Unless otherwise approved by Council, the design of the landform shall be in accordance with the IDC and supplemented by the standards and informative section DS-10 – Appendix E: Assessment and Reporting.
DS-10.10.1 Minimum Requirements
The following shall apply:
- All earthworks for a development shall:
- Comply with the design criteria for completed landforms.
- Allow for the reduction of overland flow velocity and the concentration of overland flow for the completed landform geometry and gradients.
- Provide for the permanent management of stormwater overland flow across landforms (e.g. contour drains, cut-off drains, permanent overland flowpaths exclusive of pedestrian or vehicle accessways etc).
- Provide for the construction of man-made structures to create safe landforms and designed building platforms.
- Where necessary, provide for the installation of debris protection devices.
- Define the earth fill and subgrade construction standards, define testing frequency, volumes and outline methods of testing to be used.
- Allow for observation, quality assurance and review of the earthworks design during the construction phase as covered in CS-3 Earthworks.
- Engineering appraisal and design are required:
- Prior to detailed planning, which will involve some form of subsurface investigation.
- During the review of and advice on design concepts.
- During construction to ensure the adequacy of the bulk filling and the execution of the earthworks design.
- Prior to constructing infrastructure or services (public or private) on land identified as having soils prone to settlement, the Geo-Professional shall advise Council in writing, the degree of consolidation that has occurred and the amount of residual settlement expected throughout the life of the services. Council may at its discretion, require:
- Further monitoring prior to construction
- A service level agreement be agreed and entered into.
DS-10.10.2 Safety in Design
Designers, Contractors, Building Companies and Council are all Persons Conducting a Business or an Undertaking (PCBU’s) and therefore have responsibilities under the Health and Safety at Work Act to consider safety and the lifecycle of structures.
All landform or engineering designs shall include a Safety in Design section of the design report that demostrates health and safety hazards can be minimised, isolated or eliminated during construction and operation.
DS-10.10.3 Temporary Works and Design / Build Elements
Temporary works and design build elements are building works as defined in the Building Act. While these may be the responsibility of the contractor, in many instances these will need to be consented building works and shall be included in the building consent documentation.
Where site works for a building is undertaken prior to the consent being issued, Council may provide for enforcement under the Building Act. Enforcement can made be against the owner, the persons undertaking the works and any consultant overseeing the works where they are considered to be unsafe or have potential to place other property at risk or require a building consent.
DS-10.10.4 Groundwater Seepage Pressures
The following 2 scenarios shall be undertaken when designing for groundwater:
- Scenario 1 - a design condition that shall reflect seasonal fluctuations (i.e. average winter seepage state).
- Scenario 2 - an extreme condition that shall reflect a credible worst-case seepage state that could be expected to occur for a low return period.
Both scenarios shall reflect potential transient conditions that may occur in slopes. This shall include allowance for water filled tension cracks or saturation of soils such as the Rotoehu Ash.
Groundwater is a generic term that encompasses the naturally occurring soil moisture condition (normal and worst case), perched water tables, pore water conditions (both normal and worst case), wetting front etc.
The use of the term seepage state is technically more accurate in the fine-grained volcanic ash soils in the Tauranga area where the soil is not dry above the water table.
DS-10.10.5 Site Subsoil Classification
The Tauranga area is typically Site Subsoil Class D (deep or soft soil) and some areas may be Class E (very soft soil). Site subsoil class C (shallow soil) or intermediate C/D will only be accepted with a supporting site specific geophysical study that includes shearwave velocity testing and deep drill holes with rock strength testing in accordance with NZS1170.5.
Some sites may exhibit characteristics of more than one Site Subsoil Class and it is not definitive which should be adopted. In these instances, it is appropriate to adopt the more conservative case and different site subsoil classes would be used in the structural and geotechnical analysis.
Opus International Consultants Report Microzoning for Earthquake Hazards for the Western Bay of Plenty, January 2003, states that any area where minor, moderate or widespread liquefaction is predicted, this shall be considered as being of sufficient risk to require specific investigation and design by a Category 1 Geo-Professional to address the risks of both liquefaction and lateral spread.
Liquefaction shall be assessed in accordance with latest New Zealand practice (currently given in the NZGS/MBIE Earthquake Engineering Series of Modules 1 - 4, and other relevant documents).
