Dry stone retaining walls

 

Over the past few weeks I have been asked many questions regarding dry stone retaining walls. These questions have varied from – Are they safe to How high can I build one. As dry stone walling is quite common in this area I thought that I would try to explain the basics of what to ask your builder for if you are looking to build one at your property and what defects to look for if you think you have a problem with an existing wall.
Dry stone walling (bancals) are a common sight in this area as in many other parts of Spain; they were first constructed hundreds of years ago. Their original use was to allow the formation of level areas on hillsides on which produce would be grown - an area where the vegetation would not be washed down the hillside each time they had a heavy rainfall, as experienced with the Goja Fria on the 12th.October 2008. Because the majority of these small bancals are less than a metre high, fairly thick and have the necessary weight on their side, they have stood the test of time. Modern day Spaniards have perfected the art of constructing similar walls on a grander scale, where each stone is cut to fit perfectly with the next, the end result being an attractive wall with very tight joints, which resembles a jigsaw.
These walls look spectacular, but are they safe, the answer has to be no - how often after heavy rainfall have you seen a section of someone’s wall that has collapsed, or a bulge appear - bulges are future accidents waiting to happen. In the 10 years I have lived in this area there have been several people injured and at least one person killed by the collapse of such walling.

A general myth – Over the years I have heard many builders stating that if they construct a dry stone wall and slightly slope the wall backwards, filling the void behind the wall with concrete, their wall will be there forever - this is rubbish. The only thing that is achieved by sloping a wall backwards is to alter its centre of gravity and reduce the necessary vertical weight on the foundation. As the maximum pressure behind a retaining wall occurs at approximately 1/3 its height (this being measured from its base), by backwardly sloping the wall you are actually encouraging the wall to fail at this point. Failure is also more likely to occur with the introduction of a concrete backfill which prevents the free flow of ground water through the wall, the water behind the wall builds up and the weight of this water forces the wall outwards.

In the design of any retaining wall there are generally two paths that can be followed - firstly a brute strength method, where a wall is built of a solid construction and heavier than the material it retains - secondly, a more technical and intelligent method, where a thinner and lighter wall can be constructed, these walls are designed to have sufficient lateral strength to achieve the same end result. This type of wall usually takes advantage of the high tensile strength of steel reinforcement embedded in concrete giving it the necessary rigidity. Constructional preferences can vary due to price, site conditions and even the capability of the builder.

If you are thinking of constructing a dry stone retaining wall at your property, be smart, firstly construct a substantial retaining wall, then invite the clever Spanish guy to face the wall with the stone, the finished wall will look the same and will be there forever. Also don’t forget the need for some form of drainage through the wall allowing ground water to escape.

Finally if you have an existing dry stone wall at your property take ten minuets to look for the signs of failure, ten minuets today may one day save your life or the life of an unfortunate passer by. Failure signs are - individual loose stones - sections of the wall protruding further forward than others – bulges - vertical cracking. If you see any of these failures signs get your wall checked out, these walls can fall down at any time without warning.


John Phillips (F.I.A.S. F.A.B.E. M.I.B.C.O. F.F.B. M.R.S.H. C.M.W.O.B.O. M.B.I.M)

Construction Buena Vista

 

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