Dry Stone Walling Cornwall. An engineering solution for bank stabilisation & drainage.

Dry Stone walling in Cornwall is enjoying a renaissance at the moment. Principally due to the costs compared with alternatives. An additional factor is that as the actual mechanics of these historic structures are being researched properly, (mainly due to Engineers in North America recognising the attributes of the walls), and subsequently walling is being re introduced into an engineering schemes as the preferred method of mitigation.

The completed wall above looking out over the Carrick RoadsThe 'Cornish Stone Hedge / Wall' is recognised internationally and does differ considerably from other forms of dry stone wall in the UK. This is due principally to the slate formation stone used.

This 12 tonne pile is ex Tredinnick, (Trevose Slate Formation) and at £250 / per load is one the cheapest available in Cornwall. It is also the nearest match to the slate formation bed rock found in the region of the discussed project below.A steep bank shelving immediately off from a slightly downward sloped garden. A few years previously storm water run off caused a large part of the garden to slip onto the shoreline, (In the area of this photo - the edge of the turf is where the proposed wall is to be constructed)

The resulting backfill had to be held using rock netting with gabion blocks on the upper level part of the garden. Such engineering whilst practical and useful is difficult to work off in terms of boundary construction, (including fencing and hedging). Also drainage is a serious issue. whilst storm drainage systems have been implemented for road surface water run off significant amounts of water still need to be able to drain through the area between the rocknetting and whatever is constructed.A dry stone wall acts as a drainage unit in its own right. Digging shallow foundations onto the top of the netting with backfill of loose stone enables a channel which feeds to the quoin at the lowest end of the wall.

The layer above the loose stone backfill, just above the surface, requires heavy stone, which interlocks with both faces. This keeps weight onto the smaller foundation stone. Above this the stone wall is constructed with a mix of predominantly smaller stone, but with some soil backfill. The walls are constructed at an angle of 85 degrees, just enough to ensure a downward thrust of weight directly onto the base.

This particular wall differs in so far that the capping will be of dry stone also - without an allowance for planting.As the wall bends in to hit the slope, it is absolutely essential to maintain a horizontal plane at the base. The stone allows for some change in pattern, (not wished for here), in so far as introducing bee boles or verticals.Metal pegs together with a small concrete plug, (9 in total - 2 bags of mastercrete), allow for the base of the wall to fit onto the sloping rocknetting. A straining point is also built as a 'belts and braces' measure between the lower level stretch and the climb up the hill.

The resulting finish will be preferential to neighbours / local authority and the of course the owner and will assist in preventing build up of pressure points by water, as well as weighting down the rock netting at a downward angle reducing risk of further erosion.

Finished wall 48 metres at a cost of £4800.00www.cornishgardenlandscaping.co.uk Cornish Stone Wallswww.paysagedurable.com

Many of the other quarries can supply better stone, (I would love to be able to use some of the other stone Delabole / Calliwith / lantoom, very easy to work with which can save costs on the labour and be contra'd against the supply costs). I should have clarified that the above stone selection was due to the fact that the Trevose slate formation is not only the closest match to the stone of the area, but as the many constraints and covenants in this AONB, the NT and local authority insist on a match. The huge variation between the slate formations allows for distinct boundaries in the Cornish landscape highlighted better than anywhere else in the walls and hedges.

It would be great to be able to post a map showing the slate formation lines and other underlying rock to demonstrate the differences, but copyright won't allow.

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Comments

Andy Stone January 28, 2010 at 5:12pm

12 Tonnes of slate hedging stone from Lantoom Quarry will cost you about the same amount, depending on the haulage distance however the quality of our hedging stone is superior to that shown in your photograph. For more information visit: http://wwwlantoom.co.uk
Sustainable Land Management January 28, 2010 at 5:58pm

Many of the other quarries can supply better stone, (I would love to be able to use some of the other stone Delabole / Calliwith / lantoom, very easy to work with which can save costs on the labour and be contra'd against the supply costs). I should have clarified that the above stone selection was due to the fact that the Trevose slate formation is not only the closest match to the stone of the area, but as the many constraints and covenants in this AONB, the NT and local authority insist on a match. The huge variation between the slate formations allows for distinct boundaries in the Cornish landscape highlighted better than anywhere else in the walls and hedges.

It would be great to be able to post a map showing the slate formation lines and other underlying rock to demonstrate the differences, but copyright won't allow.