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Choice of a ground improvement/reinforcement technique
2

Choice of a ground improvement/reinforcement technique

Choice of a ground improvement/reinforcement technique

(OP)
Hello Everyone!

I have the task of suggesting a suitable ground improvement/reinforcement method for onshore wind turbine foundations with known dimensions according to a specific soil type and its properties. The methods I study are piles, micro-piles, rigid inclusions, stone columns and vibrocompaction. I have to make a software guide where the user must insert values as input and the output should be the suggested technique(s). My problem is that I am unsure of what input values should I look for and how to connect them to a specific technique. The soil properties are given from a pressuremeter test. During my research I found out that the grain size is important when it comes to selecting a technique. However, I am sure that the Menard modulus, the limit pressure and maybe some other parameters that I am unaware of are important too. I have read multiple geotechnical reports but the authors just give a method, they don't explain why they chose this method. I would be very grateful if some of you geotechnical engineers out there could help me because I feel a bit lost.

Kind regards,

Boris

RE: Choice of a ground improvement/reinforcement technique

It would help if you told us the ground conditions, size of the foundations, loads etc.

Grain size is important when assessing stone columns and vibro-compaction.



Stone columns work best in soft clay. Cu from 5kPa to 50kPa.

RE: Choice of a ground improvement/reinforcement technique

(OP)
Thank you for the quick response!

I will do my best to give you all the missing data. The current wind farm that I am studying has two types of foundations:

Foundation type 1: Circular foundation with :
  • a slab diameter equal to 23.7m
  • pedestal diameter equal to 6m
  • slope equal to 2%
  • slab concrete - C35/45[/b]
  • pedestal concrete - C45/55[/b]
The foundation has a weight of 14443.06 kN and the weight of backfill is equal to 9683.65 kN, the water pressure is 7000kN. It has to be placed 2.3m - 4m maximum below the ground level. The soil in that level is limestone - marl type with a limit pressure (pl*)> 1.2MPa.

The geoengineer responsible for this project suggest a substitution of soil in that situation.

Foundation type 2: Circular foundation with :
  • a slab diameter equal to 16.5m
  • pedestal diameter equal to 6m
  • slope equal to 2%
  • slab concrete - C35/45[/b]
  • pedestal concrete - C45/55[/b]
The foundation has a weight of 9022.75 kN and the weight of backfill is equal to 3304.44 kN, the water pressure is 3500kN. It has to be placed 5m meters below the ground level. The soil in that level is mainly limestone.

The geoengineer suggest pile reinforcement in that situation. More precisely - 18 piles with a diameter equal to 0.8m Kv=500000 kN/m, Kh = 45000 kN/m. I believe this was changed after the cost study. The new choice was rigid inclusions because it was the cheapest solution. However, I would like to know why both methods can work in detail.

On top of both foundations a wind turbine with the following proprieties be mounted:

  • Kφ, dyn = 100000 MN.m/rad
  • Kφ, stat = 50000 MN.m/rad
  • Kh, dyn = 1000 MN/m
I hope I gave all the necessary details.

RE: Choice of a ground improvement/reinforcement technique

I'd check my insurance coverage. This idea of "cook - book" construct8ing where, apparently, there will be no geotech involved deciding what to do out there on the individual jobs or in the design phase, is open to all sorts of professional ethics problems. Leaving the geotech out of the final decision making, using only one form of site testing really is asking for big problems. I'd look for other clients rather than being stuck with this form of engineering. If there is no change in what appears to be a program leaving you open to all sorts of problems, don't say you have not been warned.

RE: Choice of a ground improvement/reinforcement technique

These arn't nice to have in your records.

RE: Choice of a ground improvement/reinforcement technique

Boris69 a few more questions,

can you tell us what the worst case vertical load, bending moment and horizontal load are and applied bearing pressure? I have too many questions to ask in order for me to do it my self.

