Concrete piles against lateral loading 1

[hungk]

Would appreciate opinions on this matter. We are currently building a sewage treatment plant and has recently driven 200mm x 200mm x 6000mm length square concrete piles 30 meters below the ground level (pile to length). The contractor has unfortunately excavated the sides of the piles 10ft down, exposing the piles to 10ft of lateral pressure. As a result, some piles has cracked unacceptably, most has tilted and suspected to be dislocated at the joint area, another 10ft below the exposed surface. The soil profile is :- silty clay from the ground level to about 4ft. down, and from there towards the toe of the piles is soft marine clay.

My question :-
1) Is concrete piles 200mm x 200mm of Grade 45 strength unable to take the lateral pressure of such capacity?
2) Is there a case of inefficient welding at the joint since most of the piles breaks at the joints?

Regards.

 

[Focht3]

Converting to U.S. common units:

The piles are nominally 8 inch square precast concrete piles 20 feet long.

If they are holding up an excavation that is ten feet deep, then they are probably broken and will have to be replaced. To confirm this, assume that the piles are fixed about 3 feet (~0.9 meter) below the cut, and that the soil pressure is created by an equivalent fluid with a unit weight of 20 pounds per cubic foot (about 0.32 kg/l - ~one-third the weight of water alone.) This assumes that the cut was freshly made; the equivalent fluid pressure goes up with time. My quick estimate of the force per pile width is (10'*20 pcf)*10'*0.5*(7.85"/12) = 654 lbf (2.9 kN). The moment arm is about (10'/3) + 3' = 6.33 feet (~1.9 meters). So the applied moment is at least 654*6.33 = 4, 140 lbf-ft (around 5.5 kN-m). If the section modulus is 81 in³ (~1.32*10⁶ mm³), then the outer fiber stress for the gross section is on the order of 4,140*12/81 = 613 psi (about 4.2 MPa). They'e okay so far -

You will also need to factor in the spacing of the piles: if they are one meter apart, then you will have to multiply the load per pile by (1,000/200 =) 5. I wouldn't use a factor larger than about 10 for your situation, though - the effects of soil arching will only extend so far. And the unit weight of an equivalent fluid will increase to 40 to 50 pcf in 30 days. Using &[ignore]gamma[/ignore];_equivalent = 50 pcf and an arching factor of 5, the outer fiber stress will be about 613*(50/20)*5 = 7,700 psi (~53 MPa) - which would fail most (all?) conventional PCC piles.

"They're toast" by now - assuming the cut existed for at least 30 days, and the piles are more than 1 meter apart.



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[hungk]

Thanks FOCHT3 for your info. Those piles near to the cut are physically seen as broken. But when we randomly carry out Pile Integrity Test on those about 10 metres away from the slope, it shows dicontinuity at the first 6 meter. Physically these piles are still seen straight and not swayed. Is the movement of the soil affected these piles as well? And of course it has been more than a month now since they excavated the soil. Does the discontinuity shows weak welding at joints or is it really the slope that has mess up the piles?

 

[Focht3]

I'll go out on a limb and say that the piles are broken - by slope movements caused by the contractor's excavation. But you should expose a few of them to be sure -



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[BigH]

Have you been able to measure any lateral spread of the slope? Shear strength of the soft clay? You should be able to do a quick analysis to see if the slope, in fact, is shearing. Soft clay may be sensitive so that the operable Su may be a fully softened value (somewhat larger than remoulded). See Tschebotarioff for a discussion on sensitivity discussion for foundations. As for the lateral problem, I leave that to Focht3!

 

[Zambo]

I don't know where you are from, but on my travels I have noticed very poor details on some precast concrete pile joints. In some countries there are mechanical type interlock joints which give considerable confidence, in other countries there is just a cast in steel channel to the end of each pile which is then tack welded (a full length tack weld around the perimeter, but a tack weld none the less)The strength of the joint depends on the quality of the welder.

Secondly the units in your post are mixed. Please confirm the piles are 30m long - I'm not a design engineer but this seems a considerable length for 200 x 200mm piles.

Assuming the units are correct then the contractor has excavated 3m to expose the piles (presumably at cut off level) This doesn't seem unreasonable as the piles must be cut off at the correct level at some stage. Was his work interupted which meant that the piles were supporting the face of the excavation for an extended period?

You are now concerned that at a further 3m below the excavation there is a deflection in the pile which could either be broken piles, or broken joints.

I recommend that you drive some sheet piles (or a 1.8m diam. casing) and carry out an excavation to the joint (a further -3m is not so deep) on a sample number of piles.

It seems likely that the piles which are near the excavation face and are now out of verticality need to be repaired with a 3m splice to bring them back to cut-off level. The piles which are nearer the centre of the foundation and are still vertical should also be randomly inspected, but it appears that the joints could be the problem with the Pile Integrity Test.

Zambo

 

[MeGE]

If the piles were driven into the ground, you should look into the possibility of the pile joints being broken during the driving process. We had several piles broken this way while driving through a hard soil layer into a softer layer. The large vibrations induced during the driving process caused the pile to go into tension in the soft soils. The piles cracked as a result. The piles may have been cracked before the contractor excavated the soil.

BD

 

[dicksewerrat]

Concrete does not do well in tension. 600 psi tensile force would destroy most standard concrete mixes.