Founding conditions for high res microscope

[soileater]

Does anyone have any advice for the founding conditions for a high resolution electron microscope. We have encountered shallow quartzitic rock in test pits. The conceptual design is to place a 1m thick RC base on 350mm sand. Anything we sould be aware of? How do we determine severity of ground vibrations?

 

[BigH]

Why do you want to place 350 mm sand? I always found that a sand base - if river sand - will turn to ruts when trying to pour concrete on it. Of course, it can be done if careful. But if you have good bedrock so close, why not found directly on it? Use dental concrete to develope a flat surface for your formwork and chairs for the steel.

 

[soileater]

Thanks for reply BigH...The idea behind the 350mm sand is to reduce vibration from the rock. Surely if you found on rock, the ground vibrations will interfere with the microscope??

 

[steve1]

A long time when I was in college I assisted a professor who was determining the acceptatability of putting an electron microscope in a building. I recalled that we set up accelerometers on the building and recorded the response to various external inputs (passing of a subway, passing of a tractor trailer, etc). Based on the response that we obtained he determined that the building was not acceptable. I remeber that he compared the accelarations to a set of allowables but I don't remember the details.

 

[kslee1000]

BigH:

I agree, without confinement, the sand is a no no.
I have no clue about the hardness of the rock he has encountered. However, if it possible to excavate an area larger than the foundation pad, and fill the excavation with sand, or lean clayey material, do you think this might work? Could drainage be an issue?

 

[soileater]

Here is an attachment showing what is proposed. the loads are small (single storey buiding) but the vibrations are the main concern.

 

[kslee1000]

paton: see attached for some suggestions.

 

[hokie66]

What is the source of the vibrations you are concerned about? If this microscope is in an open field, it would be best to found on the rock without any isolation. If it is adjacent to a major vibration source such as a railway, you may be able to use vibration isolation in the form of rubber bearings. But you will need advice from someone who knows more than I do. I don't think your sand layer does anything beneficial.

 

[soileater]

Ground vibrations are very low amplitude waves that are transmitted through solid mediums and travel particularly well through rock. They are continually present and not easily detected. These vibrations affect high resolution electron microscopes which are focussing on things in the nano-scale. The idea behind the sand is to introduce air voids which will attenuate the vibrations, but I need more ideas. Aren't there any vibration engineers out there??

 

[kslee1000]

I think your problem is more to geophysics than engineering. One suggestion is like others have suggested -set foundation directly on the soild rock, and find/use suitable material in between microscope base and the soild foundation to eliminate/dempen the vibration. That should be more practical and easier to chieve.

 

[csd72]

This is at the extreme end of vibration control and I would strongly recommend that you talk to a vibrations expert. Otherwise your company will have problems.

Convince the client to get someone on board as a consultant.