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Infrastructure Development Code

DS-10 - Appendix B - Landform Classification System 

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DS-10 - Appendix B.1 Landform Classification System

Tauranga City is located on land subject to various natural hazards. The natural hazards range from complex soils related issues to flood prone land and coastal erosion and inundation issues.

DS-10 - Appendix B.5 Table 1: Geo-Professional Requirements for Geohazard Risk Assessment and Mitigation  provides a relationship between landform category, the likelihood and consequence of failure, the impact on the development and the professional required to investigate, define and report on solutions for development associated with the most commonly experienced hazards in Tauranga City. This is represented by a combination of opinions published by various Geo-Professionals.

DS-10 - Appendix B.2 Geotechnical Hazards

Geotechnical studies have been undertaken within and adjacent to the City dating back to the 1970's as listed in DS-10 - Appendix A.1.  

These plus other studies provide valuable information on the issues associated with the geology Tauranga City is located on. They should be used as reference documents when undertaking work within the City.

It is clear from the various reports that the 2H:1V upslope failure zone and the 4H:1V downslope runout zone be used as a minimum for slope category definition in all parts of the City. These failure zones correlated strongly with the slip observations during the Civil Defence emergency associated with the storm event of 18 May 2005, which affected over 300 sites.

Other geotechnical hazards that have been identified as being present in the Tauranga area include (but are not limited to) include slope instability, peat, settlement and liquefaction. Those that have not historically been identified as typical include the likelihood of fault rupture and expansive soils.

DS-10 - Appendix B.3 Other Hazards

Over the years, Tauranga City has undertaken hazard management research that has assisted in defining the issues associated with many other natural hazards. The work is on-going especially in the area of catchment and flood management. 

This research is clear on the need for appropriate risk definition and solution as part of the management of the natural hazard. The most common other hazards have been placed into DS-10 Appendix B.5 Table 1: Geo-Professional Requirements for Geohazard Risk Assessment and Mitigation  to provide guidance on reporting and whom undertakes that reporting. 

DS-10 - Appendix B.4   Referencing Information

Council displays hazard information on its GIS layers.
For further advice, please contact a Council Development Engineer.

DS-10 - Appendix B.5   Risk Classification

The Geo-Professional categories are summarised in DS-10 - Apx B.5 Table 1: Geo-Professional Requirements for Geohazard Risk Assessment and Mitigation.

DS-10 - Apx B.5   Table 1: Geo-Professional Requirements for Geohazard Risk Assessment and Mitigation 

Category 1 Geo-Professional

Category 1 Geo-Professionals are required to undertake assessments when the following conditions apply:

1.1 - Where structures are proposed to be located within the 2H:1V upslope zone or the 4H:1V downslope zone. (Both sets of zones are defined on Council’s GIS system)

1.2 - Where structures are proposed to be located within the 2H:1V and 3H:1V upslope zones but where combined with various other factors i.e. relic slip geomorphic features.

1.3 - Where sites include possible/probable slope movement feature with:

  1. Clearly or poorly defined headscarp. 
  2. Indications of recent or current activity.
  3. Hummocky debris.

1.4 - Seismic ground hazard including soil liquefaction and mitigation of effects

1.5 - Lateral spread issues

1.6 - Seepage issues from slope at any height

1.7 - Where a soakage method for stormwater disposal is proposed within the areas defined as “Specific Design” and within 150m of a relic slip geomorphic feature or from the outer extent of the displayed 2H:1V zone and not within a soak hole decommissioning zone.

1.8 - To certify fill where placed on any ground defined above (i.e. where original slope is greater than 2H:1V  or where evidence of instability is present); or elsewhere where fill thickness exceeds 3m.

1.9 - Undertake geotechnical investigations for retaining walls in any area where the height exceeds 3m or where slope exceeds 2H:1V .

1.10 - Assessment and design of boulder or rock-faced slopes.

Category 1 Geotechnical Engineers are required to undertake engineering design when the following conditions apply:

1.10 - Where difficult settlement problems may exist in either:

  1. Low lying areas.
  2. Areas subject to old uncertified filling and/or horticultural fills.
  3. Estuarine silts and compressible soils.

1.11 - Low strength soils

1.12 - Any engineering design associated with a Building Consent where geotechnical analysis of:
- Retaining wall forces
- Pile capacity
- Foundation settlement and bearing
- Deep ground improvement 
is required to mitigate any hazard defined above.

1.13 - Specific design of foundations  compressible soils or uncertified/horticultural filling exist that are deeper than 2m

Category 2 Geo-Professional

Category 1 or 2 Geo-Professionals can undertake the following:

2.1 - Where site exhibits no evidence of relic slip geomorphic features within or in close proximity to the proposed development.

2.2 - Where proposed structures are to be located outside of the 2H:1V upslope and 4H:1V downslope zones.

2.3 - Areas not covered by Categories 1 or 3.

2.4 - Where a soakage method for stormwater disposal is proposed within the areas defined as “Specific Design” but is outside of the distance of 150m from a relic slip geomorphic feature or from the outer extent of the displayed 2H:1V zone and not within a soak hole decommissioning zone.

2.5 - To certify fill where placed on ground sloping between 18⁰ and 26⁰.

2.6 - Elsewhere where fill thickness exceeds 1.5m.

2.7 - Specific design of foundations where limited extents of compressible soils or uncertified/horticultural filling exist. (“limited” is defined as being less than 2m in depth where the extent of poor bearing capacity soil is easily determined).

2.8 - Geotechnical investigations for retaining walls sites not defined in Category 1 or Category 3.

A Cat 1 or Cat 2 Geotechnical Engineer is required to undertake the following:

2.8 - Specific design of foundations where limited extents of compressible soils or uncertified/horticultural filling exist. (“limited” is defined as being less than 2m in depth where the extent of poor bearing capacity soil is easily determined).

2.9 - Any engineering design associated with a Building Consent where geotechnical analysis of:
- Retaining wall forces
- Pile capacity
- Foundation settlement and bearing
- Deep ground improvement 
is required to mitigate any hazard defined above.

Category 3 Geo-Professional

Category 1, 2 or 3 (SQE) Geo-Professionals can undertake the following:

3.1 - Proving bearing capacity for foundations not included in Category 1 or Category 2 areas.

3.2 - Where building on flat sites or sites sloping less than 14⁰ (4H:1V).

3.3 - Where building adjacent to steeper slopes, beyond the 3H:1V  upslope zone.

3.4 - No compressible soils, uncontrolled filling or peat.

3.5 - May certify fill less than 1.5m thickness.

3.6 - Soakage reports where the surrounding ground stability does not need to be proven.

3.7 - Geotechnical investigations for retaining walls on gently sloping sites where the height is less than 2m.

 

Explanatory Note:
Uncertified filling and horticultural land uses are recognised by the Ministry for the Environment (Oct. 2011) as Hazardous Activities and Industries List (HAIL) activities with the potential to cause contamination likely to affect human health and the environment. As such, land development activities where such situations exist and where associated soil materials are retained on site, these are required by the National Environmental Standard (NES) for Assessing and Managing Contaminants in Soil to Protect Human Health to be assessed for contamination by a SQE Professional experienced in such assessments.

Definitions in this section

Council

Development engineer

Geotechnical hazards

Geo-professional

GIS

Other hazards

Relic slip

Stormwater

 

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