Stone to Underside Wind Turbine Foundations.

[eoinzy2000]

We have been designing wind turbine foundations for some time now. I have seen some design of late, say in the last 12 months, where a 4-12 inch layer of Type 3 hardcore has been specified under the base of the wind turbine. This confuses me as I don't want to allow rocking to occur across my foundations. I can only imagine that placing stone with a bearing capacity of 50kN/m² over a ground with bearing capacity >100kN/m² would be foolish. There must be a reason however as I am seeing more and more competitors doing this stone detail. Is it at the behest of the turbine manufacturer so that less stress is developed in the turbine structure and more rocking/flexibility allowed in the foundation??? I would think that in wet conditions, the stone would penetrate the ground and eventually cause rocking and ultimately instability. I don't like it, but I also need to know what I am missing? If something was to fail, I would want the tower to fail, and not my foundations. Can someone explain the logic behind a layer of stone under the foundation for these towers?? I would greatly appreciate this.

 

[racookpe1978]

Only thing I can think of is that dumping loose stone into a deep hole is cheaper than pouring high-quality concrete into the bottom of the same hole.

 

[hokie66]

Where do you get the 50 kPa for hardcore? I would think if you only have 100 kPa in the natural soil, that is why the stone is being used...to improve the bearing layer.

 

[BigH]

4 to 12 inches of "hardcore" would not improve in any way shape or form the bearing layer.

You have not defined "hardcore - type 3" so I have no idea what it is but I would presume it is a crushed gravel or rock that is likely being used to "level" the foundation as the foundation would be quite wide. This would be as protection against precipitation/construction damage to an approved foundation and facilitate the placement of the steel mat for the reinforced footing; likely in lieu of using a mud mat (blinding concrete). As for "rocking" effects - if the "hardcore" is suitably compacted to 100% Modified MDD (which is what I would specify under an important foundation), I would be very doubtful that this thin layer of well compacted stone would have any detrimental effect.

My thoughts . . .

 

[hokie66]

Just to clarify, I didn't mean the stone would improve the bearing, but rather provide an improved working surface...as BigH said. But I still can't figure where the 50 kPa came from.

 

[eoinzy2000]

Thanks for your thoughts and responses. I get the fact that compacted stone may be used in lieu of lean mix/concrete blinding, but in this instance, I would be inclined to place a geotextile under the stone to avoid penetration into ground during wet periods. It is definitely a potential problem in the small 8-10m diameter bases on smaller turbines. Probably less of an issue on the big 20-25m bases. We will probably not be changing design in light of what I think is generally a poor detail, even if the potential for movement is deemed negligible. A mud mat/ concrete blinding layer eliminates all risk, as small as it may be with 2 inch stone anyway!
Also, I never specify hardcore under foundations. I either have founds cast on hard ground and dead build up, or lean mix of hard up to foundation formation level. I would specify compacted hardcore (max 800mm dp) up to u/s floor slabs. Maybe I'm over-cautious.
Thanks.

 

[cvg]

unless your "hard core" is large, open graded rock, than I would not see a need for a separation layer of geotextile. assuming more likely that "hard core" is a well graded crushed rock material commonly used as base for road construction, it will not penetrate the soil below during wet periods, which you already described as being very competent material capable of supporting >100kN/m². In fact, if the subgrade is well drained, granular material, I see no reason for any gravel or blinding layer. But if you feel better with a blinding layer instead of base material, by all means that is another very good option. This sounds like an option for contractor to do only if it rains hard and not necessary to be specified by the engineer.

 

[hokie66]

>100kPa is not all that competent. About 2000 psf. Probably not well drained, granular material.

 

[cvg]

I would say it is very competent subgrade.

the following materials usually have a code allowable foundation pressure of 2,000 psf and they are generally considered to be well drained granular material:
[ul]
[li]sedimentary and foliated rock[/li]
[li]sandy gravel[/li]
[li]gravel[/li]
[/ul]

 

[hokie66]

Things must be a bit different now than they were when I worked in the US. In Australia, we don't typically use "code allowable" or presumptive bearing pressures, but we generally consider 2000 psf as not too good, maybe a firm clay or loose sand. From geotechnical investigation recommendations, sandy gravel would typically be 4000 psf, and gravel up to 8000 psf. The most common prescriptive pressure found on engineering drawings would be 150 kPa, about 3000 psf, typically obtained in stiff clays.

 

[cvg]

I noticed that for instance, older versions of the Uniform Building Code list sandy gravel at 2,000 psf minimum however, some newer codes go with much higher values. these are allowable values with a built in FS

ICC - 3,000 psf
new york city building code - 8,000 psf

http://publicecodes.cyberregs.com/st/ca/st/b200v07/st_ca_st_b200v07_18_sec004.htm

http://www.nyc.gov/html/dob/downloads/bldgs_code/bc27s11.pdf