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I have a building that is 7 story height (which 5 stories are basement floors which means they are underground) My water level is at 1.5 m from the surface. the stories have typical rectangular shape of 30 m x 36.2 m with a grid of columns as shown in the picture below. When modeling the raft foundation that is subjected to gravity loads and water pressure (opposite to gravity loads) and since the raft is at a level of 19.5 m from the ground surface it is subjected to a lot of uplift the overall gravity load is of 108858 KN as for the uplift is (19.5-1.5)*10*30*36.2 = 195480 KN which is greater than the gravity loads so their will be an uplift of the building.

How to solve this problem ? I mean how many solutions that are available today for this problem and what is the most economical one ?

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  • $\begingroup$ How is it possible that the water pressure could exert a greater overall force than the gravity load of the building? Something is not right with your calculations $\endgroup$ – Angus Murray Jun 6 '17 at 0:49
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    $\begingroup$ Why couldn't it? A building is mostly empty space, so the average density of the enclosed volume can often be quite low, perhaps even lower than that of water (in which case you have excessive uplift, which is apparently this case). $\endgroup$ – Wasabi Jun 6 '17 at 2:08
  • $\begingroup$ Need ballast in the bottom like a sailing ship.Fill the lowest floor with steel scrap. $\endgroup$ – blacksmith37 Apr 27 '18 at 16:29
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There are a few possible solutions.

You can use pile foundations, where the piles dig deep into the soil and resist the uplift. They'll work entirely in tension, instead of the usual compressive load, so point bearing capacity is non-existent, only consider lateral capacity. Depending on the soil type (and remembering it is saturated), this may or may not be viable, especially if you need to handle seismic loads. Regardless, it is probably not the best idea.

The usual solution is mass. Lots and lots of mass. Your walls and bottom basement slab are under massive loads, so just go crazy and make them monstrously thick. This will lower your steel reinforcement and reduce cracking. And, if you put in enough (including a safety factor), your structure will weigh more than the water it'll displace (weight > uplift).

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  • $\begingroup$ I think it is a good idea to make the slabs thicker, I was using a 25 cm thick slab (post tension) thinking it more economical than the solid slab (35 cm) but in this case it will cause indirectly an uplift in the building and will cost be to have piles, I think I will go with the solid slab I am sure that it will help me a lot . Thank You, very good idea ! $\endgroup$ – J.Daou Jun 6 '17 at 5:11
  • $\begingroup$ Unfortunately even if the slab is thickened the uplift is still greater than gravity loads, can you please refer to how to design a pile that is subjected to tension (any article or document would be nice) I know that reinforcement is As = Pu/(phi*fy) for pile in tension but what do i need to check ? $\endgroup$ – J.Daou Jun 6 '17 at 8:13
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    $\begingroup$ @J.Daou That's another question all of its own, but simply put: in geotechnical terms, a pile under tension is basically identical to one in compression, only there's no bearing capacity, only lateral capacity. When designing the pile itself, just consider it a column under uniaxial tension. If it's a concrete pile, that means ignoring the concrete and simply throwing all the force to the rebar, such that $A_s = F_d/f_y$. That's mostly all there is to it. $\endgroup$ – Wasabi Jun 6 '17 at 13:07
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    $\begingroup$ @J.Daou That being said, take another look at thickening the slab. Even if it ends up being an inefficient design (far thicker than necessary from a design standpoint), it'll probably be cheaper to simply make it a meter thick than to add the piles. $\endgroup$ – Wasabi Jun 6 '17 at 13:07
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    $\begingroup$ I note you say that you can't thicken the slab, but it's only an extra 3m ish thickness you need... However, another option is to extend the slab beyond the walls, so you have a toe jutting out into the soil. Your excavation is then bigger, and you have to do it by cut-and-cover, but that lets you mobilise some weight of soil outside the building perimeter as beneficial weight. Some rough guesses say so your toe needs to project 4m - probably too much in isolation, but some by this method and some by extra slab weight might be economic. $\endgroup$ – achrn Jun 6 '17 at 14:48
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In such a case following multiple solutions are suggested.

  1. Tension piles are the best solution. Raft often creates uplift pressure if the water table is high or may vary season to season.
  2. Now the cheapest one: Place pipes at various and multiple positions in the raft vertically to release the pressure. These pipes shall be interconnected above the raft in an appropriate slope towards a harvesting tank. Water will rise in the pipe which can be dropped in the tank from where can be pumped out for gardening. No pressure shall be created under the raft. This pipe network can be embedded under the floor of the basement.

One more important point: place a thick layer of stones and stone dust underneath the raft. It will reduce the capillary action in the soil and water will tend to flow horizontally instead of vertically. Allow a horizontal passage of water below the raft. It can be easily done by placing half perforated pipes network below the raft in the layer of stones a d guide the water outside the building area.

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I believe you could have also considered rock bolts/anchors. I don't know how thick your raft foundation was going to be, but sometimes making the slab thick enough to resist uplift forces can cause problems:

  • It may be unnecessarily thick, and as such quite expensive to oversize
  • It may not be possible to dig deeper to pour a thicker foundation

Rock bolts also will most likely be concealed, so it won't affect architectural elements. They can be pricey however.

Local economics/prices/availability may have a big effect on the viability of any of these solutions.

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    $\begingroup$ Rock bolts only anchor into er... rock. I'm pretty sure no-one's blasting rock to construct a basement 19.5m deep - that sounds expensive. Far more likely is a soil of some sort. In which case you'd want the soil equivalent of a rock anchor - a tension pile! Which has already been suggested in the accepted answer. $\endgroup$ – AndyT Apr 27 '18 at 10:13
  • $\begingroup$ I've actually seen some ridiculously deep parking lots dug out of bedrock. Not that it seemed reasonable, but you never know. Back to the tension piles, it's possible the bedrock is close enough to the foundations that the piles might be too short to develop enough friction, in which case anchors might be more practical. $\endgroup$ – PMasterfunk May 14 '18 at 14:23

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