Safety Safety is also important. The use of stone setts for carriageways is often criticised or avoided on the grounds that such surfaces are slippery and dangerous due to their low polished stone value. Studies of the relative safety of setted and asphalt road surfaces have shown, however, that when two identical streets with the same geomet1y and traffic are compared it is evident that considerably less accidents take place on the setts. This has been explained by the fact that the vehicle speeds on the setts tend to be much lower than on the asphalt due, in part, to driver awareness of the potential clanger. This conclusion justifies the specifying of stone for both traffic calming and conservation/aesthetic reasons. Mortar versus Traditional Specifications Even where stone is specified, there remains a fundamental difference between modern British practice, where the setts and slabs are laid in mortar, and the traditional dry methods employed in the past and still extensively used throughout Europe. In the traditional method, stones are laid as closely together as possible in ash or course sand which is brushed into the joints until they are filled. For a period, this requires regular 'topping up' but in time the fill material is joined by other small debris which contributes to a natural set. As the stones are subjected to pressure from moving traffic the flexibility of the fill encourages them to rub together and binding becomes tighter. Removal and replacement is easy, quick and seamless and the detailing allows, indeed encourages, a degree of water penetration Context 56 December 1997 Above left: The Cross, Linlithgow. New Scottish diorite cubes and Caithness paving. Above right: Kirkgate, Linlithgow. Reusedwhin setts on roadway, shared surface in Caithness flagstones with lines of diorite cubes, new granite kerbs with 'horonizing' (stone off-cuts) in diorite and Caithness paving. rather than run-off to the drainage system. Laying setts in mortar produces an inflexible, impermeable surface which can crack under heavy traffic, and repairs and relaying are impossible without obvious scarring. Often there is cliscoloration and staining clue to mortar spillage and any surface irregularities become permanently fixed. The use of mortar requires that the setts be laid with joints up to 50 mm wide to allow filling with semi-cl1y mortar, and this results in an appearance which contrasts greatly with the traditional close laying found in the best surviving old schemes. In some projects the water content of the mortar is increased to allow the mortar to be brushed into narrower joints as a liquid but this can reduce the mortar strength and increase the clanger of staining The perceived convenience of mortared construction is understandable but there are currently few national specifications or codes of practice relating to the pavement design, laying and testing of natural stone products in streetscape works apart from some British and EU standards for quarrying and production. Guidance can be taken from European publications but these are seldom available in English. Most designers have therefore developed their own specifications although these have met with varying degrees of success. There are many grey areas such as a fundamental disagreement as to whether the rnoitarecl pavement should be regarded as flexible or rigid and therefore whether concrete or bitumen construction should be used as a roaclbase. Furthermore, with setts or cubes regularly subjected to the impact of traffic there is little understanding of the performance of different mortar mixes and the bonding between the setts and the mortar. The comparative curing times of different products is another issue which requires to be better understood since the waiting time required may result in traffic delays and an extended implementation period which can increase the contract costs dramatically. If we are to advocate appropriate detailing and practice then we need to understand how our streets were constructed in the past and carry out more practical work on the ground to adapt the old ways to the demands of modern towns and cities. Contractual Issues The selection of contractors for the tender list can be problematic for streetscape schemes as the laying of cubes and setts is normally carried out by sub-contractors. The success or failure of a scheme is largely dependent on the sub-contractor and there is a problem in the lack of sufficiently qualified and experienced firms in Britain. The main contractor should be fully aware of the standards and skills associated with the laying of natural stone products and should not assume that a block-paving or slabbing subcontractor will be adequate. Furthermore, there is a tendency for the contractor to submit rates which are too 37
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