Also, where does the water pressure of 7000 and 3500kN come from?

limestone - marl type with a limit pressure (pl*)> 1.2MPa - what is limit pressure, geotechnical allowable pressure (i.e. ultimate bearing capacity divided by a factor of safety, 3 for example or may be an allowable pressure based on a settlement tolerance)

The geoengineer suggest pile reinforcement in that situation - do you mean piles are to act as rigid inclusions or piles to act in the typical manner as just transferring load to a more competent layer(s). I would think that piles to 4m deep where rock is wouldnt be too expensive? It would probably offer the best foundation option in terms of settlement and bearing capacity performance?

RE: Choice of a ground improvement/reinforcement technique

(OP)
@PEinc I know this site. I found it during my research. However, I was wondering if the selection is applicable for wind turbine foundations? When I was reading the information on it I thought that the selection is suitable for road/railway application only. It would be great if you could clarify that for me.

@oldestguy Don't worry. Let me explain my situation a bit more. I am just a student who is doing an internship in an agency that is not specialized in soil improvement/reinforcement. The agency wants from me to suggest a suitable technique for whatever type of soil at first, then I have to make an estimate of how, let's say piles or columns should be used, what length and diameter and so on according to Eurocodes 2 and 7. Finally, I must suggest a final cost for each technique. The method with the lowest price is chosen in the end. I cannot be super precise in my calculations and suggestions because if I ask geoengineers/ engineering consultants in my area how exactly do they choose a technique and how they calculate the dimensions and so on they would either give me some super vague answers or they would just say "It's a company secret". Having said all this, I am sure that you understand that my suggestions and calculations simply cannot replace the final word of an authorized engineering consultant. I believe that my agency just wants a sort of a guide to "double-check" the work of the consultant they work with when it comes to soil improvement.

@EireChch I will give you the loads as soon as I can.

I am not 100% sure about the water pressure. I believe it comes from the load case testing with partial factors at the tower base.

The limit pressure is the rupture pressure of the soil. It goes hand in hand with the Menard's modulus. Those values are measured during the pressuremeter test (photo).

At first,the geoengineer suggested piles as in the typical manner. However, I believe an engeneering consultant suggested rigid inclusions instead as the project developed to reduce settlements. The geotechnical and cost study showed that rigid inclusions will indeed work and that they are the cheapest option so the client choose that method.

In general could you please tell me what type of data do you look for when it comes to soil improvement method selection? I have plenty of information but it is difficult for me to choose the most important points to make the selection.

RE: Choice of a ground improvement/reinforcement technique

(OP)
Thank you all for the pointers.

RE: Choice of a ground improvement/reinforcement technique

Each ground improvement method has its limits in terms of magnitude of load, thickness of layer to improve, grain size distribution, etc, etc. Each site is a particular site. Hence, each project is a prototype and you will not be able to replace engineering judgement by an Excel spreadsheet. Ground improvement contractors will be more than happy to provide you with good free advice for each project !

RE: Choice of a ground improvement/reinforcement technique

This program you have going sounds very fishy. I'd put a big capital lettered statement on every page that "the writer makes no claim as to the safety of these statements or conclusions". Even as a "student project", this can result in misuse.

RE: Choice of a ground improvement/reinforcement technique

Without any disrespect intended, it sounds like the company is just giving you something to keep you occupied. That's good, a lot of time we have students and they spend more time photocopying and filing because people are too busy to try and explain stuff to them. Hopefully you will get some mentoring.

While this task will give you an opportunity to get an introduction to ground improvement, I cant imagine that you will be able to develop something that will be of much use. You are essentially undertaking preliminary design of a number of ground improvement and undertaking a cost analysis. Thats a pretty big ask for a student engineer.

Getting to a level where you are able to undertake a preliminary design and have confidence that your design is "close enough" would take a lot of learning time. In order for you to assess whether a ground improvement method is suitable for a given soil/site you need to understand the method fully.

We all probably sound a bit negative but the responses you have got are from experienced engineers so they are of of value.

It may be more advantageous if you post a soil profile for one site, we can then comment more specifically on the suitability of ground improvement applicable to that site. If you start undertaking some preliminary designs you would most likely get some good feed back.

Hope that helps.

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