Risks Associated with Mobile Crane Operations
29 March 2018
Posted by: Tasveera Singh
Factors that will affect the ability of the ground to provide adequate support include the following:
a. the presence of water, including when water is mixed with the soil as mud, and where water is present under the surface (for example, underground springs or streams);
b. the type of ground (for example, clay, sand, rock or a mixture of these);
c. backfilled ground that was previously an excavation or trench;
d. cavities or penetrations in the ground that have been covered but still exist; and
e. continued operation of the crane in one location.
When a mobile crane is being set up, the crane operator can only make a decision based on the surface of the ground. Generally, rock provides the most stable supporting surface for a mobile crane. However, although rock may be present on the surface, it may not extend far below the surface. One way to establish how far rock may extend below the surface is to examine nearby excavations or trenches at the workplace. Rock that extends far below the surface provides a good indication of the ground’s integrity. However, this will only provide a reasonable indication of the ground’s strength when the excavation is not too far from the crane. Additional risks must be managed when outriggers are positioned too close to an excavation.
Care must also be taken with ground that has a ‘crust’ on its surface. The surface of this type of ground is usually firmer than the ground underneath. The firm surface may give the perception that the ground is more stable than it actually is. If the ground is punctured by an outrigger, or the end of a crawler track, the softer ground will be exposed, which may cause the crane to overturn.
Where a mobile crane is continuously operated in one location, the ground underneath the outriggers will compact. Additional care must be taken to ensure that the crane has not compacted the ground to the extent that the minimum overturning moment of the crane is reduced (that is, the crane is more likely to overturn).
Crane Proximity to Excavations and Trenches
When cranes are set up close to excavations or trenches, there may be an increased risk of the sides of the excavation or trench wall collapsing, causing the crane to overturn. This risk increases with softer ground, and the presence of groundwater. Additionally, the risk of collapse is greater for vertical cuts in the excavation wall in comparison to walls that have been battered back at an angle. The presence of ‘slippery back’, where there is a naturally occurring slip plain such as a fracture in the ground, can also increase the risk of excavation or trench collapse.
Generally, the following principles should be applied when setting up mobile cranes near excavations:
a. Where the ground is compact and non-friable (that is, not crumbling), the distance of any part of the crane support timbers from the excavation should be at least equal to the depth of the excavation (1:1 rule). For example, for a three-metre-deep trench in compact ground, the outrigger timbers or pads should be a horizontal distance of at least three metres away from the closest edge of the trench wall.
b. Where the ground is loose or backfilled (that is, crumbling), the distance of any part of the crane support timbers from the excavation should be at least twice the depth of the excavation (2:1 rule). For example, for a three-metre-deep trench in backfilled ground, the outrigger timbers or pads should be a horizontal distance of at least 6 m away from the closest face of the trench wall.
Timbers, Pads And Bog Mats
A variety of materials can be used to distribute the mass of the mobile crane, and the suspended load to the ground. Lengths of timber (timbers) with rectangular cross sections are the most common form. However, timber and plastic pads are also provided for some cranes. For heavier lifts, bog mats, usually consisting of steel plate, are often used under mobile cranes. Timbers and pads are usually provided under outrigger feet, while bog mats may be used under the tracks of crawler cranes or where larger lifts are carried out.
Timbers, pads and bog mats should be of dimensions and materials as specified by the crane manufacturer. If the manufacturer has not provided this information, a competent person should specify the minimum size of the material to be used.
Generally, the following principles should be applied to timbers, pads, steel plates and bog mats:
a. Timbers should have a minimum width of 200 mm and minimum thickness of 75 mm
b. Timbers should be laid together so that the width of the timber pad is wider than the outrigger foot with no gaps between timbers
c. Pads should have a minimum thickness of 75 mm
d. The dimensions of steel plates and bog mats should be determined by a competent person, based on the type of mobile crane.
Performing Heavy Lifts
The likelihood of a mobile crane overturning is greater when the crane is used to lift heavy loads. It is extremely important to ensure the ground has adequate bearing capacity to support the crane when performing the following lifts:
a. bridge beams;
b. tilt-up panels; and
c. other heavy lifts where the load is 50 tonnes or more.
