Nicklin Surveying & Resource Management - Land Development Consultants

Why is a Geotechnical report required on all subdivisions?

Most Council’s now require some certification that a suitable building site can be created on each new lot in terms of NZS 3604:1999 ‘Timber Framed Buildings’.  For an engineer to certify this, a site visit and field testing is required.  The engineer will examine surrounding slopes for stability and assess the bearing quality of the subsoils.  The inherent strength of Taupo ash derived sub soils in the Cambridge area is generally assessed as being weak and need remedial work prior to house building.  If a slab foundation is required, the engineers will often specify that subsoils are removed, replaced or re-compacted to bring them up to the strength requirements of NZS3604:1999.

The engineers tests for slope stability may require a setback from the tops of banks and specify that special foundation design is required if buildings are designed over the setback area.  Usually, this means that piles are placed at a deeper level, below any possible failure plane allowing for a suitable factor of safety.

Stormwater disposal is another of Councils concerns that was not attended to on past subdivisions.  The modern philosophy is to have the subdivision shedding stormwater at the same rate that would exist if there was no development on the site i.e. the “greenfields” flow.  This means that stormwater falling on roofs, sealed access and other hard standing areas is passed through a small orifice to restrict the rate of flow to equivalent rate greenfields rate.  This is usually accomplished by the use of stormwater tanks that are either attached to the side of dwellings, or may be buried in the ground.  

The outflow from these tanks is directed away from effluent fields and potential bank scouring.  As well as attenuating flows to the greenfields rate, the engineer will identify stormwater “overland flow paths” for larger storms that may have an impact on adjoining floor levels or other infrastructure.  This is the "stormwater management plan" requirement for subdivisions.

As well as geotechnical matters and stormwater control, part of the engineers briefing is to design a suitable effluent disposal system for the residential use.  Again, the surface soils are analysed and often a percolation test is carried out.   This results in a design capacity of the soil and a suitable effluent disposal system is selected in terms of AS/NZS 1547:2000 "On-site domestic-wastewater management".  Effluent disposal control in the Environment Waikato region must be in terms of their District Plan Water Module rules 3.5.7.4-3.5.7.7.  In many cases, an “advanced” effluent disposal system is required.  The advanced systems treat the effluent in special tanks to a much lower BOD content suitable for disposal by surface dripper lines and other means as an alternative to the traditional septic tank and rock-filled effluent trench.

Whilst some clients may balk at the additional costs of these geotechnical reports, in the light of building stability, earthquake engineering and flooding problems in recent years, it is little wonder that Councils have tightened their compliance levels for subdivisions. 

A geotechnical report is now an essential part of any subdivision application and most Councils will reject an application under s88 of the Act if they consider the report does not cover the necessary detail.

Nicklin Surveying & Resource Mangement can arrange a geotechnical report to support your subdivision.

NZ Land Transport NZ (NZLTA), have committed to start the Cambridge Bypass section of the Expressway or "the motorway".  The site following note has been taken from their website.

Construction of the SH1 Cambridge Bypass was indicated in Transit's current 10-year State Highway Plan and Forecast as starting between 2011 and 2016. This is subject to community consultation, Resource Management Act approvals, land purchase, and funding. Construction is expected to take three to four years.