Requirements for geotechnical reports

 

The purpose and applicability of this information is to:

  • ensure that development in any area of potential instability or landslide has proper regard to factors affecting land stability (eg. land in Council’s opinion that may be subject to land instability)
  • provide guidance on the preparation of geotechnical reports for certain types of development
  • provide guidance on the geotechnical certifications required for certain types and stages of development
  • identify sites that are subject to instability (see the landslide hazard overlay - Part 8, 8.2.4 within the planning scheme).

Development on steep and/or potentially unstable land may be adversely affected by:

  • earthworks (excavation and filling)
  • the erection of buildings and other structures (like swimming pools, tennis courts, retaining walls, roads and driveways)
  • underground services near retaining walls and any structure
  • on-site disposal of wastewater
  • other significant changes to natural drainage patterns

Inappropriate development on steep and/or potentially unstable land may have significant risks for property and human safety on the site, and in areas above and downslope of the site. To ensure such risks are avoided or minimised, the planning scheme require geotechnical reports to be prepared where development may affect or be affected by land instability.

Expertise required in preparing a geotechnical report

The preparation of a geotechnical report requires specialised skills. Geotechnical reports prepared by an appropriately qualified and experienced geotechnical engineer (Registered Professional Engineer of Queensland (RPEQ)) – will only be accepted by Council. It is highly desirable that the person has local experience with steep and unstable areas, and demonstrable general experience with steep and unstable land. The geotechnical engineer must hold and maintain professional indemnity insurance for any one occurrence of at least $2 million and have geotechnical investigations listed as an activity on the policy.

Requirements of a geotechnical report

Geotechnical reports must be addressed to Toowoomba Regional Council and clearly state that the report is for the use and reliance upon by Council and must not contain any limiting clauses in this regard. A geotechnical report must document:

  • a description of the site and proposed development
  • a description of the existing conditions of the development site, including assessment of land stability and geotechnical constraints to development, the suitability of the site for the proposed development, having regard to the existing geological and topographic conditions. This includes an assessment of likely effects or impacts of the development upon slope stability and landslide potential
  • measures recommended to mitigate impacts, including siting, engineering and other measures required to ensure a satisfactory form of development. Such measures must not require high whole-of-life cycle costs, particularly deep soil drainage within single residential lots or public land
  • conclusions and recommendations

The extent and detail of investigation will depend upon the particular site characteristics and the nature of the development being proposed. Council will require each report to demonstrate a scope and depth of investigation appropriate to the specific proposal. The extent of the work carried out is to be determined by the Geotechnical Engineer, provided the investigation concludes that the site, house, retaining wall or other features under assessment have a stability risk acceptable to Council, as described here:

  • Contour plans are to have 1.0m contour intervals indicated obtained from survey or low level aerial photographs using ‘objective’ photogrammetric techniques
  • Geotechnical reports are to adopt the risk assessment methodology given in the Landslide Risk Management (2007) guideline documents by the Australian Geomechanics Society (AGS). These documents includes some 'Guidelines for Hillside Construction’ which should be included in the geotechnical report

The preferred format of the geotechnical report is outlined below:

1.  Introduction

1.1 Details of development

1.2 Site location and description (including survey coordinates/coordinate system)

1.3 Method and scope of investigations

1.4 Qualifications of responsible individual(s) and/or company

2.  Description of existing conditions

2.1 Geology (local and regional)

2.2 Topography

2.3 Groundwater

2.4 Surface drainage

2.5 Vegetation

2.6 Buildings, other structures etc.

3.  Assessment of land stability

3.1 Existing conditions

3.2 Geotechnical constraints to development

4.  Description of proposed development

4.1 Site layout

4.2 Proposed development components

4.3 Potential geotechnical effects

5.  Assessment of development impacts

5.1 Site layout

5.2 Roadworks, driveways and other pavements

6.  Measures recommended to mitigate impacts

7.  Summary and conclusions

8.  Site plans

Appendix - Field and laboratory test results

Previous geotechnical reports

Where a geotechnical report has already been provided as support documentation to Council for previous development applications over the site (e.g. reconfiguring a lot or material change of use), these documents must be clearly referenced in the report. The reference should state the report was prepared as support documentation for the subsequent application (e.g. operational work or building work). Support documentation must be made available to Council and should be current and relevant to the proposed development i.e. not for an alternate form of development on the same site.

