Pile foundation remedial works-any good idea ??? 1

[22222222]

Dear All,

Looking for any information, experience, literature on pile foundation remedial works. We may have less friction on few our piles according to the recent loading tests and it will be probably necessary to improve pile capacity against uplift.Settlement is no problem.Unfortunatelly some piles are already built in the ground so Is there any cheeper soulution except the more pile installation within pier and extending pile cap? Perhaps additional concrete pour on the pile cap or drilling of deeper anchor through the pile cap.Any comment will be greatly appreciated.

 

[PEinc]

For uplift design of piles, refer to Bowles' Foundation Analysis and Design, 1968 Edition (probably in newer editions also). Bowels presents a few different equations to determine the static uplift capacity of piles. I've used the Ireland method successfully when a geotech insisted it was not correct and wanted twice as many uplift piles. The contractor installed the extra tension piles and successfully ran the pile load test. Then he cut loose the extra piles and reran the load test with the number of piles I originally designed using Ireland. The fewer piles also held the uplift test load. The bottom line is: Different equations or methods do work. Find one that gives you the answer you need and that is acceptable to the regulatory agency, if any.

 

[PEinc]

PS, If load tests show that you may not have enough friction, you could consider compaction grouting around the piles. This could improve the surrounding soils and increase the friction.

 

[Focht3]

Different analytical methods are of no use if load tests indicate you may have a problem. Do study the "problem" results carefully and be sure that you really do have a problem.

What kind of piles are you dealing with? Dimensions? Soil conditions? Design loads - and expected capacity "as is"? Is the pile cap already in place?

 

[22222222]

Thanks for all your information!
Piles are 2m in diameter,bored pile and cast in place using casing and oscilator for excavation. Ground is composed of mudstone,silty mudstone,muddy samdstone interbedded with very dense sand/silty sand . SPT values are more than 100 already from 4 m bellow the surface all the way down. Uniaxial compressive strength of MS is about 5 Mpa. Ground water level is very high and artesian also, but pile are not as deep as artesian level observed during site investigation boreholes up to 45m.Rock mass is fractured with vertical joints spaced less then 20 cm with slickensides surface. Pile lenght is 23 m, working load 850 tone and pile cap is not built yet in the location of pile load test, but already done in some other places.Expected ultimate capacity in compression is 1900 tone and in tension 470 tone.Is it true that friction in tension is allways less then in compression??Some people say that it maybe at least 50% less. I think that it is to much, but have some difficulties to separate end bearing and shaft friction.
I am not sure that compaction grouting would be helpfull here since fracturation of rock mass.I will try to get some info on this Ireland method but so far I did nor here about it. Thanks

 

[boo1]

Check link:

http://www.usace.army.mil/publications/eng-manuals/em1110-2-2906/toc.htm

 

[BigH]

I've mentioned this before but it has been put forth to me by a learned type: Try plotting dimensionless settlement (i.e., settlement/pile diamter) vs load. At somewhere between 1 to 2% you should see a break; this is the takeover from friction to bearing. Try it; some results I've seen look encouraging in this regard.

Did you do a tension test? If not, 420 tonnes is a lot - is this "ultimate" or working load? Same with compression.

Would like to see the test data.

Best regards

 

[Focht3]

I'm not sure that I understand the excavation technique; but your problem may be due to weak materials that were trapped between the concrete and surrounding ground as a consequence of the concrete properties and casing extraction sequence. (Was the concrete slump less than eight inches (20 cm)?) If trapped spoils are your problem, then high pressure grouting (PEinc's suggestion) is one possible solution and has a good chance of working. (The French have done some interesting work in this area.) But it has to be a high pressure grouting job - to build up lateral stresses around each pile.

Uplift skin friction is not smaller than compressive skin friction per se; but the consequences of exceeding the uplift friction are often catastrophic, so the uplift friction capacity often is treated with more conservatism than the compressive skin friction. A 50 percent reduction looks pretty severe; 30 percent is more common (in my area.)

I agree that your uplift load looks very high; what is causing the load? Does the uplift force already include factors of safety?

Your problem can be handled with mass concrete, but that seems like an awful lot of brute force.

 

[PEinc]

Instead of adding mass concrete or grouting, you could consider adding drilled and grouted tiedown anchors to resist the uplift. They could easily be added where the pile caps have not yet been poured. Where the caps are already in place, you could install the tiedown anchors through the cap and lock them off against the top of the cap, if you have vertical clearance.

