Guide to loss prevention
© Getty Images/ Ibrahim

Mobile cranes overturning on construction sites

A guide to loss prevention

    alt txt



    Each year, a number of mobile cranes overturn on construction sites causing serious accidents. This often involves personal injury, significant property damage and potential business interruption claims.

    This guidance document aims  to provide supporting  information to assist in  preventing the overturning of  mobile cranes. There are different  types of mobile crane. The  specific operating instructions  for a particular manufacturer and  model of crane should be referred  to. This guide does not attempt  to cover the design of cranes,  nor does it cover all aspects of  lifting operations.

    Further industry best practice  guidance is included towards  the end of this guidance  document and the reader is  strongly recommended to refer  to BS 7121 and the guide C703  when planning mobile crane  operations.

    The hazards which increase  the risk of an overturning  occurrence are due to many  various factors. This includes:

    • unstable working platforms
    • subsurface voids
    • high winds
    • poor level of routine maintenance
    • inexperienced crane operators and/or supervisors
    • excessive loads
    • underrated crane capacity
    • poor crane operation such  as use of outriggers and  bearing plates

    Most incidents are as a result of inadequate planning, unclear responsibilities or unsafe use.

    Common terms and definitions

    Mobile crane: a self-powered crane  with a boom, which may be fitted on  a mast (tower). It is capable of  travelling laden or unladen, without  the need for fixed runways. It relies  on gravity for stability, with the  chassis of the crane not having any  capability to carry goods.

    Crawler crane: an off-road mobile  crane which travels on site. They are  mounted on a tracked chassis. Lifting  capacities are typically greater than for  mobile cranes and movement around  site is possible fully rigged. Crawler  cranes are generally employed for  longer durations and for undertaking  routine lifts and movements over  relatively short distances.

    Lift plan: The appointed person who  is to plan the lift should have adequate  practical and theoretical knowledge  and experience. The plan will address  the risks highlighted within the risk  assessment, resources required to  complete the lift, and the procedures  and responsibilities needed to ensure  the safety of the lifting operation.

    Permit to Lift: a formal process  recording checks and approvals prior  to confirming a lift may proceed.

    Appointed Person: a key person with  the training, practical and theoretical  knowledge and experience required to  plan and manage a lifting operation.

    Crane supervisor: Appointed Person  who controls the lifting operation and  ensures that it is carried out in  accordance with the Appointed  Person’s safe system of work.

    Crane coordinator: person who plans  and directs the sequence of operations  of cranes to ensure that they do not  collide with other cranes, loads and  other equipment.

    Crane operator: person who is  operating the crane for the purpose  of positioning loads or erection of  the crane.

    Slinger: person responsible for  attaching and detaching the load to  and from the crane and for the correct  selection and use of lifting accessories.  They are also responsible for initiating  the movement of the load.

    Signaller: person responsible for  directing the crane driver to ensure  safe movement of the crane and load.

    Working platform: temporary  geotechnical structure providing  a stable working surface for mobile  cranes, piling rigs and other heavy  construction equipment.

    Working Platform Certificate:  a certificate which confirms the working platform has been properly  constructed in accordance with the  design, and that it will be adequately  maintained to ensure its ongoing  integrity. The certificate requires the  signature of the main contractor on  site and must be handed to the  Appointed Person before lifting work  commences. The certificate introduces  a system for defining specific  responsibilities, increasing safety   awareness and highlighting the  importance of maintaining the  platform in good condition.

