Ordinary Council

28 April 2009

 

 

 

Urgent Business

 

General Manager's Report No. GM19/09

 

 

Subject:                  Alison Road, Coogee - Sandstone Retaining Wall

Folder No:                   PROJ/10483/2009

Author:                   Mark Shaw, Manager Technical Services     

 

Introduction

 

There is a significant sandstone retaining wall on Alison Road, Coogee located on the southern side between Arcadia Lane and Brook Street.

 

The wall was constructed approximately 80 – 90 years ago and is now in poor condition.

 

The reconstruction of 70m of this retaining wall has been listed in the draft 2009-10 Capital Works Program.  This report requests that funds be borrowed from reserves to allow the work to commence in the 2008-09 financial year and then repaid with the proposed 2009-10 funds for this project.

 

Recently, a collapsed drainage pipe has resulted in damage to the retaining wall and the land behind the wall outside of 343 Alison Road.  The repairs to this section of the retaining wall are being carried out as emergency works utilising infrastructure reserves.

 

Issues

 

Site Description

The properties on the southern side of Alison Road between Arcadia Lane and Brook Street are approximately 2m - 3m below the level of the road.  A sandstone retaining wall up to 2.4m in height retains the upper level footpath and road.  There is a lower level footpath along the base of the retaining wall that permits access to the properties.  See the photographs below.

 

Wall Description and Condition

The sandstone wall is approximately 124m in length. It varies in height from 0.8m to 2.4m.  The tallest part of the wall is 50m in length and includes 2 sets of stairs for access between the road and the properties.

 

The wall consists of sandstone blocks with mortar joints.  The mortar has deteriorated over time and cracks have formed along the mortar joints.  Along its tallest section, the wall has horizontal displacement, at the top away from the road, of over 100mm.

 

The movement and settlement of the stairs has resulted in a slope across the step. 

 

The wall is a segmental wall that has the potential for sliding.  The wall has commenced overturning as is indicated by the horizontal displacement at the top. 

 

The wall was structurally assessed by consultants in December 2003. At the time, they noted the same damage and concluded that remedial measures would need to be undertaken at some time.

 

In the meantime, we have monitored the wall for any changes.  Up until recently, the extent of movement (deflection) had stabilised.  However, with the recent rain, there are now signs that the movement has increased.

 

 

 

 

 

 

 

 

Risk Assessment

Should the wall fail and collapse, it would cause damage to private property and could cause injury to a pedestrian. If the wall did collapse, it would block the footpath, closing access to the properties in this section of Alison Road.

 

The lower footpath provides access to private property. This prohibits the closing of the lower footpath.

 

To assess the stability of the sandstone retaining wall, we have engaged Structural Engineers.  The Structural Engineers will first assess if short-term measures are needed to stabilise the wall and then produce a design for the reconstruction to the wall.  Once the design is completed, it is proposed to engage contractors and commence work immediately.

 

Design Options

Modern retaining structures that retain ground over 2m in height are generally reinforced concrete structures.

 

The sandstone texture and look provides an aesthetic that is a prominent feature of this area.

 

On this basis, it is proposed to construct a retaining structure that will have a similar look to the existing sandstone wall.  It is Council’s intention to retain the existing sandstone wall if structurally possible.

 

Relationship to City Plan

 

The relationship with the City Plan is as follows:

 

Outcome 6:        A Liveable City.

Direction 6a:       Our public assets are planned, managed and funded to meet the community expectations and defined levels of service.

 

Financial impact statement

 

There is funding in the 2009/2010 budget to repair this wall.

 

This report requests that funds be borrowed from infrastructure reserves to allow the work to commence in the 2008-09 financial year.  The borrowings will be repaid from the project funds proposed for this project in 2009-10.

 

Conclusion

 

The condition of the wall is poor.  We have engaged Structural Engineers to assess the stability of the wall and recommend measures to temporarily stabilise the retaining wall. 

 

The reconstruction of 70m of this sandstone retaining wall was proposed in the draft 2009-10 Capital Works Budget.  However, it is proposed that the work commence immediately to avoid any potential collapse due to recent deterioration of the wall due to heavy rainfall.

 

 

 

Recommendation

 

That:

 

a)     funds be borrowed from the infrastructure reserve fund to allow commencement of planning and work on the Alison Road Retaining Wall.

 

b)     the borrowings be repaid from funds allocated in the 2009-10 Capital Works Budget.

 

 

Attachment/s:

 

Nil