12TH INTERNATIONAL /BLOCK c O NF E RE NC E

OUTLlNE OF DRY-STONE RETAINING CONSTRUCTION IN BRITAIN AND

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 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 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 . 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, , , Portugal, , , UK), Southern Africa, Asia (including China, Hong Kong, India, Turkey), America (in­ cluding Columbia, , 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 Although in many regions construction of 'engineered' dry-stone retaining walls has been superseded by other forms, dry­ stone remains a contemporary form of retaining wall construction along hill ro­ ads in India for example [Arya &.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 walls in a disused in , 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- structures at Great 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 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.

Dry-stone retaining walls are gravity structures, relying on their self-weight to re­ sist lateral pressures. The most efficient forms of construction were shown by ex­ periment in 1834 to have an inward sloping outer face with either a vertical or parallel inner face [Burgoyne]. The overall proportions of existing retaining walls vary considerably. A base thickness:height ratio of 3:10 has been recommended [Jones, 1990]. The dimensions of some slender walls are clearly inadequate for gravity action. In such cases the walls provide a weather protection facing to re­ latively stable natural deposits cut during construction. Walls are generally built with an inward batter (slope) of between 1:5 and 1:10 on the front face.

Dry-stone walls have always been built using local stone. Stones should be dura­ ble, have sufficient strength and be available in sizes easy to handle. Parallel sided blocks, such as exfoliated granite [Walker &: Dickens, 1995], should make more stable walls than rounded highly weathered stones. Sedimentary rocks, such as Ii­ mestone, and mudstone, igneous rocks like granite, and metamorphic rocks such as slate are ali common wall materiais. In recent British surveys relati­ vely few dry-stone walls were found in areas where the solid geology is more re­ cent than the Carboniferous period [O'Reilly et ai, 1999]. The main concentrations

' 914 Figure 5. Distributian af French dry-stane walls.

of British dry-stone walls are found in the South-West of England (Cornwall, De­ von, Mendips, Cotswolds), The Pennines (Derbyshire, Yorkshire, Laneashire), West and North Wales, Cumbria, and throughout Seotland. The totallength of highway retaining walling in the UK is estimated, eonservatively, at around 10,000 km, with a replaeement eost in the region of f1 .2 Million per kilometre [O'Reilly et ai, 1999]. In Franee dry-stone walls are to be found throughout the south and west of the eountry, including regions sueh as Provenee, Pyrennes, and the Alps, as well in Corsiea and Brittany, figure 5 [Pare Naturel Regional du Luberon, 1994]. Though no national survey data is available, the totallength of dry-stone wall along Freneh roads is likely to be of a similar order to that found in the UK.

MODES OF FAILURE

As gravity earth retaining struetures dry-stone walls are assumed to behave in the same manner as rigid masonry and mass eonerete struetures. However, analysis has repeatedly shown that these walls often laek the proportions required by modern design codes [Jones, 1979; Jones, 1990; O'Reilly et ai, 1999]. The assumption that dry-stone walls aet like rigid gravity walls would seem an oversimplifieation of their

191 5 Figure 6. Bulged wall.

behaviour. Geotechnical aspects, such as soil/structure interaction, and their inhe­ rent flexibility, as demonstrated by bulging deformations, c1early play an important role in determining wall stability. Walls are capable of sustaining considerable de­ formation for a number of years without significant 1055 of stability. Out-of-plane de­ formations in the order of 50 mm are generally not considered significant. Their fle­ xible unbonded nature allows the walls to re-adjust to disturbance without over-stressing individual components whilst the structure resumes a new equili­ brium condition. Jones [1990] has likened their behaviour to reinforced earth.

Modes of failure of dry-stone retaining walls are broadly c1assified as toppling, sli­ ding (shear) and bulging. Walls often exhibit significant leaning, rei ative shear displacement or local bulging, figure 6, for many years. In contrast walls can fail suddenly with prior little warning. Whilst checks for overturning (toppling) and sliding (shear) form part of conventional simple stability analysis for retaining walls, reliable analysis of bulging requires more complex analysis. Convex bulging deformations of wall faces are typically Iimited to relatively small sections, and

1916 normally found within the lower half of the wall. Bulging is likely to be initiated by a variety of factors, including build-up of water pressure, settlement of fill or foundations, vegetation growth, or block deterioration [Cooper, 1986; Walker & Dickens, 1995], giving rise to thrust line eccentricity within the wall.