Where there has been a significant change in assessment methods since the time of the original assessment, an updated assessment may be requested by Council, particularly where the updated methods are likely to predict increased liquefaction risk.
All major alterations or replacement buildings (Importance Level 2 or higher) within the areas mapped as having a minor, moderate or widespread potential liquefaction risk within Figure 12 of the above Opus International Consultants report, shall provide a site specific geotechnical assessment by a Category 1 Geo-Professional as part of the building consent application to define the need (or not) for specific design.
DS-10.10.7 Retaining Walls (Subdivision)
Retaining wall design and construction as part of a subdivision shall:
- Comply with DS-10 - Appendix G.2 Table 1: Retaining Wall Surcharge Requirements as though the boundaries already exist.
- Be located entirely within a future lot unless party wall easements are agreed and provided for.
- Be of sufficient height that further excavation below the walls or fill above the walls will not be undertaken for single level concrete floor construction, unless the geotechnical design report outlines how this will be managed.
- Address the durability of the selected materials, the lifecycle of the wall and the potential implications of building above or below a retaining wall constructed several years prior to the lodgement of the consent for the habitable building.
- Address all maintenance issues e.g. access, frequency, type, responsibility (e.g. who).
- Identify and address risks and issues for replacement of the wall at the end of its lifecycle.
- Demonstrate compliance with New Zealand Building Code clause F4, Safety from Falling.
- Consider the requirements of DS-10 - Appendix G Geotechnical Considerations for Building Consents.
Items d) – f) above shall be identified within the Safety in Design section of the report as referred to in DS-10.10.2 Safety in Design.
DS-10.10.8 Protection of Trees or Other Features
All designs shall include the provision for protecting existing trees where required by the City Plan. All site activities including clearing, storage, cutting and filling shall be kept away from the root zone of such protected trees (defined as the extent of the drip line of the canopy). If there is any doubt, then the advice of an arboriculturalist shall be sought.
DS-10.10.9 Preparation for Fill
The stripped ground surface prepared by the Contractor shall be inspected by the Geo-Professional before any fill is placed thereon. Confirmation of this inspection having occurred is required as part of QA-5 Final Sign Off (s224c) Application Requirements.
Earth fills on or against sloping ground shall be constructed with horizontal benching of the subgrade in accordance with NZS 4431.
DS-10.10.10 Temporary Drainage and Erosion Control
Measures shall be taken to prevent excessive water-logging of surface materials yet to be excavated or compacted or both and to prevent fill material from being eroded and re-deposited at lower levels.
All dust, sediment, erosion control and temporary stormwater discharge shall be undertaken in accordance with Bay of Plenty Regional Council’s Erosion and Sediment Control for Land Disturbing Activities publication.
Issues for consideration include:
- Construction of temporary drains at the toe of steep slopes to intercept surface run-off and lead away for treatment where required, prior to discharge.
- Surface water diversion away from, or prevented from discharging over, batter faces and other areas of bare earth by use of bunds. Bunds shall be formed to intercept surface run-off and be treated where required, prior to discharge.
- The upper surface of fills shall be shaped and compacted with rubber–tyred or smooth wheeled plant when rain is impending or when the site is to be left unattended to minimise water infiltration.
- The completed battered surfaces of fills shall be topsoiled and vegetated (or the like) to reduce run-off velocities.
- Control of erosion and sediment discharge may require planting, environmental matting, hydro seeding, drainage channels or similar measures at an early stage in the earthworks construction phase
- The surfaces of fills and cuts to be graded to prevent ponding
- Sediment retention ponds to be constructed where they are necessary. They shall be maintained to ensure that adequate sediment storage is maintained. A process for decommissioning shall also be provided.
- Temporary barriers or silt fences using silt control geotextiles shall be used to reduce flow velocities and to trap sediment.
- Sections of natural ground to be left unstripped to act as grass (or other vegetation) filters for run-off from adjacent sites
- Dust control may require frequent watering, mulching or use of environmentally friendly sprays during construction along with establishment of the permanent surface at an early stage in the construction phase
DS-10.10.11 Subsoil Drainage
The following shall apply:
- Subsoil drainage systems shall be designed to lead groundwater away from all springs and potential areas of concentrated seepage under or adjacent to fills in order to prevent:
- Saturation of the fill before construction of the fill is completed.
