Skin friction for rock socket in Manhattan Schist

[structured]

We are designing a retaining wall for support of excavations in Manhattan. the wall will consist of soldier piles with rakers and wood lagging. The bedrock is very shallow, so the soldier piles are being socketed a few feet into the rock. Due to the steep angle of the rakers, there is a large uplift force on the soldier piles. We are discussing the use of the skin friction on the surface of the rock socket in order to resist the uplift forces on the soldier piles.

Do any of you have experience with this? Are there any publications which discuss the allowable skin friction for drilled piles in bedrock?

I found one reference online which referenced an "allowable skin friction on rock socket" value of 12.5tsf (~170psi). Unfortunately, it was only a scanned page from a book with no information on the book, authors, references, etc., so I am hesitant to rely on that value.

 

[PEinc]

There is no uplift force on a soldier beam due to an inclined raker brace. The earth pressure pushes the beam laterally. The raker brace is in compression. For the wall to rise up, the raker brace would also have to rise and go into tension. Not lilely. Never saw it happen in over 30 years.

http://www.PeirceEngineering.com

 

[structured]

Thanks for the response. I agree that it is not likely, but for different reasons (i.e. weight of wall, friction on lagging, friction on pile, etc.). If you draw a simple free body diagram of the system you can see clearly that the compression force in the raker requires a tension force in the soldier pile in order to satisfy equilibrium.

 

[msquared48]

Which would be resisted by the dead load of the shoring system, and the downward component of the angle of friction against the wall of the soil mass the system is resisting.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[PEinc]

The brace is not pushing upward. It is always in compression, pushing down. The brace is a reaction, not an applied force. It cannot push the wall up.

http://www.PeirceEngineering.com

 

[PEinc]

For example, if what you are worried about were true, then bridge slab overhang form brackets would push the cantilevered slab upward. This does not happen and the bottom end of the bracket is only a rounded, unconnected "knuckle" bearing in compression against the web of the facia girder.

http://www.PeirceEngineering.com

 

[structured]

Your example is a good one but you are looking at it the wrong way. The bridge formwork is essentially the same type of system as the soldier pile with rakers, it has only been turned on its side. Instead of soil pressing against the soldier pile, you have concrete pressing on the form. The form must have a tension connection where it meets the girder/abutment/etc in the same way that the soldire pile must have a tension connection where it meets the rock.

This is an overturning issue. Globally, the system is not stable unless there is a vertical (downward) force at the soldier pile. Sum the moments at the bottom of the raker and you will see.

Also, the vertical component of the soldier pile force must be equal and opposite to the vertical component of the raker reaction. The raker is in compression, therefore the soldier pile is in tension.

 

[PEinc]

Structured, theoretically you are correct. However, as msquared48 said, if you consider the friction force on the back of the wall, the net upward force is pretty small and any reasonable amount of sheet pile or soldier beam embedment in soil or rock below subgrade will be able to resist the upward thrust on the sheet pile or soldier beam. I have designed and built many sheeting walls with inclined raker braces. In addition, I have seen many designed and built by others. I have never seen a soldier beam rise upward and have never designed a soldier beam for this upward force. Also, I have never seen a reference book that indicated, or even discussed, the need to design a sheeting wall for the upward component of the brace load.

For hard Manhattan schist, you could have an ultimate bond of concrete to rock of several hundred psi. With a value that high and with a rock socket big enough to accomodate a soldier beam, I would have next to no concern about any upward force on the soldier beams.

http://www.PeirceEngineering.com

 

[cap4000]

structured

You concern about uplift I agree with. Make sure your raker connection to the soldier beam is welded plate. Your condition is similar to a one sided concrete pour for a wall with no ties. With inclined braces the brace will move upwards. What is your height and soldier spacing? Is the rock solid?