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Design of concrete slab at the bottom of braced excavation
3

Design of concrete slab at the bottom of braced excavation

Design of concrete slab at the bottom of braced excavation

(OP)
Hi Everyone,

I am new to earth retaining structures and I am working on the design of a braced excavation with a sheet pile, wales and a concrete slab at the bottom.

The concrete slab will have a lateral earth pressure load on it and needs to resist it. order of magnitude is about 400KN/m.

How do you guys design the concrete slab? is there a specific way for doing it in a braced excavation? is it the same as slab on grade? is it a one way slab? two ways?

Thanks alot for your input.

RE: Design of concrete slab at the bottom of braced excavation

Where is the water table compared to the bottom of the excavation?

www.SlideRuleEra.net idea

RE: Design of concrete slab at the bottom of braced excavation

(OP)
the water table is 3 below below grade and the excavation is 9 m below. so the water table is in the bottom of excavation.

RE: Design of concrete slab at the bottom of braced excavation

Ok, I hope you are working with an engineer experienced in this type work, it is not for beginners... and 9 meters is pretty deep. The structural aspects and the geotechnical aspects along with construction techniques are interrelated and blend together. You need to be well versed in all three. For example, the slab you are working on may have to simultaneously:

1) Resist lateral pressure on the sheeting (pressure from both earth and hydrostatic)

2) Prevent flotation from hydrostatic uplift of the dewatered excavation.

3) Be a foundation for whatever goes in the excavation.

Design of slab can depend on all of these factors, and there are not hard and fast answers.

In the most general terms, you will need a good idea of what the ground water's potential rate of flow into the dewatered excavation will be:

1) If it is "slow", may be possible to dewater the area (as, or before, excavation begins) with pumps or well points (although vacuum well points are limited to a practical lift of about 7 or 8 meters). This will allow the slab to be designed in a more or less conventional manner.

2) If potential flow is "fast", the slab may have to be designed and constructed, underwater, as a concrete cofferdam seal (possibly with a structural slab constructed later on top of the seal) using a tremie. I expect you are not familiar with these terms; I just trying to point out this is a complex situation and work on it very closely with an experienced engineer.

www.SlideRuleEra.net idea

RE: Design of concrete slab at the bottom of braced excavation

(OP)
Thanks alot of the great insights.

I am working with an experienced geotechnical engineer and he has done this many times but i am trying to get into it on my own.

From a pure structural perspective, do you design the slab as a one way or two way shear slab? Or is there something special about these types of structures?
Is there any resources i can refer to to understand this better?

RE: Design of concrete slab at the bottom of braced excavation

Quote (HanStrulo)

The concrete slab will have a lateral earth pressure load on it and needs to resist it.

From a pure structural perspective, do you design the slab as a one way or two way shear slab? Or is there something special about these types of structures?

Very special. The terms "one way slab" and "two way slab" often are used with elevated slabs, which this type foundation is not... except perhaps under one certain loading (we will get back to that).

To start, the loads are continuous, high and "real", not theoretical probabilities like typical live, wind, & seismic loads. The slab needs to resist horizontal compression from all sides at the same time... that's pretty special, and neither a one-way nor a two-way design will help.

Since the slab is in contact with the sheeting, it may (structurally) need to temporarily resist uniformly distributed hydrostatic uplift load... in that way the slab is similar to an upside-down elevated slab... which seems to call out for a two-way slab, for that loading.... that's pretty special. (If a thick concrete seal is used gravity does the work, reinforcement not typically used).

For permanent (gravity) loading, the slab may need to be designed as a mat foundation... or perhaps a pile cap instead (if permanent piles are driven inside the excavation).

The "devil is in the details", which only you know.

Work below the water table is potentially hazardous, you have to be conservative and produce an well thought out design. If your make a mistake, the laws of physics (primarily involving water pressure) will find it, and possibly very suddenly destroy the braced excavation... sometimes with loss of life.

About references, NAVFAC DM 7_02 will tell you quite a bit about braced excavations (geotechnical), but that want help a bit with (structural) design of the slab at the bottom of the excavation.


www.SlideRuleEra.net idea

RE: Design of concrete slab at the bottom of braced excavation

Also, do some research on tremie seals in cofferdams. A thin concrete slab will not be able to resist any significant hydrostatic uplift unless it is anchored down and probably reinforced, all anchoring and reinforcing being done under water. It would not be unusual for a tremie seal to be anywhere from 5 to 20 feet thick if unreinforced and not anchored down. If your "experienced geotech" does not have good experience with tremie seals, you better get another who does have this experience.

www.PeirceEngineering.com

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