How can I model a weak rock in sheet pile retaining wall software 2

[gordoncraigbrown]

I am undertaking the analysis of a Sheet pile combi-wall. The site is underlain by a weak layer of sandstone and I wish to model the wall as being driven in 1 metre into this layer to act as a keyed toe. I am looking for some information on how i can correlate a phi and c value for weak sandstone to input into the software. I am using WALLAP to model the wall. Thanks in advance.

 

[PEinc]

And how do you propose to drive the sheet piling into the rock? If you can drive into it, it isn't rock.

 

[gordoncraigbrown]

It's a combi-wall, so tubular piles with sheet piles. We will be able to get a set into the rock with the tubes but I am after some info on how i can model a weak sandstone layer as a soil for the last metre or so.

 

[PEinc]

As I said, if you can drive into it, it is not hard rock. Model it with soil properties for very dense sand. Use a fairly high unit weight and friction angle. Ignore any cohesion.

 

[fattdad]

Here's a copy of my thesis, which may provide some insight on the values of phi and c for various rocks. I'm somewhat in disagreement with PEInc (not to start anything, that is), but soft rock (I actually took a graduate class in soft rock) is a shade of gray that is in there between soil and hard rock. I could imagine pile penetration into soft rock and I could imagine relying on some unconfined compressive strength in design. Then again, all I have for your project is a little glimse from cyberland.

http://home.comcast.net/~fatt-dad/thesis.pdf

Enjoy!

f-d

¡papá gordo ain’t no madre flaca!

 

[DRC1]

I agree with PEInc. Even weak rock, you probably will not be able to penetrate a meter. Even if you were able to drive a spoon, driving a tube is different. N>50 in weathered rock grading into rock to me idicates the high probability of excessive driving stresses. Pile damage is bad enough for axial piles, but for lateral piles it can be very serious.
I have in the past year or so seen quite a few projects call for penetration farily deep (1.5 meters or so) into weathered rock. Generally the piles fetch up in the soils overlying the rock.
I would suggest taking a look at the practicality of driving so deep into this layer.

 

[Doc09]

we did a job few years back where we had sand over marl. the marl's unconfined vaues ranged between 4700 and 13000 KPa, we were able to penetrate the marl anywhere from 75 to 2m using an ICE 815 vibro hammer and PU16 sheet piles.

 

[gordoncraigbrown]

Thanks guys I will have a read through your thesis fatdad.

 

[PEinc]

Doc09, did you ever get to see the tips of the sheets after they were driven?

 

[Doc09]

[ First let me start with a slight correction. In my earlier post I wrote 75 to 2m, it should read 0.75 to 2m ]

Actually yes, very little damage believe or not. That's the nice thing about sheet piles, you can pull them out and check them again if you have to before reinserting them into the ground, and we did that with all of them. It was a relatively slow operation, but we did not have another choice. Piles were 12 to 17m long.

Furthermore, out 347 piles driven we had only three that experienced significant damage which was mainly due to unclutching caused generally when the tip of the pile encountered a relatiely harder element and started deviating to end up with a twisted pile.