Outline of Dry-Stone Retaining Wall

12TH INTERNATIONAL BRICK/BLOCK Masonry c O NF E RE NC E OUTLlNE OF DRY-STONE RETAINING WAll CONSTRUCTION IN BRITAIN AND FRANCE 1 2 2 P.J. Walker , J.c. Morel and B.villemus 'Dept. Architecture & Civil Engineering, University of Bath, Bath, BA2 7AY, UK ' DGCB, ENTPE, Rue M.Audin, 69518 Vaulx en Velin, FRANCE ABSTRACT During the nineteenth and early twentieth centuries a large number of dry-stone re taining walls were built as part of new road networks in Britain and France. Though many of these walls continue to perform quite satisfactorily, many fail simple stability checks. Maintenance authorities are typically confronted with a large number of ageing and distressed walls and an apparent increasing rate of deterioration in recent years. Initially the paper outlines the form of construction, distribution and extent of these walls in Britain and France. Failure mechanisms, including bulging, and causes of deterioration are discussed. General guidance for inspection and assessment of walls are included. Methods of repair and strengthening, which include pressure poin ting, soi! nailing, reconstruction, and buttressing are summarised as well. The Univer sity of Bath and ENTPE are currently undertaking on-going research programs aimed at improving structural integrity assessment of dry-stone walls. Initial findings from field work, model testing and numerical mode/ling are also included in the paper. Key words: Assessment, dry-stone, retaining walls, maintenance, repair. 1909 INTRODUCTION Dry-stone walls are built largely without the use of mortar by stacking uncut sto ne rubble blocks. Built by skilled masons and they rely on careful selection and positioning of stones for their integrity. Though occasionally cut or sawn, stones are generally left rough except for occasional dressing using a hammer. Dry-sto ne walling is a widely distributed form of construction, with examples of free standing, load-bearing and retaining walls found throughout Europe (including Austria, Czech Republic, France, Greece, Italy, Portugal, Spain, Switzerland, UK), Southern Africa, Asia (including China, Hong Kong, India, Turkey), America (in cluding Columbia, Peru, USA) and Australasia [Walker &. Dickens, 1995]. Dry-stone walls are mostly found in hilly and arid regions where there is a plenti fui supply of the basic raw materiais. Although in many regions construction of 'engineered' dry-stone retaining walls has been superseded by other forms, dry stone remains a contemporary form of retaining wall construction along hill ro ads in India for example [Arya &. Gupta, 1983]. In Britain the vast majority of new dry-stone work is limited to construction, repair and conservation of free-standing field walls. During the nineteenth and early twentieth centuries a large number of dry-stone retaining walls were built as road and rail networks expanded in Britain and Fran ce. Although walls vary in height between less than 1.5 metres to over 15 metres, most walls are less than 4 metres high. As previously mentioned walls are found in upland areas where there has been a plentiful supply of materiais. Typically ma teriais used for dry-stone work has tended to be of poor quality, since the best quarried stone was reserved for new buildings and bridge structures. Though a large number of these walls continue to perform quite satisfactorily, more than a century after construction, they often fail to meet safety criteria of modern codes [BS 8002, 1994]. Maintenance authorities are faced with an ageing stock of walls and an increasing rate of deterioration in recent years. Current assessment met hods, unable to consider the impact of deformations such as bulging on stability, are in the main qualitative. University of Bath and ENTPE are both currently undertaking on-going research programs aimed at improving structural integrity assessment. Work at University of Bath, funded by EPSRC, includes investigations and monitoring of existing walls in Gloucestershire, Somerset and Wiltshire, laboratory characterisation of materiais and numerical (distinct element) modelling of wall stability. To date work at ENTPE has centred on physical large-scale model testing and develop ment of simplified stability analysis. Future collaborative work will include further characterisation of material behaviour and developing the numerical analysis. Initially the paper outlines the form of construction, extent and distribution of dry-stone walls in Britain and France. Mechanisms of failure, including bulging, and causes of deterioration, including material deterioration, ground movements and heavier traffic loads, are discussed. General guidance for inspection and as- 19 10 sessment of such walls are included. Methods of repair and strengthening, which include pressure pointing and soil nailing are summarised as well. BRIEF lITERATURE REVIEW In stark contrast to their widespread use there have been very few engineering studies of dry-stone earth reta ining walls. The Royal Engineers undertook the first experimental investigations over 150 years ago. In two separate studies full-scale