Bases & Sub-bases
When designing an area designated as paving, either pathways or patios, due consideration must be given to the type of soil and nature of the ground. Clay soils (see Clay Ground), chalk and sandy soils all have different properties, which may affect the specification required for a successful, long-term project.
In general, areas to receive paving slabs of any type, will require a basic design, with an excavated depth of around 200mm from desired levels. Remember to ensure that any existing undulations, dips and uneven ground should be ignored, as ‘soft’ landscaping should not dictate ‘hard’. In other words, any grading of the localised soil should be made to suit the formation of the paving, not the other way round.
Having firmed the bottom of the excavation with a suitably heavy compactor plate or roller, lay non-woven (Terram) geotextile fabric over the whole area, taking care to extend it beyond the area by 20cm or so to ensure complete cover.
Spread 125mm MOT Type 1 compacted to a depth of 100mm, then, depending on the depth, size and type of paving required, lay either a 30mm deep bed of mortar as a laying bed under paving, or 30mm of sharp sand under block paving/setts. This is known as an unbound base construction.
Moisture content of MOT Type 1 and Type 3 should be 8 – 13%. If it is too wet, or too dry, sufficient compaction may not be achieved to meet the California Bearing Rate.
In some areas, it may be advisable to reduce dig to 300mm below finished level, especially on soft ground, or where you suspect newly laid drains may have been installed. Often, these areas only become apparent during excavation works, and additional costs may be required under Variation Orders as part of your contract documentation.
Better by far to err on the side of caution, as remedial works can be very difficult to assess and control once a project is complete.
