One third of all Dangerous Occurrences reported by FPS Members are related to working platforms. Every Dangerous Occurrence and every ‘near miss’ involving the platform is a potential fatality. Working Platforms must be designed, properly constructed, regularly inspected and maintained for the plant which will use them.
The FPS Working Platform Certificate is explained below, along with further information and guides for Working Platforms.
The FPS Working Platform Certificate confirms that the working platform has been properly designed, constructed in the accordance with the design, and will be regularly inspected and adequately maintained to ensure the integrity of the platform is maintained. It requires the signature of the Principal Contractor and must be handed to the Piling Contractor before piling (or other work) commences on site. The WPC has the full support of the Health and Safety Executive.
The design of the working platform must take account of the bearing pressures of the plant which will use it and the ground conditions on which it will be constructed. It should be undertaken by a qualified designer with appropriate geotechnical expertise.
The HSE has worked closely with the FPS to develop the working platform initiative and supports the principle of reducing accidents by the certification of properly designed, prepared and maintained working platforms.
The guide to good practice, Working platforms for tracked plant, was prepared by BRE under the direction of an FPS Steering group, and was published in June 2004 as Report BR470. Available evidence indicates that the guide has proved satisfactory in its principal objective of improving safety by promoting the implementation of minimum design, installation and maintenance standards. However, some concerns have been expressed about the way in which the effect of ‘structural geosynthetic reinforcement’ is evaluated and it has been pointed out that the contribution of geosynthetics can be represented in ways other than that depicted in the guide. Pending a full review of the guide, BRE has reviewed this one issue and its findings are summarised here.
The performance of working platforms reinforced with geosynthetics is complex and a wide variety of geosynthetics with very different properties may be utilised. In addition, there are several possible failure mechanisms along with the commonly considered punching-type failure. It is therefore unlikely that a single simple design method can be developed which will have a universal application and which will preserve both safety and economy. The calculation method in the guide for geosynthetic reinforced working platforms has two stages. The second stage of the calculation, which in many cases will be the decisive consideration in determining the depth of the platform, applies a control on how much contribution can be obtained from the inclusion of geosynthetic reinforcement.
BR470 acknowledges that there will be many situations in which alternative design approaches are acceptable. In section 3.1 it is stated ‘On some sites, where it may be more economic, or where particularly difficult conditions are encountered that are not covered by this guide, a more sophisticated approach is warranted.’ In practice a number of other approaches have been adopted for the design of geogrid reinforced working platforms. These include the load spread model which is based on an assumed redistribution of the applied load through the reinforced platform, the bearing capacity improvement model in which it is assumed that a reinforced working platform has the effect of increasing the bearing capacity of the subgrade by a certain multiple and the load factor model which is based on an experimentally determined relationship between the bearing capacity of reinforced and unreinforced platforms.
Such alternative methods should only be used in situations for which they have been validated by past experience and by appropriate experimental testing. Experimental laboratory testing should be carried out at an appropriate scale. For example, the load should be applied through a loading plate of suitable shape and of a size commensurate with the geosynthetics and the particle size distribution of the granular material used to represent the platform. Ideally the overall scale of the test should be able to incorporate materials used in platform construction and a plate width or diameter of at least 0.3 m is recommended. A test enclosure should be of sufficient size to ensure that boundary effects are not significant. Measurements should include real-time monitoring to demonstrate the development and distribution of internal stresses and strains within the platform and subgrade. The measurement of lateral strain within the geosynthetics would be of particular value. The application of the experimental results to a particular design approach should be undertaken by a competent person.
Geosynthetic reinforcement for a working platform should be designed by a competent person and a number of issues need to be considered including the position, extent, type, stiffness and strength of the reinforcement. Another critical feature in design calculations is the assignment of design values to ground properties, geosynthetic reinforcement and loading conditions and, since most calculation methods are likely to have an element of empiricism, it is important that design values incorporate factors which have been found to be appropriate for the particular method of calculation.
Manufacturers of geosynthetic products may have wide experience of the behaviour of their products in working platforms and be able to offer a cost-effective design. Where a geogrid reinforced working platform is designed by the geosynthetic manufacturer it is particularly important to identify clearly where responsibility lies for the design, placement and operation of the platform. The basis of the design calculations should be stated and reference made to credible research that validates the method.
The guide emphasises that the results of calculations should be critically appraised by a competent person. Furthermore, all forms of guidance can only be effective in so far as they are actually followed and careful supervision, control and monitoring of the platform on site under appropriate contractual arrangements are essential during placement of the platform and in its subsequent use. A particular consideration for a reinforced working platform is the vulnerability of the geosynthetic reinforcement to damage due not only to inadvertent site activities involving excavations through the platform but also to the piling operation itself.
