guidelines advise against positioning cranes close to the edge of a trench as it can collapse without warning. And many floors, cellars and basements are simply not designed to take the weight of a mobile crane.” And weather can be a crucial factor: “Heavy or prolonged rain can alter ground conditions and cause sinking. Also make regular checks when frozen ground is thawing out: it can appear to be much stronger than it actually is.”
Grizzle has designed software on the Outriggerpads website to help: you enter the load per outrigger, the type of ground— cohesive or non-cohesive, soft clay, or compact sand, and it comes back with the area of pad that you need.
Iowa-based DICA make engineered crane pads and cribbing blocks from engineered thermoplastics and composites.
“DICA recommends identifying a standard, everyday outrigger or crane pad for each mobile crane in your fleet based on a defined outrigger reaction force and target ground bearing pressure,” says Kris Koberg, their CEO. “Additional matting can be brought in when the situation calls for it. The objective is to create a safe setup and effectively manage ground bearing pressures.
“Understanding how the pads and ground influence each other is critical to understanding actual pad performance. It is a misconception to assume that wood timbers or engineered crane pads evenly distribute outrigger loads to the ground or always distribute the load over 100% of the pad,” he says.
“The harder the ground is, the more the ground resists deflection, which reduces the pads ability to distribute the load. To improve load distribution on hard ground (such as concrete, asphalt or brick pavers), you should consider using an intermediate buffer layer of softer material between the pad and ground that will not break down under high pressures. DICA recommends using neoprene in a minimum thickness 0.75in (20mm) thick. The idea is to soften the ground to enhance load distribution versus making the pads stiffer (heavier and more expensive) to overcome the hard ground challenges.
“Neoprene does two things beautifully: first, neoprene will fill small voids and irregularities between the ground and pad, which reduces point loading.
Second, neoprene allows the pad to deflect as if the ground were soft, which enhances load distribution, and reduces ground bearing pressures. If the ground is uneven, the pad will simply sit on the peaks and will not be in contact with the low points. Adding the buffer layer helps improve pad to ground contact to help smooth out the ground bearing pressure.”
One size fits all?
“Selecting a pad based on the specific crane make and model you are using it with is critical to safe operations and effective ground bearing pressure management,” says Koberg. “Identifying the particular crane and its maximum outrigger reaction force and outrigger float size and shape is essential. A 250t-capacity Grove and a 250t-capacity Liebherr may have quite different outrigger reaction forces. It is not a case of one size fits all based on the lifting capacity of the crane.
“Making assumptions or using rules of thumb is not a safe or effective practice. Crane manufacturers are doing a fantastic job of making outrigger reaction forces available for crane owners.
At DICA, we use this information and provide customers with a FIT Assessment. Our FIT Assessment begins with the crane make and model and provides customers with a process to select the crane pads that are right for the crane and their specific requirements.
“DICA’s lightweight engineered Mega Duty Pads come in many different sizes. The pad thickness and construction dictate the usage ratings,” he says. “A ten-inch-thick FiberMax Pad is rated for outrigger reaction forces up to 325,000 pounds (150t), and it has a crush rating (maximum allowable pressure) of 1,000psi.”
A crane operator should not have to be a geologist but should have some understanding of site ground conditions: if there has been heavy rain on the site, or it is beside an excavation, soils will behave very differently. “DICA suggests customers establish an ‘everyday pad’ that should address 80-90% of the lifting a crane is going to do; for critical lifts when more mat area is needed, customers can bring in additional matting to increase the area of contact and further reduce ground bearing pressures. Always evaluate additional matting to determine if the matting has the strength to support the load and the rigidity to transfer the load to meet the stated requirements.”
Pads can be made from wood, steel, aluminium, plastics, or composite material. The suitability of wood depends on the species it comes from, the condition of the timber, and the construction of the mat. “An issue that gets overlooked is the use of wood crane mats constructed from wood timbers and joined together by through bolts. The purpose of through bolts is to hold the assembled timbers together. This design works well for crawler cranes that traverse across the pad in a perpendicular manner where the crawler tracks are in contact with all of the timbers.
However, when using bolted timber pads under outriggers, it is crucial to know that through bolts do not effectively transfer load to adjacent timbers: which is a common misunderstanding when using wood timbers under cranes or equipment with outriggers.
