Pipe Pile Driving 5

[Fuzzy]

We are looking at project that specifies an 8" pipe pile driven closed end. The stratum is generally silt and clay with SPT values of between 2 to 12 (blows) about 120 feet thick. Then there is what we believe is a dense till layer, 30 feet thick, overlaying shale bedrock. The testing company did DCPT testing on assumed till with N values of 40 to 70.
The contract indicates that they want the piles driven to the bedrock. Based on the density of the till layer I do not believe it will be possible to drive a closed end pipe pile throught the till layer.
We are looking at either driving open ended or switch to an 8" H-pile.
Does anybody have any information to back-up or refute our claim that it may not be possible to drive closed end through the till layer? Any other suggesting for completing this project more cost effectively?

 

[BigH]

First off - and I am sure Focht3 will cover my "misses" - you ain't gonna get no pipe pile through 30 ft of till to the bedrock closed end driving. Not an 8" one; not any one. I think that the designer may be a bit off in his expectations. I can't imagine why the design would go for driving an 8" pile through 120 ft of clays/silts, 30ft of till just to reach shale - not the strongest rock either and one that is quite like fractured at the surface due to the ice forces. Also near surface shales might have clayey seams that would affect bearing. Queenston Shale in Ontario usually is fractured/weathered to 5 ft or so in the locations I've hit it - sometimes less. - And, to boot, 8" in pretty small for such a long pile (slenderness ratio will be exceedingly high).

What is the load to be carried by the pile? What "area" are you in - Ontario? Upper New England? You never said - I've done 10 and 12 inch closed ends for more than 100 tons in till. I like closed end tube piles; I've used them many times and they are wonderful (my opinion) driven to till. In groups, you may have to check uplift on driving adjacent piles and retap.

For consideration of other pile types - you could use "H" piles. Again, I wouldn't drive to bedrock - only into the till. With these "knife and butter" piles, you might have to worry about your driving in the till. Our company did a study years back; H piles were driven into till, then load tested. Then they were retapped and went another 10 to 15ft! - then load tested again. Another round too. In each case, after achieving the set, retapping was able to re-enable driving. In the load tests - the "near" failure loads were all relatively the same. H-piles are used extensively - or had been - in Ontario tills (highway bridges). Where the tills are thin, they did reach the underlying shale formations.

Did the designer ever consider friction piles in the "stiff" - N=12 clayey zone? Might need more piles, but they wouldn't have to be so long. Use of step-tapered piles might be advantageous.

One other aspect bothers me - the 120 ft of N=1 to 12 clays/silts. Are you going to place any fills? If so, you need to take into account the negative downdrag forces. If on the shale, this is relatively rigid and would be detrimental (see some of the 1970s papers on driving pipe piles to bedrock and then they failed as the tip wouldn't budge but fill settlements overloaded the piles). In the tills, this would be less of a problem but would seriously need to be taken into account. 120 ft of the clay and silt is pretty thick.

Anyway - the above gives some early thoughts on your problem. Keep us informed.

 

[VAD]

Hello Fuzzy:

I would not be surprised that the pile can reach the shale layer. The design of the pile re size and depth of embedment is interesting if one looks at the issue of downdrag.

 

[Focht3]

[blue]BigH[/blue] is right - those "toothpick" sized piles will not make it through 30 feet of till. Not if it'd really till. And using open ended or H-piles won't make much of a difference.

You indicated the first 120 feet is exceedingly weak. Low blow counts in waterbearing silts can be quite deceptive; often the silts are quite competent from a capacity and settlement standpoint. Did you do much testing in the first 120 feet? I would be particularly interested in CPT and dilatometer results -

And I agree with [blue]BigH[/blue] on the presence of soft, fractured zones at the top of the shale. If it were my design, I'd stop 6 inches into the till.

 

[Fuzzy]

A little more information on the site. The site is in Northwestern Ontario. The underlying clay layer is considered low to medium plasticity, average 30% moisture content, therefore subject to future consolidation under loading. Experience on the same site has seen numerous pile tests driven to bedrock with no additional set at 200 ton loading, H-pile and precast.
We have been tossing around the idea of reducing the number of piles as the pile loading is very small, listed in spec's as 18 ton/pile, and the design drawing indicate a minimum pile capacity of 65ton (taking into account downdrag). This could be a touchy issue as it would be making significant changes to the engineers design. Experience has shown that they can get their backs up when we present a alternate to their design. Note we provide engineering and contracting services.
With regards to friction piles, I cannot think of a single installation in the area due to the presence of the plastic clay layer. Though simply driving to into the tile layer is a very real option as the load is low.

