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The sustainability and assessment of drystone retaining walls
Le développement durable et l'évaluation des murs de soutènement en pierres sèches
Warren L., McCombie P.
University of Bath
Donohue S.
Queens University Belfast
ABSTRACT: The art of drystone walling is a highly sustainable traditional practise which uses local materials and craftsmen. As no
mortar is used they have low embodied carbon, and much repair work or rebuilding can be carried out using very little if any new
materials. However local practices developed to suit local materials, leading to a range of construction styles, making them difficult to
assess. This paper examines a range of construction styles of drystone retaining walls in use across the United Kingdom.
Understanding of the substantial variations of construction style is essential to enable proper assessment of these structures. Different
frictional and weathering characteristics, and the naturally occurring shapes of stone found in an area, all affect the ways in which the
stones have traditionally been assembled into walls. Ease of construction also plays a part, as the craftsman will naturally wish to
achieve a robust construction in a way that is economical of time and effort. Aesthetics may be very important, for both client and
craftsman. It is also shown that construction style is influenced by the location and function of the structures, with harbour walls
particularly likely to have unique characteristics, and the reasons for this are explored.
RÉSUMÉ : L'art de la pierre sèche est une pratique hautement durable traditionnelle qui utilise des matériaux et des artisans locaux.
En l'absence de mortier, elles ont une faible carbone incorporé, et les travaux de réparation ou de reconstruction bien peut exiger très
peu ou pas de nouveaux matériaux. Cependant les pratiques locales développées pour répondre à des matériaux locaux, conduisant à
une gamme de styles de construction, ce qui les rend difficiles à évaluer. Cet article examine une gamme de styles de construction de
murs de soutènement en pierres sèches utilisées dans l'ensemble du Royaume-Uni. Compréhension des variations importantes du style
de construction est essentielle pour permettre une évaluation adéquate de ces structures. Différentes caractéristiques de frottement et
aux intempéries, et les formes naturelles de pierre trouvés, affectent la façon dont les pierres ont traditionnellement été assemblés.
Facilité de construction joue également un rôle. L'esthétique peut être très important, à la fois pour le client et l'artisan. Il est
également démontré que le style de construction est influencée par l'emplacement et la fonction des structures, avec des murs du port
particulièrement susceptibles d'avoir des caractéristiques uniques, et les raisons de cette situation sont explorées..
KEYWORDS: Drystone, Construction styles, Assessment
1 INTRODUCTION.
Throughout the UK there are many different styles and types of
walling to be found. Most styles can be categorised as
horizontal construction, vertical construction or random
construction (Figure 1). Each has its own unique features and is
often associated with certain areas of the country and
corresponding rock types. Horizontal construction is often
found with more blocky types of stone such as limestones,
which can be stacked in a more conventional fashion; the stones
are also sometimes worked to give a better fit. Horizontal
construction is probably the most common type of construction
within the UK. Vertical construction is mainly associated with
slate type stones that can be tightly packed and may be less
successful in a horizontal configuration. Random construction is
mainly associated with more granitic type rocks which are hard
to work and are often irregular in shape. These styles are
sometimes found in combination in a single wall with a single
type of stone, and there are variations such as the herringbone
construction, found in parts of Cornwall.
It could be argued that every drystone retaining wall works
in essentially the same way, as every wall is essentially a
gravity retaining wall that relies on the frictional forces between
the stone for stability. However, each style is likely to use the
mechanical properties of the stones in different ways to achieve
the required coherence. The failure and movement of
horizontally constructed walls is comparatively well
documented and understood (Mundell, 2009) when compared to
the other construction styles. The mechanical differences
between the styles are explored below.
Figure 1. Different wall styles found in the United Kingdom: horizontal
construction, vertical construction, random construction.
1.1
Horizontal Construction
Experimental testing and numerical modelling (McCombie et al.
2012, Mundell et al. 2009, Colas et al. 2010, Villemus et al.
2007) have shown that the overturning moment due to the
horizontal component of earth pressure is resisted in part by the
vertical component acting in downwards shear on the back of
the wall, with very small deformations for well-constructed
walls in a normal condition. As walls become overloaded the
deformations can become considerable, often showing the
classic bulged profile as the walls adapt their shape to the higher
applied load (Figure 2). Ultimately, the walls will typically
overturn, though if a smooth stone is used the wall or a part of it
may slide forwards first. If the wall is constructed in a well-
bonded manner, with stones overlapping, then a tensile strength
can develop along the length of the wall which greatly assists in