CAUSES OF DETERIORATION

Form of construction

A common source of deterioration is problems associated with poor workmans­ hip and weaknesses with the original form of construction. For example, inade­ quate provision of through and tie stones will make the wall more prone to bul­ ging failure. Poor selection of blocks during construction can result in high voids ratio and high contact stresses between adjacent blocks, leading to fracture and encouraging weathering. During construction the backfill material probably re­ ceived minimal compaction. With subsequent imposed loading and passage of ti­ me the fill will have settled, possibly inducing bulging of the wall. Selection of po­ or quality cohesive backfill material is likely to have led to problems in a number of walls due to its retention of water and subsequent expansion.

Water

Previous studies have recognised the importance of water drainage to the con­ tinued stability of dry-stone retaining and facing walls [Arya & Gupta, 1983; O'Reilly et ai, 1999; Walker & Dickens, 1995]. Monitoring of retaining walls in Zimbabwe has demonstrated close correlation between movement and seaso­ nal rainfall. Collapse of a slender 10.6 m high wall in Hong Kong in 1994, ki­ Iling five people, has been attributed to a preceding period of intense rainfall (547 mm of rain fell in the 48 hour period to collapse) [Wong & Ho, 1997]. Though initially free-draining poor selection of backfill material and blockage of joints with fines or morta r can significantly inhibit free water flow. Build-up of moisture within the backfill material has a destabilising effect through increased pressure on the wall, a weakening of the retained material and loss of soil suc­ tion. Soil suction in overconsolidated clays increases on excavation, taking a century or more to reach equilibrium in backfill soils [Vaughan, 1994]. This gra­ duai loss of soil suction has been attributed to the number of collapses in recent years. However, wall collapse is frequently limited to the front face, with the co­ re material behind remaining integral with the fill. In addition to rainfall back­ fill water may originate from natural groundwater flow, burst water mains, lea­ king sewers and poor land drainage. Water services are frequently buried behind walls.

Figure 7 shows the seasonal variation of 23 reported retaining walls collapses re­ ported in Gloucestershire, England between 1990 and 1998. Interestingly nearly half of ali wall collapses occurred during the winter (21 December - 20 March),

1917 Figure 7 Seasonal variation of wall collapses

Summer Spring 4% 17%

30%

Winter 49%

when ground water conditions are at their most adverse and ground strengths at their weakest. Only one collapse was reported during the summer months. A ty­ picai collapse of a highway retaining wall in the Cotswolds is illustrated in figure 2. Overloading seems unlikely since the road pavement has remained intacto Da­ mage from vehicular impact was not responsible either. Although it is always dif­ ficult in such cases to provide a conclusive diagnosis, build-up of water pressure behind the wall seems the most likely trigger for this winter collapse. A large num­ ber of face blocks were also weathered, simultaneously reducing the effective wall thickness and impeding the flow of water.

Weathering

Deterioration of materiais due to weathering is commonplace. Unfortunately walls have often been built with the poorer quality materiais considered unsuita­ ble for building and other construction work. Loss of material with weathering is likely to lead to greater thrust line eccentricity [Cooper, 1986]. Freeze thaw de­ cay of blocks is commonplace in both Britain and France, figure 8. 5to­ ne in sheltered sites and/or covered in vegetation may remain damp throughout the year. Deterioration of stone by salt attack, both from road treatment and per­ colation from the backfill, is also attributed to wall deterioration [Jones, 1990].

Construction work

New construction work within the close vicinity of the wall can increase surchar­ ge loading, impair water drainage or directly disturb the wall fabriCo Provision and maintenance of buried services may also have an adverse effect on the already shallow foundations at the base of the wall. Like other historic highway structu-

1918 Figure 8 Weathering damage.

res, such as masonry bridges, current traffic loadings far exceed those envi­ saged at the time of construction. Poor maintenance practices, and in particular the practice of mortar pointing, has ali too often increased the rate of deteriora­ tion by impeding the flow of water through the wall.

Vegetation

In sheltered sites walls are often damp, thereby encouraging the growth of vege­ tation. Roots can cause direct disturbance of the wall structure. Climbing plants, such as ivy, are likely to inhibit free water drainage and further encourage mois­ ture build up within the walls. Extensive vegetation growth is often indicative of inadequate maintenance.