- Internal erosion (piping).
- Internal ground water pressures (normal and worst case) which may reduce shear strengths in the earthfill.
- Effects on the adjoining land or structures from an alteration to the normal ground water conditions. e.g. groundwater table drawdown.
- Subsoil drainage shall discharge to an outlet such as a stable watercourse or a piped stormwater system.
- The location, type and any other attributes required by Council of all subsoil drains shall be recorded on the As-Built plans. Permanent subsoil drains shall be specifically designed taking into account the need for long term monitoring and maintenance and who will ultimately own and maintain them. These matters shall be addressed at the time of:
- Development Works Approval; or
- Building Consent (as appropriate).
A consent notice or encumbrance will likely be used to inform future lot owners of their obligation to complete periodic or regular monitoring and maintenance as required.
DS-10.10.12 Compaction Requirements
Minimum standards of compaction for fill materials are outlined in this section in alignment with NZS4431 Earthfill for Residential Development. These requirements are applicable for both large-scale earthworks and single house sites. The Geo-Professional may specify alternative requirements depending on the end use of the land or where lab testing supports an alternative criteria.
Testing frequency, both vertically and horizontally, shall be sufficient to demonstrate to Council that the fill has consistently met the testing criteria throughout the fill area and is appropriate for the scale of the project.
- Cohesive Soils
There are two alternative methods for controlling soil compaction as follows:
- The standard Proctor method as presented in NZS 4402. This produces a water content versus dry density curve from which the optimum water content and maximum dry density are determined. The compaction specification then stipulates a minimum dry density and water content range, usually 95% of maximum dry density and about 2% each side of optimum water content. If the soil is variable, then a compaction test is required for each fill type
- With controls on shear strength and air voids. A common specification is minimum undrained shear strength of 150kPa and a maximum air voids of 10%, to be determined per NZS4402. The shear strength of the fill material shall be determined using a shear vane test as prescribed in NZGS Test Method for Determining the Vane Shear Strength of a Cohesive Soil.
Method a-ii) is ideally suited to variable soils because the same specification applies regardless of variations. It is therefore appropriate in the Tauranga area. The use of an assumed solid density is generally not acceptable as minor variations between the tested and assumed value can result in fill being accepted that does not meet the air voids criteria.
- Cohesionless SoilsThe maximum and minimum dry densities, using the most suitable method for the material, shall be obtained from laboratory testing as prescribed in NZS4402 then either:
- The dry density of the fill material is expressed as a relative density in terms of the laboratory tests. This can be done by using a Scala Penetrometer calibrated adjacent to a test bed as follows:
- Prior to commencement of earthworks establish the maximum dry density and optimum moisture content of the cohesionless soil. After completion of a portion of the earthworks prepare a test bed and re-measure the dry density and establish the percentage compaction. The Scala Penetrometer can then be calibrated adjacent to the test bed.
- This test shall not be used as a substitute for the maximum density and % compaction tests unless they are correctly calibrated. This method is for use on cohesionless soils only. Monitoring of dry density by using a "Clegg" style hammer can also provide acceptable results if calibrated in a similar manner.
- The dry density of the compacted fill material shall not be less than 95% of the maximum dry density of the fill material obtained from testing required by NZS4402.
- Where fill volumes are less than 250m³, a target of min 5 blows per 100mm is considered appropriate unless otherwise designated by the Category 1 or 2 Geo-Professional.
- Mixed Soils Mixed pumiceous sandy silts and silty sands are common in the Bay of Plenty region. Where fill material comprises a mixture of sands and silts, more than one test type may become necessary i.e. a combination of air voids, shear vanes and Scala penetrometer tests.
- For the purpose of clarification, filling within perimeter walls / foundations (i.e. subfloor filling) shall also meet the criteria defined above, regardless of foundation type.
DS-10.10.13 Density Acceptance Criteria
The minimum acceptance criteria for test results shall be as follows:
- For less than 10 tests all tests shall meet or exceed the specified minimum standard.
- For more than 10 tests the average of 10 consecutive tests shall exceed the specified minimum standard.
- Only 1 in 10 (consecutive) test results:
- On the same fill material.
- Placed in the same weather conditions.
- Undertaken within a 24hr period.
may be less than 90% of the target specified.
Definitions in this section
Bay of Plenty Regional Council
Development works approval