|
IS DRY STONEWORK EARTHQUAKE-RESISTANT? Christian Lassure On the subject of whether non-load bearing dry stone walls are earthquake-resistant, two conflicting schools of thought exist, admittedly a somewhat embarrassing situation. Walls that are unshakeable? It is no secret that the Internet site of the US society going by the name of "The Dry Stone Conservancy" boasts the following assertion: "If correctly designed, they [i.e. dry stone walls] are earthquake-resistant." (1) The same unwavering optimism is found in another assertion which adorns the site of the "Swiss Occidental Leonardo" (or S.O.L.) society: "their adaptive structure enables them to sustain the flow of water, frost and even such earthquakes of minor amplitude as occur sometimes in Switzerland." (2) Adding weight to these statements is an anecdote related by Carolyn Murray-Wooley in her article "The Stone Age, still with us. Dry stone masonry in the United States", published in the site of the US society "The Stone Foundation": a wall recently built by Scottish wallers near Seattle, Washington, survived totally intact a magnitude 7 earthquake that shook that area in March 2001 (apart from the fact that the walling had settled slightly in places). (3) In the same vein is a report in which dry stone specialist Richard Tufnell highlights the outstanding behaviour of non-load bearing walls that were built under his guidance to protect tree plantations in North Uttar Pradesh, India, in the early 1990s: "Just how strong may be judged from the fact that some four months later, an earthquake measuring 6.9+ on the Richter scale struck the area, with the wall almost exactly at the epicentre. Surrounding damage included 10,000 houses completely destroyed, yet not a stone moved out of place on the trainee wall". The story was meant to illustrate a previous assertion that "dry stone walling (...) has reasonable seismic resistance, which can be improved markedly with the incorporation of a few simple methods". (4) Or walls that do not pull their weight? One would likely be swayed by all these revelations were it not for the existence of contradictory opinions like the one posted in a forum dedicated to geotechnical engineering: "Such walls [gravity walls] can go quite high if there are no earthquakes but their mass means that they are very vulnerable to high horizontal acceleration." (5) In the same way, if would seem that the capacity of load-bearing dry stone walls (i.e. walls used in buildings) to resist earthquakes is even more questionable. For example, on the site of the Aga Khan's Planning and Building Services in Pakistan (6), one can read that "binding of stone walls with cement mortar is the best measurement for making dry stone masonry earthquake resistant." If there was ever a damning piece of evidence, this is it. Nor is it the only one. In the FAQ maintained by the Southern California Seismic Network (or SCSN), one is told that "earthquakes [in the M5 to M6 range] typically don’t cause much damage here, but in some areas of the world such a quake is a major disaster due to traditional dry stone construction or lack of building codes or enforcement of existing building codes." (7) In the light of these pronouncements, there doesn't seem to be any point in trying to pass off both load bearing and non-load bearing dry stone walls for the ultimate in resistance to earthquakes. Besides, common sense dictates that elements not bound by mortar are bound to come apart when struck by an earthquake. What recent history says... in Provence The fragility of dry stonework is borne out by the evidence supplied by a number of Provençal dry stone huts that were damaged by earthquakes in the late 19th and early 20th centuries. In his brochure on the dry stone shelters of the Aix-en-Provence and Salon-de-Provence areas in the Bouches-du-Rhône department, Louis Mille ascribes the "ruins and truncated roofs of some bories" to the June 11th, 1909 magnitude 6 earthquake whose epicentre was located in the ancient volcano of Beaulieu near Rognes.(8) It is also a well-established fact that the large dry stone cabanes that today make up the so-called "Village des Bories" at Gordes, Vaucluse, were damaged not only by the 1909 earthquake but also by a previous one in 1886 that had destroyed part of the village proper and hastened the decline of its leather industry. (9) A last example of the fragility of dry stone structures is provided by the damage sustained by a score of retaining walls in an area of agricultural terraces called Guglione in Menton, Alpes-Maritimes, during the Ligurian February 23rd, 1887 earthquake. But curiously, it is less the tremor itself than its indirect consequences that were involved: huge stone rocks fallen off a cliff came rolling downhill, flattening the walls of an agricultural property and tearing down olive and citrus trees in their wake. (10) As a conclusion, the overall impression given by this collection of data garnered here and there, is that dry stone masonry walls - bearing and non-load bearing alike - have no propensity, by virtue of their being mortarless, to be more earthquake-resistant than mortared walls. Anti-seismic recipes To all appearances, certain features have to be taken into account to explain the varying degree of resistance to earthquakes offered by dry stone walls. It is obvious that a wall built according to the book (with throughstones in sufficient numbers to bond both faces, carefully inserted pinnings within the wall, staggered joints) will provide a higher resistance to tremors than a poorly bonded contraption (no through stones, no staggering joints, voids in the core).
