AN EXPERIMENTAL AND ANALYTICAL STUDY OF DRY-STONE RETAINING WALLS

Lead Research Organisation: University of Bath
Department Name: Architecture and Civil Engineering

Abstract

Dry-stone walls are formed by carefully stacking blocks of stone rubble, without the use of mortar. Found throughout the world, dry-stone walls form the distinctive character of many areas of the UK, including the Cotswolds, Peak District and Lake District. Dry-stone retaining walls are engineering structures used to support road, railway and canal cuttings and embankments. The walls are commonly about 0.6m thick and are comprised of a bonded masonry face with stacked rubble stone behind. They were mostly built during the 19th and early 20th centuries. There are about 9000 km of these walls along the UK road network alone, having an estimated replacement value in excess of 1 billion. Though the ageing stock of walls is still performing very well, their deteriorating condition and occasional sudden collapse is a major problem for highway maintenance authorities.There is uncertainty about how these walls actually behave under load and what the factors of safety against collapse are. This current lack of understanding of real collapse mechanisms including three-dimensional effects, combined with the factors of safety required by modern design codes and uncertainties over design parameters such as soil properties, wall dimensions, groundwater conditions and loading, leads to the unnecessary replacement of satisfactory walls and the failure to identify walls that are in danger of imminent collapse.Even though dry-stone walls have distinct advantages over more modern earth retention methods (such as the use of local materials combined with a free-draining and flexible structure), the engineering uncertainties are such that new and replacement construction is rarely in dry-stone masonry. The unnecessary replacement of satisfactory walls, often by concrete structures, results in high costs associated with construction, traffic disruption, increased risk of damage to property or life, and potentially adverse environmental impacts. The current lack of understanding of the real mechanisms of dry-stone retaining wall behaviour is perhaps unsurprising given that no significant experimental investigation of dry-stone retaining walls has been carried out since a limited study undertaken over 170 years ago. The resulting lack of direct quantitative data concerning dry-stone retaining wall behaviour is not only a problem in its own right, but has also hampered validation of modern computer-based numerical analyses.Increased use of dry-stone walling for repairs and new construction, and prolonging the service life of the existing stock, can only happen with a proper, validated, theoretically based understanding of how these structures work, and the development of suitable design methods that are applicable in the modern engineering environment. The two main areas of uncertainty currently hindering the efficient and accurate assessment of dry-stone retaining walls are bulging and wall thickness. The objective of the proposed research is to develop a greater understanding of these two key issues by means of an experimental study combined with parametric three-dimensional discrete element analyses, and the further development of limit equilibrium analysis methods for the design and analysis of existing dry-stone retaining walls.

Publications

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C Mundell (2009) 'Testing of drystone walls'

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C Mundell (2008) Large scale testing of drystone retaining structures in The Structural Engineer

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Mundell C (2010) Behaviour of drystone retaining structures in Proceedings of the Institution of Civil Engineers - Structures and Buildings

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Mundell C (2009) Limit-equilibrium assessment of drystone retaining structures in Proceedings of the Institution of Civil Engineers - Geotechnical Engineering

 
Description The detailed and precise experimental observations, combined with the limit equilibrium modelling, have confirmed the hypotheses of the research team regarding the mechanisms of failure and deformation of dry-stone retaining structures, and have proven that these structures are not only amenable to relatively straightforward limit equilibrium analysis, but are also highly ductile and efficient. Prior to these tests, designers have not had the confidence to trust the predictions of any numerical models. An intention is to provide the limit equilibrium analysis program developed free of charge to designers and assessors of retaining walls. This will be done once a final version is fully validated and debugged.
Detailed consideration of the failure mechanisms, and the mechanisms of load transfer within the structures which give rise to their extraordinary ductility, and parametric studies, have allowed an understanding to be developed of which aspects of 'best practice' wall construction really matter. A particular aim of the project was to obtain an understanding of how bulging develops and can be analysed. It has been shown through both numerical and experimental work that bulging deformations do not necessarily indicate imminent failure which will have a significant impact on assessment and repair of these historic structures. Thermal imaging has been explored as a means of investigating the degree of connectedness within a wall and between wall and backfill, and especially to identify the presence of through-stones, which are critical to wall stability. A small number of structures showing varying degrees of distress have been investigated, and the concept has been proven.
The project has been very successful in delivering all of its objectives. The importance of masonry voidage, block rotation and joint wall friction to bulge formation has been experimentally confirmed. In addition to validating the limit equilibrium analysis procedure, the experimental data has been used to validate 3D numerical models developed by project partner (University of Southampton).
Exploitation Route The project has set the basis for further work using the full-scale test facility to investigate: the efficacies of existing repair techniques; novel repair and strengthening solutions; numerical modelling techniques in support of repair solutions; and, further develop practical field investigation techniques, such as thermal imaging. Project findings have been successfully disseminated to key stakeholders and further information on this dissemination is available in the associated documentation.
Sectors Construction

 
Description The project has set the basis for further work using the full-scale test facility to investigate: the efficacies of existing repair techniques; novel repair and strengthening solutions; numerical modelling techniques in support of repair solutions; and, further develop practical field investigation techniques, such as thermal imaging. Project findings have been successfully disseminated to key stakeholders and further information on this dissemination is available in the associated documentation.
First Year Of Impact 2009
Sector Construction
Impact Types Societal,Economic

 
Description Bradford Metropolitan District Council 
Organisation Bradford Metropolitan District Council
Country United Kingdom of Great Britain & Northern Ireland (UK) 
Sector Public 
Start Year 2006
 
Description Cornwall County Council 
Organisation Cornwall Council
Country United Kingdom of Great Britain & Northern Ireland (UK) 
Sector Public 
Start Year 2006
 
Description Gloucestershire County Council 
Organisation Gloucestershire County Council
Country United Kingdom of Great Britain & Northern Ireland (UK) 
Sector Public 
Start Year 2006
 
Description Network Rail Ltd 
Organisation Network Rail Ltd
Country United Kingdom of Great Britain & Northern Ireland (UK) 
Sector Private 
Start Year 2006
 
Description Surrey County Council 
Organisation Surrey County Council
Country United Kingdom of Great Britain & Northern Ireland (UK) 
Sector Public 
Start Year 2006
 
Description Wiltshire County Council 
Organisation Wiltshire County Council
Country United Kingdom of Great Britain & Northern Ireland (UK) 
Sector Public 
Start Year 2006
 
Description Annual Surveyor Conference and Exhibition 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact Presentation of research outcomes followed by open forum discussion afterwards.

Informed practical approaches to dry-stone retaining wall maintenance
Year(s) Of Engagement Activity 2009
 
Description British Geototechnical Association Annual Conference 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact Oral presentation followed by questions and discussion of practical significance

Gloucestershir County Council followed with application of technology in maintenance project.
Year(s) Of Engagement Activity 2009