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?

 

[r13]

You need a experienced geotechnical engineer to assist you in determining subgrade conditions, including ground water control. Are you the plant engineer?

 

[enriko12]

Considering dead load only, I am getting SF=2.6 against overturning, 1,900 psf / 0 spf on the edges (4.2 ft bearing length). This is with 0 soil cover on the inside, and zero lateral earth pressure. Adding the moment from lateral earth pressure on the outside would somewhat help to reduce the eccentricity. The wall which was cast together with column pier should also take up some of the load.

The footing under the wall appears to be 3' 6" wide (blue color on the cross section), 1'6" projects outside. I am not very concerned about the wall itself, should I be? By excavating on the inside, we are removing active pressure from the inside and adding a smaller active pressure on the outside, which helps (insignificantly) to counteract thrust from the column.

"What are the consequences of staying within budget and having a project that is a failure?" Depends on what kind of "failure" are we talking. I do believe it will be a big improvement over the old slab, and it is the best what can be done giving the space constraints. Now, the last thing anybody wants is for the building to collapse or sink on 1 end. So, as long as we can prevent every remote possibility of this happening, I am happy.

The biggest issue I see as of now is removing earth cover on the inside of the footing, which will increase the eccentricity and pressure at the outer edge. Am I missing something?

P.S: retired13, yes, I've done a fair share of "simple" and right out of the code structures, such as residential, reinforced concrete, mezzanines, etc. Geotechnical is definitely not my strongest point.

 

[r13]

The reason for seeking geotechnical input is the fluctuation ground water, though it may have been considered in the original design, it could compromise the bearing strength of the soil when subjected to not so well planned excavation and temporary dewatering. The building can settle due to change in pore water pressure. Consult one, if you don't have one on your team.

 

[SlideRuleEra]

Without documentation of the existing building's foundation it's not reasonable to say that one feature or another is either "ok" or "unsatisfactory". The excavation is "large", considering the space, there will be loads on foundation components that they were never intended to see, and work in the excavation will take a while.

Working below grade always has uncertainty. This is as much a construction management project as it is design. "Someone" with experience who has authority to step-in needs to be present when the "unexpected" happens. There is already a good example on this project of not having real time oversight: The concrete contractor who removed the existing slab, creating the hairpin problem.

http://www.SlideRuleEra.net

 

[enriko12]

The pit is at 24" now, the guy removed not just the slab, but also sand and some of the dirt along with the concrete. Actually, I suspected possible hairpins, so told him "no cutting" in that area and to watch out for possible rebar and to carefully preserve it. I was hoping not to find them, that was wishful hoping, not sure what I was thinking.

We tried all possible channels to obtain the original plans, and failed. Remember, it was 40 years ago, no computers, AHJ destroyed paper prints 1 year after completion, and general passed away along with the original owner, and nobody even knows who the EoR was.

Not sure how much anyone can do without plans and soil reports, and if I had those, I could estimate bearing pressure more accurately and see if we are in any trouble. I already measured the frames, and analyzed them for the reaction. Even if we excavate at every corner of the footing and measure it precisely, still no details on reinforcement/connection/and whats underneath. Everything looks stable now, no movement, cracks or differential settlement. I might be overcautious and worried too much, not only the pad under the column, but the entire strip footing (3'6" wide) under the wall takes the load, so actual bearing pressures are much less. But its better to be cautious and think ahead instead of wishfully hoping (e.g. hair pins).

Any possible measures to take before and during excavation to mitigate risks? I am mostly concerned about the side with 6'8" wall. Front side (4' wall) has much lower eccentricity and bearing pressure, and we don't even need to dig all the way to the wall, and most earth cover can stay.

 

[enriko12]

The only risk mitigation thing I came up with so far is doing it in sections, excavating on 1 side of the column and back filling, and then excavating the remaining part. I am worried about compaction process even more than excavation itself, would not the force from plate tamper cause additional loads and potential settlement of the soil under footings?

Am I being paranoid and over conservative? Kick out with the dead load is only 2.1 kip, foundation wall also transfers the thrust to the adjacent columns with intact hairpins, so not all of the lateral load goes towards loading the footings and increasing eccentricity.

 

[SlideRuleEra]

Am I being paranoid and over conservative?

There comes a time when no amount of analysis or design skill can anticipate and solve field problems in advance. The fact that you are asking this question indicates the design has reached that point. It's time to hand control of field activities over to a qualified, full-time construction manager. If you have doubts it's because the budget does not allow for obtaining the geotechnical info needed. Instead of being "conservative", I believe you are taking too much risk.

http://www.SlideRuleEra.net

 

[r13]

Can you set the equipment on raised mat instead of underground?

 

[enriko12]

Not enough height clearance, that's why the idea was to go deeper

 

[enriko12]

We seem to have trouble with sourcing GW type soil in Indianapolis area. No one ever heard of it. Here is an example of whats available:

https://irvmat.com/AggBoardPrices.pdf

Is there a different name for it? Would #53 or #73 limestone work?

 

[SlideRuleEra]

seriko12 - That is a question for the geotechnical engineer.
For the last time - it's time for you bring in others who have experience and background you don't.

Good Luck.

http://www.SlideRuleEra.net

 

[enriko12]

Now you are giving me a really hard time. Yes, soils are outside of my area, that's why I am asking. Is not it what this board is for, so engineers can exchange ideas and experience to help each other out? Neither contractor no suppliers know what GW, GP, A-1-a etc stands for. They speak a different language and I need to specify the material they understand and which is available. And preferably I don't need to compare sieve sizes etc just to find that out.

Btw, I am still trying to convince the owner to approve budget for soil borings, so at least we know at what depth the good soil is at, and the bearing capacity near footings. Believe me, I don't want to do extra work, especially something I am not 100% comfortable with and with lots of unknowns. But sometimes its not up to us.