 

[Fuzzy]

I have to qualify the reference to the thickness of the till layer. The soils consultant stopped taking samples when he initiated DCPT testing. Up to that point the soil was silt/silty clay. So silt/silty clay may extend farther before till layer (assumed based on experience). DCPT numbers climb liniarly from start of testing, 15 to 100. Would assume till layer once blow count reaches around 30, which would be about 17 feet from the point of refusal.

 

[VAD]

Hello Fuzzy

I suggest that you ask to review the design so that you can fully understand the premise of the small piles and issue of downdrag. Again, I feel that the downdrag issue is the reason for the type pile design coupled with the fact that the pile is required to reach the shale layer.

A larger pile to reduce the number of piles at face value is reasonable but requires more energy to get it to its proposed toe depth and as well attracts more downdrag load.
My suspicion is that the proposed pile size takes those factors into consideration as well as other factors -structural and lateral loading issues.

If you can demonstrate to the Client that your proposed pile type is going to be suitable with respect to the premise of the design that was used by the designer and it would save them money and be suitable re engineering aspects, then the Client would be hard pressed to reject your alternate.

I suspect that you are dealing with the MTO who would have had the design done by consultants and properly peer reviewed before approving and setting the work out to Tender.

It is interesting to know that the piles could achieve the specified depth. If one reviews the SPT blow counts properly and with experience it is possible to predict the depth of toe embedment quite reasonably. To do this one has to review the spectrum of values.

Good Luck

 

[DRC1]

1. Get a good subsurface profile. Driving over 100ft, it is worth it to do a few more borings to see excatly what you have. I would also get a few tubes of the clay and have it tested to establish soil parameters. If this sound expensive, rember a pile driving crew is going to cost a couple thousand dollars a day while everbody tries to figure out later what is going on.
2. Visit

http://www.unisoftltd.com

for very good information on drag down.
3. Since your loads are light, you may want to consider fewer larger piles. They offer the following advantages: Higher alowable driving enegy, less drag down surface area per bearing area cross section, and probably lowered installed cost.
4. Decide what blow count you need for clay or till and drive to the blow count. PDA testing can be used to confirm.
Once you achive blow count you have the resistance you require. Settlement may be an issue in the upper layer, but this can be evaluated (especially if some one took the tubes in step 1.) It sounds like the till should be able to support the load with out significant settlements, however the complete final design should always be reviewed by the engineer of record.
5. Open end piles tend to form a hard plug at the tip and therefore do not offer a signficant advatage over closed end pipe. Closed pipes allow the piles to be filled with concrete which: allows inspection of the pile after driving, and allows placement of rebar and conrete to increase pile capacity or privde a second form of support if the pile corrordes.
6. Final word of caution, overdriving piles can be worse than under drivivng the piles. Trying to drive a pile, especially a small pile 100 feet plus then 17 feet into till is a situation wher the pile may easily be damaged.
Good Luck

 

[Focht3]

Remember that a pile 120 feet long and 8 inches in diameter will have a L/D of 180. That is an exceedingly slender member. [blue]DRC1[/blue] is right - overdriving is a much bigger concern than underdriving. The pile is so slender - and small - that you probably would not see a change in driving resistance when the pile tip is damaged -

Depending on how the calculations shake out, driving closed pipe piles to the top of the till might be a feasible approach. You might offer to inspect the inside of the pipes with a sewer camera to document the final condition of the piles. (The results are videotaped - so that all can review the pile's condition.) This will ease a lot of concerns on the part of the original designers.

 

[PHEs]

Hello Fuzzy


I am not a geotechnical, so I feel free to come up with a perhaps crazy idea.
The idea is to have a small steam pipe with a one-vay valve in the closed end pipe. When adding some amount of steam, a steam bubble, the steam bubble lift the above water column and condense. When the steam bubble condense, the above water column will come down with a very high velocity and acts as a hammer, a water hammer.

If you make a simple test with a small pipe 3 or 4”, so please let me know the result!

With kind regards,

PHEs

 

[clayR]

Yes, an 8" pipe can be driven to rock, but will require predrilling.

Open ended pipe will require same energy to drive as closed
end pipe. Both will refuse long before you reach t/r.

You could Predrill the hole to rock, install an 8" can in the upper 20 foot to stiffen the pile up. Drop a sheathed #18 bar down the hole and grout. Negitive skin friction goes away and you reduce the number of pile required by 3 or 4. This is actually a micropile. Of course the lower 20 ft or so would be bonded (unsheathed) in the till for the load transfer. Not designed for end bearing.