    Hirer and contractor responsibilities

    Hired Crane (hired and managed) Contract Lift (fully contracted)
    The employing organisation (hirer) is responsible for: The employing organisation (hirer) is responsible for:
    - Carrying out all work in accordance with BS7121 and Lifting Operations and Lifting Equipment Regulations 1998 (LOLER). - Allowing access to site to site for a full survey to be carried out for the risk assessment and method statement(s).
    - The safety and welfare of crane operator. - Informing crane owner of hazards which are not evident during site visit.
    - The safe operation of the crane. - Supplying accurate information of the load to be lifted.
    - Supplying a qualified and competent Appointed Person.
    - Providing a qualified and competent slinger/signaller and crane supervisor.
    - Planning the lift and operate a safe system of work.
    - Producing risk assessment and method statement(s).
    - Ensuring that the crane hired is of a suitable type and capacity.
    - Ensuring suitability of working platform.
    - Verifying the credentials of the crane hire company and certification supplied.
    The crane owner (contractor) is responsible for: The crane owner (contractor) is responsible for:
    - Providing a suitable crane that is properly maintained, thoroughly examined, tested and certified. - Providing a suitable crane that is properly maintained, thoroughly examined, tested and certified.
    - Providing a qualified and competent operator. - Providing a CPCS qualified Appointed Person.
    - Providing a qualified and competent slinger/signaller and crane supervisor.
    - Providing a qualified and competent crane operator.
    - Providing risk assessment and method statement.
    - Organising and controlling the lifting operation.
    - Supplying the appropriate certified equipment and competent persons.
    - Planning the lift and operation of the safe system of work.
    - Organisation and control of the lifting operation.
    - Carry out all work in accordance with BS7121 and Lifting Operations and Lifting Equipment Regulations 1998 (LOLER)

    Key components in preventing the overturning of mobile cranes include:

    Management and planning

    Many overturning occurrence root  cause can be traced back to poor  management and planning of the lift.  The following elements are vital to  ensuring a lift is carried out safely and  without incident.

    1. Safe system of work

    A safe system of work should be  established and maintained for every  crane installation operation and lifting  operation. A safe system of work  should include:
    • Planning of the lifting operation  including preparation of the lift  plan. Selection, provision and use  of a suitable crane(s) and  work equipment.
    • Any necessary preparation of a site  for the lifting operation which  may include entry/exit routes for  the crane(s).
    • Any rigging/derigging or setting  up of crane, Inspection,   maintenance,  thorough examination and testing  of the crane(s) equipment ensuring  reports are available.
    • The provision of properly trained and  competent personnel who have been  made aware of their responsibilities  under the Health and Safety at Work  etc. Act 1974.
    • Adequate supervision by properly  trained competent personnel having  the necessary authority.
    • Prevention of unauthorised  movement or use at all times.
    • Coordination of crane movements to  avoid collisions between cranes and  other type of plant or structures.
    • The safety of persons not involved in  the installation or lifting operation  and the need for the operator, unless  in imminent danger, to remain in  control of the crane throughout the  lifting operation.

    2. Selection and duties of personnel

    All those involved in a lifting operation  must be competent, adequately  trained and aware of their duties which  is detailed in BS7121-1. All crane  operators should hold a CPCS card.

    3. Appointed Person

    The Appointed Person has overall  control of the lifting operation and  their duties should include:
    • Planning of the lifting operation  (including approval of risk  assessments, lift categorisations  and method statements), crane  and lifting accessory selection,  instruction, supervision and  consulting with responsible bodies.
    • Ensuring the outcomes of planning  process are recorded in the lift plan.
    • Ensuring adequate pre-operational  checks, intermediate inspections,  maintenance and thorough  examination of equipment is  completed.
    • Taking responsibility for the  organisation and control of the lifting  operation.
    • Ensuring the crane supervisor and  other members of the lifting team are  fully briefed on the contents, scope  and limits of the method statement.
    Being familiar with the relevant parts of  the project health and safety plan  where the lifting operation is being  carried out on a site where the  Construction (Design and  Management) Regulations 2015 apply.  Some of the duties, but not the  responsibilities, may be delegated for  simple lifts. The Appointed Person  should consult with other experts  including temporary works engineers  and crane suppliers.

    4. Crane maintenance

    The crane should be in a satisfactory  operating order at all times. Planned  maintenance should be undertaken and  thorough examinations performed, with  Reports of thorough examination, test  certificates and documentation in  place. Detailed requirements are  included in BS7121 and within Lifting  Operations and Lifting Equipment  Regulations and Provision and Use of  Work Equipment   Regulations.

