Water table above basement 2

[Gus14]

I am learning the design of raft foundations below the water table. I have a couple of questions about the design of the basement slab when the water level is above the basement slab. 1) Should I design the mat for the soil reaction + uplift force? For example, if the mat is sized so that the soil reaction on it is 300 kn/m2 should I add whatever uplift force is ( which let's say might be 20 kn/m2 and design the footing for 320 kn/m2 ) 2) For raft with slab thickened below columns should the slab length be measured between columns or between footings? 3) Should I design for the water table height only or should I design for the full basement height in case the water table rises due to rain in the future?

Sidenote, Question#2 will help me understand the slab design methodology should I model it as an upside-down flat slab supported on columns under the uplift force, or would the footing thickening act as a support? Similar to the following sketch

https://files.engineering.com/getfi...099-970f-01454e26c110&file=slab_thickned_.pdf

 

[SlideRuleEra]

Gus14 - I can offer you some suggestions.

When uplift (buoyancy) is involved it very easy to get confused. For example, if the soil reaction is 300 kn/m² (dry) and uplift (from buoyancy) is 20 kN/m² the soil reaction becomes 280 kN/m², not 320 kN/m². Suggestion: Don't try to combine buoyancy with other loads. Design assuming the water table elevation is not a factor, then apply buoyancy to check the design for flotation.

Keep the mat design really simple. Never forget that the contractor will be building this mat in a wet hole in the ground. He will have use "means & methods" to dewater this below-the-water-table hole... that is a messy job at best. If the water table is high enough, these "means & methods" may have to be quite elaborate and a design project itself (for the contractor). Suggestion: Forget the mat build downs for the columns, make the mat a constant thickness... which will allow simple concrete forming and a basic rebar cage.

For buoyancy, design for the worst credible conditions. If the water table can rise so that the basement is below the water table, even for a little while (buoyancy never takes a holiday)... design for that condition.

Below the water table, hydrostatic force on both the walls and hydro static uplift on the slab have to be checked. It's possible that load on the columns keeps the mat from "floating" but hydrostatic uplift ruptures a mat that was not designed to resist it.

I would not model the slab upside down. That would work, if done correctly, but could easily get even more confusing since all forces are now reversed.

http://www.SlideRuleEra.net

 

[HTURKAK]

1-You SHALL design the mat for the probable worst conditions.. If the soil reaction is 300 kn/m2 , the structure should be in the range of 25 storey bldg etc. and you probably need some basement storeys. You should model the mat supported on Winkler springs etc. rather than using average soil stress. If the GWL could vary , you shall consider the extrem cases for uplift .
2- If for hand calculation , you may use strips and the span of slab btw. columns.

-In order to model the raft , supported by columns for uplift and average soil pressure , the raft should be reasonably rigid. Otherwise, the soil stress would be high under the columns.

 

[Gus14]

Thank you SlideRuleEra, and HTURKAK for replying.

Suggestion: Forget the mat build downs for the columns, make the mat a constant thickness... which will allow simple concrete forming and a basic rebar cage.

Although this might increase the cost of material it will certainly decrease the labor cost and it will provide significant resistance towards upward deflection, so I will stick with it. However, For the mat builds down below columns method I will increase the slab thickness based on the arrangement of the columns and will heavily reinforce the slab top layer so It might not be very economic. I don't trust deflection results from FEM software so I will specify it manually and check for it with FEM.

If the soil reaction is 300 KN/m2, the structure should be in the range of 25 story bldg, etc.

Sorry for misleading you I was suggesting it as an arbitrary number probably should have chosen a better one to be fair. I know it's impossible to achieve a uniform soil pressure reaction of that magnitude below a raft.

After further review, I recall from my hydraulic course that it's impossible to increase the water table in a significant manner from rain. So unless the soil around the basement is a fill soil, it may be just fine.

 

[SlideRuleEra]

I am learning the design of raft foundations below the water table.

For the mat builds down below columns method I will increase the slab thickness based on the arrangement of the columns and will heavily reinforce the slab top layer so It might not be very economic.

It's your project and, of course, you can do it your way... but you asked for advice on how to do this right. Stop penny-pinching on material cost; the "big money" is the work below the water table. There are few contractors who are competent to perform this type work. The idea is to complete work below the water table as quickly as practical... saving labor doing so is just a fringe benefit.

Also if column spacing of "700" means 700 cm, a slab 25 cm thick is nowhere near thick enough to be considered a mat that will, more or less, uniformly distribute corner column loads over the entire mat. Even 60 cm thick is probably marginal thickness for an effective mat.

http://www.SlideRuleEra.net

 

[Gus14]

Yes, I completely agree with you. I meant that I prefer your method as it reduces the labor cost required to dig the additional thickenings below columns so the increase in concrete and steel quantities is justified. And the original method I have mentioned might not be as economical as I will increase the slab thickness and top layer reinforcement.