DS-10 - Appendix G - Geotechnical Considerations for Building Consents

DS-10 - Appendix G.1 General

The following issues are recommended for consideration when designing geotechnical features at the time of subdivision and for Building Consent purposes.

DS-10 - Appendix G.2 Stability and Retaining Walls

Prior to issuing a code compliance certificate (CCC), all site works and retaining walls shown on building consent plans or present at the time of CCC inspection shall comply with New Zealand Building Code clauses:

Council may apply discretion when existing works were undertaken for previously consented buildings and the new building works do not rely on the stability of those works.

Walls that are low risk and do not require a PS1 Design Producer Statement shall have the construction details shown on plans. Council may ask for more information regarding compliance with the building code during consenting or following construction.

Construction contractors shall keep construction records and issue a PS3 Construction Producer Statement (refer Schedule 6 NZS3910 or Schedules E1/E2 NZIA SCC 2011) for these walls.

Retaining walls shall be set back from boundaries the distances shown in the MBIE Schedule 1 guidance document or designed for the surcharges in DS-10 - Appendix G.2 Table 1: Retaining Wall Surcharge Requirements, unless an assessment by a chartered engineer demonstrates to the satisfaction of Council that the wall is sufficiently setback from the boundary that loads beyond the boundary will not add a surcharge and failure of the wall will not affect other property.The surcharges in DS-10 - Appendix G.2 Table 1: Retaining Wall Surcharge Requirements are to be taken as dead loads due to uncertainty regarding load and load duration unless otherwise indicated in DS-10 - Appendix G.2 Table 1: Retaining Wall Surcharge Requirements.

DS-10 - Appendix G.2 Table 1: Retaining Wall Surcharge Requirements

Situation	Surcharge Requirement	Reason
Sloping ground < 4 degrees above or below the wall	None required	Nominal slope unlikely to have significant effect
Sloping ground > 4 degrees above or below the wall	Increase coeffcient of active earth pressure using soil mechanics equations if slope above the wall. Decrease coefficient of passive earth pressure using soil mechanics equations.	Will increase horizontal load on the wall or reduce the soil resistance respectively.
Boundary of other residential property (undeveloped)	5 kPa dead load.	Does not restrict single level dwellings or driveways
Boundary of other residential property (developed)	2.5kPa dead load if no parking or building area present. 5kPa minimum deadload if surcharged from building or vehicle areas.	Site conditions are known
Road carriageway	12 kPa (normal) 24kPa (overload) in accordance with the NZTA Bridge Manual unless otherwise agreed with the Asset Manager.	Consistency with NZTA Bridge Manual 3rd edition
Boundary supporting council road reserve	10 kPa dead load.	Large vehicles may be parked above wall
Boundary supporting all other council land	5 kpa dead load.
Boundary supporting private right of way	7.5 kPa dead load.	More frequent traffic than a private driveway
Residential parking / manoeuvring	5 kPa dead load.	Modern SUV's / boats and campers can exceed the maximum weight for light vehicles given NZS1170.
Residential building (NZS3604 type construction) including freestanding garages	5 kPa dead load.	Specific assessment for buildings higher than 1 storey
Commercial parking / manoeuvring	12 kPa dead load.	Allows for truck and goods deliveries
Boundary of other commercial property (undeveloped)	12 kPa dead load.	Does not restrict the development of adjacent property

All retaining wall designs that require a PS1 Design Producer Statement shall have a site specific geotechnical assessment by the appropriate category of Geo-Professional and have investigations to sufficient depth to characterise soil and groundwater conditions within the depth range providing support to the foundations. The geotechnical design report shall summarise GIS information, site observations, the proposed works and geotechnical design parameters. The structural design report shall provide the basis for the design and address all load cases.

The design engineer shall provide a letter confirming that they have been engaged to inspect the retaining wall construction and a schedule of inspections. If the design relies on a geo-professional and structural engineer from separate companies, then two letters will be required.

The certification shall cover: foundations conditions, structure, drainage, backfill and finished landform with respect to B1 and B2.

Following the completion of works, the engineer(s) shall provide Council a summary of the inspections undertaken, as-built information records, any calculations to support minor design changes and a PS4 Construction Review.

Explanatory Note:

The NZ Building Code requires that retaining walls are designed for the combination of loads that may occur throughout their life. Walls adjacent to existing or proposed boundaries need to consider potential surcharges that may occur but are not under the control of the owner. Please refer to the Ministry of Business, Innovation and Employment (MBIE) Schedule 1 guidance document for surcharge zones.

DS-10 - Appendix G.3 Foundation Design

Foundation design shall be accompanied by sufficient geotechnical information from the appropriate Category of Geo-Professional to provide council reasonable grounds to issue the building consent. This will vary depending on the structure type, bulk, location and site conditions.

In some cases the information in the original Geotechnical Completion Report may be sufficient, however where a site is reworked the Geotechnical Completion Report may no longer be sufficient. Where no information is available, a site specific assessment will be required.

Explanatory Note:

It is intended that the Geo-Professional will include limits on the applicability with consideration to future works, building types, foundation types etc.

DS-10 - Appendix G.4 Site Works

Where further site earthworks are required as part of the building works the applicant shall provide council an assessment from the appropriate Category of Geo-Professional with regards to fill induced settlements, batter stability and any drainage requirements. A specification for the proposed works shall be provided that addresses fill materials, compaction and testing requirements, and inspection requirements. The specification shall be in accordance with the IDC and NZS4431.

Prior to the Council’s foundation inspection the Geo-Professional shall confirm in writing that they have undertaken the works as per the specification and also tested the cut platform. A letter outlining the works, inspections and testing records along with a statement of suitability shall be provided prior to a code compliance certificate being issued. It is important that the statement of suitability applies to the whole building platform i.e. both cut and fill areas. It would also include the geotechnical suitability of any retaining walls within the zone of influence of the building platform. Where relevant, this shall also include the stability of the accessway, stormwater and wastewater provisions.

Definitions in this section

As-built information

Asset manager

Council

Geotechnical completion report