It also stands to reason that a wall made of large stones will be less likely to fall to pieces than one of smaller stones, or again that a wall built of flat stones will be less susceptible to crumbling than one of rounded fieldstones with minimum frictional resistance (11). Only a systematic investigation of the damage inflicted on dry stone walls in relation to the quality of their craftsmanship can provide better information on the subject. NOTES (1) Address : www.drystone.org/history/ (2) Address : www.s-o-l.ch/inherit/french/ (3) Address : www.stonefoundation.org/stonexus/01_issue/28-stone-age.pdf. Quote: "Scottish masons recently completed a dry stone wall near Seattle, Washington. Local observers kept asking what kept the wall together. They could not understand strength without cement. These questions were resoundingly answered in March of this year with the devastating 7.0 Seattle earthquake when the wall survived totally intact, merely settled more firmly into place (Aitken and Rippingale)". (These two names refer to the following article: Aitken, Nick, and Neil Rippingale, Shaken, But OK, in The Waller and Dyker [Periodical of the Dry Stone Walling Association of Great Britain], Spring 2001, p.3). (4) Richard Tufnell, Three Case histories of dry stone walling as transferable technology, in La pedra en sec. Obra, paisatge i patrimoni, IV Congrés internacional de construcció de pedra en sec, Mallorca, del 28 al 30 de setembre de 1994, Consell insular de Mallorca, Mallorca, 1997, pp. 193-206, in particular. p. 196. Quotes: "Just how strong may be judged from the fact that some four months later, an earthquake measuring 6.9+ on the Richter scale struck the area, with the wall almost exactly at the epicentre. Surrounding damage included 10,000 houses completely destroyed, yet not a stone moved out of place on the trainee wall". (5) Address : groups.yahoo.com/group/geoengineer/message/556?viscount=100. Quote: "Such walls [gravity walls] can go quite high if there are no earthquakes but their mass means that they are very vulnerable to high horizontal acceleration". (6) Address : www.icimod.org/focus/risks_hazards/wiremesh1.htm. Quote : "binding of stone walls with cement mortar is the best measurement for making dry stone masonry earthquake resistant". (7) Adresse : www.scsn.org/faq.html. Quote: "Earthquakes in the M5 to M6 range typically don’t cause much damage here, but in some areas of the world such a quake is a major disaster due to traditional dry stone construction or lack of building codes or enforcement of existing building codes". (8) Louis Mille, Les bories des terroirs d’Aix et Salon-de-Provence, 1993, 27 p., in particular. p. 21. (9) See Pierre Viala, Histoire d’une restauration : le « village des bories » de Gordes (Vaucluse), in L’architecture rurale en pierre sèche, tome 1, 1977, pp. 151-153. Quotation: "Deux tremblements de terre ont ébranlé le site, en 1880 [lire 1886] et en 1909. Sans doute faut-il leur imputer des linteaux cassés, des lézardes et quelques tassements" [The site was shaken by two earthquakes, in 1880 [in fact 1886] and 1909, hence presumably the few broken lintels, cracks and settlings to be seen]. See also www.beyond.fr/villages/gordes.html. Quote: "Gordes was a center of wool and leather industries in the 17th and 18th centuries, including weaving, carding and tanning. In 1886, part of the village was destroyed by an earthquake, which tended to hasten the already declining local industries". (10) Address : www.azurseisme.com/SeismeLigure.htm (11) "Large stones and through stones are essential to providing stability to a dry stone masonry". This quotation is taken from a report published on the site of the "Engineering for the World" society (India ’05: Technical report. A glimpse into some of India’s traditional structures - address : www.engineering4theworld.org/UrbanHazard/INDIA TECH REPORT’05.pdf). A letter from Scottish waller Nick Aitken Date: Tuesday, 16 December 2008 To print, use landscape mode © CERAV The above article will be referenced as follows: Christian Lassure page d'accueil sommaire publications
|
