Isolated Slab/Footings - Urgent help required please

I hope this is not a break of protocol, but I urgently need some guidance on a question I asked in my other thread on Saturday (New music room extension – Hobart, Tasmania), but I have had no response (not viewed yet! :( ) I suspect that may be because the experts in isolated slabs may not have read past the early part of my thread where I originally planned to use a wooden floor (I have since been convinced by Stuart to use an isolated slab). My urgency is that I have a builder ready to start in a month and I have yet to get engineering drawings to council. I need to clarify the details of slab isolation before I go to the engineer. I have read everything I can on the subject, including THE book. My questions relate to one raised in the thread: Different concrete for different rooms? by sword9 In that thread, cadesignr posed this problem:

The only thing I wonder about is if the slabs have to be "keyed" to keep individual slabs from sinking or misaligning height wise over time maybe. I don't have a clue how you would do this though, and keep them isolated.

As far as I can see, that question has not been answered. This is very relevant to me because I will have one side of my room on very stable rock and the other side on about 1 metre of black clay, which has already contracted somewhat due to drought in Tasmania and may yet contract further. The peripheral foundation strip will require piers to a solid base below the clay, but the slab will be floating on the clay itself, so the prospect of relative movement (settling of the slab) is real. The problem is that anything that could stop the slab from settling would end up with a considerable proportion of the weight of the slab and the inner room on it, so it would have to be a seriously engineered structure. I came up with the following idea where the slab sits on top of the perimeter foundation but isolated by a layer of sand between them. (My builder originally came up with the different idea of creating an isolating void by dissolving a styrofoam layer between the slab and the perimeter footing. This doesn’t solve the settling problem but may be a useful contribution to the forum, since I have seen burning of styrofoam suggested here. Both ideas are detailed in my other thread. I won’t duplicate them here. ) Questions: 1. Has the problem of relative movement already been solved elsewhere? 2. Will my plan work i.e. will the sand will provide acoustic isolation when under compression with a slab on it? 3. Is there a better way of doing it? and I suppose I should add: 4. Is there a better (more appropriate) way to get urgent help on an unanswered question on this forum? (Still a newbie) Cheers Simon

Hey . Ive been in the process of building my own studio. Small amount of space but im happy with whts coming off it. Im working on an isolated slab principle which i kinda fugured out myself in conversation with my contractor after having got some good advice ont he forum from very helpful ppl. I moved from my idea of having a 2 foot deep concrete reinforced floor with steel rod to a 10" layer of clay covered with about 4" layer of sand + a 3" layer of brick and about 8" of concrete reinforced with stone chips with a 1" thick layer or thermocol (as we call it here in india) all along the perimeter of the floor to isolate it from the external concrete wall and foundation. Its a strong floor as my entire studio is built with brick and concrete walls and the new floor seems to take the weight of the inside brick and plaster walls with ease. U can see a few pics i posted on my thread here: viewtopic.php?f=2&t=11498 what i suggest and plan on doing my self is building the control rooms and the recording booths on seperate isolated floors with each rooms walls built on its own floor. how i think u might be able to implement that in your design, without creating a difference in level in both the floors is (its just a thought dont know how well it will work but i think its logical) u can build ur entire floor as one floor and let it rest while u get other work done on your studio and when ur ready to start building ur interior walls for your different rooms then u can cut a channel all the way to the foundation in your new floor to disconnect the floors for all the rooms by then i suppose the new floor would have settled down as much as it has to and there wont be a difference in level. Hope it helps.

OK - first the isolation qustion - no the slabs cannot pin together as this will still couple them. Beyond that point - you really need to get together with a local engineer who understands the conditions with your soils. You're correct that as lay dries out it will setle - and quite considerably - BUT - you also can have the possibility of the reverse - it's called expansive soils - and in seasons with heavy rains you could have the entire slab lift. I do not know anything about the soils in your area - and so I am not qualified to give you advice relating to it - and seriously recommend a local engineer. I did run across a situation in a studio I designed in Canada where we ended up having to build the exterior foundation on piers - and then installed piers - with interior grade beams to create a structural slab for the inner structural frame. There was no choice in this as the soils were not capable of handling the load without severe settlement problems - not unlike your situation. The idea you have for multiple layers of slab still will not address the issue of settlement from what I can gather about your ground conditions from your post. And it sounds like much more money than needs to be spent to acheive a sound structual condition. I wish you luck...... Rod

Thanks Rod, Ankit, for your speedy replies, That's pretty much what I was afraid you'd say. I was going to consult the engineer with whom my architect has been dealing anyway, but I'm not sure how much he knows about acoustic isolation of slabs. I really wanted to get some expert input from you guys who understand this in depth and to see if this problem had already been solved by another method. But I thought I may have to go to a supported slab. It seems as if that will be necessary whether I go for an isolated slab or a monolithic one. It will just require a few more legs to support 2 units. Anyway, thanks very much for your input. I will now see the engineer as soon as possible. Best wishes Simon

Hi all, Some further questions about slabs I'm afraid. The engineer didn't show up to our meeting, but relayed some suggestions through the designer. I am pretty clear on construction of the room: Concrete slab, double stud walls, outer with 2 x cement sheet, inner with 2x plasterboard with green glue, corrugated iron roof with cement sheet below, suspended ceiling hanging from isolation mounts, sealing to inner stud wall. I have to choose a slab design, but I'm still not clear on the relative performance benefits. The design is complicated by the clay layer under one side of the room. This is the "best" version as far as I understand it, with isolated slab. However, I have some reservations about this idea, mainly the long-term stability over the reactive clay (with the possibility of relative movement betwen slabs), as well as the complexity of the site preparations, given my difficult site access and the fact I've already injured myself digging rocks out of clay. The Australian distributors of Green Glue (http://www.ultrafonic.com.au) suggest an alternative whereby the double stud walls sit on a single slab but are isolated from it by Shearflex pads. This is clearly a cheaper and simpler arrangement and looks attractive IF the performance is similar. To add to the confusion, the guy that sells the Shearflex pads suggested that I should use a floating floor (damped) above the slab, to kill ground-borne noise from the highway nearby. A similar arrangement is also shown on the Green Glue site. Jeff Cooper's book suggests that this would add 12 dB isolation (by implication from his wooden deck example). This is what was suggested. (I am not sure how much noise is actually coming through the ground. As mentioned in my earlier thread, I have 88 dB max from log trucks, by airborne sound. However it is a smooth road on bedrock and there doesn't appear to be much vibration. I could hear some noise through the bedrock by putting a contact mic on it, but I'm not sure how well isolated it was from airborne noise. Tomorrow I will try again, encasing it in an isolated sand box.) My questions relate to the relative performance of these designs. I have asked the Shearflex guy for performance figures on his design, but I have been unable to find any performance data on the isolated slab design suggested at this site and in Rod's book, with which to compare them. Questions: 1. Does anyone have isolation figures for isolated slab vs simple slab? 2. Does anyone have any experience with wall isolation via the Shearflex pads, or similar? 3. If ground-borne noise is an issue, is the damped deck any use? Cheers Simon