    5. Contract lift vs crane hire

    An organisation that requires a load  to be moved, and does not have its  own craneage, has two options. These  are; Crane Hire or the employment  of a contractor to carry out the lifting  operation (Contract Lift). If an  individual or organisation does not have  expertise in lifting operations, they  should not hire cranes but should opt  for the contract lift option. Insurance  arrangements should be clarified. A  summary of the options is included in  the following table.

    6. Complexity of lift

    The duties and extent of planning  involved for a lift will vary depending  on its complexity.

    Lifts are categorised into;  Basic, intermediate or complex.  The categorisation is a function  of the level of hazards present  within the area of the crane  operation, which are established  through the risk assessment  process. Detailed guidance is  further provided in BS 7121-1.

    7. Risk assessment and method statement

    An essential element of any crane  operation is the production of a risk  assessment and method statement.  The risk assessment should be  carried out by the Appointed Person  and identify the hazards and risks  associated with the lifting operation.  Reference should be made to the  overall site risk assessments included  in the CDM Construction Phase  Plan CIC80. The Appointed Person  should ensure that a full method  statement is prepared, detailing  the safe system of work and control  measures for the lifting operation.  A ‘Permit to Lift’ is a useful  mechanism to ensure all pre-lift  checks are undertaken.

    8. Crane selection and access

    The choice of a suitable mobile  crane is governed by a number of  factors including the characteristics  of the load to be lifted (weight and  dimensions), radii, heights of lifts,  areas of movement, frequency  and types of lifting operations,  length of time on site, ground  conditions and space available  for access, erection, travelling,  operation and dismantling.

    9. Crane siting

    The siting of the crane should take  into account the crane standing and  support conditions, the effect of wind  and the adequacy of access to allow  the placing or erection of the crane  in its working position. This will also  include dismantling and removing  from site.

    Particular care needs to be taken when  siting mobile cranes in or near the high  risk areas, for instance at the edge of  open trenches and excavations as  these are likely to collapse without  warning (see C703). An engineering  assessment by a competent  geotechnical engineer is required  before setting up in such a location.

    10. Checklists

    The management, planning and  execution of a crane lift has to take  into account many aspects. It is  recommended that the use of  checklists and pro-formas are  employed to ensure all aspects are  addressed. These may include (but  not limited to) key elements of the  safe system of work, the load and the  lift, the location, specifying and  operating teams, the crane, the lifting  gear and any temporary works. Sample  checklists are included in C703.

    The mechanism of overturning is  often as a result of a failure of the  ground or supporting structure  beneath the crane or the crane  outriggers. The following aspects  are key to ensuring an adequate  foundation is provided.

    11. Site categories and underground hazards

    Sites can be split into a number of  categories to highlight the most  likely hazards that need to be  considered and risk assessed. More  attention is required to establish  the strength of the ground where  ground conditions are poor or where  there is a lack of data on the nature  of the subsoil.

    Typical categories include:

    • Greenfield sites - particular problem  areas are adjacent to rivers, estuaries  and floodplains where soft alluvial  deposits and high groundwater  tables can be expected.
    • Beaches - low sand density and a  variable groundwater level can  create difficult conditions.
    • Brownfield sites - unknown previous  use including basements, storage  tanks, poorly-filled open pits and  poorly-compacted fill, etc.
    • Paved areas - tarmacked or paved  areas can appear deceptively strong  but lead to outriggers perforating  through weak surfacing. Lightly-trafficked  car parks, estate roads  and footpaths should be scrutinised.
    • Town centre sites – consider  underground hazards including  services, drainage pipes, buried  cables, basements and tunnels  beneath paved areas. This can lead  to outriggers perforating in to the  void below.

    12. Ground investigation

    Before a crane arrives on site, existing  information on the nature of the  soils should have been investigated  Reference should be made to any  existing site ground reports, and  particular attention paid to the  character of the ground at shallow  depths where the mobile crane(s)  will be sited.

