11.2 Ground conditions and crane support
11.2.1 Ground factors
11.2.2 Crane proximity to excavations and trenches
11.2.3 Timbers, pads and bog mats
11.2.4 Performing heavy lifts
11.2.5 Cranes on outriggers (or stabilisers)
11.2.6 Calculating pressure applied by outriggers
11.2.7 Crawler cranes
Ground conditions can vary dramatically from one workplace to another, and even within the one workplace. Failure to address poor ground conditions to ensure crane stability may cause the crane to overturn resulting in serious injury to the crane operator and other persons in the vicinity of the crane.
11.2.1 Ground factors
Factors that will affect the ability of the ground to provide adequate support include the following:
- the presence of water, including when it is mixed with the soil as mud, and where it is present under the surface (e.g. underground springs or streams);
- the type of ground (e.g. clay, sand, rock or a mixture of these);
- backfilled ground that was previously an excavation or trench;
- cavities or penetrations in the ground that have been covered but still exist; and
- 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—refer to section 11.2.2 of this code for further information.
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 (i.e. the crane is more likely to overturn).
11.2.2 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:
- Where the ground is compact and non-friable (i.e. 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.
- Where the ground is loose or backfilled (i.e. 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 six metres away from the closest face of the trench wall.
11.2.3 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 (see figure 3) are the most common form. However, timber and plastic pads are also provided for some cranes. For heavier lifts, bog mats (see figure 4), 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.
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| Figure 3 - Crane outrigger foot on timbers | Figure 4 - Crane outrigger foot on bog mat |
Crawler cranes will generally apply considerably less point load to the ground than a crane on outriggers with no timbers. This is because of the large area of tracks in contact with the ground, in comparison with the smaller contact area of the outriggers, on cranes of similar capacity. However, for heavy lifts, and where the ground has poor bearing capacity, bog mats or other supporting materials may be required.
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:
- Timbers should have a minimum width of 200 mm and minimum thickness of 75 mm.
- Timbers should be laid together so that the width of the timber pad is wider than the outrigger foot with no gaps between timbers.
- Pads should have a minimum thickness of 75 mm.
- The dimensions of steel plates and bog mats should be determined by a competent person, based on the type of mobile crane.
11.2.4 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:
- bridge beams;
- tilt-up panels; and
- other heavy lifts where the load is 50 tonnes or more.
The bearing capacity of the ground is usually estimated by the crane operator when lifting smaller loads. However, certification of the ground bearing capacity must be obtained from a geo-technical engineer17 before performing a heavy lift (see section 11.2.6 for further information).
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 (i.e. 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.
11.2.5 Cranes on outriggers (or stabilisers)18
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.
11.2.6 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. Table 1 identifies the maximum permissible ground pressure according to the ground type.
Ground type
|
Maximum permissible ground pressure, |
Hard rock |
200 |
Shale rock and sandstone |
80 |
Compacted gravel (with up to 20% sand) |
40 |
Asphalt |
20 |
Compacted sand |
20 |
Stiff clay (dry) |
20 |
Soft clay (dry) |
10 |
Loose sand |
10 |
Wet clay |
Less than 10 |
Table 1: Maximum permissible ground pressures for various ground conditions.
The greatest force applied by any outrigger to the ground will be:
- at the point of tipping, just as the crane is about to overturn; or
- when the crane boom is located directly above an outrigger foot.
If a crane is designed in accordance with AS 1418.5 Cranes, hoists and winches - Mobile cranes (non-Queensland Government link), the 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
| Pout = | 0.65 x (total crane mass + lifted load) |
| (individual outrigger area) |
| Pout = | 0.65 x (CM + L) |
| area |
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
| area = | 0.65 x (total crane mass + lifted load) |
| pressure applied by outrigger feet |
| area = | 0.65 x (CM + L) |
| Pout |
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.
Worked example 1
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
| Pout = | 0.65 x (CM + L ) |
| area |
| Pout = | 0.65 x (40 tonnes + 20 tonnes) |
| 0.64 m2 |
| Pout = | 39 tonnes |
| 0.64 m2 |
| Pout = | 60.9 tonnes per m2 |
Worked example 2
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
| area = | 0.65 x (CM+ L ) |
| PMAX |
| area = | 0.65 x (25 tonnes + 10 tonnes) |
| 40 tonnes per m2 |
| area = | 22.75 tonnes |
| 40 tonnes per m2 |
| area = | 0.569 m2 |
Dimensions of outrigger timbers: √0.569 m2 = 0.754 m
Therefore, length x width of timbers required = 755 mm x 755 mm.
11.2.7 Crawler cranes
The ground pressure applied by crawler cranes is different to that applied by a crane on outriggers. It is sometimes assumed that the ground pressure will be the same at any place where the track is in contact with the ground. However, in practice this is rarely the case.
When the crawler crane is being used with a suspended load, the ground pressure will be greater towards the front of the crane. If there is no load suspended on the crane, the ground pressure will be greater towards the rear of the crane.
The distribution of ground pressure applied by a crawler crane will vary according to the working radius, load mass and counterweight mass.
17. See appendix 1 for definitions.
18. Stabilisers are the devices used on a vehicle-loading crane to increase the vehicle's stability.
Last updated August 3, 2006