Caving of Bored Pile Construction 2

[Vporrazzo]

This is about a problem we are having for a 1.5m bored pile construction The following is the situation:

1. Boring data shows loose silty fine sand from surface to depth -5m, underlain by soft to medium stiff silty clay (N value=2 to 6) to -25m depth after which depth the clay becomes stiffer and eventually quite hard (N>50) at about -35m. Water table starts about 3-4m down.

2. 1.5m diameter permanent steel casing (6mm wall) was set to 12m depth and then the hole was advanced to depth -35m using a 0.9m diameter bucket. Contractor planned to later lean out hople to 1.5m diameter. Contractor used Supermud for drilling fluid. Upon reaching -35m, it was decided to extend the hole to -40m depth because it was reported that bottom was still soft and samples of soil had high moisture content. I subsequently concluded we were looking at caving material and not actual bottom of hole conditions.

3. After reaching -40m depth, contractor backfilled hole with sand since no decision on acceptance bottom condition had been made and we were afraid of caving. Backfilling operation took a lot more sand than anticipated so we concluded that caving had been occurring right along.

4. Contractor now proposing pulling casing out and placing new casing to depth -30m using 10mm thick wall. He claims caving zone is at depth of 12 to 20 m but I suspect it is full length of uncased hole.

I believe this is a terrible idea to pull casing as we are likely to worsen the caving problem by pulling casing and could very well knock down the existing bridge as we are right at the abutment area. I am sceptical if they will be able to even get the casing out at all without damage to abutment.

I believe they should redevelop the hole to the required 1.5m diameter by drilling short sections, placing bentonite-cement to stabilize the section and then redrilling thru the hardened mass. Is this a reasonable approach or should they just redrill it using appropriate drilling fluids and complete the pile with a larger diameter in the caved areas?

Your comments would be grealy appreciated.

 

[22222222]

Hi Vporrazzo,

Only simple advise.
I would also use the casing all the way down from the top to the bottom. Backfill your hole within the casing level and excavate again.We had better ground on our site but still had a problems with high GWL, including artesian.If your site is an industrial area have a look on the water wells there to avoid future ground settlement problems and make your pile design adequate to this.

 

[BigHarvey]

A polymer mud should be adapted to the type of soil encountered. Nevertheless I would make a try with bentonite mud and a short temporary casing as already used.
If caving still happens, you should look if artesian conditions are not present because that would be the only explanation. If you have an artesian water level then the steel casing shoud be permanent.
The 10 mm diameter casing seems too thin for me : it will deform a lot ( it is usual to take 1/100 th of diameter or here 15 mm ).

 

[jheidt2543]

Vporrazzo,

A very nicely detailed question. As we all learned in Engineering 101, the first step to a solution is to "State the Problem". I'm sorry I don't have a solution for you but, it is nice to follow a thread that starts out so well.

 

[Vporrazzo]

Thanks for all the quick and useful replies. By the way, the site is by the seaside and there is no evidence of artesian pressure but I wouldn't totally discount this either as the inspectors are not very experienced in this type of construction.

I agree with the reply of BigHarvey about use of bentonite. The contractors here (Philippines) have become overly relient on use of polymers and casing instead of using bentonite in my opinion. Bentonite requires proper support equipment while Supermud and casing does not. The work gets sloppy and they don't maintain sufficent head in the hole and they become careless or use insufficient polymer (stuff is expensive!) and probelms develop. Standard solution from contractors side is extending casing at owner's expense. My usual reply is "If we wanted a steel pipe pile, we would specify a steel pipe pile!" Traditionally, good foundation contractors have been able to work without using permanent casing under pretty poor ground conditions. I don't consider these conditions as I described really terrible, running loose sands are much worse to handle in my view.
As BigHarvey also mentioned, the casing is a bit thin, a design blunder I'm afraid as we usually use at least a 10mm casing. Fabricating much thicker than 15mm is a problem here. We let it go with 6mm for the 1.5m pile as the ground is pretty soft and getting it down wasn't a problem. We changed it to 10mm for the piers where we have larger diameter piles.