“Patented FiberMax Crane Pads more effectively distribute the load over the intended area of the pad because of the engineered unibody construction and bi-directional shear web design. The ground does influence distribution and the pads effectively and predictably distribute the load in all ground conditions. Actual performance data in different ground conditions is available from DICA. You don’t have to hope that the load will be distributed—the pads are engineered to do it, and we can provide the data to support it.”
“Mega Duty Pads can be thinner than timber constructed mats and may be thicker than steel constructed pads. Our pads generally weigh one-third the weight of traditional wood/ steel and steel crane pads. In many cases using FiberMax Pads can save enough weight to eliminate an entire truck from your loadout—which saves customers a tremendous amount of money over the expected twenty plus year life span.” That is the case not only for the largest cranes and projects: “It applies across the board, up to around 500t capacity.”
“All materials, be they wood, steel or aluminium, have their strengths and weaknesses,” says Grizzle at Outriggerpads.
“The correct pads or spreader plates are selected based on their performance in a particular situation. We use Ultra-High Molecular Weight (UHMW) polyethylene because we believe its all-round performance is better in the type of applications that our customers work in. It is lightweight relative to its strength, which is an advantage. There are many different types of polyethylene and ours has its own unique signature.
We have subjected our material to several tests and simulations to ensure its quality, strength and performance are second to none which is why our pads were qualified by the British Antarctic Survey on their Halley VI base, by FMGRU for the Genoa Bridge reconstruction, US military, NHS Romford hospital, and many others.”
Outriggerpads also have a product they call Multi Mat. “It is popular and well liked both in the UK and overseas. It has been used on projects such as HS2 and Crossrail. Its versatility allows the user the flexibility to configure the mats to reach the desired area and load bearing capacity. The system is available in three sizes (1,500, 1,800 and 2,000mm) and two options (standard and interlocking), which allows you to build a strong and stable base, reducing the overall pressure directed at the ground to safely and effectively support your equipment. The mats have been tested and analysed, are widely used in several industries, and come with a warranty.”
Bigfoot Construction Equipment is a US customer manufacturer that specialises in outrigger pads, cribbing and crane mats in both composite and wood. “We have options with interlocking outrigger pads and cribbing,” says co-owner Jeff Steiner. “Our ‘Big Grip’ product bites into the ice, mud or snow when used face down, but also the teeth interlock when used face to face. Knowing the ground bearing capacity, the size of the float on the outrigger, the maximum outrigger force and the type of crane and outrigger system on the crane is key to get the best pad for a safe set up.
“Bigfoot’s custom composite is guaranteed for life, and with its load and crush ratings you do not have to have as thick a pad as you would with wood. But the preference is up to the end user. Wood is more rigid than composite, but it has also a more limited life span, usually around ten-plus years.”
Steiner also has a rule of thumb: “A known figure for ground-bearing capacity may not be available for every job site or equipment setup. And in a number of applications—including line work, tree trimming and some other crane work—determining the capacity may not be practical, particularly during storm response work where you need to act quickly. To address the issue, I suggest following a rule that I learned from my father and have been using for more than 20 years.
An outrigger pad should be at least three times the square surface area of the outrigger shoe or float, provided you are working on Grade A soil conditions. As soil conditions worsen, continue to create a bigger footprint using outrigger pads and cribbing.
“And after the equipment is set up and level and outrigger pads have been deployed, make a dry run of the boom. Use a spotter to ensure the outrigger pads are not shifting, sliding or sinking into the ground. Then check to see that the equipment is still level or within grade. If any issues are identified, stop and stow the boom. Now is the time to add cribbing, blocking or more outrigger pads.
“In terms of materials, I have found only three that I believe should be used with any critical pick or lift with a load of 10,000 pounds or more. They are American custom composite, premium birch and American steel.
Our custom composite can carry 3,000psi and maintains its value for a lifetime. This material is engineered to perform in the most extreme conditions and typically will outlast the equipment it is being used with. Premium birch has been battle-tested in the harshest conditions with some of the largest equipment. These outrigger pads have a typical life span of ten years though some have been used in the field since 1991. And American steel crane mats will also outlast the equipment they are being used with.”
Keeping the crane safe and stable is paramount. The choice with all support pads is down to the user; but don’t wait for the horse to bolt before closing the stability door.