Field Investigation for a Wind Turbine

[YounEng]

For a wind turbine with about 150m length, and a planned foundation B x L = 20m x 20m, is a drilling depth of 40m enough? I looked into field investigation notes online but didn't find a specific one for such a structure. Any engineer that has experience with such field investigation?

 

[EireChch]

I am guessing here but i presume you would be pilling such a tall structure, as it may have some very high tension, compression and moment loads?

Eurocode 7 requires investigations to extend to 5m beyond the base of the pile. 35m deep pile for a 150m high turbine sounds ok to me? Again its hard to know. What are your loads, expected geology etc?

 

[MotorCity]

What does your foundation analysis indicate is the required depth? Surely you cannot base the depth based solely on what you find online, right? What are the recommendations of the geotechnical engineer?

 

[YounEng]

Weak rock is mostly expected, but gravel and clay could also be faced. The moments should be around 10000 to 20000 kNm, while the vertical force should be around 10000 kN.

 

[YounEng]

The engineer hasnt had his say yet, however the client requested an investigation depth of 15m, which is very unreasonable.

 

[SlideRuleEra]

YounEng - Find out what type foundation the client has in mind. A rotor diameter of 150m is a fair size machine, maybe 6 to 8 megawatts. On good soil, shallow spread footing foundations, maybe 2m thick, are used for wind turbines. Perhaps a field investigation to 15m depth is not too unreasonable a suggestion. I have not worked on a wind turbine project; there are photos from the web:

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[jcm1890]

YounEng,

Having worked on a number of wind farm projects with base sizes of the order you suggest I would note that the depth of investigation required is highly dependant upon the ground conditions and whether there are any perceived underground risks e.g. cavities due to mining or highly erodible rock masses.

On the assumption that such cavities are not expected and the risk is therefore sufficiently low:
I would suggest that boreholes are specified to depths of up to approximately 40m as you suggest (2xDiameter) this should be sufficient to determine the required settlements and stiffness's with confidence. Where rockhead is encountered before this depth, such a depth would not be required - our current practice is to terminate boreholes after 5m of continuous rock coring, if prior to the specified termination depth - at this point it is reasonable to assume that rock is continuous beneath this (i.e. not a boulder).

Should no rock (or deep) be encountered within the borings, it may be that some boreholes could be terminated at shallower depths, with deeper strata properties inferred from nearby locations in order to reduce the GI costs.

Hope this helps,
J

 

[oldestguy]

My "rule of thumb" for any footing type foundation is investigate to the depth where the pressure increase due to the footing is 10 percent of the existing pressure. The geotech should be able to provide that figure. Knowing the approximate depth to the highest water table elevation helps. In temperate areas take that as at the ground surface if in doubt.

 

[Okiryu]

OG comment is the basis for your initial found of drilling to 2B. But you should be careful if there is a potential for deep soft deposits at your site. I would check with local drillers to have an idea of the site geology.

 

[fattdad]

the OP doesn't provide sufficient information for any opinion other than 2B or 10%.

As a geologist first and geotechnical engineer second, I just want to say, we are building these things in real places in the real world. Geology is essential to answering such questions. Is it in an area of prior glaciers? Is it underlain by limestone? Is it in the Coastal Plain? Are prominent structures typically placed on piles? Is it in the Piedmont?

I mean, how should the internet answer such OP?

2B or 10% is quite consistent with general practice; however. . .

f-d

ípapß gordo ainÆt no madre flaca!

 

[oldestguy]

Looking at this in another way, if a wind generating structure tips over, will any one be injured or killed? Other structures damaged? Likely not. Sure some expense, but same goes for any electric utility feature such as transmission lines. Where do you go ahead and get it going?