Others

Other destabilising factors include: burrowing animais; accidental damage (vehi­ cular impact of parapet walls); storm damage (scour); mining subsidence; earth­ quake. INSPECTION AND ASSESSMENT

Highway retaining walls under the care of maintenance authorities will normally undergo general inspections every 1-2 years and principal inspections (to assess their capacity) every 5-6 years. Existing walls will often fail to meet the safety re-

19 19 quirements expected by modern design codes [BS 8002, 1994]. Unfortunately unnecessary and costly intervention work may be undertaken when a wall fails to demonstrate the required factors of safety in checks largely developed for rigid walls, even though walls often appear quite stable and continue to fulfil their function more than 100 years after their construction. Some walls exhibit consi­ derable bulging for many years without little apparent further deterioration in their condition. Meanwhile other apparently sound walls collapse with little or no warning. Consequently prediction of capacity and failure is largely a matter of ins­ tinct and experience rather than rigorous mathematical analysis.

Inspection of dry-stone earth retaining walls in Britain is guided by Highways Agency document BA 16/97 [1997]. Assessment is qualitative, relying on visual inspection and engineering judgement based on a comparison with adjacent structures. Inspectors are guided to record : • Size and location of trees and large shrub growth likely to influence stability; • Wall dimensions (overall height, retained height, parapet height); • Wall thickness; • Location of drainage outlets, the carriageway, services, vegetation growth and ground slope behind and in front of the wall; • Evidence of movement, bulging, deformation, cracking, adjacent ground mo­ vement and pavement cracking, partial wall collapse; • Identification of the stone and extent of weathering/deterioration; • Investigation of wall drainage.

Following inspection and assessment a variety of remedial techniques are availa­ ble. Quantitative analysis is difficult since conditions (stone quality, age, subsoil conditions, geometry, local expectations) vary greatly. If the assessment is incon­ clusive regular monitoring, using tell-tales, demec gauges or survey techniques, may be undertaken.

Reasons for failure of quite stable dry-stone retaining walls to meet basic stability requirements of modern codes are certainly numerous and complex. However, it is likely that uncertainties regarding characteristics of the backfill and pressures exerted on the wall comprise some of the most important shortcomings in our current understanding.

The distinct element method offers a promising and alternative approach to nume­ rical assessment of dry-stone walls. The Unified Distinct Element Code (UDEC), de­ veloped by Cundall (1971) to model discontinuous media in mechanics, has be­ en applied to model dry-stone walls. Initially used to model bulging of free-standing walls at Great Zimbabwe [Walker & Dickens, 1996], UDEC has more recently been used to successfully model instability of retaining walls [Wong & Ho, 1997; Harkness et ai, 2000]. Research developing the model is currently underway at the University of Bath . Though unlikely to be used as part of routine wall assessment, UDEC provi­ des a powerful research tool to develop understanding of wall behaviour.

7920 REPAIR AND REPlACEMENT

A variety of remedial and replacement methods are currently used in Britain and France. These are briefly outlined below.

Pointing and grouting

Conventional mortar pointing of joints along the wall face has been widely used in the past with varying success. Mortar fixes unstable sections, but unless ade­ quate drainage is maintained (using weep holes) this repair may actually accele­ rate deterioration. However, once applied subsequent cracking of rigid mortar joints can act as tell-tale indicators of further movement. Whereas trowel applied mortar rarely penetrates wall joints more than 100 mm, and often does little mo­ re than block water drainage, low pressure pumping (pressure pointing) of ce­ ment mortar fills joints up to 500 mm into the wall. This is a more effective me­ ans of stabilising loose sections or lengths of wall. Pressure grouting has been used with success and has the effect of transforming the wall into a rigid gravity structure. Grouting work must be undertaken at low pressures to minimise the risk of collapse.

Buttressing

Masonry buttressing may provide a successful repair for unstable sections but the technique relies on sufficient space being available in front of the wall. Alternati­ vely have proven ideal for temporary support to unstable sections. A per­ manent earth buttress or embankment may be used to entirely cover an unstable wall, effectively removing it altogether. Material should be free draining and the earth bank must be suitably graded to provide usable agricultural space for the adjacent landowner. Masonry walls may also be locally thickened using a concre­ te or cement mortar render, stabilising loose blocks and protecting stonework from further weathering, figure 9.

Soil nailing

This technique has been used successfully on walls in Bradford [Bruce & Jewell, 1986 and 1987] and more recently along a 340 m length of Telford's walls on the A5 road in North Wales. Dry-stone walls are at first pressure pointed or grouted, cored and then anchored back into the ground behind the wall using soil nails in­ clined at 10° - 20° on a 1-3 square metre grid. Nails can be galvanised mild ste­ el, stainless steel or glass fibre reinforced plastic.

Reconstruction

After collapse the most common reconstruction technique involves replacement with masonry faced mass or reinforced retaining wall. For walls up to 4 - 5 m mass concrete walls are widely used in Britain, figure 10. The concrete is

1921 Figure 9 Concrete repair, Southern France.

Figure 7O . Mass concrete repair.

