Monolithic river bridge 3

[1967pradyot]

I have a problem in hand for which I wish to hear a solution from our experts.
The structure is a monolithic river bridge. The foundation proposed is 1.5m bored cast-in-situ pile of around 30.0m deep. The sub-soil in general consists of alternate layer of sand and clay.The top layer is 10.0-11.0m thick sand, followed by 3.0-4.0m thick silty clay. Silty sand of 4.0 - 5.0m thick is underlain to silty clay. Below which 8.0-10m thick stiff clay and 4.0-5.0m thick silty coarse sand. Foundation tip is in silty coarse sand. Use of Bentonite is not permitted.
I would like to know whether this pile can be bored without permanent casing? If so, how? If temporary casing of 30m is used, how to extract it without using vibrohammer. Or, any other suitable method which can stabilise the hole without casing. Geomud is used but of no use. If you have any earlier experiences please share the same.

Thanks

 

[BigHarvey]

You won't be able to guarantee the stability of the hole unless you use bentonite or polymer mud. In coarse grained materials, it is often necesary to add some bentonite to the polymer ( around 7 kg/m3) in order to have enough fines to build a cake on the walls of the excavation ( otherwise you experience a loss of fluid which endangers the stability of the excavation ). If you need to use a temporary casing and do not want to use a vibro hammer to remove it, you can use a casing oscillator ( louvoyeuse in french )

 

[BigH]

Praydot - I agree that holding the hole open will be dubious at best. Why is bentonite not permitted?

How deep is your bottom layer of coarse silty sand below your pile tip or are you soon into another layer of clay?
The reason I was thinking is have you ever thought of using a Franki-pile (also known as expanded base pile). These, I have used and the company for which I worked in New Jersey used many many times in Atlantic City and around the Philly area. Good capacities. Might be able to use in your case - get higher capacities for smaller diameter.

We found, on our project here, that 1.2m dia bored piles were grossly overdesigned (1 to 3mm total settlement after the 24 hour hold). Could it be similar in your case??



p.s. package on its way - advise on receipt.

 

[1967pradyot]

Bentonite is not permitted as per contract document in view of environmental pollution as regards to its disposal (world bank recommendation). BigH, right now I cannot change the foundation type - then contractor will come out with lot of variations and claims. This design was proposed earlier by design consultant. Even if we go for enlarged base pile, to prevent the side collapse we have to provide some kind of protection works. Below the pile tip, the silty coarse sand layer is extended upto another 7-10m.
By the by if we use permanent liner, what is the % of friction can be generated along the pile shaft?

BigHarvey, can you please tell me how this casing oscillator performs? Information regarding supplier of the same? Actually, we do not want to impose any vibration in the casing during withdrawal since we have observed severe adverse effect on reinforcement, sub-soil and green concrete.
Any further suggestions/ opinions to solve the above problem?

 

[BigHarvey]

A casing oscillator is generally a crane attachment ( hydraulic power from the crane/drilling rig )which can apply both a crowd force and a torque on a casing to force it in the ground and then withdraw it once the pile is concreted. For short piles you can use a standard steel tube as casing, for deeper piles such as in your project , you generally use special casings which can be assembled and disassembled easily ( generally double wall to allow room for the assembly bolts ). You can see this type of equipment on

http://www.bauer.de,

then choose BAUER MASCHINEN, machine programme and then casing oscillator.
This kind of equipment does not generate any vibration. The drilling method can then be either rotary or grab and you cn work without bentonite, just with water.
Bentonite has been close from being banned by european laws, nbut the construction industry could persuade the technocrats that there was sometimes no economical alternative ( ie diaphragm walls ). It is possible to work with bentonite and respect the environment. in order to get rid of the mud you have to use separation plant which will separate the water from the clay particles and limit considerably the volume of waste material ( which by the way is just clay !). Alternativel, as I suggested you can use a mixture of polymer and bentonite and treat it as above explained.
I believe it might be cheaper to use a permanent casing ( european codes contemplate the design of friction piles with steel liners )because yo will certazinly have more offers from contractors if casing oscillators are not used in your area

 

[VAD]

Some comments:

Generally in the conditions that you have a pilot hole is always required so that one can observe whether the hole will collapse etc.

