Wind farm job on 100m fill? 8

[martyno1]

Hi I am new to this so bear with me.
My company has been asked to look at the possibility of designing wind farms on an infilled open cast coal mine. Approximately 100m of granular backfill has been used and we have been informed that the fill has been compacted in 3m layers (unlikely).

Anyway all text and publications relating to turbine foundations discuss shallow pad foundations or shallow piles which are unlikely to work in this case. Has anyone heard of a job similar to mine or have deep foundation plans for wind turbines?

Thanks

Martin

 

[ishvaaag]

If they are asking you to rely in a badly compacted rock debris with ordinary footings, say no, you are there to protect the investment and safety, not to risk the bussiness. In all, go along with the design only if the bussiness plan allows for a proper foundation. The nature of ground will make difficult even for piles to be built. And ask from the turbine structures designers what amount of lack of verticality their structures can stand, maybe they can adjust for horizontality at the head etc, but neither ruin it by excesive inclination.

 

[hollowstemhead]

Have any test borings been performed through the body of fill?

 

[cvg]

from my experience working with alternative energy developers, both solar and wind powered units are generally proposed where either sun or wind is sufficient for economic power generation. In addition, the high cost of land acquisition and environmental mitigation can be a deal breaker for the developer. From this standpoint, landfills, old mines, floodplains and other areas that are not optimal for normal development are the chosen locations for alternative energy development sites. Virtually every site I have worked on so far was less than ideal for normal development. The cost of additional geotechnical investigation and / or large spread footings or deep piers may more than offset the cost of buying better land for development. Ishvaag brings up an important point regarding allowable tilt. This is an important design criteria for both wind and solar to maintain tracking of either the sun or the wind. Seismic stability is also quite important and vibrations must be kept low to prevent damage to very costly equipment.

 

[msquared48]

"Approximately 100m of granular backfill has been used and we have been informed that the fill has been compacted in 3m layers (unlikely)"

To what soil bearing value? What compaction value? What was it "compacted" with? All rock and no fines? Apparently no documentation...

Sounds like a lawsuit in the making if you accept this one.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[TDAA]

Get in there and engineer it. Just set the project up so you get the information you need.

cvg has it right. Consider it like an infill lot in a city. There is a reason that no one has built there (assuming the ground sucks), but now it is worth the effort to put something in, so you need to engineer around the problem.

If all sites had non-expansive, non-liquifiable, very dense sand, we wouldn't have much for geotechnical engineering, would we?

 

[martyno1]

Thanks for all the replies!

Unfortunately I have no information whatsoever regarding SI, strength characteristics or loadings for turbines. The land owner merely asked the question of my bosses and my task these past few days has been to research the topic. I see there have been a few cases most notably in the US.

I suppose with the furore over climate and renewable energy I will be working on thse projects more frequently within the coming years so all your input has been greatly appreciated. Especially TDAA wouldn’t be in a job if it wasn’t for problems like this!

Thanks again,

Martin

 

[civilperson]

Cast in place drilled piles/caissons will work if the horizontal subgrade modulus can be determined. Wind foundations are moment controlled, (a vertical beam with horizontal load).

 

[concretemasonry]

The wind turbine/generators are huge machines designed to last for many years. Even a single blade requires one long semi to deliver them to the site, let alone the entire structure.

Turbine manufacturers have very specific requirements (deflection/tilt) to insure the life of the long term investment and performance in the equipment.

Make sure you get the criteria from the turbine supplier, since that is the major investment.

Dick

 

[msucog]

wow could be very ugly especially if seismic potential and liquefaction are a possibility. if you move foward, i suggest utilizing deep profiling via geophysical surveys in addition to borings being performed. good luck! (and i suggest you have all structural/performance requirements provided to you in advance of making too many recommendations or even performing the exploration for that matter)

 

[cntw1953]

I think your client had simply asked: Can I build winfarm on a decommissioned surface coal mine in the costal area.
I would be surprised to hear "NO".

You shall do a research on the ground formation - contact local geological society to see if there is a pre-exist geological survey, the data can provide you with a rough idea on what type of action, and what kind of foundation system is anticipated. Best of all, try to locate the geologist for the mining company, and bring him on board.

Not necessary true, but usually you would have shallow rock bed rather than thick weak soil layers, this is your task to confirm.

 

[cntw1953]

Then, for 100 m of gravel fill, drilling could be difficult, but can it be strengthened by injection, or play the card of dummy soil theory to minimize the potential for settlement, cost of each, these shall be included in your research as well.

 

[Drumchaser]

Exhaustive subsurface site characterization would be in order for this potential barrel of quandaries.


Several full sized test excavations and a litany of borings/pens would be a good starting point...

The only foundations for windys I recall hearing about, other than spread footing/inv T, are the ones designed by AVEC in Alaska or was it Canada.

 

[Zambo]

Of course a very thorough SI is required, starting with investigation of the history of the site and boreholes.

If the SI is showing reasonable results then the next step could be 3 or 4 test bored piles. These could be 100m long if thats what the SI suggests is needed and can be instrumented and tested to failure.

 

[hokie66]

Zambo has it right. No point in guessing. A comprehensive site investigation and deep foundation testing program is essential.

 

[DRC1]

I have seen referencee to case studies for deep foundations for wind farms, but can not recall the reference. I would start with DFI. They may have papers in their journal, or can point you in the right direction. Some other sources would be Piledrivers.org (PDCA) Pile.com (GRL, large literature collection) and fellenius.net (also a large collection of papers)

 

[ranlgre]

I work with a small windpower company out of Reno. We've recently completed a job in N.C. with soil blow counts below 15 for the first 30'. Our solution was a foundation repair company based out of Oklahoma. They have dealers everywhere, but their headqaurters has a host of engineers that specialize in cases like this.

http://www.ramjack.com

http://www.solarwindfoundations.com

 

[fattdad]

with the correct site characterization and site improvement methods, you can do it just fine!

f-d

¡papá gordo ain’t no madre flaca!

 

[sunny7uc]

Loike fattdad said, site characterizxation and appropriate ground improvement method will do fine. You can insllaanchors, helical piers, micropiles, Dynamic Deep Compaction, etc. depending on the soils and loads. check out

http://www.haywardbaker.com

for different site improvement methods.

 

[chicopee]

Check out this article on the webb titled "Master's Thesis-Design of Wind Turbine Foundation Slab" authored by Pekka Maunu