Working platforms for tracked plants.
Serious incidents involving the failure of working or crane platforms have occurred, which has resulted in serious injuries in death. This guide is intended to promote safety in the design, installation and operation of these platforms.
The federation of Pilling Specialist (FPS) has initiated a drive to improve practices related to the use of piling and associated specialist plant; the article has been prepared following the recognition of the need for safety initiatives to improve the approach and the provision of working platforms. In this article, it should be recognised that these platforms are subject to complex loading conditions and are difficult to design; specialist advice is always needed from a qualified Geotechnical Engineer.
The expression working platform is restricted to ground-supported working platforms, for a tracked plant, constructed of granular material. No other type of working platform is considered. The working platform is taken as including not only the platform itself but also the associated ramps and accesses. The guidance has sought to avoid being over-prescriptive as this might limit the scope for innovation and the development of cost-effective solutions. The guide is an enabling document and does not form a code of practice. The guidance does not in any way limit the responsibilities of those parties involved in the design, specification, installation, operation, maintenance and repair of a working platform. While the guide describes good practice in general terms it cannot deal with every eventuality and site condition. Any site activity that could affect the integrity of the working platform, such as an excavation through the platform, should only be permitted if it is supervised by competent site staff and adequately reinstated to the original specification. All empty bores should be backfilled with self-compacting granular material. Backfilling around guide walls requires particular care. Damage to the platform should be repaired promptly so that the platform is reinstated to the required specification. Irrespective of who designs the platform, the principal contractor is responsible for safety on site and should be in overall control of all other contractors working on the site. The principal contractor is therefore the body with sufficient control to be responsible for ensuring that the platform is adequately maintained and repaired. A formulation of good practice can be of value only where it is applied with careful supervision, control and monitoring of the platform on-site under appropriate contractual arrangements. All parties have to exercise their knowledge, experience and judgement.
Objectives of working platforms
Working platforms are critical for plant stability, and safety is a vital issue. Most working platforms perform well but the overturning of rigs has occurred more frequently than it should. The principal objective of this guide is to facilitate the design, specification, installation, operation, maintenance and repair of working platforms so that an acceptable level of safety is achieved. While the principal objective of the guide is safety, a secondary objective is that safety should be achieved without unnecessary or excessive expenditure. Although this is a secondary issue from the standpoint of this guidance, it is certainly not a minor issue since in many applications the cost of the working platform is a significant proportion of the total cost of the process, such as piling or ground treatment, for which the platform has been installed. In many cases, a good-quality platform also improves the performance of the construction processes using it. For the construction of working platforms, the tenets of sustainable construction require the excavation of natural materials to be minimised and the use of waste materials to be maximised. These objectives can be achieved in several ways. Working platforms should not be larger than they need to be, and material specifications should not prevent the use of those recycled or secondary materials which will perform satisfactorily. Many working platforms are used in subsequent construction phases and the design should be checked to ensure that it is appropriate for these uses and conditions.
Causes of failure
The objective of enhancing safety without excessive expenditure can be achieved only by identifying the significant hazards and determining where the primary risks lie. Otherwise, there will be much unnecessary expenditure improving matters that are already satisfactory. Identification of the most significant hazards is a crucial first step in the design procedure (Figure 1). Experience has shown that it is far more likely that rigs will overturn owing to localised problems rather than to a generally inadequate platform thickness across the whole site. Localised weaknesses can be associated with the existence of ‘soft spots’ in the subgrade under the platform or with weak areas within the platform formed by inadequate backfilling of holes that have been excavated by other contractors working on the site. Similarly, ‘hard spots’ caused by old foundations or basements can cause difficulties. Thus, the identification of weaknesses in the subgrade and the prevention of weaknesses being formed in the platform is likely to do more to improve safety than a refinement of design calculations. Other hazards such as open excavations and the edges of the platform and access ramps should be identified and marked. The problems experienced with rig instability are often associated with failure adequately to maintain and, where necessary, repair a working platform rather than with defects in the design or installation of the platform. Inappropriate operation of rigs can lead to serious incidents involving rig instability.