Summary
1. BR 470 has proved satisfactory in its principal objective of improving safety.
2. BRE has reviewed the single point about the use of alternative methods to represent the contribution of geosynthetics instead of that depicted in the guide.
3. In principle, BR 470 can embrace alternative approaches to the design of mechanically stabilised working platforms providing:
a) the objective of safety is preserved,
b) alternative approaches are based on credible and representative research which is interpreted and formulated according to the geotechnical discipline and validated by well documented case studies,
c) a person competent in both geotechnical engineering and geosynthetics is made responsible for the design.
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For Further Information contact Ken Watts: © BRE 2011
Piling rigs are likely to become unstable on poorly designed platforms and on well designed platforms that are not rigorously managed by a maintenance and repair schedule.
A working platform should provide a safe and sufficiently durable working surface from which construction plant, such as piling rigs and cranes can operate. The expression working platform in this guidance is restricted to ground supported working platforms, including not only the platform itself but also the associated ramps and accesses.
This guidance covers the key issues related to working platforms constructed using stabilised materials. These comprise hydraulically bound materials (HBM), mixtures that set and harden by hydraulic reaction. They also include CBM (i.e. mixtures based on the fast setting and hardening characteristics of cement) and hydraulically bound mixtures based on slower setting and hardening binders made from industrial by-products such as PFA and blast furnace slag.
Contractual arrangements should ensure that the platform is properly designed, installed, operated, maintained and, as necessary, repaired throughout its working life. The respective roles of the various parties should be clearly understood and the responsibilities and liabilities of all parties should be defined in the relevant contract. This guide does not extend or affect such responsibilities and liabilities.
Installation, operation, maintenance and repair of the platform should ensure that it performs according to the design over its design life. Systems should be in place to ensure that;
The design of a temporary platform requires information about each load bearing soil layer. The subgrade is in general characterised by an undrained shear strength (cohesive, cu) or angle of shearing resistance (granular, f’); representative values should be identified for the areas of platform to be designed. Later applications for the working platform may require further information for design purposes. Whilst in general volume stability of a temporary platform will not be an issue, the sulphate content of soils to be treated may well need to be addressed if a permanent works design is also being considered.
Site drainage requirements may vary throughout the year and the need for any additional drainage should be assessed and incorporated into the platform construction programme. Weather, particularly temperature, may adversely affect the setting of the HBM binder.
Last minute project design changes can lead to changes in construction, and hence working platform levels. The potential implications of such changes, particularly when there is cut and fill on a site, should be recognised and the working platform designer informed.
The design of a working platform is a geotechnical design process and should be carried out by a competent person. The designer should have full access to the health and safety plan, including the desk study and all other site investigation information. This guide is not intended to replace or reduce the designer’s input, but rather to promote the implementation of minimum design, installation and maintenance standards.
HBM materials are likely to have very low permeability. Hence drainage design is critical, to ensure that the platform remains suitable for use and does not become waterlogged, potentially creating plant instability and hazards to operatives. In general this can be carried out by careful construction of the platform layer and/or the subgrade. Falls can be constructed into the platform by means of grading during final compaction. Large areas may require falls to intermediate drains as plant stability necessitates a maximum gradient of 1 in 10.
In all cases the requirements of the construction timescale should be taken into account. Winter working may require drainage design for heavier rainfalls and the water table beneath the platform may also rise. Similarly ice forming on the platform due to the presence of standing water can render construction activities dangerous. In the worst cases it may be necessary to place a granular topping layer.
Due consideration must be given to the piling process employed. Some piling techniques will cause groundwater to be deposited on top of the platform and this quantity must be added to that from rainfall and any rising groundwater table. Also if the piling technique employs a support fluid then this must also be taken into account. Driven piling, whether precast or cast insitu will probably require pre-boring of pile positions prior to driving to minimise damage to the platform.
Disclaimer: Although every effort has been made to check the accuracy of the information and validity of the guidance given in this document, neither the FPS or its members accept any responsibility for mis-statements contained herein or misunderstanding arising herefrom.
The recently published BRE design method for calculating the thickness of the working platform for tracked plant, e.g. piling rigs and cranes, requires the input of realistic parameters for the properties of both the platform material and the underlying stratum. Use of conservative parameters will result in the calculation of conservative, and hence uneconomic, platform thicknesses.
The plotted examples shown below demonstrate the sensitivity of the calculated platform thickness for a range of input parameters, for typical types of piling rigs.
The figures below are general examples. Platform designs must be based on the actual rig loadings supplied by the piling contractor and the FPS can take no responsibility for any use made of the example information shown.