This article outlines matters to be addressed in a geotechnical report, on the basis that such support documentation (earlier geotechnical teport) is not available. In the event geotechnical reports and certifications for the previous applications are available, items already covered in these earlier reports/certifications may be referenced and covered in less detail.

Investigation of existing conditions

The geotechnical report must include an investigation of existing site conditions. This should include an assessment of the existing stability of the site and details of geotechnical constraints on building and/or other development works on the site. The investigation of existing conditions is to include descriptions of:

  • Existing geology (surface and subsurface materials, soil/rock stratigraphy) and geomorphology (slopes, ground contours, natural features, terrain analysis, landslip features) both locally and regionally in the area of interest. This may include a review of information available from published materials, including the landslide hazard overlay map of the planning scheme, aerial photography, geological maps and reports (e.g. the Geological Survey of Queensland Record Series)

Field investigations and tests using methods such as excavators, drill rigs and/or seismic techniques will be required, particularly to assess the following factors:

  • Subsurface profile within proposed works areas (including roads, infrastructure, building sites, potential swimming pools, tennis courts, garage, access driveways, underground services and the like)
  • Classification of surface and subsurface materials:
    • Erosion potential
    • Foundation conditions that could affect structural performance
    • Suitability for wastewater disposal
    • Any other site characteristics relevant to slope stability
  • Evidence of previous instability (eg. irregular contours, hummocky topography, scarp faces in area of tension crack(s), curved and/or non-vertical tree trunks, broken kerbs and gutters, cracked or uneven roadway surfaces, distressed houses or other buildings). The classification of any existing slips (type, severity and likely mode of failure) should be determined
  • Extent and type of any existing occurrences of erosion;
  • Surface drainage patterns and characteristics (rapid surface runoff, presence of pools/ponds), sub-surface drainage characteristics (e.g. presence of water table, springs, swampy areas, wet grass types, groundwater (such as presence of, depth to and any special conditions (artesian)), and possible presence of confined aquifer beneath site) and the likelihood of this occurring as a result of high rainfall events
  • Existing vegetation cover
  • Any existing site improvements (e.g. buildings, other structures, earthworks)

The results of all field and laboratory tests must be included in the geotechnical reports, including the location and level (including datum) of field investigations such as boreholes and trench pits.

Conclusions

The geotechnical report must include conclusions about the overall suitability of the land for the proposed development. These are to include clear statements on whether:

  • all existing/proposed lots are presently stable
  • all lots, and associated completed buildings (e.g. detached house) and infrastructure, will have an acceptable stability risk if constructed in accordance with the geotechnical recommendations - that is, a loss of life risk no greater than 1 x 10⁻⁵ per annum, and no more than a "low" risk to residential property in accordance with AGS 2007 Guidelines
  • any conditions need to be placed on the development of lot/s to maintain an acceptable risk of long term stability i.e. a factor of safety against slope failure of at least 1.5Leon15

Recommendations

The geotechnical report must include recommendations that outline the following:

  • whether the site has any history of landslips or instability
  • whether the proposed development (including all applicable lots and buildings) will alter the present state of stability of the site
  • whether any portion of the site should be excluded from the development and included in natural, undisturbed or rehabilitated areas
  • whether the proposed development (including all lots and buildings where applicable) will adversely affect the current state of stability of adjoining land
  • whether the proposed development (including all lots and buildings where applicable) should allow cuts and fills and if so, to what depth
  • whether retaining structures are required and if so, provide necessary foundation design parameters, including drainage and subsurface drainage requirements
  • whether any special design features are required to improve or maintain the stability of the site, or portions of the site (including each lot where applicable)
  • whether any special surface and/or subsurface drainage measures need to be taken to improve or maintain the stability of the site, or portions of the site (including each lot where applicable)
  • whether on-site disposal of liquids should be allowed
  • whether any follow-up inspections are required by the geotechnical engineer during construction

