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Saturday, 26 April 2014

Corbelled chambers


One of Tim Daw's excellent photos from his "Neolithic" burial chamber at All Cannings.  There is a whole gallery of photos here:
http://www.thelongbarrow.com/gallery

and here we have much more info about Tim's project:
http://www.thelongbarrow.com/about-the-long-barrow

This is another nice illustration of the way in which the seven chambers are being constructed:


The technique is not quite "dry stone corbelling" because there is a lot of concrete used, out of sight, on the outside of each dome.  The flattish stone slabs are also much more standardised in shape and size than those used in corbelled chambers constructed in the Neolithic.  Health and safety, and cost, have obviously had an influence on the methods used by the builders.......  But it's great to see this piece of engineering skill being used again, and for a practical purpose.

For comparison, here are some other corbelled chambers:

Knowth in Ireland, made with much larger flattish slabs

L'Ile Carn in Brittany, made with a collection of stones of all shapes and sizes.  It's a miracle that it still stands after all these years.......


Newgrange in Ireland -- also made with large flattish slabs to provide considerable stability.  No concrete, but small stones used as packing...

This photo of from the inside of one of the Sardinian Bronze Age "proto-castles" called "nuraghi" -- very similar indeed to the one being built by Tim and his friends in Wiltshire.

15 comments:

TonyH said...

There's no argument about these man-made corbelled chambers.
The so-called Rhosyfellin "alcoves" seem to be much harder to see (your Post, April 10th).

BRIAN JOHN said...

Not even I would claim that these cambers are made by glaciers! I find them rather impressive..... but as for that "alcove" it's a complete red herring......

BRIAN JOHN said...

oops -- should be "chambers"......

Timothy Daw said...

Thanks Brian for the kind words. That is the smallest chamber roof, we are starting on the first large corbelled roof next week. And I must point out, as the stonemason is very passionate about it, that it is lime mortar he is mainly using.

BRIAN JOHN said...

Sorry for the mistake, Tim. Three cheers for lime mortar!

Jon Morris said...

Lime mortar is a traditional material but, importantly, allows a much larger degree of settlement (by comparison to modern Portland cements) without adverse structural effect. The reason for this is that lime cracks are 'self healed' by water flow. This is very important for longevity in the type of construction that Tim is trying to achieve.

Constantinos Ragazas said...

Jon,

This is very interesting! Thanks for that information re: “lime mortar”.

Which raises this question. Chalk being essentially limestone can “lime mortar” be made with chalk? Perhaps mixing ground up chalk with mud or clay or gravel, forming a “putty-like” substance?

Kostas

Jon Morris said...

Lime is made by 'burning' crushed chalk (and some other natural materials) Costas. When chalk (the mineral calcite) is burnt it produces calcium oxide. This forms a mortar with low strength and a long setting period (adding water produces calcium hydroxide). This mortar gradually reverts by the process of carbonation (or sometimes called carbon sequestration). This sequestration produces calcite: Chalk.

So lime mortar is effectively chalk. This is why it is self-healing.

Constantinos Ragazas said...

Jon,

I appreciate the lesson. ”So lime mortar is effectively chalk”. Can “lime mortar” naturally form? A recent discussion prompted this question.

Kostas

Anonymous said...

Lime mortar is made from crushed limestone not chalk.

Jon Morris said...

Lime mortar is made from crushed limestone not chalk.

Wrong. Lime is made from chalk or limestone:


http://en.wikipedia.org/wiki/Lime_%28material%29

I thought you weren't taking these anonymous comments Brian?

BRIAN JOHN said...

Thanks Jon. Very true -- I try to keep the discussions honest and open, but sometimes one slips through when I'm not concentrating....

Anonymous said...

Hydraulic lime is a variety of lime, a slaked lime used to make lime mortar. Hydraulicity is the ability of lime to set under water. Hydraulic lime is produced by heating calcining limestone that contains clay and other impurities. Calcium reacts in the kiln with the clay minerals to produce silicates that enable the lime to set without exposure to air. Any unreacted calcium is slaked to calcium hydroxide. Hydraulic lime is used for providing a faster initial set than ordinary lime in more extreme conditions (including under water).

BRIAN JOHN said...

Anon -- thanks for the info. I don't like Anonymous entries -- if you send in any other comments, please add your name in the message, as Pete G does. You should still be able to use the "Anonymous" button if you don't want to go through the rigmarole of signing up or signing in -- not sure how these things work on other people's computers.....

Jon Morris said...

Hydraulic lime is a variety of lime, a slaked lime used to make lime mortar.

Again, this is not entirely accurate. Another variety of lime mortar uses non-hydraulic lime. It has a slower setting time, relying on the carbonation process to set.

Traditionally, non-hydraulic limes were used in construction (typically as a putty which was combined with other materials such as horse-hair).