TYPICAL RIGS |
Track width |
Example Case 1 loading |
Example Case 2 loading |
||
Design pressure q1 |
Effective track length L1 |
Design pressure q2 |
Effective track length L2 |
||
(m) |
(kN/sqm) |
(m) |
(kN/sqm) |
(m) |
|
| Heavy hydraulic |
0.800 |
212 |
2.90 |
633 |
1.24 |
| Crane mounted |
0.915 |
347 |
1.73 |
365 |
1.09 |
| CFA |
0.700 |
103 |
3.15 |
206 |
1.85 |
| Mini-pile |
0.400 |
92 |
1.53 |
188 |
0.79 |
For further details refer to BRE Report 470, Working Platforms for Tracked Plant: good practice guide to the design, installation, maintenance and repair of ground-supported working platforms.
This guidance has been produced by the Federation of Piling Specialists It is not intended to be used as a design method. Platforms should be designed according to the actual rig loadings supplied by the piling contractor. If expert assistance is required, the services of a competent professional should be sought. Although every effort has been made to check the accuracy and validity of the above guidance, neither the authors nor the Federation of Piling Specialists accept any responsibility for mis-statements contained herein or misunderstandings arising herefrom.
September 2005
Last reviewed June 2013
This document sets out the basic procedure for calculating the track bearing pressures for a crane or piling rig for use in the working platform design process set out in the BRE Report BR470, Working Platforms for Tracked Plant: good practice guide to the design, installation, maintenance and repair of ground-supported working platforms.
It should be noted that the track bearing pressures calculated by the appropriate method for use in the BRE design method are commonly much higher than given by a simple calculation of the total rig weight divided by the total track area.
The calculation of the track bearing pressures in an appropriate manner consistent with the BRE design method is a fundamental requirement of its use, and the use of bearing pressures calculated by any other means could potentially lead to an unsafe working platform design.
All of the figures in the document are general examples. Platform designs must be based on the actual rig loadings supplied by the piling contractor and the FPS can take no responsibility for any use made of the example information shown.
The FPS have a Rig Loading Spreadsheet tool that is referenced in this document. We recommend undertaking the training workshop which are held by the FPS before using the Spreadsheet. For more information about the Spreadsheet or the workshop, please contact the FPS at FPS@FPS.org.uk.
FPS Rig Track Pressure Calculation Tool v3.1 (revision Oct 2024)
A working platform is the foundation for a piece of machinery that may weigh anything from 5 tonnes to more than 150 tonnes. Although stable when on a firm surface, a 1m2 soft spot can be sufficient to unbalance the rig and cause it to topple over. Every year piling rigs fall over. ‘Near misses’ due to the site surface are even more common.
The FPS has developed the WPC in order to raise awareness of the importance of providing a proper platform for the plant which will work on it – and the importance of maintaining it during the course of the contract. Signing the certificate confirms your awareness of these requirements.
Attempting to assess the adequacy of a working platform by visual inspection is one of the failings of the historic practice that the WPC is addressing.
The WPC does not seek to transfer any responsibilities. Both the Principal Contractor and the Piling Subcontractor already have responsibilities under CDM and the Health and Safety at Work Act. The use of the WPC is a way of highlighting these responsibilities by increasing awareness of platform safety and the importance of maintaining the platform in good condition throughout the contract.
The WPC does not impose any additional responsibility, it merely confirms that the Principal Contractor has fulfilled his obligations under CDM, which does have statutory force. In this respect it is not unlike a permit to dig, which is widely used but is also not a statutory requirement. The initiative has the support of the HSE.
The principal contractor has a responsibility under CDM regulations to ensure that all activities are carried out by suitably qualified persons, and this includes the platform design, installation and maintenance. The Working Platform Certificate is to be signed by the principal contractor to confirm that the design, construction and maintenance has been / will be carried out by competent persons. It does not state who will carry out these activities.
This is a matter for agreement between the main or principal contractor and the specialist contractor whose plant will be working on the site. The introduction of the Working Platform Certificate has not changed the previously existing commercial or contractual situation with regard to who actually has responsibility for the design of the working platform, other than ensuring it is clarified instead of being a grey area as often used to be the case.
The working platform can deteriorate due to use over time. Additionally, excavations, trenches or other holes dug in the platform surface must be properly backfilled to avoid creating a soft spot that might give way under the tracks of a piling rig or other plant and equipment. The edge of the platform must always be clearly defined and should be regularly inspected to ensure that there has been no degradation over time.
Some FPS members design working platforms and some do not. Some are prohibited by the terms of their PI insurance from carrying out the design of working platforms. Some do not have the facility to carry out the installation. For these reasons, the WPC does not define who should be the platform designer. However, the piling specialist will always provide the loading information for the envisaged equipment.
The platform will continue to exist after the piling contractor leaves site and may be used by following trades so maintenance may need to continue beyond the end of the piling contract. Even during the piling contract it might be used by other trades.
The design, installation and maintenance of working platforms should be the responsibility of the organisation that has continuous control of all site activities.
