Basement Raft Heave/Hydrostatic Uplift - Pile Stiffness 2

[JohnnySm]

Dear All,

Could someone please kindly provide me with a step by step process on how to attain the uplift pressure from a clay based subgrade on a single basement Raft and the general design procedure for such a structure?

Please note this is purely educational as i am soon to be using this technique in an up and coming project, so i have no true specifics.

Otherwise please assume the following: 1. the construction uses a piled raft with propped cantilever rc retaining walls up to ground level slab. with super structure over.

I assume a water table must be present, or at least 'could' be present, with its position above that of the basement slab? otherwise there would be no hydrostatic pressure correct? For conservative design should i assume this anyway? Is heave only an issue if there is water table present?

Furthermore, piled rafts seem relatively complicated. In order to assess the raft i know i have to model the piles as springs with a certain stiffness to allow for settlement etc. i assume this just a process to assess raft stresses thoroughly, whats to stop someone designing the piles as simple supports (I.e ignoring stiffness), similar to traditional ground beam design in smaller construction? How do you actually calculate the correct k stiffness value to use in the FE model?

i appreciate this is a long question, as a grad engineer, i should be asking my superior. but sometimes i like having all the answers. thanks guys any comments welcome.

JS

 

[SlideRuleEra]

Hydrostatic uplift force does not "care" about the type subgrade, soil properties, or the design. The answer comes from physics, Archimedes Principle.

1. The structure, or at least part of it, must be below the water table.

2. Compute the volume of structure that is below the water table. This volume includes any waterproof, enclosed space, such as the interior of the basement (air). Piling are included if they are attached to the raft to resist uplift.

3. Compute the weight of a water that is equal to the volume calculated in Step 2. (Fresh water at 62.4 lb/ft[sup]3[/sup] or sea water at 64 lb/ft[sup]3[/sup]).

4. Compute the dry weight of the structure below the water table, including piling as appropriate. (Reinforced concrete at, say, 150 lb/ft[sup]3[/sup]). This does not include any enclosed space (air).

5. Subtract Step 4 from Step 3. If difference is the hydrostatic uplift force. The answer may be either positive or negative.

If positive, other forces such as additional dead weight, pile friction with soil, anchors, etc. are needed to resist hydrostatic uplift.
If negative, the self weight of the structure below the water table resists hydrostatic uplift.
Note: In either case, additional uplift resistance may be needed to meet prescribed flotation safety factors.

There can be other hydraulic forces, such as artesian pressure, but that goes beyond hydrostatic uplift you asked about.

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[jayrod12]

SlideRuleEra hit the nail on the head regarding hydrostatic pressure.

With clay sub-base, you also want to be cognizant of swelling potential. If it is truly a piled raft, you should make sure there is some sort of void form under the raft to prevent uplift pressures due to the swelling.