dry jointed retaining walls were built in progressive stages and their response to backfill pressures noted [Corps of Roy~1 Engineers, 1845; Burgoyne, 1853]. Lieut General Burgoyne built four full-scale granite walls in a disused quarry in Ireland, two of which collapsed due to lateral pressure when 5.2 m high, whilst two ot hers remained stable up to a full height of 6.1 m high. Remarkably Burgoyne's in vestigation remains the most detailed full-scale experimental work on dry-stone retaining walls carried out to date. Ienes has written a number of articles on dry-stone walls, documenting details of their construction, associated problems, and outlining typical maintenance prac tices (Jones, 1979; 1990; 1992]. Similarly Gupta, Indian Public Works Depart ment, and others in 1982-83 outline construction, maintenance and repair of dry stone retaining walls along highland roads in northern India [Gupta & Lohani, 1982; Arya & Gupta, 1983]. In 1986-87 Bruce and jewell reported on the then novel use of soil nailing to repair an unstable wall in Bradford [1986-1987]. In 1999 O'Reilly et ai reported on the results of wall surveys in England and Wales . Many residential buildings in Hong Kong have been built on hill terraces supported by dry-stone retaining walls [Wong & Ho, 1997]. Wall collapses, which have led to significant 1055 of life, have often been associated with nearby construction work or ingress of water from heavy rainfall or leaking pipes. Examples dry-stone retaining walls in sub-Saharan Africa [Walker & Dickens, 1995], South America [Maldonado & Gonzales, 1989], Korea and japan [Kim, 1975] are reviewed elsewhere. Commonly observed failure modes are bulging, toppling and shear, of which bul ging is the least well understood. The degree of bulging that any given wall can safely accommodate, without immediate fear of collapse, remains unknown, hampering effectiveness of assessment and maintenance programs. In 1986 Co oper examined the deflections and failure mechanisms of dry-stone retaining walls, considering the ground pressures and foundation resistance acting on the walls. He used a sim pie analysis to explain a mechanism of bulging arising from induced thrust line eccentricity within the wall face . Walker & Dickens have reported on the appraisal and conservation of dry-stone wall structures at Great Zimbabwe National Monument [1995]. During this work they pioneered to use of the distinct element method [Cundall, 1971] to model stability analysis of free-standing and retaining dry-stone walls [Dickens & Walker, 1996]. Using a relatively simple UDEC model [1996] they were able to simulate 7971 bulging deformations observed during full-scale tests. Subsequently other inves tigators have applied the technique to retaining walls [Wong & Ho, 1997]. Most recently Harkness et ai [2000] have used UDEC successfully to simulate numeri cally Burgoyne's test walls. FORM OF CONSTRUCTION The typical cross-section of a dry-stone retaining wall is shown in figure 1 below. Walls typically comprise one outer face of 'coursed' blocks behind which there is a more random graded core, figure 2. However, some 'engineered' walls in France have a more regular dressed stonework face, figure 3. Whilst generally the largest and best quality blocks are retained for the face, the core is compri sed of poorest quality material packed together with quarry waste chippings. So me walls comprise two faces with a core stacked between. Similar in form to field walls the two faces are connected horizontally across using larger 'through-sto nes'. Face blocks are normally laid horizontally, with their longest dimension ideally placed into the wall to enhance 'bond' w ith the core. However, some co astal slate block walls are built with vertical bedding planes instead, figure 4. In many walls large coping stones provide a small pre-compression to restrain the uppermost blocks. Alternatively a cement mortared coping may be used to pre vent toppling. Figure 1. Cross-sectian af typical dry-stane retaining wall. 9 12 Figure 2. Collapsed Cotswold wall. Figure 3. 'Engineered ' wall, Southern France. In general dry-stone walls have not been built to modern standards, and indeed variations in workmanship are apparent along single lengths of walls. Walls are normally built with very shallow footings. Larger blocks may be placed at the ba se of the wall to help spread the pressure. Base blocks rarely extend 500 mm be low initial ground leveI. To help balance uneven blocks during construction small pieces of stone, known as pins and wedges, are widely used . 191 3 Figure 4. Coastal wall, Cornwall. New walls are inherently porous structures, allowing free flow of water from the backfill without the need for weep-holes. Depending on the material used and quality of work, the proportion of voids is estimated at between 10% and 20%, though during coring voidage has been measured at 50%. In the period after construction fines material from the backfill and core may be deposited in the joints between blocks, subsequently inhibiting the flow of water.

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