1922 Figure 77 Shear box test set-up.

typically placed by hand and may be no fines to encourage drainage, though we­ ep holes must also be provided. Walls are faced with stonework with recessed mortar joints to give the appearance of dry-stone masonry. Higher walls may be replaced using a reinforced concrete cantilever retaining wall faced with stone­ work, ground anchored concrete walls, or reinforced earth. walls provide a further alternative solution [Jones, 1990]. Except for walls less than 2 metres high collapses are, unfortunately, very rarely replaced with new dry-stone ma­ sonry despite the proven durability of this solution.

EXPERIMENTAL STUDIES

Recent experimental work has been undertaken by researchers at ENTPE to con­ sider internai sliding (shear) deformation and strength of dry-stone walls built using limestone blocks. A large-scale shear box and a model retaining wall test have been completed to date. Initial findings from this work are briefly outlined in the following section.

Shear characteristics of joints between blocks were investigated using a direct shear box testo To allow shear charaeteristies of full-size blocks to be assessed, and also minimise other seale effects, a one metre square box test was undertaken using LlRIGM facilities at the University J. Fourier of Grenoble, figure 11. The she­ ar box was filled by placing varying sized blocks, similar to wall construction, in sue h a manner to provide a horizontal joint coincident with the experimental she­ ar plane. Given the irregular nature of the blocks, however, the shear plane was

7923 Figure 72 Model tesTo

non-planar. The normal stress was provided using four hydraulic jacks, applied uniformly through a steel plate and polymeric membrane onto the top layer of blocks. The normal stress was varied in four increments between 30 and 130 2 kN/m • The hydraulic jacks were adjusted for dilation effects during shearing to maintain a constant normal stress. Shear displacement was controlled using a ma­ nual single hydraulic ram applied to the upper half of the box, figure 11. The ra­ te of shearing was less than 3mm/min. The sample was sheared through to ma­ ximum in stages at increas ing normal stress (the normal stress was removed then re-applied between each increment).

After testing the block surfaces across the shear plane were noticeably damaged by friction along discrete contact points. The frictional characteristics of the sam­ pie conformed to the classic Mohr-Coulomb law, with an internai friction angle of 36° (± 4°) and an apparent cohesion of 8 kN/m2 (attributed to block fracture during shearing, though further work is required to consider this effect). Results

1924 of shear box tests have been used to check sliding strength of various wall geo­ metries ([Villemus, 1999].

Following the shear box test a 1 m high (1 m long and 0.6 - 0.7 m thick) model dry-stone wall was built and subject to hydrostatic pressure, figure 12. Following the shear box tests the wall was predicted to fail at a hydrostatic pressure of 10 kN/m2, though experimentally the wall remained quite stable at this pressure. Thereafter the water pressure was increased in 1 kN/m2 increments (per hour) un­ 2 til failure at 13 kN/m • Observations during testing noted that stones rotated, as they were no longer in compression at failure. Contact between the blocks de­ creased, consequently reducing the shear resistance. Failure of the wall was even­ tually in shear. From the failure pressure the predicted friction angle of the wall joints was 38°. At 1 m the test wall was not high, but the test was primarily in­ tended to complement the shear box test. Further shear box tests, on different stone materiais, such as granite, and larger model walls are planned for the futu­ re .

SUMMARY AND CONCLUSIONS

The paper has outlined the form of construction, extent and distribution of dry­ stone walls in Britain and France. Though little new construction work is under­ taken the large stock of existing walls is a very significant maintenance problem for highway authorities in both countries. Despite the widespread use it is clear that little previous engineering study of these walls has been undertaken.

The original form of construction is inherently flexible and porous. Though a gra­ vity earth retaining structure, simplified stability checks, which assume rigid body action, seem inadequate for assessment and conservative for designo Modes of failure include bulging, toppling and sliding failure. The principal agents of decay have been outlined. Of those discussed water, weathering, and inherent weak­ nesses in the form of construction appear to be the most common significant.

Inspection and assessment of dry-stone retaining walls is largely qualitative, rel­ ying heavily on past experience and engineering judgement. To date a lack of empirical data on the performance of dry-stone retaining walls has undoubtedly limited our understanding of their behaviour and restricted further development of numerical techniques. For calibration of analytical models recent work has had to resort to data collected from tests reported 145 years ago. Further model tes­ ting is on going at ENTPE.

