New to Old Raft Foundations, is it Possible?

I do not understand some of your description about what you are attempting. This maybe because I'm not a structural or civil engineer, so please take this as a layman's comment.

I looked at a few web pages about a raft foundation. A raft foundation is intended to evenly distribute a load onto a soft soil. (To act like a raft resting on top of a fluid surface.) So to change the orientation of the structure that will be sitting on top of this foundation can significantly change the loading onto the foundation. This sounds to me like a method to defeat the foundations design.

I would not do it. I certainly would not do it based on any opinion I got from a public web site that anyone could comment on. I would only do this if a licensed civil engineer surveyed the site and signed off on the load change and redesign.

It all depends on the original virgin soil Load Bearing characteristics, Comparing it to the actual loading from the structure.ucture

If the Loading per unit of area of the structure as transmitted trough the Raft is much less than the load bearing of the soil, the settlement will not be much of a problem. IF you remove the old raft, clear the soil to a relatively lower level, removing the compressed top (maybe few inches), then making a new Raft for all the structure it might even be better if the load bearing was too near to the raft loading originally.

These are only informative advices and a local civil engineer will be more competent to judge the case.

Adding to and joining to seem same to me. What am I missing about the difference?

Seems to me you are answering your own questions.

I am not there. Still it would seem of no great problem to overcome one way or another, and not a particularly mysterious procedure considering the long history of your area.

Hasn't it been done before? What was the outcome for similar events?

By possible do you really mean possible at an acceptable cost, or simply possible?

And this thing about the tree? You are building to account for expected tree growth? When will the tree grow to its maturity and fall over?

Thank you for the replies.

Its not without minor frustration that I have posted this as I have been researching for over a week, and have found no similar situations or reference to this type of issue, in fact its how I found this forum and the previously mentioned topic.

When we originally built, the borough engineers were not really interested and had no suggestions, only when I proposed the split foundation did they agree with some conditions such as minimum 1 meter depth apron to the raft and reinforcement to both the raft slab and every course of bricks up to dpc level, rather serious for a single thermalite block building I thought, however I cant argue as it hasn't moved!

The new building will be 9" brick/ block with tiled roof (old was flat roof, never again) and therefore heavier. At this time I am not sure of the soil loading spec but the idea of leaving the new raft floating is looking more favorable as what I didn't mention is that there will be 2 door openings directly opposite each other in the long walls bang in the middle and exactly over the join in the footings. Therefore I am thinking that as this is always going to be a weak spot, perhaps its simply better to just let them move and repair as and when as suggested.

Cost isn't really an issue with the rebuild and I am quite happy to do what's necessary if it provides a solution. As far as the tree goes, it has no preservation order as it's a fruit tree, although I have been advised that there is also no benefit from removing it as the ground will shrink and possibly cause more unfavorable ground conditions, plus its 100 years old and part of an historic orchard.. been looking at it for 27 years and there's still no sign of it falling over yet J

I am sure also that the solution isn't mystical.. just elusive and your interest so far is very much appreciated.

David

I can see that this would cause some frustration. The original building has already gone through 20 years of settlement. By contrast the new building will be on soil that has not been comparably loaded. If the two rafts are connected the unsupported end will settle quite a bit different than the supported end. I know that the apron is an attempt to minimize this, but settlement will still happen, since the new building is heavier than the original. I worked on a team that had to design a building in a similar situation. I believe the soil was preloaded for a period of time to try and move the soil through its primary settlement. The rebound had to be accounted for as best we could, and then the secondary settlement that the soil engineers believed would occur. Even with that we designed the building in such a way as to place a wall on each side of the joint and connected the two with a "flexible" connection that gave a natural appearance. If you want to put doors in this area however, that will not work. (I would find a way to relocate those doors.) I never saw pictures of the final product, so I can't say how it looked. I think through that process we were able to minimize the shift between the original building, and its addition. The new building still shifted a very small amount, but was nearly unnoticeable. In any case though the soil and structural engineers played an important part in the design of the building from the beginning. Our project was in a high seismic zone, and so we did quite a bit more to take that into account than I could go into now. I am not sure where you are located, but it seems that since you are still in the planning stages, hiring a structural engineer to work with your soils engineer would be a good investment. It sounds like a pretty interesting challenge. Good Luck

I enjoyed omw7s answer, for I wondered about the influence of the tree.

I'm not sure if apron means vertical barrier, as done for bamboo isolation.

I am equating slab, with raft as nomenclature issues.

In international discussions here I wonder sometimes if Earthing, and Grounding are really the same thing to everybody?

The flex of wood in relation to foundations is of interest.

I had a case of farmhouse or barn foundations for a home renovation land build with a choice where the foundation for the barn was so superior to the foundation for the house, that we were forced to see the house, original draw for the property, burned down for fire department practice.

I have done some pile driving for piers run out into the ocean, and think about how to build in sand and water on occasion.

Many Thanks omw7s for your reply it was very encouraging and helpful and also your thoughts Transcendian.

The new building will indeed remain a single storey 20' x 10' workshop detached from any dwelling, and the difference between the old and new is that "new" will be a double wall (9"brick/block) and a tiled roof as opposed to a single block and flat roof of the old one.

Although this will clearly increase the weight on the old portion of the raft, the original raft was quite substantial with (I believe) enough redundancy for the revision. However I am not an engineer in this field and therefore will be reminding myself of the exact spec of the old raft with some measurements (including below ground) so as to match like for like, clearly I don't want to over or under build the new raft and provoke difference.

The original has a four sided apron (as you would expect) and I am not sure of the effect of adding another four sided apron along side the old resulting in a "double" apron in the middle. In addition the external vertical side of the middle apron below ground has a clear rough cast protruding step, no doubt caused by the soil trench wall compressing slightly under the weigh of the concrete when originally cast. If We therefore cast the new middle apron against this (as is) it will clearly act as a key and attempt to restrict movement or cause an undesirable movement (perhaps even push away). I therefore wonder if it would be beneficial to smooth the face or even provide a gasket between the two rafts?

I am very open to linking as suggested , however I am assuming this would only be practical via the horizontal surfaces, as I fear any vertical attempt would require considerable excavation (in respect of drilling) which could eventually compound "difference". I will see what I can find out as I might simply be ignorant to methods at the moment.

I have now cut the old workshop in half (adding a dividing wall) and removed that portion of the existing building from the raft end that we intend to re-use so I can now start taking measurements and outlining the new addition.

I realize this maybe a minor project for many who read, but your thoughts and advice are very much appreciated.

David

Sorry I forgot to log in on the last post and to thank Idrivetrains for his reply