Retaining Stone Walls in Cornwall - Failure in Older Structures - LJN Blog Posts

Following the recent adverse weather some older Cornish retaining walls have collapsed and there have been several enquiries relating to this, there have also been some real problems for clients with regards dealing with the insurance companies. Assessors tend to have a knowledge only within the construction industry which consequently leads to confusion and then often refusal of payment, this subsequently leads to the loss of a historical and traditional feature of the region. However often the cost of minor repairs can be less than the excess involved and certainly cheaper than other forms of retention, which may be chosen in the misguided believe that it will be cheaper than the dry stone work.The following may assist in determining some of the major problems associated with the walls. It is a heavily abridged version of some work by Jamie Flattery, prior to his leaving to live and work in Canada.Mechanical Failure in Cornish Dry Stone Retaining Structures:As the use of Dry Stone, principally slate formation or granitic stone, comes from ancient traditional techniques much of the engineering principles and standards have been acquired through the dissection of ancient structures. Structures built using slate formation are considerably younger than granitic structures and tend to be concentrated in coastal sites on the North Cornish Coast. However there is evidence of some ancient slate formation walls constructed using a horizontal pattern, similar to the Cornish Hedgerow standards of today, in the Falmouth area. These walls have seemingly bonded together due to weathering to create near vertical structures, which have survived due to a solidification of the back material also and run off flowing over the top.Dry stone structures in the main, fall under the remits of soil mechanics in engineering terms rather than standard structural engineering principles, although in the US it has been widely used as a standard system for seperate level boundary structures and for sub urban landscape engineering. The principal academic research comes from edaphology studies as the combination of the vegetation within the structure is a significant element of it’s longevity.Such research is somewhat lacking in availability and these walls when constructed within engineering principles are now principally sought as a means to create a faced drainage system. However the majority of all walls now built are due to aesthitic choice and more recently for cost savings due to the increases in the cost of pre fabricated or cement based products, many of which in turn were borne out of the need for such products after WWII, when there was a shortage in the natural stones due to a combined shortage of suitable mechanical quarrying equipment and labourers.Failure tends to occur soon after construction, which can be easily remedied, or after a significant change in the surrounding soil make up; which can occur anytime in a period of between 70 and 350 years after initial construction. The changing soil profile due to natural processes can lead to dramatic failure, often occuring after heavy periods of rainfall as the changed soil profile blocks the natural drainage available in the structure.Alternative intervention to walls leads to the same problem:Walls which have been removed of their vegetation or have been pointed or repaired using mortar are highly susceptible to damage.The overlaying of impermeable surfacing in front and at the base of the structure leads to a deepening of the foundations of these structures, which are often built in shallow footings. The foundations are built to a depth which allows for the displacing of both groundwater seepage and surface water runoff over a suitable soil profile. In Cornwall this layer is often shallow, (commonly found between 400 and 600mm depth) and is generally the surface of a compressed sub soil clayey shale and shillet layer, which lies undisturbed and is significantly less permeable.Modern construction practices have often been practised with an ignorance for the correct soil profile to be reinstated after operations to the building, which usually only see surface remediation by way of re laying a top soil over the disturbed soil. This can often lead to a weak profile within the soil which is easily disturbed in adverse weather leading to poor drainage, (this is often all too evident in the planting of such gardens and is a common landscape engineers gripe) and sometimes serious mechanical failure of existing dry stone structures within <200m of the new build.By far the most common failure in older structures is alterations in the groundwater seepage, following significant rainfall. The soil profile is continually changing and clay particles will accumulate on the inside of the wall structure building up to levels which block the drainage through the wall, thus leading to serious mechanical failure by way of pressure build up.Dr J Flattery Phd, MSc.Cornish Stone Walls

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Comments

Richard Clegg November 21, 2009 at 12:46pm

I do traditional dry stone walling. I am an artist and a craftsman. Do the job right and the wall will be both a thing of beauty and long lasting. Please take a peep at my gallery at www.richardclegg.co.uk and say hello.
Sustainable Land Management November 21, 2009 at 1:58pm

Hi Richard, your stone structures are jut amazing, one of my favourites is your 'stonehenge' gateway. Superb.
Jane Dorning Cornwall Allotments March 23, 2010 at 1:08pm

Love your work Richard, totally amazing! Jane