SOCIETY OF CHIEF OFFICERS OF TRANSPORTATION IN SCOTLAND
The use of natural stone materials in high quality streetscapes has made a major contribution to the current move to regenerate our town and city centres. Natural materials have a quality and longevity that greatly enhances the attractiveness and aesthetic appeal of these centres which in turn can lead to greater economic activity.
The use of natural stone in pavements has not been without problems and many of the pavements have not performed as expected, resulting in a loss of confidence in the use of stone. Recognising this problem the Society of Chief Officers of Transportation in Scotland (SCOTS) in conjunction with the Scottish Enterprise Network and the Scottish Executive promoted a programme to study, research, evaluate and understand the issues inherent in the use of natural stone in pavements. The First Edition of this document was one outcome of that programme and gave guidance to practitioners working in this field.
Natural stone materials are generally used as surfacing in conservation areas and prestigious streetscape projects. Roads and footways paved with stone can enhance the townscape by providing a traditional, high quality appearance. Stone can also offer sustainable construction and provide a more environmentally sensitive approach to the design of traffic calming measures.
Today, town and city roads are required to provide access to vehicles, public transport and retail service deliveries, often within restricted road widths. The stresses imposed on these roads and shared areas have increased owing to traffic growth, higher vehicle weights and new vehicle configurations. Appropriate design procedures are therefore essential to ensure the finished product is functional as well as looking good. Engineered construction that integrates maintenance regimes and future statutory undertakers’ requirements is also critical.
However, a gap exists in engineering knowledge of how natural stone units behave as surfacing in pavements subjected to heavy loading. Throughout Europe, most of the constructions used are based on semi-empirical principles or tried and tested local specifications. These constructions vary from place to place, and there is no consensus on how to design a durable stone surface.
This guide is the result of a major research and testing programme, which investigated the fundamental engineering principles of the behaviour of stone unit surfaces, developed possible constructions and tested these at both small and full scale to establish loading capabilities.
Concurrently with the research programme, BS 7533 “Pavements Constructed with Clay, Natural Stone and Concrete Pavers” was being drafted and the initial results of the programme published in the first edition of this document were used to inform the drafting of the British Standard.
Feedback and comment on the First Edition and responses during the consultation process for the British Standard have lead to this second edition of the guide being produced. This incorporates the recent thinking on design, specification and construction techniques.
It provides updated advice on good practice associated with sourcing materials and maintenance.
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This Section of the guide discusses key issues, which have to be considered when embarking on the installation of stone pavements. It is intended to provide an overview of the issues and introduces new concepts, which have been developed from the research undertaken. These are then discussed in more detail in later sections, where fuller guidance is given on all aspects of pavement design and construction.
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Stone materials can account for up to half of the cost of a natural stone streetscape contract.
Consequently the selection of stone that is fit for the purpose is a critical part of the project process. Desirable properties that need to be considered relate to the aesthetic, dimensional and physical characteristics of the material comprising the sett element and of the sett element itself. Three European Standards specify the performance requirements and the corresponding test methods:
a. EN 1341:2000 Slabs of natural stone for external paving – Requirements and test methods.
b. EN 1342:2000 Setts of natural stone for external paving – Requirements and test methods
c. EN 1343:2000 Kerbs of natural stone for external paving – Requirements and test methods
Combined, these standards cover most of the common elements of natural stone surfacing used in streetscape contracts. Each has the status of a British Standard, is mandatory, and takes precedence over all other standards and test methods. Formal specification, evaluation and acceptance of manufactured slabs, setts and kerbs must therefore be in terms of these three standards. However, they do not cover the effect of de-icing salts, an issue likely to be of particular significance in streetscapes.
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This section of the guide presents a number of design options for the structural design of pavements surfaced in natural stone. These options have been developed after extensive research and a series of tests and trials undertaken for this guide and after subsequent consultations on this guide and on the emerging BS 7533 - Pavements Constructed with Clay, Natural Stone or Concrete Pavers. Before discussing these methods, several important parameters, which influence the structural performance of such pavements, are reviewed. The categories of traffic loading are discussed along with a methodology for assessing the design traffic load for particular situations. The options for structural design are then discussed in detail before considering how the risk assessment model developed in the first edition of the guide can be applied to specifications produced by the structural design options. Lastly, several non-structural issues of design are considered.
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European experience shows that the use of skilled and properly trained personnel is of paramount importance in streetscape projects involving stone surfaces. The tendering procedure (see 1.4) should ensure that the Main Contractor has appropriate skills and experience. Designers should obtain references and consider visiting completed schemes. Guarantees that experienced craftsmen will be deployed during the works should be obtained.
Training for the construction industry is the responsibility of the Construction Industry Training Board (CITB), which has been established since 1966. CITB is the National Training Organisation (NTO) for construction and is responsible for developing qualifications, training systems and educational materials.