Soft Spots and weaker areas
The term ‘soft spot’ is often used to denote any small, localised part of a site where the subgrade is weaker than the surrounding ground. The soil may not necessarily be ‘soft’ in the sense that a geotechnical engineer would normally understand the term; that is to say, a soft spot is not necessarily formed of a cohesive soil with low undrained shear strength. The significant feature is that, in the soft spot, the ground is weaker than allowed for in the design calculations, which have been based on a cautious assessment of the ground conditions generally encountered across the site. Consequently, in this guide, the term ‘weaker zone’ is used.
Design Calculations
The Geotechnical Engineer is required to undertake design calculations for site-specific soil and groundwater conditions. The platform design method follows a logical sequence from assessment of plant loading through to platform thickness. It is not appropriate to apply only part of the methodology in isolation. Recommendations are given on design loading conditions. The choice of suitable characteristic values for soil strength parameters of the platform material and subgrade will depend, among other matters, on the quality of the site investigation information and the quality of the proposed granular platform material. Design values are derived by applying factors to characteristic values. The design calculations indicate the minimum thickness of the working platform that is required for the given platform, subgrade and loading conditions. However, the design method contains many simplifying assumptions and there is likely to be considerable variability in near-surface ground conditions. It is important, therefore, that the results of calculations are critically appraised by a Geotechnical Engineer. An appropriate tensile strength of geosynthetic reinforcement is required in the design calculations. The ultimate tensile strength of geosynthetic reinforcement often occurs at a very high strain. The factor applied to this ultimate strength should reflect the strength available at low strain and should also depend on material type, design life, ambient temperature and exposure to mechanical, chemical and biological degradation.
Design Life
The intended duration of the working platform is a significant element in its design. The bearing resistance of the platform will be reduced by degradation under repeated loading. With some platform materials, bearing resistance can reduce significantly with the number of loading cycles. This could be an important issue on heavily trafficked sites and should be taken into account in evaluating the characteristic value of angle of shearing resistance. Where the working platform subsequently has other temporary or permanent uses, the extent of degradation during its use as a working platform becomes quite critical. Piling or other works on the platform may leave it contaminated. However, the incorporation of the platform into the permanent works can save on cost and programme. Furthermore, there is a tendency on-site to take greater care of the platform if it is seen as part of the permanent structure.
Operation and Maintenance
Throughout its working life, the platform should be under the day-to-day control of competent site staff. Contractual arrangements should ensure that the platform is adequately inspected, controlled and maintained. Regular observation of platform performance is essential to ensure that the design assumptions are reasonable and remain valid throughout the platform life. The working platform has been designed and installed to be safely used by a certain plant. Site control should be adequate to ensure that it is not used by the plant for which it has not been designed and that only designated working areas are used. Problems may arise where working platforms are also used as haul roads. Many factors related to the usage of the working platform will affect its performance and hence the required maintenance:
- Type of plant used and dynamic or vibratory effects due to their operation
- Frequency of passage across the platform of service cranes with heavy loads
- Frequency of passage of concrete trucks/supply vehicles which may be restricted to routes where the platform has been strengthened
- Scale and duration of works
- Drainage, contamination, degradation of the material
- Acceptable minimum thickness stated in the design
Inadequate performance of working platforms may be due to their poor maintenance and many of the problems experienced with rig instability can be associated with failure adequately to maintain and, where necessary, repair a working platform rather than with inadequacies in specification or installation. The integrity of the working platform, including ramps and accesses, should be preserved and the originally designed standard should be adhered to throughout the working life of the platform. Problems with performance may require areas of the platform to be increased over and above the specified minimum thickness. The working platform should be reasonably level, well-drained and kept in good condition for the duration of the works. Timely and appropriate maintenance relies mostly upon adequate monitoring and remediation on-site by the contractor responsible for the platform. Careful surveillance can detect a deteriorating situation before it becomes a major hazard. Simple checks on platform thickness can be made using profile boards. Maintenance of heavily-trafficked areas such as access routes and ramps require special attention. Problems can be caused by plant spragging, heavy concrete wagons and other traffic. Contamination may be caused by mud from the site, from machine wheels or tracks, from specialist operations involving spoil generation or specialist materials such as bentonite. Poor drainage can cause difficulties, and the flow of water across the surface of the platform should be avoided. Suitably graded, stable material, similar to that specified for the installation of the platform, should be used for the maintenance of the platform to reduce deterioration due to traffic and weather.