ANSWER
TO RIGGING QUIZ No. 25

There are really two parts to this quiz. First is determining where to safely set up the crane relative to the shoulder of the slope and second is making sure that the outrigger float loads are distributed down through the blocking or load spreaders such that they don’t exceed the allowable soil bearing pressure.

**QUESTION No. 1: Safe distance from crane to excavation**

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**Case A **The crane **is not positioned **far enough away from
the shoulder of the slope.

According to CIRIA (Construction Industry Research and Information Association, Westminster, London), the horizontal distance “D” should be greater than 4 times the width of the outrigger float blocking “B”, without having to consider soil type.

Therefore “D” = 4 * 2.5 feet = 10 feet

Using cut slope criteria from OSHA (Occupational Safety & Health Administration, USA), it is recommended that for this type of soil, the horizontal distance from the edge of the blocking under the outrigger float to the toe of the cut slope be a minimum of a 1.5 Horizontal:1Vertical. This would be applicable to fill slopes as well. The criteria is also applicable to crawler cranes with or without crane mats.

Therefore “D” = 15 feet – 10 feet = 5.0 feet (with a minimum “D” distance of H/2).

See the supplemental information on page 3 for a graphic layout of recommended minimum “D” distances based on different types of soil defined by OSHA.

Per CIRIA, the criteria for this situation is that “D” should be the larger of 4*B or 1.5*H

Therefore “D” = 10 feet

Again, using OSHA criteria, it is recommended that for this type of soil the horizontal distance from the edge of the blocking under the outrigger float to the edge of the retaining wall be a minimum of a 1.5 H: 1V. This criteria is also applicable to crawler cranes.

Therefore “D” = 6 feet (with a H/2 minimum “D” distance).

**QUESTION No. 2:
Ground condition checklist**

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Five things that should be checked for in regards to ground work prior to setting up a crane near a slope are:

1. Are there cracks in the asphalt/concrete paving? Have there been any new patches? This could be an indication of settlement below the pavement. Also watch the pavement during the lift to see if the cracks enlarge.

2. If the soil has been recently placed, check for compaction.

3. Are there underground utilities or hazards such as culverts, electrical duct banks, water lines, sewer lines, steam lines, gas lines or underground cavities (such as a slow leak in a underground steam line).

4. The presence of a water table. The higher the water table, the less stable the ground is.

5. Loosely compacted soil close to new buildings or foundations is susceptible to settlement.

**QUESTION No 3: Outrigger blocking size**

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From a past article entitled “Outrigger Expertise” printed in International Cranes (IC), a guideline was presented for determining the area of blocking or load spreaders required under the outrigger floats for compacted sand clay aggregate soil: The area of blocking in square feet that is required under each outrigger float is equal to the maximum capacity of the crane in US tons divided by five, equivalent to a ground pressure of 70 pounds/square inch (10,080 pounds /square feet (psf)). The area of blocking per the above is 45 tons/5 = 9 square feet.

Therefore, the size of the blocking required under the outrigger floats = 36 inches square

= 40 inches in diameter

It is recommended that for cranes up to 100 tons in capacity to size the blocking or load spreader so that it is 4 times larger than the area of the outrigger float. In this case 4 times the area of a 24 inch diameter float is 12.6 square feet.

Therefore, the size of the blocking required under the outrigger floats = 42 inches square

= 48 inches in diameter

Using outrigger blocking of 48 inches in diameter and the OSHA criteria for locating the crane next to a slope, the operating radius for Case A = 33.25 feet. For a 81 foot boom, 35 foot radius and a 14,000 lbs. load, Grove calculates that each front outrigger will have a vertical force of 35,000 lbs. This yields a soil bearing pressure under each outrigger blocking of 35,000 lbs/12.6 square feet = 2,780 psf < 3,000 psf ===è good.

Using outrigger blocking of 40 inches in diameter per the criteria in the IC article yields a soil bearing pressure of 35,000/9 = 3,890 psf > 3,000 psf ===è no good.

If the load was 18,650 lbs., the load to each front outrigger would be 40,000 lbs. and the soil bearing pressure under the outrigger blocking would be 40,000 /12.6 = 3,175 psf > 3,000 psf ===è not good. Some settlement of the blocking could be expected.

In this quiz, the load of 14,000 lbs. was chosen because as a non-engineered lift, The author would restrict the lift to 75% of the load capacity chart. Note that the actual lifting capacity for a 35 foot radius is 18,650 lbs.

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**QUESTION No. 4: Maximum outrigger loadings**

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The answer is “**A**”

With a 34 foot boom over the front of the crane at a radius of 10 feet and with a 90,000 lb. load, the loading to each of the front outriggers is 60,000 lbs.

With a 81 foot boom over the front of the crane at a radius of 35 feet and with a 18,650 lb. load, the loading to each of the front outriggers is 40,000 lbs.

**SUMMARY:**

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**Safe distance from
crane to excavation**

The guidelines presented for placing cranes next to a slope are somewhat conservative, with CIRIA being more conservative. If the user needs to set the crane closer to the shoulder of the slope than is indicated in the guidelines, then an engineering assessment should be made to determine the crane location relative to the shoulder of the slope.

The criteria used by the author for sizing the blocking area under the outrigger floats is applicable

to structural fill compacted properly within a plant site where the allowable soil bearing capacity is between 3,000 and 4,000 psf and the load being lifted is not more than 50% of the load capacity chart. If the allowable soil bearing capacity is less than 3,000 psf or the load being lifted is greater than 50% of the load capacity chart or the lifting capacity of the crane is greater than 100 tons, then an engineering assessment should be made. The study should determine the outrigger loadings, the allowable soil bearing capacity of the soil and then the sizing of the blocking under the outrigger floats so that the actual soil bearing pressure is less than the allowable.

Type A Soils - Cohesive soils with an unconfined compressive strength greater than 3,000 lb./square foot (psf). Examples are clay, silty clay, sandy clay and clay loam. Cemented soils such as caliche and hardpan are also considered type A.

Type B Soils - Cohesive soils similar to above but with an unconfined compressive strength greater than 1,000 psf but less than 3,000 psf. Also granular noncohesive soils including angular gravel, crushed rock, dry rock that is unstable such as shale, previously disturbed soils, and soils that are fissured.

Type C Soils - Cohesive soils with an unconfined compressive strength of 1,000 psf or less. Also granular soils including gravel, sand and loamy sand, submerged soils, soils from which water is freely seeping, and submerged rock that is not stable.

NOTE: These categories are to be selected by a competent person based on an analysis of the soil properties, the excavation performance and the environmental exposure.