    13. Ground bearing capacity

    An assessment of the ground bearing  capacity is required to determine  the size and type of crane foundation  required. This can be calculated with  reference to the ground investigation  reports and should be carried out by  a competent geotechnical engineer.  It should be noted that the presence  of water tends to reduce the strength  of soils and can lead to a reduced  capacity, since the initial bearing  capacity assessment.

    14. Settlement

    Settlement must be kept to a  minimum to avoid the slewing ring  being out of the horizontal resulting  in the jib not being in a vertical plane.  This can result in side loads on the  jib and possible failure of the jib.

    Excessive settlement can also attract  additional loading onto the outriggers  or track that is settling. Level  indicators and inclinometers should  be employed. If settlement occurs,  then the foundation will need to  be reassessed.

    15. Working platforms and design

    A working platform may be required  to provide a designated area of the  site over which mobile and crawler  cranes can travel during their delivery  and movement around the site, lifting  operations and removal. The design,  installation, maintenance and repair  of the working platform should be the  responsibility of the main contractor.

    The appointed person should consult  with a temporary works engineer on  the detailed requirements. The  platform should be free-draining to  prevent the build-up of water. In  certain cases, separation/filter  membranes may be required beneath  the platform. Appropriate factors of  safety should be employed in the  design and regular checks undertaken  to ensure the platform is not disturbed  by other construction activity.

    Excavations, trenches, or other holes  dug must be properly backfilled and  repaired to avoid creating soft spots.  The edge of the platform needs  to be clearly defined and ground  preparation should extend beyond  the working area required.

    16. Working Platform Certificate

    Consideration should be given to  using a ‘Working Platform Certificate’  to ensure that the correct procedures  have been followed, the ground is  adequate to support crane activity  and that there are no irregularities  that could result in local subsidence  or toppling. Further detailed guidance  on working platforms for tracked  plant and a sample working platform  certificate is available from the  Federation of Piling Specialists.

    The issue of a ‘Working Platform  Certificate’ should be an aspect of  the ‘Permit to Lift’.

    17. Outrigger foundations

    Outriggers to mobile cranes should  always be fully extended and all the  tyres lifted clear of the ground.

    The outrigger feet are relatively small  and transfer high pressures on the  ground. The pressure can be reduced  by the provision of suitable spreader  mats which, depending on the  allowable bearing pressure of the  subsoil, could consist of timber mats,  timber and ply plates, proprietary  mats, steel grillages, concrete pads  or piles (for high loads in week soil  conditions). Calculations will be  required and the Appointed Person  should consult with the temporary  works design engineer and crane  supplier. Outriggers should always  be positioned central to the spreader  mats, which should be in contact  with the ground over its entire  surface area.

    A useful best practice guide and  pro-formas on producing a risk  assessment and method statement  for a contract lift is available from  the CPA Crane Interest Group  (

    Case studies

    Case study 1

    Two construction companies were  prosecuted by the HSE following the  overturning of a 35-tonne truckmounted  telescopic crane on site.  The collapse occurred after one of the  outriggers, which supported the crane,  sank into the ground.

    The crane driver was forced to leap  to safety and the 5.7-tonne beam that  was being lifted into place narrowly  missed two employees as it fell. The  principal contractor and crane operator  were both fined and ordered to pay  substantial costs.

    The incident could have been avoided  had the planning and supervision of the  lifting operation not been so deficient.  The crane overturned because it was  being operated, with the knowledge of  both companies, in a part of the site  that had not been prepared for such  activities. The roadway was not wide  enough to accommodate the outrigger  spread of the crane. Clear warnings  were ignored in the run-up to the  incident about the ground bearing  capacity for the use of cranes on the  site. The risk assessments which had  been produced were inadequate as they  only considered use at a completely  different part of the site.

    Case study 2

    The principal contractor and crane  hire company were ordered to pay  a substantial fine and costs after a  serious communications failure led to  a mobile crane toppling over on site.  The extended 50-metre jib fell across  the site, narrowly missing workers and  a nearby busy road.

