Nominated building platforms shall be prepared in consideration of the following criteria:
- The performance and functional requirements of the following sections of the New Zealand Building Code:
- B1 Structure.
- B2 Durability.
- D1 Access Routes.
- E1 Surface Water.
- Works recommended in the geotechnical and other hazards sections of the Development Evaluation Report have been completed and certified within the Development Completion Report.
- Slopes deemed to be complete shall:
- Have a minimum factor safety against slope failure of 1.5 (normal or average winter seepage condition).
- Have a minimum factor of safety against slope failure of 1.2 for extreme groundwater or other low return period temporary load conditions.
- Be assessed for seismic conditions by either of the following methods:
- Achieve a factor of safety of 1.0 for an appropriate design acceleration for structures likely to be constructed on slopes within the 3H:1V downslope failure zone to 4H:1V downslope runout zone or on land potentially subject to liquefaction.
- Design may be undertaken with consideration to the potential residual displacement of a slope with a factor of safety less than 1.0 under the peak ground acceleration used for design. If this approach is to be used, the design shall include a specific assessment of the potential impact of strain softening on the predicted displacements. This is particularly important given the occurrence of highly sensitive soils in the region.
- Nominated building platforms shall be located clear of the 3H:1V upslope failure zone or the building restriction line determined appropriate for the slope. The Geo-Professional shall determine this set back and define an appropriate building restriction line that will ensure that the nominated building platform achieves the factors of safety in c) for the required annual exceedance probability of extreme events.
- Nominated building platforms located within the 4H:1V downslope runout zone have been protected by way of a debris protection measure where slope stability factors of safety have not been achieved.
2H:1V, 3H:1V and 4H:1V failure zones are recognised as general failure zones based on the observation of numerous failures within the Tauranga area.
The 4H:1V zone is the limit of debris runout zone for most failures involving sensitive ash soils.
The 2H:1V defines the limit of a high proportion of slips. Land within the 2H:1V limit could be designated as high risk.
The 3H:1V defines the limit beyond which slope failures do not usually occur.
A smaller proportion of slope failures do occur between the 2H:1V and 3H:1V limits. Land between the 2H:1V and 3H:1V could be designated as moderate risk. For difficult soil issues (refer DS-10 - Apx B.5 Table 1: Geo-Professional Requirements for Geohazard Risk Assessment and Mitigation), Council requires the geotechnical reporting to be compiled by either a Category 1 or Category 2 Accredited Geo-Professional.
- Residual differential settlement is no greater than 25mm over 6m lateral distance as currently specified in the New Zealand Building Code.
Where any change is made to the New Zealand Building Code or any relevant industry standard / text, Geo-Professionals are urged to contact Council to discuss. Changes to the IDC will be made where the consequences of the Building Code change are understood by Council.
Until formal change to the IDC occurs or Council sends an advisory to the industry, the IDC shall take precedence.
Consideration shall be given to potential settlement of both engineered fill and foundation soils of building sites. This assessment shall consider both total and differential settlement potential. Differential settlements shall be no greater than those stated above. Any estimated differential settlement exceeding this range shall be specifically identified within the Development Completion Report or the site specific geotechnical assessment along with building development recommendations to manage the estimated differential settlements.
- Where the failure of a slope may impact a high importance structure or facility (i.e. Importance Level 3 or greater as given in NZS 1170 or Dam Structures) then the slope shall be designed to ensure that the factor of safety criteria referenced above are appropriate for use with the particular high importance structure or facility.
- Roads, service lanes or accessways vested in Council shall meet the performance criteria designed by the Geo–Professional at the time of Development Works Approval.
- Settlement monitoring shall demonstrate that the settlement has attained t90 prior to s224 being granted. If monitoring is ongoing on part of a development, the Resource Consent stages shall be amended (potentially requiring a variation to the Resource Consent) so that the monitored lots are not included in the s224 application.
Lots that are undergoing monitoring cannot be included within the Geotechnical Completion Report as being complete because they are not complete. It would therefore be inappropriate and potentially unsafe for any construction of infrastructure or structures on these lots.
Designers may need to consider reliance based design and may be required to consider larger seismic events. Documents such as the NZTA Bridge Manual and NZSOLD Dam Safety Guidelines should also be referred to by the designer.
Definitions in this section
Development completion report
Development evaluation report
Development works approval