Make sure that, besides friction bond capacity of individual piles, you check mass stability. The bond value times the bond area for the piles or tiedown anchors may be greater than the total resisting mass of soil or rock that could be mobilized. Sometimes the tension piles or anchors need to be longer from a mass stability standpoint - especially when you have buoyant soils.

 

[Focht3]

Okay, grouted anchors will work - theoretically, at least. But 22222222 has over 400 metric tons per pile to make up; that's a lot of anchors! Another answer would be to add more drilled piers to each group; enhancing both the anchor's weight and uplift resistance. Not an ideal or elegant answer, but it may be the fastest and cheapest overall solution.

Buoyant soils are not a problem with this site (sands, sandstones, mudstones.) The 20 cm spacing on vertical fractures is a concern; but my gut tells me that construction technique is the likely culprit in this case. I never use casing with a rock hole - unless I don't plan to use skin friction in my design.

 

[PEinc]

You are correct that 470 tonnes ultimate (1036 kips) is a lot of load per bored pile (caisson). However, we are talking about supplementary anchors to make up the design load shortage with a suitable safety factor - not to replace the entire, ultimate, uplift capacity.

I said, "when you have buoyant soils." My comment about buoyant soils was a general comment, not necessarily applicable to a particular project. Besides, fractured rock could have significant water in its joints and therefore could be buoyant.

 

[Rawand]

I think that 30% reduction in uplift friction is rather sever, Uplift friction is equal o the sattelment friction but
1- resisting uplift the weight of the pile is considered
2- Reducing total area of contact between the soil and the pile decreases the fixing resistance
3- I think You houldnt go for Piping the first place, sepecially for such strata, Deep foundation was a better solution

 

[Focht3]

Regarding the 30 percent reduction: this is applied to the design ultimate skin friction value - not the total resistance. Put another way,

(skin friction * 0.7)/F.S. = (Uplift Force - Dead Weight)

It may seem too severe to some, but that is how these calculations are typically done. The soil F.S. may range from 1.2 or 1.3 for very infrequent short term loads (like a tornado) to 2.5 or 3 for sustained loading on a critical foundation element. Again, these are not my recommendations - it's what is customarily done.

A 2 meter diameter 23 meter long (79 inch by 75 feet) drilled pier is a large, deep foundation; I would imagine that these were hard enough to install. (The mudstone had a tested compressive strength of 5 Mpa - about 104 ksf. At 100 ksf, silica sand grains crush.)

PEinc.: It appears that I have misread 22222222's capacity figures.

22222222: Do you have uplift test results? What are they? If you don't have uplift test results, what are the compression test results? Were the piers instrumented? Can you separate bearing from skin friction?

 

[PEinc]

It's probably too late for this project but it might have been a good idea to perform one or more O-Cell (Osterberg Cell) tests to determine the friction (side shear) and end bearing capacities for these bored piles (caissons). It's cheaper than a load test and cheaper than remediation.

For information on O-Cell testing, see

http://www.loadtest.com

They have offices in Gainesville, FL and Baltimore, MD.

 

[Focht3]

Osterberg cells are great - but are out of the question unless the owner is willing to install one or more new test piers. But a good suggestion for future projects -

 

[22222222]

Thank you all for your valuable comments.
Only just for repeat I want to explain the method of excavation of our bored piles.We used the oscilator with 360 degrees rotation. Casing was inside the oscilator that squeezed the casing and push him down.In the mean time the few tone heavy steel grab was dropped and lifted to break the ground and remove the soil/soft rocks.Steel rebar cage was later lowered down and all piles were then cast in situ.Concrete slump was OK. WE did of course pile loading test, which showed skin friction in mudstone layers between 15-23 tone/m2, but for design we used only 12t/m2. We have concluded that pile did not fail in compression even if larger settlement occured, but friction in tension is less then foreseen.We have also checked anchor pile uplift which was between 5-10mm, but not the same for each pile. A 50 percent reduction on uplift skin friction is too much, but this what some people think so.40 percent is what we used.We have also made the plot settlement/pile diamter) vs load that give us ultimate skin friction about 1200tone. Less than expected.It is difficult to say how much load was taken by end bearing since the pile did not fail.Now we will try to do pull out tests on smaller diameter piles say 300or 500 mm diamter and install devices to measure skin friction again in neighbour areas of previous test.Casing in soft rock is importan because of high water level, which may cause soil collapsing to the hole bottom and cleaning is very difficult.In between the mudstone layers we have also dense sand but almost with no cementation so it is also possible that water may migrate between the ground and concrete and be a one cause of decreasing our friction.