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  • James Brokenshire

The Why of Engaging a Geotechnical Engineer

Introduction


Councils require applications for building consent to provide sufficient information to enable Council to make a decision on granting a consent.


This information also often requires geotechnical documentation to support the consent process. 


Background


Geotechnical engineers carry out shallow investigations, testing the ground and reporting on the suitability and bearing capacity of near surface soils to support building loads.


NZS 3604:2011 is a guidance document for timber framed buildings and has an emphasis on establishing “good ground,” which is soil with an ultimate bearing capacity (UBC) of 300 kPa.  “Good ground” is not the minimum criteria to support building foundations, however soils with lower bearing require appropriate investigation and specific foundation engineering design. 


NZS 3604:2011 has other criteria which must be considered, such as is there uncertified fill underlying the development area, are expansive soils present, is sloping ground close to building foundations etc


Ground Testing


The geotechnical engineer will undertake ground testing suited to the ground conditions encountered.


Two in ground test methods are described below and it is likely that a combination of both tests will be used to test the soils.


Coarse grained soils

In coarse grained, granular soils, such as sands, in ground testing comprises of a Scala penetrometer testing.


Fine grained soils 

In fine grained, cohesive soils, such as silts and clays (including Ash soils), in ground testing comprises of hand held shear vane tests.


Objective of Site Investigation


The ground on which a building is built can be quite variable.


A well-devised, specific site geotechnical investigation is a key requirement for good building performance and mitigates the risks of unsatisfactory ground performance.

The understanding of site soil conditions is an essential input into robust foundation solutions.


A ground investigation serves two primary purposes, by:


Providing data for analysis and design and reducing uncertainty about ground conditions, thereby reducing the risk of construction cost variations.


Recognise that site investigation is usually money well spent and is at a modest upfront cost compared to the cost of the proposed development. 


Also, often an appropriate site investigation will lead to a more economic foundation design solution, reduce risk of  project delays and reduce risk of post-development building performance issues, so the geotechnical report will pay for itself.


Use an appropriately qualified Geotechnical Professional to tailor the site investigation objective(s) to the project’s requirements and risks.



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Shire Geotechnics Ltd     /  Auckland /  info@shiregeotechnics.co.nz

Ground Testing