The recommendations must also provide guidance on appropriate measures required to make the site suitable for the proposed development, including:

  • preferred locations for buildings, other structures, driveways, etc
  • footing requirements such as bearing pressures, pile design parameters, special techniques for expansive clays, etc
  • pavement types and design
  • construction methods to avoid problem areas associated with loose materials and groundwater seepage
  • preferred excavation/retention/stabilisation techniques and suitability of excavated materials for use in on-site earthworks
  • surface and subsurface drainage requirements. Deep soil drainage within single residential lots or public land is not acceptable to Council
  • preferred methods of wastewater disposal
  • vegetation protection and revegetation requirements

Required certifications by geotechnical engineer

Formal certifications by a geotechnical engineer will be relied upon by Council to make judgements on the suitability of developing land for residential purposes and on approving stages of developments.

Certifications must be prepared by a geotechnical engineer (RPEQ), be addressed to Toowoomba Regional Council and be in the form of an engineering design certificate which include a description of project components covered by the certification, the basis of certification, reference documents, any exclusions and details and signature of the RPEQ.

Additional certifications may be required by Council for the following circumstances:

Reconfiguring a lot

With the development application

Certification that:

  • a stable building area exists on each lot
  • stable driveway crossovers/driveways can be constructed for each lot in accordance with Council’s Standard Drawings or AS 2890
  • each roadway cutting or fill can be retained or treated to maintain long-term stability
  • all necessary services (water mains, stormwater drains, sewer lines and the like) can be installed within the natural slopes or fills without detrimentally affecting the long-term stability of the natural or altered slopes
  • the proposed roadworks, services and house development earthworks will not adversely affect the natural seepage of water from the slopes
  • the proposed works, including provision of services, will not adversely affect the long term stability of the stable building areas

Operational work

With the development application

Certification that:

  • the operational work plans address all likely geotechnical risks associated with construction works proposed for the site, and that the works undertaken in accordance with the plans will not pose any significant risk to the stability of the site or adjacent properties
  • all cuts have been designed and analysed using appropriate soil parameters and the net impact of the cuts will not adversely affect the stability of the site or the surrounding areas
  • geotechnical assessments for all works covered by the application show no more than a ‘low’ risk to residential property

Building work/planning scheme works

With the development application

Certification that:

  • the building plans address all likely geotechnical risks associated with construction works proposed for the site, and that the works undertaken in accordance with the plans will not pose any significant risk to the stability of the site or adjacent properties
  • all cuts have been designed and analysed using appropriate soil parameters and the net impact of the cuts is such that they will not adversely affect the stability of the site or the surrounding areas
  • building plans are countersigned by the geotechnical engineer (RPEQ) if any special conditions/design features are required in the design
  • geotechnical assessments for all works covered by the application show no more than a ‘low’ risk to residential property

Further information

For further specific information on geotechnical report requirements, please contact Council’s development assessment – engineering section on 131 872 or visit your nearest Customer Service Centre.

Disclaimer

The information contained in this article is a guide only. This information has been prepared by Toowoomba Regional Council to help people gain an understanding of the Toowoomba Regional Planning Scheme. Please consult the Toowoomba Regional Planning Scheme for detailed information including maps (zones, local plans, overlays and priority infrastructure plan), provisions and policies. The content of this information article is not intended to replace the provisions of the Toowoomba Regional Planning Scheme.

Related document

INFO 045 Requirements for geotechnical reports information sheet 

Last Updated: Sunday, 02 July 2017 13:44
Back to top