Can Hydrostatic Force also Resist Applied Load? 1

[mikeCTE]

We are designing a secant pile shaft that's about 100ft deep. The contractor will use it to launch a tunnel boring machine for a tunneling project. The TBM will use hydraulic jacks to advance casing in the soil. Max loads applied is 2000 kips. We are checking various components of our shaft as a result of this jacking load.

One of the engineers in my office wants to use soil pressure (static) + hydrostatic behind the secant piles to resist the jacking loads. The hydrostatic pressure is about 50% of the total resisting force.


I contend that the hydrostatic load cannot be used to resist the force from the TBM. I fully agree that the shaft itself sees the hydrostatic loads (it's a closed, sealed, rigid structure). But, if I take the shaft and put it in a bathtub, it can just float around with no apparent resistance (some, but you get the idea). Meanwhile, the shaft still sees the full hydrostatic load. It's not until you add dirt to the bathtub that the shaft gets stuck and now has to overcome resistance from the earth. The soil is a spring, the water is not. At least that's my $0.02.

Thoughts?

 

[MTNClimber]

I don't fully understand your analogy. I wouldn't see why you couldn't use the hydrostatic force to react off. I would consider discounting how much you are using by applying a factor of safety. You're saying that the shaft sees the hydrostatic load, then how does statics not apply anymore? The hydrostatic force is there so you should be able to use it to your advantage.

 

[Ron]

Hydrostatic resistance is a bit like pushing on a water balloon. It will dissipate in another direction depending on the soil permeability. I wouldn't count on it, so I agree with you, Mike.

 

[Retrograde]

I'd be curious to know how you design your shaft generally. What is your typical hoop stress limit? I have read some guidance that says you need to limit it to about 1.0ksi (or 5MPa). Also, how do you account for pile tolerance?

Are you using a thrust block to spread the jacking force?

 

[fattdad]

no.

f-d

ípapß gordo ainÆt no madre flaca!

 

[BridgeSmith]

Assuming that you have a water pressure differential due to a submerged condition behind the wall not matched by the water level in front, then you should be able to account for it. If it's actually like your bathtub analogy, without a pressure differential, then no. In that situation the pressure would be balanced and there is no net force or resistance.

 

[PEinc]

If the entire shaft is surrounded by water pressure, I would not use it to resist the jacking force. Remember, the soil is buoyant so the passive resistance must be based on buoyant weight. And, you must consider inflow of water (and soil?) when you cut the hole in your wall below the water table.

http://www.PeirceEngineering.com

 

[bpiermat]

Due to the reason Ron stated, I relented and agree that it can't be used. If the material had uniform pore pressure, in theory it could be used. However, given multiple types of soil, this is unlikely, even given a constant hydrostatic head that has been around since the glaciers left.

 

[Bobby46]

It sounds like your analogy is blurring the lines between buoyant concrete weight and passive water pressure. If you drive a sheetpile in the middle of a lake, it wont fall over due to water pressure until you pump water away from the passive side.

 

[MTNClimber]

Hydrostatic resistance is a bit like pushing on a water balloon. It will dissipate in another direction depending on the soil permeability.

Maybe I'm thinking of this wrong but here's how I'm justifying it in my head. I'll run with the balloon analogy.

It takes a certain amount of pressure to push the balloon in, right? Imagine the balloon is 50' tall. How much force would it take for you to push the balloon inwards? A fair amount I would imagine depending on what you're using to push (roughly 62.4 pcf x 50 ft = 3120 psf). Now add a 15'x15' rigid plate to the side of the balloon. How much force is it going to take to push the a balloon wall in? Quite a bit (roughly 600 kips).

I don't think the hydrostatic force (towards the dewatered excavation) disappears just because you start pushing on the wall in the passive direction. If that was the case, then the design loading on tie-backs would be significantly less.

Maybe I'm missing something but I haven't read (or maybe understood) anything that has made me change my thought process.

 

[PEinc]

A submerged submarine is subjected to great hydrostatic pressure yet requires relatively little force to propel it sideways. Therefore, the hydrostatic pressure on one side does not add to its lateral stability. Neither should hydrostatic pressure resist your jacking force. Does hydrostatic pressure make it harder to roll a submerged bowling ball around on the bottom of a tub of water? I don't think so.

http://www.PeirceEngineering.com

 

[Bobby46]

I agree if it is truly a sealed rigid structure. Then the net hydrostatic force is zero.

I was thinking of it as an independent wall designed on a per foot basis.

 

[BridgeSmith]

I think several posters are all saying basically the same thing (feel free to correct me if I'm wrong, anyone) - water pressure can be counted on as a resisting force only if a static head pressure differential between the back of the wall and the front can be maintained.

That means unless the wall is sealed so that, at a minimum, whatever leaks through or under the wall into the void space is replenished by free water migrating in from the surrounding soil, the pressure is not constant and cannot be counted on as resistance. That, to me, seems a fairly high bar to meet.

 

[MTNClimber]

I was definitely thinking on the same lines of Bobby46. PEinc's explanation makes sense to me. A "sealed" shoring system under the groundwater table is different beast all together. Thanks for the lesson!

 

[EireChch]

See below extract of a micro tunnel method statement that we are working on. The contractor included this as a reference in the calculations, its obviously from a tunneling book but i dont know the name.

Its states, groundwater should not be considered as it could fluctuate or be lowered with dewatering. It doesn't seem to argue the points discussed above (i.e. sealed or unsealed, dissipation with permeability etc) but i would be reluctant to consider it as groundwater (like most thing in geotechnical engineer) are an unknown. Use a larger jacking wall slab to reduce the force/pressure on your secant piles.

 

[PEinc]

Thanks, EireChch. I've have used a very similar method for designing jacking blocks for as much as 2500 kip jacking force. Gee, I thought I invented that trapezoidal method (LOL).

http://www.PeirceEngineering.com

 

[EireChch]

PEinc - well at least you're thinking like the "smart guys"