Drystone Retaining Walls Presentation to Bridge Owners Forum at Kings College, Cambridge 13 May 2008

Definition in our context A stone wall, built without mortar, to retain soil or weak rock specifically for use on the infrastructure and may have been strengthened after construction (modifications)

National and Local Highway Authorities – responsible for some 400,000km of roads Network Rail – responsible for some 16,000km routes British Waterways – responsible for some 3540km of canals British Rail Properties Board (residuary) Ltd. Owners of Drystone Retaining Walls

Background and project team proposed by bridge owners forum started Dec 2005 Final Draft Stage managed by CIRIA funded by – CSS, DoT, Net. Rail, DRDNI Steering Group – wide input CIRIA – Philip Charles – Chris Chiverell contractors – Dr Myles OReilly – Consultant – Dr John Perry – Mott MacDonald

The challenge drystone retaining walls are vital elements of the UKs highway, rail and canal networks almost all of these structures were built in the late 18 th, 19 th and early 20 th centuries over the intervening years many of these drystone retaining walls have been repaired most often by pointing estimates put the replacement value of the surviving structures at about £6 ± 1 billion at 2000 prices by their nature there are special difficulties with the condition appraisal and preservation of such aged infrastructure thus, the most economic and sustainable approach is to provide practical guidance to enable the residual value of this infrastructure to be maximised

Title of report Drystone Retaining Walls and their modifications: condition appraisal and remedial treatment

Purpose of the report to present best practice to facilitate knowledge sharing to provide an enabling document to have national application to recommend an auditable conservation strategy for best value for money

Contents Chapter 1. Introduction and background 2. Management of drystone retaining walls 3. Understanding the stability of drystone retaining walls 4. The inspection and qualitative assessment of drystone walls 5. Maintenance, repair and renewal 6. Repairing drystone retaining walls 7. Replacing drystone retaining walls

Contents (cont.) Chapter 8. Research and development 9. Summary and recommendation 10. References Appendices 1. Literature review 2. Extracts from Highways Agency documents 3. Examples on inspection reporting 4. Stability of existing masonry retaining walls and their strengthening 5. Stability of existing masonry retaining walls and their strengthening 6. Case histories

Main user groups Clients (asset owners and operators) engineers (consultants and contractors) managers and administrators of maintenance and repair

water presence behind retaining wall properties of retained material geometry of wall stability of foundation change of circumstances Stability of Drystone Retaining Walls

Failures

Main causes of failure changed circumstances – new construction – repair works – utilities – increased dead and live loading ineffective drainage

visual based inspection/assessment assessment based on results of visual assessment need for engineering judgement regular inspection essential information collation for improvement of inspection and assessment methods Inspection and qualitative assessment

Maintenance Repairing drystone retaining walls pointing grouting soil nailing thickening embankment rebuilding

Maintenance (cont.) Replacing drystone retaining walls mass concrete reinforced concrete other solutions – ground anchors – precast units – reinforced and anchored soil – recycled masonry

Recommendations Strategic level reactive approach inconsistent with achieving a sustainable transport network need to implement effective management procedures need to provide sufficient resources of both people and money on regular basis

Recommendations (cont.) Operational level need to develop expertise and understanding of drystone retaining wall behaviour need for complete inventory need for preplanned management policy need for well trained inspectors and supervisors need to encourage co-operative working need to recognise aesthetic and environmental value need to encourage research and development and analyse accumulated data