The particular skills of a “mason paviour” are recognised within the Construction and Civil Engineering Services N/SVQ level 2. This qualification deals with the laying of natural and manufactured materials to paved surfaces of varying sizes. It additionally covers the requirements of the Roads and Streetworks Act. There also exist stonemasonry National/Scottish Vocational Qualifications (N/SVQs) at levels 2 and 3 with content relevant to the preparation and laying of natural stone material.
Training for craftsmen can take place in a college or training centre as well as the workplace. Due to the nature of the work carried out however training is more suited to the workplace. CITB offers guidance and financial support to employers wishing to train apprentices/operatives either on or off-the job or as a combination of both.
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In writing a specification for the construction of a natural stone pavement, the following issues should be considered. Where appropriate, references are given to the relevant section in this guide.
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This section deals with the construction process and gives guidelines for the different types of stone pavement identified in BS 7533. Part 7 of that document defines these as:
Part 7 of BS7533 discusses the construction of the lower part of the pavement sub grade sub base and roadbase. It then defines materials for each form of construction. Various aspects of laying methods, which are common to both construction types, are set out, followed by more specific requirements for each form of construction.
This section of this Guide is divided into three parts. Firstly the process of sorting the stone elements and laying patterns, which are essential to the laying and performance of both types of pavement are discussed. Then each type of construction is considered with a fuller explanation of construction methods and guidance on how to achieve the required standards. Finally common features including cutting and trimming and the use of complementary fittings and special units and laying in inclement weather are discussed.
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Stone surfaces need to be maintained in order to protect the considerable civic investment, which they represent, and to ensure the continued safety of all users. This part of the Guide deals firstly with the important interaction between Design and Maintenance and then with common Defects and their repair, concluding with a short section on Cleaning. Further guidance is contained in BS 7533 part 11.
The long-term success of any streetscape project will depend to a large extent upon the priority given to maintenance issues. From the outset the future maintenance regime needs to be considered as an integral part of the Design process. The absence of consultation at this early stage has given rise to later problems when either design failures are revealed or conventional maintenance measures prove to be inadequate or inappropriate.
The need to understand each others needs and requirements points not just to timely public consultation, but to the necessity of co-ordinated training and mutual dedication of all members of the project team – including maintenance engineers and cleansing operatives.
Such integration of the project process should not imply designing only to the conventional maintenance criteria, but should encourage innovation on the part of designer, contractor and maintenance personnel that is based on sound agreed criteria, in which there is a common ownership of the project, persisting long into the future. Common problems resulting from faulty detailing, incorrect specification or poor workmanship can be reduced and misunderstandings over the cost and dedication to future maintenance that is required can thus be avoided.
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Following publication of the 1st Edition of the Natural Stone Surfacing-Good Practice Guide in October 2000, some areas for further research were identified.
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As the preceding sections have demonstrated there are many factors to consider in producing a successful natural stone paving design. A failure risk model has been developed to ensure that all of the elements of the design process have been considered correctly. The risk model can act as a check on the design. The aim of the model is to provide a rapid check on the suitability of the design for its intended setting. The model also considers some of the factors that are not quantified in the design process such as workmanship.
The risk assessment is essentially a parametric analysis in which the most significant parameters influencing the performance of natural stone pavements are considered. The calculation process employed in the model derives a value that has been termed the Natural Stone Paving Failure Index.
The model has been developed as part of the research project for SCOTS and based on the experience and knowledge of the project team. There are areas of the model that will become better defined as knowledge of the behaviour of natural stone pavements advances. However testing of the model with a range of current and past specifications has shown that it can provide a reliable indication of the risk of failure of a natural stone pavement. It must be stressed that this is a risk model and it distinguishes between high and low failure risk for natural stone pavements. It does not indicate that a pavement will or will not fail or when. By definition low risk events can occur and high-risk events may not occur.
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In July 2002, SCOTS appointed iD Consultants to study, in detail, a wide range of completed public realm streetscape schemes that have used natural stone paving materials. Each scheme was assessed through the completion of a detailed questionnaire by the local cleansing manager, site visits and in some cases interviews with key staff.
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During the past decade, towns and cities all over the country have been the subject of extensive reconstruction of their public realm streetscape infrastructure. Many of these schemes have incorporated natural stone materials for street surfaces because of it’s exceptional aesthetic quality, durability and sustainability.
As part of their ongoing national research into all aspects of street improvements using natural stone materials, SCOTS wished to evaluate the technical, economic and aesthetic characteristics of a selection of 24 completed schemes around the country. ID Consultants were appointed to carry out this evaluation in September 2002.
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For the past decade streetscape schemes using natural stone materials have been justified in preference to man made or asphalt on the basis of the durability and high life expectancy of the stone. This rather crude justification is at best simple and at worst misleading calling for a more mathematical and accountable method of selecting natural stone versus other products. What is required is a method yielding a robust argument which will guide designers and policymakers in making more informed decisions whether or not to invest in high quality streetscape schemes.
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© 2016 Society of Chief Officers of Transportation in Scotland