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Geotechnical76 (Civil/Environmental)
18 Jun 08 0:31
I am designing a sheetpile where rock is 45 feet deep. The sheetpile is anchored at a certain distance from the top (above ground water table). The sheetpile analysis indicated that the pile should be extended beyond the soil/rock interface. What is the best practical method to install sheetpiles in rock? Thanks.
Doc08 (Geotechnical)
18 Jun 08 8:45
Sheet pile in rock? Hmm, that is a tough job myfriend.  If the rock is relatively weak & fractured you could start by predrilling the hole down to the required depth into the rock.  You need also the right equipment and a smooth operator in order to minimize the damage to the sheet piles.

At one site where we had anout 6-8 of sand followed by Marlstone (unconfined compression between 4.5 and 12.5 Mpa) we were able to drive PU16 section, 15-17m long piles, with an ICE 815 vibro hammer anywhere from 60cm to 3m into the marl, without predrilling.  Incidents of declutching and damged piles were very limited (3-4 piles out 465) but we had a hell of crew performing the job.

good luck
PEinc (Geotechnical)
18 Jun 08 12:36
Sheet piling usually can't be driven into rock (with or without driving shoes) - hightly weathered or decomposed rock maybe for a short distance, but don't count on it.  As Doc08 said, you don't want to overdrive and damage the sheets.

Recheck your design. Maybe you could be a little less conservative with your soil values or your safety factor for passive resistance.  How are you applying the safety factor for embedment?  Are you factoring down your Kp or are you using full Kp and then increasing the embedment length by at least 20%?  Reconsider your coefficient of passive earth pressure.  I know this will generate a lot of comments, but try looking at using Coulomb's equations to calculate your earth pressure coefficients using appropriate wall friction.

Some projects have used steel pins drilled into rock at the toe of the sheet.  The steel pins provide a shear reaction at the sheet tip.

If you can lower your anchors a little, it might reduce the embedment requirement.
civilperson (Structural)
18 Jun 08 14:54
Your analysis is suspect when it requires sheet piling into rock.  Perhaps lowering the water table would be a better approach.
Chewi00 (Geotechnical)
19 Jun 08 13:54
I have used rock pins like PEInc suggested.  usually you attach a sleeve to the pile before driving, clean out sleeve then drill into rock, place your pins and grout into place.  If you fix your toes in the design, it should give you the shear force at the bottom you need to design the pins.  

Also, have you explored a SP&L or secant wall?  Those area easier to socket into rock.
Chewi00 (Geotechnical)
19 Jun 08 13:57
oohhh.  I forgot, we used "rock teeth"  in once instance.  These are fabricated to the bottom of the piles and help with obstructions and rock.  That was a very stiff sheet, impact hammer and rock with poor RQD though.   
muuddfun (Geotechnical)
20 Jun 08 13:19
How deep do you usually go with the pins?  How well do they work?  How easy is it really to develop the required shear?  How much shear can you develop, i.e. how stiff do they make the end of the pile?  How much does it add to the cost of the sheet pile installation?  How do you monitor the construction quality to determine that you have an addequate pin installation before excavation starts?  Iv got a bridge job comming up where I probobly need to build a cofferdam for abutment construction in the stream, and their is probobly very hard rock shallow enough that I wouldn't be able to get enough embedment for sheet pile under normal design conditions.  Do you know of any literature about rock pins for sheeting?  Thanks
mapelCE (Civil/Environmental)
30 Jun 08 18:49
Geotechnical 76,

I think PEinc and AChwalibog are right on.
You will spend a fortune trying to drive into rock if you can do it at all.
We just shredded some pretty solid sheets getting them throught stiff clay (N=40)
If you can't adjust the design assumptions with reasonable comfort, then I would recommend a secant wall as AChwalibog mentioned.
Given the price of steel and the price of dealing with rock, we would typically go to the secant option.

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