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Tension Pile Load Test

Tension Pile Load Test

Tension Pile Load Test

(OP)
I have a requirement to conduct a tension pile load test for a test load of around 650tons. I want to know the most reliable way of tying the pile to the test beams for the pull. Recently I tried using prestressing cables for a test load of 515tons. The problem in using these is the elongation. The hydraulic jack a has limited stroke and the cables are pretty elastic. I am planning to make threads on the pile reinforcement and using bolts to make the pull. but I am not sure how to design the threads and the bolts. Can anyone help?

RE: Tension Pile Load Test

I'd also post in a mechanical engineering room. They might come up with some suggestions on technique also.

add: How's about several cables at low tension each, since elongation is tied to stress.

RE: Tension Pile Load Test

Please provide more information. Is the 650 ton tension load to be applied to one test pile? Or, is this a static compression test on one pile where a compression test load is resisted by several, tension reaction piles? If you are trying to apply a high tension design load to one test pile, why aren't you using one or more tiedown anchors instead of a tension pile? If the single pile is being tested to 650 toms compression, why can't you resist the compression test load with a number of temporary tiedown anchors?

www.PeirceEngineering.com

RE: Tension Pile Load Test

I have seen DYWIDAG threaded bars used for this application.

RE: Tension Pile Load Test

(OP)
@oldestguy, that can be done, but for a 900mm diameter pile, there is not enough room to add so many cables. The cables are actually a hindrance during the pouring for the tremie pipes.

@PEinc, it is a 900mm diameter pile RCC pile, the depth of the pile is 20m. It is a single pile which is to be tested for a test load of 650 tons. the design is not ours. We are contractors who are asked to conduct the load test only.

@Retrograde, Yes, that is also one method of doing it. But unfortunately, the DYWIDAG threaded bars are not available here.

RE: Tension Pile Load Test

You can use O'cell technology if the pile is not already cast.

RE: Tension Pile Load Test

That's a lot of tension applied to one, 20m long pile. A 3" diameter, Grade 150 (GUTS = 150 ksi), threaded bar has a temporary allowable load of 822 kips = 411 tons = 0.8 x GUTS x Area. Therefore, for a 650 ton (1300 kips) test load, you would need two bars embedded sufficiently into the 900 mm concrete pile. Embedded and fully bonded into the pile, the threaded bars would have very little elastic stretch. Also, check that the pile can mobilize a cone of soil that weighs 650 tons PLUS a factor of safety. You may find that the 20m long pile is not long enough. Frequently, with tiedown anchor design, the controlling factor for tiedown length is mass stability and cone height, rather than bond length of tendon to soil or rock. Buoyant soil will increase the pile or tiedown length as the soil weighs less when saturated.

www.PeirceEngineering.com

RE: Tension Pile Load Test

If the pile is not cast yet, you could cast in at least 28 0.6-inch diameter Grade 270 post-tensioning strands. Rent a hollow jack and wedge plate used for stressing high-capacity rock anchors. It should have sufficient movement capability.

Hire a pro to run the test. There is some danger here.

RE: Tension Pile Load Test

mbilal89

for a 20m length, prestressing strand extension should be in the order of 160mm. Multistrand jacks should have stroke of 200 - 300mm.

And you could anchor and grab again if you have to so that ytou can increase the stroke..

And this is assuming the strand is debonded so the force reaches the bottom of the pile. Does it need to or can the prestress tendon be bonded.

RE: Tension Pile Load Test

(OP)
@BigHarvey, that would be expensive.

@PEinc, what do you mean by the mobilisation of the cone of soil? The stratum is dense sand up to 4m depth, followed by claystone of 2-5 MPa unconfined strength. I think the pile can take the load. I am thinking of prestressing strandwires for this test. I am only worried about the elongation.

@Fattdad, thanks for the contact.

@aeoliantexan, I guess 28 wiress would not be enough. The cross-sectional of a 12.7mm dia strandwires is 98.7mm2 and the breaking strength is 1.8MPa. So each wire would take 18 tons of load before breaking. that would be atleast 37 wires; more if we consider an FOS. End blocks, jack and wedges are available.

@rapt, The tendons would be bonded with the concrete. I am planning to insert two bunches of strandwires, 25 wires each. It would be embedded to 10m into the pile.

RE: Tension Pile Load Test

mbilal89, the pile will be bonded to the soil and/or rock by side friction. As the pile is pulled up in tension, the pile wants to also take the soil or rock up with the pile. Theoretically, the pile can pull up a plug of soil or rock, the plug shape usually being assumed to be a cone shape. The uplift-resisting mass of the cone of soil or rock needs to be equal to the tension load multiplied by a safety factor. If the rock were solid, without any fractures, you might have more than the volume of a cone but, with fractures, a cone is the usual assumptions when designing tiedown anchors. It is not uncommon that the length of a tiedown anchor is controlled by the required cone height rather than by the required frictional bond length, especially when the ground water table is high. I realize that the tension pile is not a classical tiedown ground anchor but it is performing the same function and requires a similar design procedure. See attached file showing cone mass.

www.PeirceEngineering.com

RE: Tension Pile Load Test

1. Aside from a cost savings, is there any reason to use tendons over threaded bar? Threaded bars are much safer, and easier to work with in terms of end anchorage. I have never used tendons on a tension pile load test.
2. 1,300 kips is very high. High powered math: ignoring top 5', then eff sidewall = 60' with diameter = 3.0'. fs = 1,300 kips / 565 sq ft = 2.3 ksf, if perfectly and uniformly resisted (it isn't).
3. I'm betting a cheeseburger that the load doesn't hold, but am happy to be wrong. I would really like to see the load frame and test results.