Maybe I'm just cheap and old-fashioned but I expect the contractor to bid the job based on the design drawings and info provided. There was a also screw-up during the drilling when the Resident Engineer delayed the decision on extending the hole for 2 days because of soft bottom conditions so the contractor probably does have a claim basis for the caving. Just feel if he had used bentonite in the first place we wouldn't have had the problem.

Anyway, the basis problem is where do we go now: extend casing or drill-grout-redrill. If casing, I think we need to telescope the size down within the 12 m casing section. I don't want to try pulling it, we are next to an existing abutment. Can't extend the present casing either as it is admittedly too thin. So telescope and extend may be a solution but rather expensive plus we would need to grout the space between the old and new casing I think.

Feel free to throw some other options at me. Also I hope I am not too long-winded with my comments. I am used to kicking this stuff back and forth with the USA office so I tend to get into a lot of information and detail that may not be appropriate for this forum.
Anywany, thanks again.
Vince Porrazzo

 

[Focht3]

Congratulations on presenting an exceptionally well stated problem. Our comments will be much more on target as a result.

While the water table is fairly shallow, I doubt that you have artesian conditions. It sounds like the site conditions are just tough to deal with when installing a fairly large drilled pier (bored pile.)

My first reaction is that the caving was occurring within the cased zone, falling down into the hole created by the 0.9 m auger. Pulling the casing may or may not cause problems for the existing foundations; that evaluation will depend on where the collapse was actually occurring and the design and construction of the existing foundations. Did you talk to the actual driller, or just the driller's manager? If the driller is very experienced, he will likely have a good idea about where the collapse is occurring. Give his opinion some weight -

Can you get a larger, longer casing? You could use a 1.7m casing and drive it around the existing casing, then extract the 1.5m piece. If you go this route, be sure that you prepare the 1.5m casing for extraction before you drive the 1.7m section - or you may have hell getting the 1.5m section out! I'll call this approach the "strong arm" method. I agree with [blue]BigHarvey[/blue]: 10mm is too thin; I'd prefer 20 to 25mm, although 15mm might be fine.

Another approach would be to use a low pressure, low strength grout to fill any voids. This has the advantage of a much reduced volume of soils to be removed. While there is no guarantee you will fill all the pockets, it should get the majority of them. You could then extract the existing "short" casing and install a new "long" section. But I don't like using the same diameter casing for the "long" section due to ground disturbance issues - I'd go somewhat larger.

You can use a drilling mud with the existing casing - provided you are comfortable that the collapse zone isn't below the existing casing section. I'm a bit nervous using drilling mud when a borehole collapse has already occurred, though. This isn't my first option...

Did you have someone on-site actually observing the pile construction? Regardless of your answer, be sure you put someone at the site as the work moves forward. I strongly recommend that you use an engineer with good bored pile design and construction experience.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[doc11]

Sounds like you have more than one problem.
You said your site is seaside which probably means the groundwater is either salty or brackish. Straight bentonite won't work. You may be able use a polymer enhanced bentonite but if you grout and redrill the cement will breakdown the polymer. Attapulgite clay will work but it can be difficult to get unless there is a lot of offshore oil drilling going on in your area.
You may be able to salvage this hole by following PDSco's Instructions for using super-mud in salt water, they are pretty specific, and can be downloaded from this link:

http://www.pdscoinc.com/foundations.htm

and adjusting the concrete mix to allow for a higher slump so that it will flow out into any cavities created if there is anymore sloughing.

A couple of more thoughts would be to rotate and pull the tools very slowy to minimize any mechanical distrbance, and keep the hole full of slurry at all times, if the whole column rises when you pull the tools you are creating a suction on the bottom and sides of the hole, you may want shorten the lenght of your drill stroke and/or change the type of bit to keep this from happening

 

[GeoPaveTraffic]

I won’t try to address the issue of what type of drilling fluid to use, no experience in that area and you already have plenty of opinions. The one comment I would like to make is that instead of trying to pull/remove the existing casing so a longer casing of the same size can be installed, why not install a slightly smaller casing through the existing casing?

