Sub-grade preparation for Vibration Isolation

[enriko12]

We are building a vibration isolated slab for sensitive scientific equipment. After reviewing a lot of literature, I see that anywhere between 1 ft to 8 ft of engineering fill were used, sometimes geo-textile fabric was also mentioned, but no details. I feel like its more of an art than a true science and would love to get some opinions on what would make sense and what would be a questionable investment.

So basically we are pouring a 12x12 ft slab, 4ft thick. Idea is to keep low frequency vibrations to the minimum. Equipment will be further installed on pneumatic vibration isolators, so that will take care of higher frequencies fairly well. Soil on site is some sort of clay with traces of organics. Main source of random vibrations is local road with occasional truck traffic, approximately 140 ft away. Also some mechanical equipment (e.g. chiller, rotary air compressor) is supposed to be installed as close as 20ft away. I planned on putting those on springs insulators and on a dedicated 12" thick slab

Would it make sense to excavate an additional 4 to 6 ft and fill with compacted gravel, geo-fabric, etc, or 1 ft of gravel would be just as good enough? Would gravel be the best material to use for the fill?

 

[HTURKAK]

A sketch and more info . regarding the sensitive equipment will be useful ..

I assume the the sensitive equipment is not vibrating ..the concern is the other vibrating eq!s such as chiller, rotary air compressor ..and truck traffic, approximately 140 ft away..

I will suggest to use PU filled trench at the perimeter of the sensitive equipment foundation. Just write ( soil vibration isolation using filled trench ) and search the web, some of the docs;

https://ir.lib.uwo.ca/cgi/viewcontent.cgi?article=1378&context=etd

https://scholarsmine.mst.edu/cgi/viewcontent.cgi?article=1547&context=icrageesd

 

[SlideRuleEra]

seriko12 - The laboratories at our generating stations (for testing coal, ash & water quality/properties) are on a portion of large slabs-on-grade that are nowhere near 4' thick. There is plenty of vibration from multi-hundred megawatt units operating at full output. What the labs do have is excellent subgrade.

Maximizing vibration isolation depends on slab "stiffness" and the best possible properties for subgrade. Slab stiffness is proportional the cube of slab depth. Also, the slab is subject to both vertical and horizontal vibration.

For a 12' x 12' slab, 4' thick concrete seems far more than needed. IMHO, going to 2' (or so) thick should be more than enough and will reduce the cost of constructing a (mass concrete) 4' thick slab.

Spend the money "saved" on concrete for plenty of top-quality subgrade. The best material for high subgrade is clean, well-graded gravel (Soil Group GW). Should be able to obtain, say, k = 500 psi/in. I would use at least 2' of subgrade under and around (see sketch below) the slab and use a geotextile fabric under the subgrade separate it from underlying soil.

Additionally, for all backfill on the sides of the concrete slab use the same subgrade material.

http://www.SlideRuleEra.net

 

[r13]

If vibration is the focusing concern, wouldn't the construction material shall have a low shear wave property?

 

[enriko12]

Yes, only external vibrations are of a concern. Room needs to meet at least Nist-A vibration criteria. Equipment is somewhat heavy (~15 tons) and placed closer to 1 side, hence another reason for a much heavier concrete slab is to minimize effects of a heavy eccentric load.
Filled trenches would not be practical in this case, as those have to be deeper than Rayleigh wavelength to be effective, so depth required to isolate low frequencies would not be reasonable.
I attached a sketch below, there is also a 2ft drop on 1 side of the slab, and 2nd side is in close proximity to the building foundation.
I planned on adding a moisture barrier on the bottom and painting sides with waterproofing mastic, do you guys see any problems with that? Slab will get a coat of epoxy paint, which does not like moisture very much.

 

[r13]

For heavy eccentrical load situation, and such close to building foundation, wouldn't pile will be better solution, ideally friction piles. I don't think gravelly fill is good idea for this case.

 

[SlideRuleEra]

Room needs to meet at least Nist-A vibration criteria.

You should consult a specialist in vibration isolation.

The proposed design conflicts with the basic principles of vibration isolation that I'm acquainted with.
Slab foot print is "small" (12' x 12')... it should be "large".
Slab is "thick" (4')... it should be rigid, but not massive.
Slab has high eccentric loading.

I want to be clear on two topics:

1) High subgrade modulus is key to vibration isolation. The highest subgrade modulus is obtained by using the correct material for that purpose... clean, well-graded gravel (Soil Group GW).

2) Unless a qualified specialist recommends piling, they should not be used. Piling can act as an antenna to collect and transmit external vibration directly into the foundation.

http://www.SlideRuleEra.net

 

[r13]

IMO, damping is more important than rigidity for this case. Expert opinion/advice on vibration isolation is definitely required. Here is an article maybe helpful, as a starter. Link

 

[enriko12]

Larger mass lowers natural frequency, so I would think that the more the better.
15 ton load 3 ft off center on 85 ton slab is not necessary a problem, but on 10 ton slab, eccentricity will probably be a huge issue
I was wondering if piles would pickup more vibrations, the only design example I saw using piles was where they were a few ft deep and reached all the way to the bedrock, which was very shallow (not the case in this area).

 

[r13]

I don't think bearing (to rock) pile is good choice, use friction pile if soil condition permits.

