NAMBUCCA SHIRE COUNCIL

 

 

ORDINARY COUNCIL MEETING - 15 DECEMBER 2015

 

LATE AGENDA                                                                                        Page

 

 

 

   

9        General Manager Report

9.15   Geotechnical Investigation - 2 Nelson Street, Nambucca Heads.......................................... 2    


      


Ordinary Council Meeting                                                                                            15 December 2015

General Manager's Report

ITEM 9.15    LF4225             151215         Geotechnical Investigation - 2 Nelson Street, Nambucca Heads

 

AUTHOR/ENQUIRIES:    Michael Coulter, General Manager         

 

Summary:

 

Council has now received the report from Coffey Geotechnics.  There are no observed indications that any action (or lack of action) has directly contributed to failure of the retaining wall which appears to have failed without any specific impact from the mass movement in the slope below.

 

This report was considered by Council at its meeting on 29 October 2015 and it was resolved that:

 

“Council write to Coffey Geotechnics seeking clarification regarding the differences between their two reports regarding the geotechnical investigations near 2 Nelson Street”.

 

A response has now been received from Coffey Geotechnics which is attached.  For the sake of completeness the delegation by Ms Leckie to Council’s meeting on 29 October; as well as the advice from consultants de Groot & Benson as well as the letter to Coffey Geotechnics seeking their clarification are also attached.

 

 

Recommendation:

 

That the information from Coffey Geotechnics concerning the geotechnical investigation and stability assessment of site NH44 (2 Nelson Street) be received.

 

 

OPTIONS:

 

Council’s position on the matter as per its resolution of 16 January 2013 is:

 

“That Council advise the Crown Lands Office that it doesn’t generally support the sale of community land however Council does recognise there is a need to reconstruct the failing retaining wall and Council supports the Crown Lands Division advertising the application for public comment”.

 

Beside the recommendation the Council could pursue the following options:

 

1        Lobby Crown Lands to proceed with the sale of the 290m2 of crown land to Mr & Mrs Leckie for between $10,000 and $70,000 as per the valuation they supplied dated 22 October 2012.

 

2        Lobby Crown Lands to proceed with the sale of the 290m2 of crown land to Mr & Mrs Leckie for a price to be determined through a valuation obtained by either Council or the Crown Lands Office.

 

 

DISCUSSION:

 

At Council’s meeting on 12 March 2015 it was resolved as follows:

 

1.       That Council engage Coffey and Partners to prepare a preliminary investigation report on the site NH44 being the cliff at the east end of Nelson Street as described in the report by Douglas Partners dated October 2003.

 

2.       That Council pay or reimburse payment for $396.00 for the use of a sewer camera and high pressure water jetter to clear several root blockages in a sewer line.

 

Council has now received the report from Coffey Geotechnics which is attached.  Coffey Geotechnics specialise in geotechnical engineering and have provided Council with many design solutions for sites where there has been or is likely to be mass movement.

 

This report was considered by Council at its meeting on 29 October 2015 and it was resolved that:

 

“Council write to Coffey Geotechnics seeking clarification regarding the differences between their two reports regarding the geotechnical investigations near 2 Nelson Street”.

 

A response has now been received from Coffey Geotechnics which is attached.  For the sake of completeness the delegation by Ms Leckie to Council’s meeting on 29 October; as well as the advice from consultants de Groot & Benson as well as the letter to Coffey Geotechnics seeking their clarification are also attached.  They have been listed reverse date order.

 

The report is a consequence of considerable correspondence and discussions between Council, Crown Lands and Mr and Mrs Leckie based partly on an argument that Council is liable for any instability of the land.

 

Council’s position on the matter is as per its resolution of 16 January 2013 being:

 

“That Council advise the Crown Lands Office that it doesn’t generally support the sale of community land however Council does recognise there is a need to reconstruct the failing retaining wall and Council supports the Crown Lands Division advertising the application for public comment”.


In relation to the site generally and the potential for future landslides, Coffey Geotechnics has found that:

 

“Any natural hill slope and colluvium soil (soil derived from mass movement) is susceptible to ongoing failure.  It is neither practical nor expected to undertake attempts to achieve a substantial factor of safety against movement in colluvium.  Measures can be adopted to help maintain the slope.  Critically this will be through control against saturation and erosion of the soil.

 

Superficial drainage should be directed to prevent ingress into the soil and rock, and reduce potential for overflow on to the slope.

 

Maintaining vegetation assists, especially by drawing water from the soil.

 

Collapsed debris helps maintain the new equilibrium and it should not be removed.  It may, however, benefit from minor action to prevent water ponding.

 

Here, there is a buffer zone of about 2.5m between the current landslide head scarp and the damaged retaining wall structure.  Any design for a replacement retaining structure must recognise the geometry of the slope beyond its toe and also the need to control drainage from the wall location to avoid comprising its foundation.”

 

Coffey Geotechnics have found that the primary reasons for the failure of the existing retaining wall are as follows:

 

·           The wall was inadequately designed to support the retained fill materials due to the lateral earth pressure behind it especially on a perhaps with the sloping surface above, against bearing capacity of the foundation beneath the footing; and potentially also water pressure.

