If the height of the foundation layer were too low the layer of the bedding mortar would have to be increased and this would affect the resistance to load and the durability over time of the paved surface.
If the foundation layer is not laid with the necessary attention paid to the inclination this may lead to a non-uniform distribution of the bedding mortar and as a result a different reaction to loads which would lead to the formation of dips and depressions in the paving.
The ideal foundation for porphyry paving, then, IS one which cannot be compressed, is at the correct height and with the same inclination as the finished paving wit have. so as to allow the run off of water (> 1.5%).
The most common systems at present are the use of roadbed and concrete.
Roadbed is made of a layer of stones, larger ones in contact with the ground gradually getting smaller until there is no Space between them near the layer of bedding mortar above.
At present the use of 'stabilized soil" is common. This is a dry aggregate of various stones, gravel and clay with cement, lime and/or bitumen. The materials and percentages of composition of this mixture vary considerably depending on the availability of the various law materials in the area of use. The roadbed must be compacted well by the repeated use of road rollers or similar.
Often used, due to its ease of laying, is a base in concrete. On top of the foundation ground a 10-15 cm layer of concrete is laid. It is a mixture of sand, fine gravel, water and cement in the ratio of 200 kg per m^2. It is advisable to immerse an electro-saIdered metal grid in the mixture (5 mm diameter, 20 x 20 cm knit). The layer of concrete, especially for large areas, will need expansion joints to be included and these must be repeated in the paving above.
Bedding Layer
The bedding layer, placed on the foundation layer, is the bed for the laying of the paving elements. It may be made of:
a. SAND: this is the most traditional and most tested with regards to the laying of 'cubes". Above the foundation, already at the correct height and consistency (even better on a layer of bedrock), a layer of sand is spread with a depth of 6-9 cm depending on the size of cubes to be used.
Due to the laying, wetting and compacting of the cubes, the bed of sand will be Compressed and will reduce its height by about 2 cm. Tables 7 and 8 show the best thicknesses for each type of cube it is essential that the sand layer is never higher than the figures quoted so as not to compromise the stability of the paving over time. Often the foundation, or part of it, is not at the correct level and, for reasons of time and deadlines, the level of sand is increased. This is one of the most frequent causes of subsidence or sagging of paving in cubes, as the resistance to wear and load is compromised.
The sand should have fairly large grain size (0-4), it should be clean and free of soil particles. Fine sand or sand containing lime should be avoided as it does not supply the necessary mechanical resistance to the paving elements. The presence of earth in the sand may cause subsidence in certain points in the paving as the earth disaggregates in water creating holes in the bedding. It is not advisable to use sand from the sea, which contains a lot of salt which dissolves in the water used when compacting. or rain water, and would result in the same above mentioned problems. It could also create ugly marks on the paving above. The bedding sand should be free of any foreign material which could inadvertently come into contact with it (pieces of brick, wood, stones, etc.) which could alter its properties.
