Authors: Brett Dutton, Petra Stock
Before building a wind farm, it's essential to consider the properties of the ground on which the turbines will sit. If the ground is sandy, or rocky, these properties must be taken into account when designing the wind turbine foundations. If there are underground caves, the site may simply be unsuitable for development.
There are two stages to geotechnical assessment. Preliminary design is done at at the desktop level and should highlight any potential issues in the general geotechnical properties of the area. Detailed design involves the drilling of boreholes and can be bundled under the main supply contract if required. It is important however that the project doesn't become too far advanced before identifing geotechnical issues that could affect the project.
Why do a geotechnical assessment
Modern wind turbines are heavy and exert both static (dead weight) and dynamic (varying three dimensional) forces on the ground on which they sit. Most megawatt class wind turbines weigh in at around 200-250 tonnes so the foundation design is critical. The type of foundation used is dependant on the ground conditions at the locations where the turbines are situated, so part of any project feasibility study will be to ascertain the suitability of the ground conditions to bear the loads of the operating turbine.
Incorrect assumptions will result in significant cost later on so the better the geotechnical information you have about your site, the more reliable you budget estimates for the project. Initially, in the feasibility phase, it is worthwhile conducting a desktop study using a qualified geotechnical expert to assess the ground conditions in the area. The result of this study will indicate if you need to allow for additional supporting structures under the turbine, or if you can go with an 'off-the-shelf' design for your foundations. Siting turbines in a swamp or near the coast is a completely different story to siting turbines in stable, consistent ground conditions.
Desktop assessment
The aim of this phase is to identify if your proposed turbine layout locations are unworkable due to ground conditions. This study forms part of the project feasibility picture. Having said that, there is usually a foundation solution for most ground conditions and solving any issues is generally a matter of the cost.
Detailed assessment
Once the project commences implementation phase, more detailed studies will be conducted to enable foundation design and these usually come under the scope of the Turbine Supply contract. There are a range of activities to be undertaken, all designed to understand the micro-level conditions at each turbine (and substation) location, the details of which are summarised in the following paragraphs.
Ground testing will need to be undertaken at each proposed wind turbine location well before construction starts. The geotechnical testing falls into two broad categories:
- Tests to determine the load bearing capacity of the subsurface
- Tests to determine the electrical properties of the soil.
Load bearing tests
To undertake load bearing capacity tests, one or two boreholes (about 6 inches in diameter) per turbine location are sunk to a depth of typically 14m (this can vary if the ground is rocky). Soil profile core samples are gathered over the 14m length. The aim is to establish the subsurface conditions in the zone of influence of the turbine foundation and this will in turn determine the type of foundation used.
The analysis of the core samples is conducted by qualified geotechnical engineers. This is a critical civil preliminary design activity. A geotech company will will typically cost between $10,000 and $25,000 per turbine location to drill the boreholes and produce 'bore logs' (the records). More money is needed for the civil designers geotech engineer to interpret the results and produce a design.
Electrical properties tests
To test for electrical properties, two studies are traditionally conducted - Earth Resistivity and Thermal Resistivity.
Earth Resistivity tests are required to establish the electrical resistivity of the soil near the surface so that the wind farms earthing system can be designed appropriately. You need a well designed earth system to dissipate energy in the event of lightning strike. There are also electrical standards which specify the resistance between points inside the substation and between the substation and the turbines.
Thermal resistivity tests are conducted to establish the thermal resistivity of the soil and this information is a factor in the correct design of the underground cabling connecting the turbines to the substation (the underground cables are often referred to as reticulation). This is a critical electrical preliminary design input. This only requires small excavations and will cost around $40,000, depending on the size of the wind farm.
Geotechnical study for the development application
Sometimes a geotechnical study is required as part of the development application documentation. A desktop study and site walkover (by a geotechnical engineer or geologist) to assess geotechnical risks is usually sufficient for this purpose, but the scope may vary depending on the needs of local and state approval authorities.
Of course, if you have already undertaken detailed geotechnical studies for the proposed wind farm, you can use these in your development application.
Scoping the job for a consultant
When engaging a consultant to write a geotechnical report it is best to follow the tips and tricks in the article Engaging specialists for environmental & technical studies. Essentially that article suggests writing a tight scope and getting multiple quotes for the job.
The scope should generally include:
- A detailed description of your wind farm including turbine types or dimensions and their locations. If this is still unknown, it's best to include a 'wind farm envelope' — the largest area in which turbines could be sited
- The reason you are having the report written (e.g. to satisfy planning conditions)
- Who will need to read the report
- What you expect too see in the report (See 'what to expect' below).
Information a consultant will need
The inputs to a geotechnical study are typically:
- proposed foundation design
- GPS coordinates of all wind turbines; or
- GPS coordinates of the site boundary
- A digital topographical map with height contours at intervals of not less than 10 m, extending a minimum of 30 kms from the wind turbines
- The type of wind turbines to be used, or at least the dimensions (hub height and blade length).
What to expect in the consultant report
A good consultant report will be a stand-alone document with plenty of description such that it can be interpreted by somebody who is not a professional in the field.
A standard geotechnical report based on a desktop study and site visit should include:
- a description of the underlying geology of the site and suitability for supporting turbine foundations
- an assessment of the risk of earthquakes and volcanic activity occurring within the project lifetime
- recommendations for further geotechnical work required prior to construction, especially in relation to site-specific issues such as karst landforms, low strength soils
- an indication of likely excavation techniques for the site, whether excavators and backhoes, or rock breaking techniques such as blasting
- an indication of excavation stability, whether holes dug for turbine foundations will require support or battering
- earthing requirements
- an assessment of available material sources on site and in the local area, including materials onsite which are suitable for use in construction (for example as road pavement materials) and nearby quarries.