    The 80-tonne crane was supplied and  operated by the crane hire company  but confusion arose as to whether the  job had been set up on a ‘crane-hire’  only or ‘contract-lift’ basis (where  planning of the lifting operation is  included).

    This meant both firms neglected the  planning of the lift and led to vital roles  for the job not being assigned.

    A competent person should have been  appointed to plan the lift, either  provided by the crane hire company if  it was a contract hire arrangement or  by the hirer if it was a hire-only  agreement. The crane hire company  should have ensured this happened  when it set up the contract.

    Important information — including the  weights being lifted and ground  conditions — were not passed on to the  workers involved. The crane was  overloaded and was being operated on  poor ground. A larger crane and ground  mats to spread the load of the crane‘s  outriggers should have been used.

    Hazards and mitigation

    Overturning hazard Controls to mitigate the risk
    Failure of the ground
    Unstable/uncompacted working surface Undertake a suitable and sufficient site investigation to determine the nature of the ground conditions. Ensure a suitably designed and compacted working platform is installed and maintained.
    Underground hazards including services, drainage pipes, buried cables, basements and tunnels, etc. beneath, paved areas Obtain existing services drawings and undertake trial pits, probing and cable detection to identify all buried services and voids.
    Open excavations and slopes Ensure crane is set up a suitable distance from the edge of open excavations and slopes.
    Poor ground conditions – adjacent to rivers, estuaries, marshes, floodplains, beaches, landfill sites and areas with a high groundwater level Undertake a suitable and sufficient site investigation to determine the nature of the ground conditions. Monitor groundwater and soil saturation levels. Ensure a suitably designed and compacted working platform is installed and maintained. Use suitable bearing plates beneath outriggers
    Management and planning failures
    Inexperienced supervisors and crane operators Ensure only competent, trained persons are employed (see BS 7121-1). Consider a Contract Lift if competence of supervisors is in doubt.
    Unclear roles and responsibilities Ensure key positions are established and individuals appointed detailing their roles and responsibilities.
    Movement or set-up in unauthorised locations Provide a sketch illustrating the permissible set-up location(s) and allowable access route(s). Prevent access to unsafe areas with barriers.
    Uncontrolled lifting operations Ensure all lifting operations are included within a Lift Plan, including a risk assessment, a method statement and a ‘Permit to Lift’.
    Incorrect factor of safety in design of working platform Ensure a competent designer is employed and suitable Factor of Safety (FoS) used. Clarify if the design criterion refers to ultimate bearing capacity (ground failure) or allowable bearing pressure (including an FoS). See C703 for guidance.
    Load too heavy Ensure the load to be lifted is not greater than permitted in the planning. The rated capacity indicator/limiter should be maintained in good working order.
    High wind loading Establish site wind speed limits (red/amber/green). Use an anemometer to monitor in-service wind speeds. Monitor weather forecasts. Adhere to manufacturers tolerances for wind speed.
    Lifting radius too far Stay within the agreed lifting location and radius. Avoid swinging of loads or travelling with high loads.
    Mechanical failure
    Poorly maintained crane Ensure crane is regularly maintained in accordance with manufacturers specification and in line with the Lifting Operations and Lifting Equipment Regulations (LOLER) and Provision and Use of Work Equipment Regulations (PUWER) requirements.
    References and guidance: BS 7121-1:2016 – Code of practice for safe use of  cranes ( BS 7121-3:2017+A1:2019 – Mobile cranes  – CIRIA Publication C703:2003 – Crane stability on  site ( BRE Report 470 – Working platforms for tracked  plant ( CPA/Crane Interest Group – Best Practice Guide  for Risk Assessment and Method Statement for a Contract Lift: 2018 ( Working Platform Certificate ( Disclaimer: The guidance in this document refers to  industry best practice loss control advice. Adoption of  the advice contained within this document does not  imply compliance with industry, statutory or HSBEI  guidelines, nor does it guarantee that related losses  will not occur.

    HSB Construction Insurance

    We have a comprehensive range of insurance products which can cover all aspects of construction projects across the UK and Ireland.