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Measuring Load/Stress

Measuring Load/Stress

Measuring Load/Stress

(OP)
I am having a difficult time finding an instrument to measure load or stress for my application.

Basically, I have a concrete pile (cylinder) embedded in soil. I would like to measure the stress, normal to the side of the pile. The pile is about 3 feet long, and has a diameter of 4 inches.

Does anyone know of some type of load cell that can be fixed onto the concrete surface for this purpose?

RE: Measuring Load/Stress

Maybe if we knew the purpose for measuring the stress in this direction, we could suggest something. Because even if you could measure the stress in the soil (if I'm understanding this right), what would you use it for. It would dissipate so quickly that it wouldn't be anything but a unique snapshot.

RE: Measuring Load/Stress

(OP)
The reason I would like to measure lateral stress is because that is the principle in which a pile foundation holds a building up. Piles use the friction between the pile and the soil to resist the load of the structure.

So if you know the interface friction angle and the lateral earth pressure against the pile, you can calculate that frictional force, which is called "skin friction" in geotech.

I just figured it may be easier to fix something to the surface of the concrete pile to measure this stress than to try to have something simply resting in the soil.

RE: Measuring Load/Stress

I assume a load is placed on the top of the pile(axial)and there is exposed pile you can place a gauge on?
Not sure which direction is "hormal to the side", is this axial? For measureing axial stress (at the surface), a strain gauge should suffice.  

RE: Measuring Load/Stress

(OP)
Yes, an axial load will be placed on the top of the pile.

This axial load will be resisted by the friction between the pile and the soil. There will be some lateral stress distrabution along the length of the pile. Please see the attached (poorly drawn) image.

I drew the stress distrabution as uniform, but this will not be the case. So I would like to measure stress at different points along the length of the pile.

By "normal to the side of the pile" I mean perpendicular to the axial direction. Hopefully this helps clear up my question.

RE: Measuring Load/Stress

First of all, your lateral stress will be very low since your pile depth is only 3 feet.  It is difficult to measure small differentials in such an application.

Next, the lateral load from the soil on the pile will not increase because of an axial load on the pile.  The lateral load will increase on the soil beneath the pile.

You might consider some interior instrumentation of the pile.  A load cell could be cast into the pile as could strain gages.  From these devices, you can gather a lot of info; however, your info will be limited because of the limited depth of the pile.  

RE: Measuring Load/Stress

(OP)
I agree, the axial load will not cause an increase in the lateral earth pressure. However I guess I should mention all the details of the application.

This is a geothermal energy pile, so it will have fluid of varying temperatures running through it. What we would like to do is see if the thermal cycling, which will cause the pile to expand and contract, will cause any relaxation at the soil/pile interface. That is the main purpose we would like to measure lateral stress. I understand this value may be small, and the stress changes due to thermal expansion will be much smaller, I just wasn't sure if this was even possible.

RE: Measuring Load/Stress

I doubt that you will get an elastic soil reaction with such small lateral loads and displacement.  A little "re-orientation" of the soil at the pile surface, but probably not measurable.

In order to get an elastic reaction from the soil in such an application, the soil will have to be highly compacted and be essentially "bonded" to the pile by contact pressure.  Not likely.

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