Waiheke Island – Ignore its geology at your peril.

Aug 18, 2020 | Insights

Obligations, requirements and hazards, with GWE Consulting

Any Development on Waiheke Island provides unique challenges at all stages of building; from concept planning, subdivision and land development through to detailed design. Our beautiful landscape of rolling hills, seafront properties or steeply sloping sites require careful consideration of the geotechnical constraints which may cause undue time or cost extensions to project deliveries if they are not identified early in the build process. But where do you start with geotechnical engineering? What are your obligations and requirements as a developer and what are the main geotechnical hazards present on Waiheke Island and the Gulf?

Waiheke Island Geotechnical Challenges

New Zealand includes some of the most challenging terrain to develop on,  subject to seismic hazards, slope instabilities and in some places, significantly deep profiles of soft soils.  Waiheke Island, like New Zealand as a country, has it all – beautiful landscapes and the full array of geotechnical hazards to go with it.  Our coastal sites are sometimes susceptible to the effects of liquefaction, while our steep slopes are subject to instabilities and ground movement.  Most likely you have seen evidence of this while travelling around the island such as terracing, hummocks or washouts.  

Our island is special and so our developments require a unique set of engineering skills to match its environment. At GWE we have provided effective and adaptable engineering built upon years of service and experience with the island.

Ok. I want to build, but what are my geotechnical requirements?

The level of geotechnical engineering service required depends largely upon the stage of your development. At concept planning or pre-purchase due diligence stage it is a good idea to understand the constraints of developing a site and what extent of geotechnical engineering works and design will be required to get out of the ground. GWE can provide preliminary advice or a written report outlining the development constraints you may encounter.

written report outlining the development constraints you may encounter.

Once you have development or subdivision concepts ready for Resource Consent, it is a perfect time to kick off your site-specific geotechnical engineering assessments.  To achieve Resource Consent you are required to provide stable building site(s) for development.  This generally requires geotechnical input such as a feasibility report, and some form of assessment such as slope stability modelling where there is evidence of instability or where sites are generally >20 ° such as within close proximity to steep gullies or drop-offs.  In this situation you may be required to build a retaining wall to stabilise a building site or let our fully qualified engineers provide advice on the best location for development with minimal cost and retaining requirements.  Sometimes this may be as simple as moving the proposed building site.  At this stage we normally adopt some preliminary geotechnical investigation principally with hand tools to minimise disturbance of the site yet guide and provide sufficient information for the Resource Consent assessment process.

Parts of the investigation may be deferrable until the Building Consent stage which generally requires detailed geotechnical investigation, reporting and in some cases design of retaining walls, ground anchors or deep foundations to ensure your dream home or development can stand the test of time.  It is a good idea to undertake this once you have finalised your development plans for Consent.

That may still sound quite complex and overwhelming, just to get your house out of the ground.  In fact, a staged approach to geotechnical assessment is a safe and relatively inexpensive way of discovering your development possibilities, limitations, risks and goals without the shock of unexpected expense down the line.  A little time to understand the site and obtain reliable data early in the process gives large gains over the project development cycle.  Our special offer for the month of July is a great way to get started with this.

Let’s put geotechnical engineering into perspective

GWE has been performing geotechnical investigations and providing reputable advice on the island since 2010.  We see the full array of geotechnics, from flat and stable sites to steep sites requiring extensive investigation and accurate modelling.  

In recent years, one of our most challenging projects was for a proposed four storey luxury home overlooking Onetangi Beach, Tobias Francis, GWE’s Senior Engineering Geologist and Geotechnical Manager says. 

The objective of the investigation was to establish the in-situ soil and rock strength parameters for the design of a combined ground anchor retaining system to stabilise the slopes above the site of the proposed home.  The work on the steep site (up to 45 ° and a 30 m elevation change from front to back) involved the preparatory work of hand digging four platforms to allow the establishment of a helicopter portable drill-rig “Heli-Rig”, at selected locations up the slope. Drilling was undertaken to recover soil and rock samples from depths of up to 20 m beneath the site.

Groundwater drawdown effects of excavation.

Establishing and disestablishing the drilling rig, generator, health and safety equipment and other resources at each of the platforms was only achievable with the use of a helicopter. Personnel abseiled into each platform site with the assistance of ropes attached to an access point higher up the slope on the ridge crest. The investigation team spent two to three days at each site recovering and testing soil and rock samples and only briefly stopping to take in the fantastic views over Onetangi Beach, and the Hauraki Gulf beyond.

The geotechnical solution required a comprehensive soil/ rock anchor system to provide geotechnical stabilisation of the large excavations into the very steep relic cliff slopes, which comprised old landslip soils and rock boulder outcrops. Due to the steepness of the site and the presence of these older geological features, a permanent debris catch fence was designed and installed to protect the building platform from the potential inundation of soil/ rock/ vegetation flowing downslope from the un-touched and un-stabilised slopes above.

The un-serviced site also required a multi-disciplinary collaboration for the treatment and onsite disposal of wastewater and for the detention and discharge of stormwater from the site, without affecting the stability of the engineered batters around the building platform. This is an example of a complex site that required equally creative solutions by GWE to develop the results required by our Client and their Architect.

A “Heli-Rig” was required for the Onetangi property.

Special Offer

We all want easy access to sufficient potable water to make our lives comfortable. But we should also have a conservation ethic that stops us being wasteful with this precious resource. This is why the clever design of your plumbing system and the correct selection of water-saving fixtures (showerheads, toilets, taps and washing machines) minimises water use while at the same time providing for all your needs. A site-specific water balance study for your development will help you decide on the volume of potable water storage required versus the unpredictability of experiencing water shortages in the future.
The consequential, beneficial effect of minimised water usage is the reduced amount of wastewater that is piped away from the house to your onsite wastewater treatment plant and irrigation system.
The less water used the smaller the wastewater system needs to be.

Wastewater volumes are generally determined by a rule of thumb method based on the number of beds in the house. But volumes can be exceeded during summertime when your city friends and family come to stay! This additional volume can be detrimental to the operation of the wastewater system and lead to partially treated effluent being discharged to the environment causing issues in local watercourses or even further afield.