Drystone Retaining Walls

Presentation to Bridge Owner’s Forum atKing’s College, Cambridge13 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 O’Reilly – Consultant– Dr John Perry – Mott MacDonald

The challenge

drystone retaining walls are vital elements of the UK’s highway, rail and canal networks

almost all of these structures were built in the late 18th, 19th and early 20th 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 Highway’s 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