Ground Conditions, Outriggers and Where a Crane Can Set Up

Four Small Pads Carrying a Very Large Load

When a crane sets up, the truck comes off its tires entirely. From that moment everything, the machine, the counterweight and whatever is hanging on the hook, is carried by four outrigger pads, each covering a modest patch of ground.

That concentration is the whole issue. A vehicle spreads its weight across four tire contact patches while it is moving and the load shifts constantly. A crane on outriggers puts a much higher and much more focused pressure into four fixed points, and it does it while the load moves around, which means the pressure at each pad changes as the boom swings.

The pad carrying the most is whichever one is on the side the load is over. Swing the boom and that changes. A setup that is fine with the load in one direction can be marginal with the load in another, which is why the operator cares about ground under all four legs and not just the two he happens to be looking at.

Full Extension, and What It Costs to Go Short

Outriggers are designed to go all the way out. Fully extended, the crane has the widest possible base and the chart it is rated against assumes exactly that.

Sometimes there is not room. A narrow driveway, a wall on one side, a neighbor's vehicle. Partial extension is possible on many setups, but it is not free: a narrower base means the machine tips more easily, so the chart that applies is a different and more restrictive one. What the crane can lift drops, and it can drop a lot.

That is a decision made against the chart, not by eye. It is also a reason to clear more room than seems necessary before the truck arrives. Six feet of extra clearance on the load side occasionally makes the difference between a job that works and a job that does not.

Cribbing, Mats and Spreading the Load

Cribbing is the timber or engineered blocking that goes under an outrigger pad to spread its load over a wider area. Mats do the same thing over a larger footprint. The purpose is to take the pressure at the pad and hand it to the ground over more square footage than the pad alone provides.

They come out for soft ground, for surfaces that mark or crack easily, and for anything where the operator wants more margin than the bare pad gives. Decorative concrete, pavers, pool decking, asphalt in summer heat and any recently poured slab all get this treatment.

Asphalt deserves a mention on its own. In July, asphalt in this town is soft enough that an unspread pad will leave a permanent dent. That is not a structural failure, it is a cosmetic one the property owner has to live with, and spreading the load prevents it.

• Pavers and decorative or stamped concrete
• Any concrete poured recently, before it has fully cured
• Asphalt, particularly in summer heat
• Pool decks, patio surfaces and coping
• Loose gravel, landscaped areas and recently graded ground

Caliche, and What Is Not Caliche

Under a few inches of desert soil across most of this area is caliche, a hard cemented layer that is genuinely good bearing material. When a crane is sitting on undisturbed native ground here, the ground is usually not the problem. That is worth saying plainly, because people assume desert soil means poor support.

The problem is everything that is not undisturbed native ground, and from the surface it all looks the same. A backfilled utility trench crossing a driveway. Fill brought in to level a building pad. A wash that got graded flat years ago. Landscaping soil over what used to be a hole. All of it can be sitting there looking like solid ground and behaving nothing like it.

Septic tanks, leach fields, cisterns, drywells and shallow utility laterals are the other category. These are not soft ground, they are voids or vulnerable structures with a thin lid over them, and a crane pad on top of one is a genuinely serious situation.

This is why the questions about buried services get asked and asked again. If you know where a trench was cut, say so. If a section of the yard was regraded, say so. Nobody can see any of it from the seat of a truck.

How a Bad Set Shows Itself

Ground problems announce themselves during setup rather than during the lift, which is one reason setup is unhurried. As the truck comes up on its legs, the operator is watching for a pad that keeps going down when the others have stopped, for the machine refusing to sit level, and for the level drifting after it has been set.

During the pick, the sign is the machine settling as the boom comes over one side. A pad that sinks progressively while the load is out is telling you the ground under it is consolidating, and the response is to set the load down and reassess rather than to finish quickly.

Anyone on site can call a stop if they see the truck moving in a way that looks wrong. It is far better to have a false alarm and lose ten minutes than to be right and say nothing.