Granite setts or “cobbles” as they are sometimes referred to‚ have been around since Roman times. Almost indestructible‚ setts can be purely functional as a hard wearing-course for roads and pathways but‚ by utilising the range of colours and sizes now available to a designer‚ they can be used for aesthetic reasons as well‚ providing lovely and varied textures.
Most standard sett dimensions are available from stock. Any other requirements made to order can normally be supplied to any specification within 8-10 weeks and often less.
Granite or Porphyry Setts Unbound for Vehicular Traffic
This describes the laying of setts in an unbound mbr.
A granite bedding aggregate and jointing aggregate with limestone dust for the topping. No cement is used.
The Principles Behind Laying Unbound Setts
The setts are held in position by the jointing aggregate surrounding them‚ acting with the setts next to them. The outside edge of the area of the setts must be restrained. This may be achieved by laying up to walls‚ buildings‚ kerbs or channels or by being haunched in concrete.
The sub-base needs to be strong enough to carry the anticipated traffic; the setts are a wearing course.
The method is suitable for driveways‚ commercial access ways and parking areas rather than public streetscape projects as cleansing of the streets by machines using suction‚ powerful water jetting and rough brushing will remove the jointing aggregate. These actions will also reverse the natural migration of fines down through the aggregate matrix and lead to failure.
Many city streets were laid unbound in this fashion‚ a century or more ago as were railway goods yards‚ docks and factory yards generally. The setts had the time to compact really thoroughly with use before the advent of today"s juggernauts so could later carry today"s loads. Newly laid setts cannot immediately achieve the same compactness to carry the heaviest traffic.
While the whole surface retains its integrity‚ it will be remarkably successful as a resilient load-bearing surface and last a lifetime. So‚ if at any stage after completion‚ the jointing material becomes depleted‚ the joints must be refilled forthwith and‚ should there be any type of minor failure‚ repairs must be effected as soon as possible.
Materials and Tools Required
150mm of compacted DOT Type 1.
This should be igneous material including an even mix of all sizes
In accordance with BS 7533 Part 7‚
CED reference IR-60-LB
You may use the bedding aggregate providing it will fully fill the joints. Alternatively‚ if the joints are particularly narrow and need a finer aggregate‚ this may be ordered in accordance with BS 7533 Part 7‚ CED reference IR-30-LJ
A hard carboniferous limestone dust in accordance with BS 7533 Part 7‚ CED reference HL-20-LT (Note - the topping aggregate may be replaced with fine soil or loam)
As heavy as a bricklayer"s hammer‚ preferably with a wide blade on the opposite end to act as a mini-shovel for manoeuvring the bedding aggregate.
To indicate levels to work to.
Kneepads & Stool
A little‚ low one-legged stool to sit on.
A designation of 40kN Energy (this is a powerful piece of equipment and needs to be so or it will not compact the finished work sufficiently)
Setts must be cropped on the sides. Sawn sides do not bind sufficiently with the jointing aggregate. The top surfaces of the setts may also be cropped or they may have a naturally flat or a sawn‚ then textured‚ surface. Ideally‚ but not necessarily in every case‚ the bottom should be rough or naturally flat rather than sawn.
There are so many possible options. For laying unbound‚ the optimum for long-lasting results in traffic conditions is to lay the setts in arcs. This spreads lateral loads over a wide area and avoids lines of weakness. If setts are to be laid in rows‚ they should always be laid with a staggered bond if there is vehicular traffic. Even in purely pedestrian areas this is sound advice to follow.
Laying in straight lines
Setts laid in straight lines should never be laid in rows that are parallel to the traffic flow. It is unsatisfactory both visually and technically and‚ specifically‚ bad for bicycles. The setts may be laid in rows that are perpendicular to the traffic flow‚ which is traditional‚ but they may also be laid at 45° or even somewhere in between.
Laying in arcs
The definition for laying areas in arcs is more complex. One should think about the arcs as arches transferring the strain on the bridge down to the supports. On the road or driveway‚ repeated strain is usually through braking and acceleration. On the horizontal with modern vehicles‚ the forces resulting from braking are generally greater than from acceleration so‚ from the driver"s viewpoint‚ he should be driving towards the heads of the arcs. If there is a slope‚ the heads of the arcs should lead up the slope‚ just as in a bridge the head of an arch supports the load and so is uppermost. The steeper the slope‚ the more critical this is.
When laying setts in curves within an area of paving‚ for visual benefit the outside line of the curve needs to be laid tidily‚ the inside line is less important.
60/80mm cubes‚ Geometric plan for the laying of overlapping arcs
Here you will find a selection of insitu images - click on an image to enlarge.
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