PIling contractors generally have the requisite skill sets. In som cases telescoping casing is used . For example, the first 10 m with sand is drilled with a slightly larfer diameter than the required 1.5 m . The casing would be sealed into the clay layer for a short distance. Then the hole would be advance using the auger to give the desired pile diameter. It is possible that the interbedded sand layers below the top 10 m would not slough out, but this is an assumption which could have been assessed re pilot hole.

Regarding the removal of temporary casing, this is done following the pour and in fact the vibrations would assist in ensuring that the pile is integral with depth which is sometimes a concern but often the height of drop of the concrete takes care of that problem. There is no way that you would want to leave the casing inthe ground after the pour.

Re friction if casing is left in ground there are valuss in the foundation books that are used and I am sure that you can find them.

Re; bentonite , I think the fear is the loss of skin friction. I ghave done small strain integrity tests on piles constructed using the Benoto system and they were perfectly okay.That was my fear at the time in 1986. However you may wish to read the paper by Dan Brown- Effect of Construction on Axial Capacity of Drilled Foundations in Piedmont Soils ASCE Journal of Geotechnical and Geoenvironmental Eng Dec 2002 pages 967 - 973.

Unless I wa sure that the soils coud be readily drilled without problems of sloughing I would not have recommended bored piles in the ground with the stratigraphy described.

The ground seems suitable for H piles or open end pipe piles as well. However, I am sure there are other factors that allowed the bored piles to be the foundation of choice.

Genearlly your contractor is made aware of the ground conditions and he may have bid the job with the realization that casing will have to be used and this would have to be left in the ground. He will be asking about that alternative re design if he starts to encounter difficulty.

Many times the contractor is the one to speak to as unfortunately we all do not have the hands on experience despite that we may be able to design a variety of foundation types. A geoengineer working with a piling contactor may however have the hands on. My motto is to discuss these types of ground with a contractor or in lieu have a test pile included during the geotechnical invesigation phase of the work.

I hope this gives some insight.

Regards

 

[Focht3]

Well, you've been painted into a corner not of your making. But there is a way out.

BigHarvey gave you a reference for temporary casing - and did discuss vibro hammers. He's right - the vibratory hammer should not affect the integrity of a properly designed drilled pier. (Although the hammers do cause some ground shaking - but it's small.)

What would I do? I'd install temporary casing to the final bearing depth, then drill out the soils inside. I would flood the casing to be sure that the water level inside the casing is higher than the river level (and/or groundwater level) - and carefully maintain the water level at all times. If the pier includes a "significant" bearing component, I would specify that a clean-out bucket be used to remove cuttings from the bearing surface before the steel and concrete were placed. The concrete would be placed by tremie to the bottom of the pier, with the casing and tremie lifted as the concrete level advanced upward. And I would specify a fairly high slump concrete (6 inch minimum, 8 to 10 inches preferred) to reduce the risk of honeycombing or voids within the concrete.

And I'd think of the vibratory hammer and temporary casing as a very large concrete vibrator.

That's what I'd do. (And I've done it frequently - with great success.)

 

[1967pradyot]

As a step towards reducing the side friction, outer guide casing (telescopic) is provided upto 12.0m. Inside which a temporary casing is installed upto the bottom of pile. The water level inside the temporary casing is always kept higher than the river water level. The proposed bridge is around 20m away from existing bridge where foundation is cassion.
From the experiences of 5 installed piles, it is evident that till the vibration is introduced to extract the casing for first 15m - 20m, no problem was met with. As soon as the vibration (2500 vpm) is applied by vibrohammer, there is a certain adverse impact on sub-soil (temporary localised liquefaction), reinforcement cage and concrete.
Please advise me what will be the skin friction if we use permanent casing (any references?)? How best we can justify the use of bentonite in this stratification? What will be the best combination to withdraw the casing?

 

[Focht3]

Well, permanent casing will work - think of it as a concrete-filled pipe pile. But that's an expensive solution.

