FAQ >> Mobile Crane Ground Support Capacity

Mobile Crane Ground Support Capacity

Presumptive soil-bearing capacities used for building foundation design are often conservative when applied to crane supports. Buildings present long-term loads to the ground, so that allowable soil capacity must reflect a degree of long-term settlement control not necessary for crane use.

Unless the crane is to remain installed for weeks or months, the initial settlement on first bringing the crane into place will be the only settlement of note; its effect will be overcome by initial leveling of the crane. For long-term installations, periodic releveling may be required. Long-term settlement is of little practical consequence in mobilecrane support design if attention is paid to keeping the crane level at the site.

When evaluated on the basis of strength alone, most soils can support higher crane loads than the building design values would indicate. Although cranes are often operated on the ground on uncribbed floats with apparent success, there is actually great risk involved in doing this on most soils. Standard outrigger floats are limited in size by weight and stowage considerations and therefore can induce very high [15 tons or more per square foot (1.44 MN/m2)] bearing pressures. Soil shear failure is sudden (Figure 5.15) and often results in tipping. Cribbing is needed in most instances.

The soil at any location is a unique blend of constituents that may include very fine particles such as silts or clays, fineto medium-sized sand particles, coarse sands, gravels, and boulders, or organic materials, as well as water or even ice. Soil makeup and content are important factors in soil bearing strength, as is initial density or degree of compaction.

Organic material in soil makes the mass resilient or spongy and is therefore an unwanted component of support soil. Most surface soils contain some organic plant materials which ordinarily do not have an important effect on crane support properties, but as the organic content and thickness of surface soils increases, bearing capacity drops. A thick organic layer is unsuitable for crane support.

Fine-grained soils tend to shear at relatively low values. As soil particle size and the variation in size within the sample increase, soil strength will increase as well. The highest capacities are obtainable in compact well-graded sand-gravel mixtures made up of rough uneven particles.

A measure of soil capacity is its angle of repose, the maximum angle of a naturally stable slope. Shear strength increases with the angle of repose, and along with it the usable bearing capacity increases. For a given soil mix the angle of repose and strength will improve as compaction, measured by density, increases. Table 5.1 lists presumptive bearing capacities for various common soil types, as well as the authors' suggested bearing capacities for crane support. These are offered to help in installation planning; evaluation of the actual soil by an experienced person, preferably a licensed professional engineer, is certainly advisable if any doubt exists about soil capacity. Ground surfaces can conceal places of potential peril for mobile cranes traveling or working. Pavements may cover vaults, poorly consolidated backfill, or voids where fine-grained soils have washed out. The variety of soil conditions encountered in crane work is as wide as the field of soil mechanics. Where there is complexity or doubt, a geotechnical engineer might be consulted. Here are some general lessons from the authors' experience.

  • Recently backfilled areas should be approached with caution, unless the crane user knows that the fill is a select material that has been placed and tamped in layers and then tested for compaction.
  • Utility trenches are often hastily backfilled and should be approached by a crane warily unless it is known that a proper backfilling job was done. The surface may look deceptively sound if the backfill is fresh; over time, runoff and pounding of traffic may cause some revealing rutting.
  • Backfilling around footings is also frequently loose and uncontrolled, a potential danger for crane support.
  • A perilous condition, especially for crawler cranes, is a support that is partly on soft backfill and partly on hard ground leading to differential settlement under the crane. Fresh backfilled areas are prone to this hazard.
  • Pavements conceal manhole chambers and utility vaults that are not fully revealed at the surface. Cellars of urban buildings sometimes extend out under the sidewalk or even under the roadway.
  • Heavily trafficked areas around a construction site are often well compacted and thus suitable for cranes. If the ground is churned up and muddy, the surface can be improved with crushed stone or brickbats.
  • Old urban streets sometimes have sewers and gas mains below that are not in good condition. The problem for cranes may be exacerbated by the disturbance caused by new cellar excavation. Sheeting and bracing may bring about lateral displacement of utility lines or the washout of fine-grained soils around them. Utility lines have been known to rupture from this combination of soil disturbance and crane loading.

A common thread to most of these warnings is that disturbed ground can be problematic. When cranes are operating at a site, excavations and backfilling should be a concern that should ideally be brought under control by sound engineering and construction practices. Some enlightened contractors and owners understand the implications of construction loads, determining in advance where heavy equipment may be placed and preparing a site accordingly.

A second theme of the warnings points is the need to survey a site to evaluate surface and underground conditions before a crane is brought in. In some instances it is not possible to determine observationally whether backfill has been prepared adequately to support a crane. Some load testing might be advisable, say by running trucks or heavy equipment across a backfill to see its effect. The crane itself could be used for a self-check by swinging the counterweight over the backfill with the boom at minimum radius and with no load on the hook.

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