I realize that this will mean that the completed size of the bored pile will be smaller than the design diameter and that the total length of the pile will likely need to be increased to achieve the required capacity. But this may be a simpler, cheaper, and safer alternative to trying to remove the existing casing.

Just an idea that you might want to consider.

 

[Vporrazzo]

Been talking to the manager, good idea to talk to the driller too. Regarding the salt water environment, we usually flush the casing with fresh water, having cased off the obvious entry points for salty water and use bentonite mixed with fresh water. If hole is completed quickly the bentonite mud seems to stay ok.

As I mentioned before, current plan seems to be to go with a smaller, inner casing extending down to 25m. Contractor seems to think caving area between the 12m to 25m depth. We will redevelop the hole from there to -40m depth, backfill with tremie concrete to -35m and set the reinforcing cage from the top to -35m and complete the concreting. We will grout the annular space between the larger and smaller casing as well. Hope the other holes go better.

Thanks for all the comments, I found the ideas useful and the discussions very interesting. Isn't the internet great?!

 

[BigH]

Some time back in the Geotechnical News put out in Canada/US there was an article about a downhole calipre that could be used to measure the "width" of the hole - you seem to be wholly unsure where the cave was - it might be possible if you had this tool (or could jerry-rig one up from the picture given in the article) to actually determine the location of the cave -

Hard to believe that soft clay caves unless the geotechnical investigation missed a series of sand seams. This could happen depending on your level of geotechnical drilling expertise. I saw a bored pile in NW China which was to be in clayey silt loess - with plasticity - and it caved horribly bad - so bad that one pile had 80m3 of concrete put into it when the design was for 20m3. No claims, though. In fact the material was a fine sandy silt. They ended up hand digging to 10m (water level) then used drilling mud for the next 10m.

I, too, give you some (my snaps) for a well laid out presentation.

 

[Vporrazzo]

Thanks for the insight on your experience. They use caliper logging in the petroleum industry wells I know but have never seen one used for a bored pile.

Still think the problem was caused by sloppy work by the contractor. I will put them & inspectors thru the '3rd degree' about the exact timing of the drilling, properties of drilling fluids such as amount of Super Mud, viscosity, sand content, and amount of backfill material used. They have been pretty vague about all of these, just wanting to add casing as the solution. Should have gotten the full documentation from contractor and inspectors on this at the beginning of the problem rather than jumping for a solution.

As Focht3 mentioned, caving probably extends above bottom of casing. This also has me concerned so I am leaning more towards a grout and drill solution right now. One of my friends (a European bored pile contractor) also mentioned I should stay away from cement-bentonite and use a very lean concrete instead. He said cement-bentonite is very slow setting and more generally used for plugging leakage than for caving problems.

Thanks for all the help.

 

[doc11]

You might want to go back and look at the samples from the test borings again, if possible, and maybe do some lab tests on the ones classified as silty clay they may actually be clayey silt especally if the person doing the logging had less than 15 or 20 years experience logging borings. I've seen this one trip up even the best. You stated earlier that you thought the boring was collapsing over the entire lenght since it took more sand than expected to backfill the bottom 5 meters of the hole. Like BigH said soft clays generally won't collapse, but soft silts are just as bad if not worse than soft sands (they remain in suspension longer and you won't know that you have a problem until sometimes it is too late).

From experience I've been able to drill out cement/ bentonite (2%) after as little as 16 hours without any problems, but never any larger than a 12in. hole. If you go with lean cement make it pretty lean or you could end up with a hole deviation problem.

 

[Vporrazzo]

Materials below 12m depth to about -35m are definitely highly plastic, marine grey silty clays, I have the samples in my office from the bored pile bucket. Upper material (0-5m depth) is a very fine sand which tends to flow given vibrations or a bit of water. Regarding the cement-bentonite mix, I have been advised that for small holes it sets up fine but larger volumes as we have for a bored pile are slow to set. I agree that the lean concrete solution needs to be 'very lean' to avoid problems with later redrilling and the possibility of deviation.

Tghanks for the comments.

 

[BigH]

For quick setting cement, there is a black cement we used to use in core holes when going through brecciated zones called "fondue". French, but . . . It sets up very quickly.