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Infrastructure Development Code

DS-10 - Appendix H - Geotechnical Requirements Summary 

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DS-10 - Appendix H.1  General

The following table summarises the geotechnical reporting requirements at each stage of the development process:

Table 1: Geotechnical Reporting Summary Table

Stage Report Purpose Typical Investigation Type Typical Document Provisions Comment
Stage 1. Resource Consent (Subdivision) Development Evaluation Report

Identify:
- Natural Hazards.
- Development Constraints.
- Appropriate Stormwater Management.
- Appropriate Wastewater Management.

Confirm:
- Land is suitable for the proposed landform, density and use.
- Safe access is possible.

- Proposal review

- Aerial photograph review

- Geological map review

- Site walkover

- A limited geotechnical assessment

- Sufficient investigation to fully assess all hazards 

- Infill subdivision may only need 2 hand augured boreholes or test pits to 2m deep.  Larger subdivision will need more tests, likely deeper tests especially if liquefaction possible.

- Regardless of scale of subdivision, liquefaction, stability and settlement risks shall be quantified.

- Overview of constraints

- Considerations for landform design (i.e. max cut / fill heights and grades)

- Preliminary assessments of stability, settlement and liquefaction

- Recommendations for additional assessments

- Statement of Professional Opinion (per 4402)

- For infill or very small subdivisions, on land with no geotechnical issues and where no earthworks are proposed, it may be appropriate to proceed to Stage 4 at the completion of Stage 1. 
Where this is the case, additional detail regarding foundation requirements is likely to be necessary.

- In cases where works are required to create safe & stable building areas or access, these shall be undertaken as part of the subdivision works so that there is a suitable area when titles are issued, Stages 2 and 3 are then applicable.

- If further subdivision of an existing lot (or lots) is proposed, additional Geotechnical assessment is required.  Refer to original report author for comment.

Stage Report Purpose Typical Investigation Type Typical Document Provisions Comment

Stage 2. 

Development Works approval

Development Works Approval

- Address information gaps from Stage 1.

- Connect the requirements of Stage 1 to the provision of infrastructure.

- Detailed design of all engineering and servicing issues.

Investigation and analyses for:
- Geotechnical issues.
- Earthworks.
- Infrastructure issues.
Provision of detailed design and calculations for:
- Geotechnical issues.e.g. retaining walls, embankments, slopes etc.
- Infrastructure issues e.g. pipes, roads etc.
 
Stage 3.
Final Sign Off (224)
Geotechnical Completion Report (GCR) - Confirmation that all Geotechnical requirements from Stages 1 and 2 have been met.

- Observations, Testing and Monitoring during the development. 

- Additional testing as determined by the certifying engineer.

- Summary of works undertaken
 
- Testing and monitoring records

- Statement of land suitability 

- Recommendations for Building Consent phase 

- As-Built Information of the finished landform and any structures.

- Individual Lot Summary table

- Statement of Professional Opinion (Form G2 or per 4431)

- Confirmation that RC Conditions have been complied with
- Limitations on the applicability of the recommendations.

- A Geotechnical Completion Report
 certifies that the finished landform is safe and stable.

- It may state foundations can be designed to NZS3604, however earthworks proposed in Building Consent phase may excavate deeper than the testing or fill deeper than 600mm, therefore specific investigation is required.

- Although NZS3604 requires 4 tests per lot the number of tests required can be less than this, depending on the volume of earthworks, the type of test, the number of lots and the variability of the ground.  

- DS-10 recommends a minimum of two tests per lot, (minimum 2m deep) with at least one of these tests extending at least a metre below the likely depth of foundation.       

Stage 4a.
Building Consent (where GCR or other existing site specific report is available)
Engineering Assessment Report Provide the Building Consent Authority sufficient information that there is reasonable grounds to conclude that the:
- Site works
- Foundations 
- Retaining walls
- Soakage systems
and the like, will comply with the Building Code and recommended outcomes within the IDC.

If:
- Building outside the nominated building platform
- Retaining walls and/or earthworks are proposed
then a geotechnical plan review and/or a geotechnical report will be required.

Additional investigations requirements are likely to be limited unless:
- Otherwise defined in the GCR
- The national standards have changed since the report was prepared 
- The testing was not sufficient for the proposed site development.

Where the GCR is older than 7 years: 
- A review is required to confirm no additional earth works have been completed and to check for other changes either on ground or in legislation since that time.

If the GCR was written prior to Dec 2012 and liquefaction is a possible risk, an assessment of liquefaction risk shalld be requested.  

Where a GCR is older than this and liquefaction was assessed, the original author shall be contacted to confirm the appropriateness of their recommendation in light of more recent developments.

If further subdivision of an existing lot is proposed, additional Geotechnical assessment shall be required. Refer to original report author for comment.

- If the only requirement is inspection or testing of cleared ground by an engineer, a letter confirming an engineer has been engaged for that work shall be supplied.

- Geotechnical plan review (unless 3604 applies) suggests
"unless complaint with existing report”.

- Results of any testing (if required).

- Refreshed GCR (where applicable) or geotechnical design report or geotechnical assessment report.

- Requirements for temporary works /  protection of other property.

- Geotechnical design report or geotechnical assessment report.

- Schedule of inspections required (if applicable).

- Where earthworks exceed the limits in DS10, a plan review by a Geo-Professional is required.  

- The GCR shall define the limits beyond which a Geotechnical review (and/or design inputs) is required.

Additional investigations are required where: 
- Specific design requirements are recommended in the GCR.
- National Best Practice Standards have changed since the report was prepared.
- The proposed design method is not supported by the content of the GCR.

This typically would include the requirements of the Joint MBIE/NZGS Geotechnical Engineering Guidance, Modules 1 – 6 that are summarised in Councils Ground Checks Guideline.

- Depending on location, proposal and  potential hazards, an accredited Geo- Professional may be required.  

Stage 4b.
Building Consent (where GCR is not available)
Engineering Assessment Report Provide the Building Consent Authority sufficient information that there are reasonable grounds to conclude that the completed:
- Site works
- Foundations 
- Retaining walls
- Soakage systems 
and the like, will comply with the Building Code and the approved plans.
Requirements will vary depending on:
- Proposed site works
- Proposed foundations
- Proposed retaining walls
- Underlying geology
- Possible hazards

- Site specific testing.

- Interpretation specific to the proposal.

- Recommendations for:
- Foundation design
- Site works
- Retaining walls
- Discharge locations / rates 
- Temporary works 
- Protection of other property 

- Geotechnical design report or geotechnical assessment report

- Schedule of inspections required

- A full Geotechnical report is required in this instance. 
This typically would include the requirements of the Joint MBIE/NZGS Geotechnical Engineering Guidance, Modules 1 – 6 that are summarised in Councils Ground Checks Guideline.


- Depending on location, proposal and potential hazards, an accredited Geo- Professional may be required.  

Stage 5.
Building Consent Code Compliance
Certificate
  Provide the Building Consent Authority sufficient information that there are reasonable grounds to conclude that the completed:
- Site works
- Foundations 
- Retaining walls
- Soakage systems 
and the like, comply with the Building Code and the approved plans.

Inspections and testing of site works and foundation, retaining wall construction

Building Consent documents will specify if inspection by an appropriately accredited Geo-Professional is required.

- Summary of inspections undertaken.

- Test results

- Plan of locations 

- Fill certification per NZS 4431 (where applicable)

- As-Built Information of any changes to the approved plans 

PS4 Producer Statement (where applicable)

- The PS4 is a supporting document.  

- The emphasis is on inspection records and testing.

 

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