 

[SlideRuleEra]

seriko12 - So your client has an objective criteria (NIST-A) to meet, and you are going to base a design on what you "think" and what someone on the internet guesses. Good Luck.

http://www.SlideRuleEra.net

 

[enriko12]

Sometimes you have to work with what you have and under strict budget constraints. It is much cheaper to add extra gravel and pour extra concrete than fly a consultant across country at a $10k+ cost who still will not guarantee that certain vibration criteria will be met, even if all of their recommendations are strictly followed.

The paper "Design and performance of an advanced metrology building for MIKES" has the following details for the vibration criteria sensitive rooms: "The floor of the dimensional laboratories on the first floor with vibration specification V1 consists of a 1 m thick concrete block (top), a 50 mm thick vibration isolation mat, a 400 mm thick layer of gravel and a 400 mm thick concrete plate which lies on rock."
"For underground laboratories with vibration specification V2 (sticter than V1) the lowest parts of the vibration isolation structure are concrete raft foundations, which were cast directly on rock. The vibration isolation consists of pneumatically suspended heavy concrete platforms. The sizes of the platforms are approximately 4.0 m x 7.7 m x 1.2 m (70 metric tons). Platforms are suspended by six pneumatic vibration isolators"

In our case 15 ton equipment will be suspended by pneumatic insulators, so it kind of plays a role of a platform used in V2 rooms. And while bedrock makes an ideal subgrade, in this area bedrock is ~ 200 ft deep, so it would not be feasible. I am sure anything will be an improvement over existing 5" slab on grade which is not insulated from the rest of the building and vibrates as people roll carts over or even simply walk nearby

 

[enriko12]

What do you guys think of an idea of keeping the slab lower and installing equipment on columns/pedestals? I've seen that done, and the lower the slab the quieter it will be, however I am not sure of the effect pedestals will have, especially on horizontal component.

 

[SlideRuleEra]

1) Sometimes you have to work with what you have and under strict budget constraints.
...in this area bedrock is ~ 200 ft deep...

2) Room needs to meet at least Nist-A vibration criteria.

3) I am sure anything will be an improvement over existing 5" slab on grade which is not insulated from the rest of the building and vibrates as people roll carts over or even simply walk nearby.

1) I can identify with those conditions.

What is the requirement?

2) NIST-A?

or​

3) "Best efforts" project?

One more question for now... Is this vibration isolation slab to be constructed inside an existing building?
If so, the floor plan of the building will be nice start to see what constraints there are. (A good dimensioned sketch should be sufficient.)

http://www.SlideRuleEra.net

 

[enriko12]

The truth is they don't know for sure as it is an experimental equipment. So "Best efforts" under budget and space constraints would be the best way to describe the requirements. It is inside of an existing building with not much room around. There is around 4 to 5 ft on each side of the proposed slab available for the backfill, and there is an exterior wall 5ft away from one side (shown on the sketch above)

 

[SlideRuleEra]

Very good, you are going need an accurate drawing of the interior of the building to define where the slab is to be located. If you don't have one, create it. Show the building's footings, including how far below the floor they are located and how thick they are.

What are the reasons for the 12' x 12' x 4' dimensions of the slab? Can those dimensions be changed, if needed?

http://www.SlideRuleEra.net

 

[enriko12]

4' depth/thickness can be changed, I might reduce it to 2'. especially if we decide to go with equipment pedestals like on the last sketch. 12x12 not so much, may be extend to 14x14 at most but not less than 12x12 due to machine size, control cabinets and operator space. The exact footing size/thickness is somewhat troublesome, as it turned out close to impossible to locate the original prints for this building.

 

[SlideRuleEra]

The exact footing size/thickness is somewhat troublesome, as it turned out close to impossible to locate the original prints for this building.

That is precisely why I've told you that an accurate drawing of the building interior/footings is needed.

Don't be concerned about thickness at this time.

Adjust the geometry of the slab to see if you can accomplish three things simultaneously (this is just the beginning, not the final design)

1) Have the centroid of the slab coincide with the 30 kip equipment. (Suggest using kips instead of tons... Engineering Tips has users worldwide. There are many definitions for "ton"... short ton, long ton, metric ton...)

2) Have the slab, plus an allowance for gravel on all sides, fit within the building.

3) None of the slab / gravel allowance of the sides is directly above the building's footings.

Projects like this have to be designed "backwards". The design is not about what you want, it controlled by what is practical, based on engineering principles.

You are going to perform the design, not me, but I can walk you through the steps to do it. I had a career doing projects like this at our electric generating stations.

http://www.SlideRuleEra.net

 

[enriko12]

#1 is nearly impossible to achieve. 30 kip machine is 6'x 12' and has to be offset to 1 side, so there is room for the operator and control enclosures
Regarding #3, the way I have it now, the slab is only 1.5' away from one of the footing, is there a problem if gravel allowance extends a little over the footing, assuming there is still ~2ft of dirt above footing before gravel?

 

[SlideRuleEra]

#1 is nearly impossible to achieve. 30 kip machine is 6'x 12' and has to be offset to 1 side...

I've been looking at the geometry, too. A "perfect" solution to #1 may not be practical, but I believe there is a way to greatly reduce eccentric loading on the slab from the 6'x12', 30 kip load.

Concerning #3, don't want load from the new slab added to the footing for the building. Best for there not to be overlap. Also, if top of the footings is not far below the surface, the slab/subgrade may have to fit between the footings. You have to think of the design in 3 dimensions... all 3 at the same time. There's a way around this... you can make some compromises, remember this is a "best efforts" project.

See if you can get the inside dimensions (length & width) of the existing building.

I'll get back to you tomorrow.

http://www.SlideRuleEra.net