Managing the Regulator s Risk from the Effect of Slope Instability on Hillside Development

Transcription

1 Managing the Regulator s Risk from the Effect of Slope Instability on Hillside Development PR Tobin Senior Geotechnical Engineer Wollongong City Council IPWEA 2011 International Public Works Conference Canberra

2 A Spectacular Setting

3 Rockfall at Wombarra 2011

4 Debris Flow at Tarrawanna 1998

5 Landslide at Wombarra 1974

6 Managing Slope Instability Council has 2 key roles: - Emergency Management - Consent Authority for Development

7 A Long History Records date to the 1880s Very rapid post war city growth Little good developable land left Marginal land becomes attractive to developers Geotechnical issues were barely recognised or understood

8 Years of both prolonged and intense rainfall Extensive hillside instability occurred Many homes were lost Litigation followed

9 As Consent authority Council had no clear guidelines for developing hillside land The result: a poor outcome in court

10 A Legal Response What must a Council do? to fulfil its legal responsibilities and duties as the consent authority Get legal advice! There are 3 must dos; And, 1 may do.

11 The Must Dos 1. Consider: is there potential for slope instability? 2. Have a policy or procedure defining land susceptible to slope instability 3. Require applications on susceptible land to be supported with specialist advice setting out the site geotechnical constraints

12 The Must Dos Without the 3 Must Dos Council is exposed to actionable negligence With the 3 Must Dos Council can rely on the submitted information

13 The Must Dos Council is not responsible to provide professional advice, BUT Council must avoid being negligent With this process in place any claim on Council would be unlikely to be successful

14 The Geotechnical Review Process Is based on susceptibility maps These maps need to be refined as new information becomes available Ideally they should be available to the public, but this is not easily achievable

15 An Example Susceptibility Map

16 The Coledale Test 1988 An intense rain storm An embankment collapse 2 people killed A Coroner's inquest

17 The Coledale Test 1988 Council was investigated for culpability and quickly cleared on the basis of its geotechnical review process The process worked, BUT

18 The Coledale Test 1988 This test highlighted that: there will be times when a process based on the 3 must dos will not be enough

19 The Coledale Test 1988 Arguments could still be put that Council was in part negligent, - BECAUSE it had geotechnical information unknown to the developer s consultant

20 The May Do The legal advice also included one May do It should not be overlooked It states Council s engineer - MAY REVIEW the basic facts of the submitted geotechnical information

21 The May Do The engineering review should: Consider: the qualifications and experience of the signatory; Compare: the submitted information with known geotechnical information; Compare: the submitted information with observations made on site

22 The May Do If satisfied with the submitted information: - the Council can then set appropriate geotechnical conditions for the development

23 Thus began The referral of applications for an engineering review Not all applications. When in doubt refer

24 Engineering Referrals As more marginal land becomes desirable for development the number of referrals should also increase

25 Number of DA Referrals Engineering Referrals Geotechnical Reporting Indicators - Referrals /95 95/96 96/97 97/98 98/99 99/00 00/01 01/02 02/03 03/04 04/05 05/06 06/07 07/08 08/09 09/10 Reporting Year

26 The Mature Process Council s Comprehensive DCP now includes a specific chapter setting out geotechnical requirements for slope instability The DCP incorporates the AGS2007 Landslide Risk Management Guidelines Development assessment considers Acceptable, Tolerable, Unacceptable risk to both people and property

27 Process Flow Chart

28 Conclusions Extreme rainfall events will still occur Hillsides will always be susceptible to slope instability BUT it can be managed Land not susceptible to slope instability can be made susceptible by development eg basement excavations

29 Conclusions The need for an engineer review of geotechnical issues of a development will increase IF Council is to: Avoid approving inappropriate development in geotechnically sensitive areas; Minimise its legal liability with respect to geotechnical aspects of a development

30 Situations to Avoid The following 4 slides show examples of what the review process tries to prevent from occurring

31 The Tide is Out

32 Where is our back yard going?

33 Footing fall over

34 Do the waves sound a little closer today?