Where problems occur remedial measures include pointing, soil nailing, recons­ truction, and construction of buttress walls. These measures are generally ex­ pensive, often difficult to implement, and can involve significant loss of land. Few are reversible and none maintain the original structural formo If incorrectly imple­ mented some may even accelerate collapse. Reconstruction after collapse very ra­ rely involves restoration of the original masonry wall, because of uncertainties re-

7925 garding stability and the high initial cost of such work. However, current research will hopefully improve understanding and encourage the future use of dry-stone wall construction for both remedial and new works.

REFERENCES

Arya, A.S. and Gupta, V.P. (1983). "Retaining wall for hill roads", Indian Road Congress joumal, 44, 1, 291-326.

BA 16 (1997). "The Assessment of Highway Bridges and Structures", Highways Agency, The Sta­ tionary Office, London.

British Standards Institution (1994). "Code of Practice for Earth Retaining Structures", BS 8002, Bri­ tish Standards Institution. London.

Bruce, DA and Jewell, RA (1986-87). "Soil nailing: application and current practice", Ground Engineering; 19 (8),10-16; 20 (1),21-33.

Burgoyne, J. (1853). "Revetments or retaining walls", Corps of Royal Engineers Papers, 3, 154-159.

Cooper, M.R. (1986). "Deflections and failure modes in dry-stone retaining walls", Ground Engi­ neering, 19 (8), 28-33.

Corps of Roya l Engineers (1845) "Experiments carried on at Chatham by the late Lieutenant Ho­ pe, Royal Engineers, on the pressure of earth against revetments, and the best form of retaining walls", Corps of Royal Engineers, 7, 64 - 68 .

Cundall, PA (1971). "A Computer Model for Simulating Progressive Large Scale Movements in Blocky Rock Systems", in Proe. of the Symp. Int. Soe. Rock Mechanics, Nancy, France, Paper No. 11-8.

Dickens, J. and Wa lker, P. (1996). "Use of Distinct Element Model to simulate behaviour of dry­ stone walls", Structural Engineering Review, 8, 2/3.

Gupta, V.P . and Lohani, N.K. (1982). "Treatment and repair of partially damaged retaining walls in hills", Indian Highways, October, 20-28.

Harkness, R.M., Powrie, W., Zhang, X., Brady, K.C and O'Reilly, M.P. (2000). "Numerical mo­ delling of full-scale test on drystone masonry retaining walls", Geotechnique, 50, 165-179.

Jones, CJ.F.P. (1979). "Current practice in designing earth retaining structures", Ground Engine­ ering, 12, 6,40-45.

Jones, CF.J .P. (1990). "Dry stone walls", in The Maintenance of Brick and Stone Masonry Structu­ res, E. & F.N. Spon, London, pp.349-360.

Jones, CJ .F.P. (1992). "The maintenance of old masonry retaining walls", in Proc. of the confe­ rence Retaining Structures, Cambridge, Ju ly, Thomas Telford.

Kim, S-K. (1975). "Stability analysis for masonry walls by circular arc method", in Proe. 4 th East Asian Conf. On Soi! Eng., Kuala Lumpur, 5.38-5.46.

Maldonado, R. and Gonzales, M .1. (1989). "Engineering of the Taironas circa 1200", in Proe. 72 th ICSMFE, Balkema, Rio de Janeiro, 1273-1275.

O'Reilly, M.P., Bush, D.I., Brady, K.C and Powrie, W. (1999). "The stability of drystone retaining walls on highways", Proceedings Institution ofCivil Engineers Municipal Engineering, 133, 101-107.

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Uniform Distinct Element Code Version 3.0 (1996), Itasca Consulting Group Inc., Minneapolis, USA.

VILLEMUS B. (1999). Etude des murs de soutimement en pierres seches, DEA, ENTPE, Lyon.

1926 Walker, P. and Dickens, J. (1991). "A study of the structural instability of dry-stone walls at Gre­ at Zimbabwe", in Proceedings of the Ninth International Brick Masonry Conference, Berlin.

Walker, P. and Dickens, J. (1995). "Stability of Medieval Dry Stone walls in Zimbabwe", Geo­ technique, 45,1,141 - 147.

Wong, H.N. and Ho, K.K.S. (1997). "The 23 July 1994 landslide at Kwun Lung Lau, Hong Kong " , Canadian Geotechnical Journal, 34, 825-840.

ACKNOWLEDGEMENTS

Dr Walker wishes to acknowledge the financiai and in-kind support of Enginee­ ring and Physical Sciences Research Council (EPSRC), Gloucestershire County Council, Halcrow (Gloucester), Somerset County Council, WS Atkins (South West), Wiltshire County Council, Transport Research Laboratory and The Royal Society.

1927