 

·           The unreinforced retaining wall has inadequate strength to support the pressures applied from the fill materials located behind the wall.

 

The existing landslide identified on the site is assessed not to have contributed to the failure of the retaining wall above it.  However, future slope movements such as likely ongoing transitional landslides or soil creep could impact on the retaining wall and any construction must not exacerbate disturbance to the equilibrium held by the slope….

 

Coffey has not observed indications that any action (or lack of action) has directly contributed to failure of the retaining wall that appears to have failed without any specific impact from the mass movement in the slope below.

 

A newly constructed retaining wall may reasonably be constructed at the crest of the slope to achieve a sufficient stability without major restoration or remediation of the slope below being necessary, but design must consider the potential to adversely affect its stability”.

 

Coffey Geotechnics state that the report does not offer an opinion as to the level of risk that may be acceptable to current and future stakeholders as this depends on many factors in addition to geological and geotechnical considerations.  They advise that the stakeholders for the site must decide on a level of risk that is acceptable or tolerable to them for both loss of life and property loss.

 

The findings of Coffey Geotechnics are relevant to Council’s previous resolution concerning the reconstruction of the failing retaining wall.

 

CONSULTATION:

 

There has been brief discussion with Council’s Engineering Designer who commissioned Coffey Geotechnics to undertake the work.

 

SUSTAINABILITY ASSESSMENT:

 

Environment

The report has no implications for the environment.

 

Social

The report does not directly address social implications.  The report does not offer an opinion as to the level of risk which may be acceptable to current and future stakeholders.

 

Economic

Any economic implications relate to the potential for the redevelopment of the land.  This is unknown.

 

Risk

The report does not offer an opinion as to the level of risk which may be acceptable to current and future stakeholders.

 

FINANCIAL IMPLICATIONS:

 

Direct and indirect impact on current and future budgets

The preparation of the report was not included in Council’s budget.

 

Source of fund and any variance to working funds

The report cost $5,000 to prepare.

 

Service level changes and resourcing/staff implications

There were minor costs to Council in the staff time required to commission the report and follow up its completion.

 

 

Attachments:

1

40349/2015 - Clarification of Differences between Draft and Final Report

 

2

35048/2015 - Request for Clarification

 

3

40357/2015 - Delegation by Kelli Leckie

 

4

40356/2015 - Advice from De Groot & Benson

 

5

31344/2015 - Coffey Geotechnical Investigation - 2 Nelson Street

 

  


Ordinary Council Meeting - 15 December 2015

Geotechnical Investigation - 2 Nelson Street, Nambucca Heads

 



Ordinary Council Meeting - 15 December 2015

Geotechnical Investigation - 2 Nelson Street, Nambucca Heads

 

Enquiries to:       Mr Michael Coulter

Phone:              02) 6568 0200

Email:                michael.coulter@nambucca.nsw.gov.au

Mobile:              0409 153 788

Our Ref:            LF4225

Your Ref:                    GEOTCOFH03259AA

 

 

 

3 November 2015

 

 

 

Mr Mark Maharaj

Project Geotechnical Engineer

Coffey Geotechnics Pty Ltd

1/18 Hurley Drive

COFFS HARBOUR   NSW   2450

 

Dear Mr Maharaj

 

SITE NH44 NAMBUCCA HEADS

 

Reference is made to your geotechnical investigation and preliminary stability assessment report dated 30 September 2015 for site NH44, Nambucca Heads.  This report was considered by the Elected Council at its meeting on 29 October 2015.

 

In response to the enclosed report to Council on the matter, the Council received representations from the owner of 2 Nelson Street setting out inconsistencies between the draft and final report and also reporting statements from yourself.

 

The Council also received some commentary on your report from consulting engineers de Groot and Benson Pty Ltd.  A copy of their commentary is enclosed.

 

The Council resolved to write to yourself to seek clarification regarding the differences between the draft and final report.  You may also care to provide comment on the advice received from de Groot and Benson Pty Ltd.

 

If you believe it to be necessary, you would also be welcome to address the Elected Council as well as or instead of providing a written response.

 

If there is a cost involved in responding to the comments from the property owner at 2 Nelson Street, could you please advise me before preeoceeding.

 

Yours sincerely

 

Description: 035374405@07042009-0537

 

Michael Coulter

GENERAL MANAGER

MAC:ms

Enc    Copy of report to Council’s meeting on 29 October 2015

          Copy of Council’s resolution

          Copy of Delegation by Kelli Lecki

          Copy of commentary from de Groot and Benson Pty Ltd


Ordinary Council Meeting - 15 December 2015

Geotechnical Investigation - 2 Nelson Street, Nambucca Heads

 







Ordinary Council Meeting - 15 December 2015

Geotechnical Investigation - 2 Nelson Street, Nambucca Heads

 



Ordinary Council Meeting - 15 December 2015

Geotechnical Investigation - 2 Nelson Street, Nambucca Heads