There are occasions, when designing a garden, especially a ‘wild’ area or site, when the planting scheme calls for a softer approach. Even if the scheme does not call for a lot of hard landscaping, there will still be a requirement for stability, either paving, steps or walling, and often a designer can find difficulty in trying to combine both worlds.
Take paving for example. Natural stone materials are the usual choice for patios and footpaths in wilder gardens, lending themselves more readily to a local appearance. Purbec stone, York sandstone, Oxford Hornton stone, Caithness, Somerset, Leicester stone – they are all very attractive in almost and setting, most certainly, in their own regions.
How often though, are they spoilt in appearance with mortar pointing. Crazy paving – definitely not the grey 1960s broken precast variety! – if laid correctly, can be very attractive, with uniform joints, either close jointed or laid with wider gaps to encourage moss or lichen.
A form of ‘soil concrete’ may be produced for use under the paving slabs, comprising dry, crumbly soil (friable good quality, weed and stone free topsoil is fine), or sandy/loamy soil – any soil that will ‘mix’ easily with cement, not chunks of clay!
Simply mix the concrete, 6 parts soil to 1 part ordinary Portland cement, not too wet, almost semi-dry. Dry enough to crumble in your hands, but wet enough to ‘ball’ together.
Use this soil concrete in the usual way, bedding each slab firmly into the material. For light pedestrian use, 75 – 100mm depth should be sufficient, but please allow for soft ground in the normal manner. You are still using the same logic as if you were playing standard concrete, only substituting ballast for friable soil.
If you need to make the ‘concrete’ more malleable to receive the slabs, make the paving wet by dipping in into water, do not make the ‘concrete’ wetter, otherwise you will end up with a muddy mess! The amount of water on the slab will be sufficient to make the base more amenable to laying the individual slab.
Planting Soil Concrete
Using normal paying methods, ensuring an even surface, with no trips, low spots to collect rainwater, to levels and falls – to repeat – exactly as though you were laying in the usual way, the only difference being that the concrete base, and pointing mix will be vegetative enough to support those plants that require little nutrient to survive and thrive.
If you wish, you could leave areas unpointed, with perhaps a 10 – 20mm recess between the slabs, in which alpines, mosses, sedums and micro leafed plants e.g helexine, arenaria and thymes may be introduced.
Use this same soil/cement material, perhaps adding common building sand with the friable soil and cement; 3.3.1.mixing ratio , to point between natural stone walling. I often use soil concrete to build walls that are drystone in appearance, yet actually backed with wet concrete to secure each stone, either left without any pointing, or with a heavily recessed joint using the ‘soil mortar’ method, which also encourages vegetative growth. (sedum, aubretia, arabis etc).
With experimentation with your local soil/s, you could devise your own specification, suited to your own choice of plant varieties and stone selection. This technique may also be used when constructing water courses and streams, although the specification will no doubt call for a butyl liner to prevent too much water loss by means of evaporation and capillary action. Enjoy working out your own specialities!