RE: Tension Pile Load Test

I agree with ATSE. Also, embedded threaded bars would have less elastic stretch than strands. Using multi-strand tendons could allow the use of a single test jack. Using two threaded bar tendos might be easier with two jacks. I would want the embedded tendon(s) to be fully bonded to the pile - no unbounded length = less elastic stretch = less jack ram required.

www.PeirceEngineering.com

RE: Tension Pile Load Test

Seeing it is bonded, how much extension are you expecting with strand. It should be about 8mm per m of strand. I would think you would only have a few metres of strand between the end of the pile and the jack, so even with strand breaking bond in the top of the pile the extension would only be in the order of 40-50mm. Easily within the capacity of any reasonable PT jack.

You can get 15.7 - 15.9mm strand which takes about 300KN ultimate, so you could go to about 240KN per strand. Any reasonable PT company should be able to supply that. Some have jacks etc for 19mm strand if you want. We used that many years ago for pile tests.

Agree with the comment above about hiring a PT company that knows what it is doing to do the tests.

RE: Tension Pile Load Test

(OP)
@ATSE, I will share the results, it should be somewhere around the mid of next month. I will be looking forward to that cheeseburger :D

RE: Tension Pile Load Test

(OP)
Attached with this post are the results of the uplift pile load test. Post your comments on the results. I have used threaded 40mm bars for the uplift here. I am amazed to see the even with method, there was alot of elastic elongation. I doubt if the load was even transferred to the pile or not.

RE: Tension Pile Load Test

mbilal89:

1.9 mm of total pile displacement is small.

You have 4 dial gauges measuring off the reference beams - was it possible that you were in the zone of influence of the tension pile, such that the dial gauge readings were not from an independent reference ?

Did you have a another EXTERNAL reference point that you surveyed to verify total pile displacement.

Did you have a load cell above the jack to verify the applied loads?

Also, what was the stroke of the jack? Was it possible that you ran out of stroke, and what you were capturing on the pressure gauge was internal hydraulic pressure with the ram at max stroke?

RE: Tension Pile Load Test

(OP)
@Ingenuity,
Yes it is small.I recon that most of the load was borne by the elastic steel.

There were 4 dial gauges only. I am not entirely sure about the influence zone of the pile. But in my opinion, if that were the case, then there would have been some visible cracks on the ground surface. Some heaving of ground or cracks would indicate that the ground is being disturbed by something. i have experienced this before, cracks and heaving, at another job site.

there was no other source of measuring the uplift.

The stroke of the jack is 150mm. It is enough i guess. and pressure won't increase if the stroke is exhausted.

RE: Tension Pile Load Test

mbilal89, I think your testing is against the ASTM recommendations that the reference beams shall be supported at a clear distance of at least 5 times the test pile diameter. The picture shows that the supported reference beams are inside the cracking zone area. So the maximum displacement of 1.9 mm is meaningless since this was influenced by the supported reference beams movements.
The clear distance of at least 5 times the test pile diameter shall be applied to the distance between the test pile and the concrete blocks as well.

RE: Tension Pile Load Test

Quote (mbilal89)

But in my opinion, if that were the case, then there would have been some visible cracks on the ground surface. Some heaving of ground or cracks would indicate that the ground is being disturbed by something.

There are ground cracks, somewhat away from the test pile, but closer examination of the area around the pile zone may reveal evidence of ground movement.



Quote (mbilal89)

and pressure won't increase if the stroke is exhausted.

Yes, it will. Internally, if the operator neglects to notice the jack stroke, the pump will continue to pressurize hydraulic fluid through the hoses, and the gauge will increase in pressure reading. It will not increase the load to the pile (as the stroke is maxed out) but the gauge reading will increase quickly. If the internal pressure exceed the max pressure (typically 10,000 psi, or 70 MPa), the system relief valve will release.

RE: Tension Pile Load Test

LOk is correct. Your reference beams moved together with the pier; instead, they should have been at least 8m long.
Your test results, if accurate, are impressive indeed, but valid for skin friction stresses only. Your deflection data should be discarded; it has no value.
Also, your compression reaction supports are way too close to the pile. ASTM D1143 gives minimum distances (usually 3m min from pile to closes support for small diameter piles), which minimizes but does not eliminate the overlapping stress cones. For this test, your closest reaction should have been no closer than 4m.
You ran a direct shear test, not a pile test.

RE: Tension Pile Load Test

(OP)
I understand that we were not actually going by the book (ASTM) when we talk about the procedures of testing. But I would like to point out that the code also suggests that the engineer may increase or decrease the 5D distance.

About the cracks, I doubt that the cracks you are pointing out to are due to ground disturbance. They may as well be, but from what I have experienced , the pattern of cracks is not like what is shown in the figure.

We had our limitations, because the pile was located on the median of a road, and we could not disturb the traffic on the road, so we couldn't use longer reference beams.

The information regarding the pressure on jack is new for me. I'll investigate that.

What do you mean when you say that we conducted a direct shear test?

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