sheet piling

[kags]

We have to excavate about 12m below the ground level.The soil is sand. The water table is about 2m below the ground level. The client had purchased the sheet piles before the design was complete. As there are buildings and roads very close (approximately) 12m from the face of excavation,the sheet piles can not be anchored back.The sheet piles are of cantilever type.
The client has the piles in 12m length. So, we designed the sheet piling in a step like pattern. It is the first 6m depth of excavation will be carried out with 6m embedment depth. And then we have to drive another sheet pile , starting 6m below the ground level and goes down by 12m. So that the balance depth of excavation 6m can be carried out.
This lower level sheet pile wall also will have 6m embedment depth. In such condition, what should be the minimum distance to be maintained between the sheet pile walls? Please expalin how the overall stability of the excation/ sheet piling can be checked ?

 

[RWF7437]

You may find this reference useful:

Engineering and Design - Design of Sheet Pile Walls (Engineer Manual EM 1110-2-2504, 1994 edition, 75 pages, PDF file format

You can get acces to it through

http://www.pdhcenter.com

Russ Faust
Salem, OR USA

 

[timmc]

This sounds like a very difficult situation to me.

The groundwater will be a big problem--you may want to check with your geotechnical expert to model what type of piping you can expect. The "step" will mean there are more locations that water can seep in, and must be controlled.

I don't know your exact layout, but you might have construction access problems with a "step"--is there an option to put some inside walers in, maybe using toe braces to the floor of the excavation?
Lots of design information and guides are available through

http://www.pilebuck.com

, I don't know which one directly applies to your situation.

 

[PEinc]

kags,

I agree with timmc. You did not say what you are building but you probably need to use a closed, braced (wales and braces) cofferdam. You could dewater, but you would have to consider the effect of dewatering on building settlement.

Even if you had enough room to install the tiered sheet pile walls, you probably will not be able to show that 12m long sheets will be sufficient in length, let alone strength, to cantilever 6m while supporting water pressure and most likely building surcharges. This does not even conside the problem of leakage and/or soil loss between the two wall levels. In addition, if you could design the cantilevered sheeting, you probably have excessive sheet pile deflections and therefore settlements behind the sheeting walls and under the building. In addition, if the two tiers of walls are now far enough apart, the upper wall will add load to the lower wall. The closer the walls are together, the closer you are to having 12m of earth pressure rather than 6m of pressure twice.

Although I always consider what materials and methods a contractor has or prefers, I would design this wall without worrying what size sheets the contractor has. I don't think 12m sheets are the way to go.

 

[jeffraines]

I think you are really asking for trouble if you don't get a registered civil (geotechnical) engineer with local experience with foundation design to do an excavation plan for you. The fact that you asked the question here sort of indicates to me that you don't really understand what you're asking (sorry). You won't get $50,000 worth of free consulting, not to mention you can't get the $15,000 in subsurface exploration you need, on an Internet message board.

Jeff