Raft foundation - semi-basement raft

[TaviB]

Hello to all,

Finally, after a while when i was only reading, now i joined the forum and start asking. I have a dilemma with a project that our company is subjected to do. It is about a 5 stories flat building, with a bearing pressure of about 100 kPa. Because of the low bearing capacity of founding soil, and low deformation modulus we went for a raft foundation. The foundation and superstructure system is the following:
- Raft with a thickness of 50 cm (L x B = 23.5 m x 13.5 m), laying on a bed of crush stone which is about 85 cm thick
- The superstructure - basement with reinforced concrete walls 25 cm thick
- 4 stories with brick load-bearing walls.
- average load transmitted by bearing walls is 100 kN/m
- The construction will be done on a hill, so the basement if actually a semi-basement. Considering this, in the back of the building, where there was excavated of about 6 m of soil, the settlement in theory have to be null, because the bearing pressure is less then the initial soil pressure. In front of the building the foundation will be at about 2 m from original ground surface...so there will be a net pressure.
My questions are the following:
1. We tested the crush stone bed and obtained a dynamic modulus of deformation of 40 MPa. Considering this, when the relative stiffness of super structure and foundation is calculated what deformation modulus of foundation soil should be taken into account? Should be done an average modulus of deformation considering the influence zone of raft foundation? But...if the raft will act in fact as a flexible one, the influence zone will be only about 2-3 meters under raft where load-bearing walls are, and not of about 19m if i consider the raft rigid. So the average modulus of deformation can differ a lot.
2. It if the method considering the raft as an inverted floor slab subjected to 100 kPa uniform pressure, where the reinforced concrete walls are the supports safe in this case? And can this method be adopted for both and flexible raft foundation? What can happened if in reality the raft foundation acts as a flexible one and is calculated whit this method?
3. I could not find anywhere an example where a raft with concrete basement walls is designed. Most examples provided in books and literature, are with columns. Could you provide me a book where i can find and example?
4. We calculated the thickness of raft from shear punch criteria. Should this raft be thicker considering above mentions?
5. Considering that in theory in the back of the building the settlements will be small (considering the amount of soil excavated), so there will be small/no differential settlements --> small stresses ---> less reinforcement (only minimal one provided by design code - EC2). Is this theory correct?

I attached a longitudinal section across the raft.

 

[SlideRuleEra]

Should this raft be thicker considering above mentions?

In my opinion, yes. Consider the proportions of the foundation:

Ratio of Length (23.5 m) to Thickness (0.5 m) = 47:1
Ratio of Width (13.5 m) to Thickness (0.5 m) = 27:1

To me, with those proportions the foundation has more in common with a slab-on-grade than a raft foundation.

Increasing thickness will quickly improve the raft's rigidity. Take a look at a thickness of 1 meter. This should be enough to approach the rigidity needed for a raft, but thin enough that mass concrete placement measures (such as careful control of heat of hydration) are not needed.

The extra 159 cubic meters of concrete for the thickened raft will be relatively cost effective. Subgrade preparation, equipment needed for concrete placement, concrete finishing / curing are all unchanged. Even concrete forming does not changed very much. The cost of buying the concrete and some extra time to place it are the only significant increases.

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[TaviB]

The problem with a thicker raft, is that our project verifier (with local minister authorization) told that a raft thicker than 0,50m is surely not needed because from his point of view, for such loads, with a 0,85 cm crush stone cushion and reinforced concrete walls at basement, and considering that a considerably amount of soil is excavated, the total settlements/differential settlements will be small.
Another problem is that because the loads are small, for 1 m thick raft foundation, the reinforcement area will be much bigger only from minimum area of steel needed for such a section.

On the site are build 2 such buildings, the other building (because of better soil condition) is build only with a 0,7m x 1,5 m (bxh) non-reinforced concrete under load-bearing walls.

And yes, surely from rigidity point of view, is more to act like a slab-on-grade, this is why we thought to analyze it as a inverted floor.

 

[SlideRuleEra]

...this is why we thought to analyze it as a inverted floor.

What results did you get?
From Question #2, it looks like that approach has more unknowns than answers.

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[oldestguy]

The upside down floor idea, with beams, etc is done a lot, except the beams are under the slab, in trenches. A grid of these covers the site. I usually recommend designing that raft so that all areas might have no support over unknown zones at any place underneath. For houses and apartments I usually suggest that those no support zones at any location, including walls could be 10 feet square. For your site maybe 15 feet on a side.

 

[TaviB]

Well, today we managed to calculate the raft also in SAP with elastic springs (determined ks value on site-25000 KN/m2/m), which is better than we expected.

As an inverted floor, we had with about 15% bigger bending moments, comparing to the ones resulted from design with elastic springs. The thickness for the raft was the same (50 cm). So, the values obtained are safe from inverted floor design.

I consider that because of the reinforced concrete walls from semi-basement, there was a big improvement in superstructure rigidity. I also read an article from Army of Corps, where they evaluated the effective thickness of raft with concrete walls, and for a 3 stories building, for a thickness of raft of about 55 cm. The resulted effective thickness was about 1,8m.