Segmental Arcs

 

The following is extracted from my article in the New Hampshire Landscape Association Newsletter, May 1999:

 

 

Classic Cobblestone Pavement

 

by Thomas Berger, Green Art

 

Installation of Granite Cobblestone in Segmental Arcs

 

 

Widespread in many parts of  Europe, but rarely seen in the U.S., are certain types of cobblestone pavements. One of the most beautiful and also the most durable is the pavement in segmental arcs. It is a very traditional pavement and so typical for the old town roads and market squares that one could call it the "Classical Pavement". With the extensive use of asphalt, these beautifully structured surfaces became rare as did the craft of creating them. It is even difficult to find a good description of how to install such a pavement. In fact, we could not find a satisfying description anywhere and so pieced our technical information together out of a number of different books and from counting and measuring stones on photographs of segmental arcs pavements.

The book told us that segmental arcs are always done with small cobblestone sizes, such as "cubes" (9 pieces per square foot) and never with "jumbo pavers" (2 pieces per square foot).

The foundation of this pavement consists of a compacted base of aggregate, such as crushed stone. Its design depends on local conditions and type of traffic. The cobblestone is set into a bedding course of finer aggregate such as a mixture of sand and stonedust.

 Another way is to set it into "landscaper's concrete." To do this, one spreads out the sand and stonedust mix and then pours cement over that, mixes the two with a rake and sets the cobblestone. Careful watering is done after all stones are in place.

This is all easy, but what about setting the stone to get these charming curves?

  

 

By analyzing photos of segmental arcs pavement we realized that:

  • The arcs had to meet at a 90 degree angle.

  • The right and left edges of the walkway would have to cut the arcs in the center (in half), so that the cobblestones along the edge would be parallel with that edge.

  • To achieve these two goals, the size of the arcs would have to be exactly calculated before starting the job.

  • In the photos, the arcs seem to have a width of about four feet.

We worked out the following calculation:

  1. Width of walkway minus edging strips equals the width of area covered by segmental arcs.

  2. Divide this width by 4 feet (approximate size of one segmental arc) to get the number of arcs. (This is only important for wider areas. In our case it was clear we would only get in two arcs, i.e. one complete arc plus two half ones along the edges.)

  3. The result of calculation #2 will most likely give you an uneven number such as 3.5. In such a case, you would have to decide if you want to go with three larger or four smaller arcs.

  4.  The width (result of calculation #1) divided by the number of arcs (results of calculation #2 and #3respectively) will give you the exact size of each arc. This size (let's call it "c") is the distance from one end to the other end of the segmental arc.

  5. Using Pythagoras' theorem  (a2 + b2 = c2) one can find the radius of the circle that forms this segmental arc: 

        The radius (a) is  the square root of c2/2

      (c is the known length. a and b are equal and are the some as the

    radius of the circle. This circle gives us the curve for the segmental arc.)

 Of course, one could do the job just with the result of calculation #4 and lay the stone by eyeballing the curve. However, Knowing the radius gave us the possibility to work with a template and ensure that the curve could always be adjusted when adding more and more rows of cobblestone.

We cut a transparent Plexiglas template in the calculated size. The template gave us the curve for half an arc. It could be guided along a string to keep it in the correct angle (see photo, page 11).

The cobblestone was placed into a mix of sand and stone dust over a base of crushed stone. It was set with a paving hammer (rawhide faces) to a height slightly above finished grade. Large stones were placed in the center, small ones at the edges of each segmental arc. Occasionally, pavers had to be split with the help of carbide tools where triangular pieces were needed. A tracer worked well for that. After jointing with our sand and stonedust mixture, the surface was compacted.

And how did it fare over the winter? Excellent. When snow on asphalt paving was melting at daytime and freezing to solid ice at night, it turned into vicious traps for unsuspicious pedestrians and drivers. Not so with the cobblestone pavement, because by the time the melted snow was freezing again, it had long disappeared through the joints between the cobblestone. Therefore: don't use concrete for the base! It doesn't need it anyway.

 

 

 

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