Did the "temporary localised liquefaction" create a depression around the pier? How deep? How wide? (Densification of the sands may be adding to the pier capacity...)

If the bearing strata is going quick during the vibratory extraction of the casing, then it must not be providing a great deal of axial capacity (since the phi angle is related to the relative density of the sand, and yours appears to have a D[sub]R[/sub] of less than 60.) Why not make the pier longer, and ignore end bearing? You can prevent the rebar cage from penetrating the sand by holding it with a crane.

Were the existing piers installed using temporary casing? How are they performing?

 

[1967pradyot]

Corrected SPT value of the end bearing strata is 35 and more which supposed to be densely compacted sand. There is no depression visible on the ground level upon application of vibration. However, the reinforcement cage is sunk only after application of vibration. I feel the frequecy applied is on very higher side which is causing this problem.
The existing foundation is on cassion. So, probably no temporary casing was used.
Even if I make the pile longer, casing extraction problem will remain there. Permanent casing may be a solution, but there will involve hefty amount of financial impact!!

 

[Focht3]

I don't think that the sands are liquifying to any significant extent. It sounds like the D[sub]R[/sub] of the sands is 80 percent (or more.)

It has always been my experience that the pier contractor fully supports the rebar cage while the vibrator is operating. Perhaps that is why I have not observed the problem you have described.

This seems to be a "contractor's procedure" issue - not a constructability problem. BigHarvey (or anyone else) - what do you think?

 

[VAD]

Hello Pradyot:

I gather from your discussions that you have installed 5 piles already. I am not sure why the "liquefaction" issue is a problem. This is perhaps only one dimensional and not in the case of earthquake induced liquefaction where the possibility exists for lateral spreading. In such a case the depressions assumed to be large will occur. Hence what is expected is some minor ground settlement which may not be readily discernible. It is not expected that you would get settlements mor than 3 to 6 inches if so much as the situation is not continuous. The settlemnt would add to some site densification as well.

This situation is typically what occurs when piles are driven into sand soils below the water table. When the pore pressure dissipates the strength is regained. The piles are generally perfect in my experience.

If you are in doubt of the pile integrity then do some low strain integrity testing on the existing piles and as a check their integrity. This is done using a hand hammer( rubber mallet) to tap the pile and instrumentation to receive signals. The PDA group generally has the equipment for this work.

By now the piles should have been installed unless you have stopped the job. If that is the case you would have had to have answers or you will be facing a hefty claim.

I would be interested to know the status of the project at this time.

Regarding the use of permanent casing I think if the 5 piles have been constructed without these then just continue the process. The rest is academic in my opinion.

If you wish better advise than you have received so far I would post tis question on geoforum where you can receive the advise of Bengt Fellenius whio would have some comments to make on this subject I would think. This is not to say that the advise given so far is not sound but it appears to me that you are not getting enough to give you the confidence on what to do or what has been done. I think the latter is more what I suspect but I could be wrong.

Regards

 

[BigHarvey]

We have to deal with a project where all parties have already been chosen and piles have to be constructed with what is in hand.
If the contractor has experience in drilling under bentonitic mud, it would be easier to work with about 6 to 9 m of lost casing to start the pile and then drill under bentonite. When drilling is completed, replace the drilling mud with new mud, clean the bottom of the hole, equip with reinforcement cage and concrete with tremie pipe. Environmental problems linked to the use of bentonite can certainly be overcome. Design problems can also be overcome by overdesigning pile length obtained from foreign codes such as the european ones if necessary. I believe that time lost must be more expensive than the above !
If the contractor is not experienced with bentonite drilling,then it would be quicker ( and eventually cheaper at the end...) to drive a permanent casing with vibro and maybe hammer ( for 1.5 m diam casing 30 m long you certainly use quite a big vibrator like PTC 50 H or bigger ). Obviously the contractor in charge has no casing oscillator.
If the piles are very close from the river you can also drill them inside the casing with a chisel fitted with an air lift. We have drilled 1.6 m diameter piles in Guyana in sands at a rate of 9 m an hour with this method ( the piles were for the intermediate piers )