The crane owner should compare the ground bearing capacity with the maximum pressure the crane will apply to the ground for the lift. The maximum pressure applied by a crane is a function of the crane mass, crane configuration (that is, boom length and centre of gravity) and the mass of load on the hook. The ground bearing capacity must be greater than the maximum pressure applied by the crane to the ground to ensure adequate crane support. If not, then appropriate control measures, such as the use of bog mats, must be in place to increase the ground bearing capacity before the lift is performed.
Cranes on Outriggers (Or Stabilisers)
The use of outriggers on mobile cranes helps to provide greater stability to the crane when lifting loads. Irrespective of the ground conditions, timbers or other means of distributing the load should always be placed under the outriggers.
Outriggers should be set according to the manufacturer’s operating instructions for the specific type of mobile crane. The outriggers should also be used to help level the crane.
Many cranes are not designed for lifting with partially extended outriggers. If one or more outriggers are not fully extended, the crane may become unstable during lifting operations. In some instances, it may not be possible to fully extend all outriggers. Only cranes that have the manufacturer’s approval to lift with partially extended outriggers should be used this way. If a lift is to be undertaken with partially extended outriggers, the correct outrigger configuration, according to the appropriate load chart, must be used.
Calculating Pressure Applied by Outriggers
A number of crane manufacturers provide information on the maximum ground pressure that is applied when the crane is at maximum capacity, in the stability range of the load chart.
Different ground types will have different ground bearing capacities. Generally, harder ground, such as rock, is capable of withstanding higher ground pressures than softer ground, such as dry sand. Where the ground consists of a combination of ground types, the poorer ground type should be used for determining the maximum ground pressure that can be applied to the ground when the crane is set up on outriggers. The table below identifies the maximum permissible ground pressure according to the ground type.
Table 1: Maximum permissible ground pressures for various ground conditions.
The greatest force applied by any outrigger to the ground will be:
a. at the point of tipping, just as the crane is about to overturn; or
b. when the crane boom is located directly above an outrigger foot.
A crane will overturn within the stability part of the load chart when the maximum safe working load (SWL) is multiplied by a factor of 1.33. In reality, a crane will not approach this condition, provided the operator does not overload the crane. However, a reasonable approximation for maximum ground pressure applied by the outriggers is detailed below.
Pressure (tonnes per m2) applied by outrigger feet
When the minimum allowable ground pressure is known, the minimum area required under the outrigger feet can be calculated as follows:
Minimum area required under outrigger foot
To find the length and width dimensions for the outrigger timbers, find the square root of the area (√area).
The following examples demonstrate the practical application of the above formulae.
A mobile crane with a total mass of 40 tonnes is lifting a 20-tonne load – 20 tonnes is the maximum the crane can lift in the stability range of the load chart. Each of the four outrigger feet on the crane are provided with timbers that are 0.8 m long by 0.8 m wide.
Calculate the maximum ground pressure that will be applied to the ground when lifting directly above an outrigger foot.
Lifted load (L) = 20 tonnes
Total crane mass (CM) = 40 tonnes
Timber area in contact with the ground = 0.8 m x 0.8 m = 0.64 m2
Pressure applied by outrigger feet
A mobile crane is to be set up on its outriggers on compacted gravel. The crane has a total mass of 25 tonnes and is to lift a 10-tonne load – 10 tonnes is the maximum the crane can lift in the stability range of the load chart. The lift plan requires the load to be slewed above each outrigger foot. Calculate the minimum required area of the timbers to be placed under each outrigger when lifting directly above an outrigger foot.
Lifted load (L) = 10 tonnes
Total crane mass (CM) = 25 tonnes
Maximum allowable ground pressure (PMAX) for compacted gravel = 40 tonnes per m2
Dimensions of outrigger timbers: √0.569 m2 = 0.754 m
Therefore, length x width of timbers required = 755 mm x 755 mm.
Source: Leading equipment Magazine.