Soldier Pile in Rock Socket 4

[cap4000]

AASHTO has had many formulas regarding the Passive Pressure developed for a H-Pile in a rock socket. I have the Kulhawy and Carter ASCE article from 1992 with their experiments in rock sockets. It looks nothing like AASHTOs formula or even their loading diagram. Does anyone know if AASHTO has ever issued an errata clarifying this very tricky issue. There have been other posts on this web site relating to this yet very unclear procedure set forth by AASHTO. Thanks in advance.

 

[jdonville]

cap4000,

The HB-17 formula for permanent walls should read as follows:

Pp = s*D*[D+sqrt(2)*b]/(1+tan(beta))

This is based on a bedrock wedge failing in shear similar to the wedge failure in clay presented in the COM624P manual and derived by Reese back in the 50's.

The trick is to determine 's' for the bedrock - especially if the rock is not largely intact. So far as I can tell, there is not a lot of guidance on determining 's'.

If you have some references on the latter problem, they would be much appreciated. I have tried Hoek's book at

http://www.rocscience.com,

but the information does not sem to lend itself to a plug-and-chug approach.

Jeff

 

[cap4000]

Thanks for the formula. The formula I haved used does not have your first "D" in it. What I haved used in the past is the "residual shear" or its cohesion value for "s" . Its derived from the typical Mohr-Columbs Circle and its Tension Cutoff. ASCE's Rock Foundation Book # 16 gives various cohesion values depending only on the RQD values. My diabase rock sockets are typically 3 to 5 feet deep so I do not really count on any deep rock overburden pressure contribution. Intact diabase or granite like rock has unconfined compressive strenghts on the order of 30,000psi or 10 times stronger than typical 3000psi concrete. I have many good rock books but they do not have any plug and chug formulas.

 

[PEinc]

If the soldier beam toe is in rock, you usually don't need very much s (shear strength) to provide sufficient passive toe pressure and a reasonable toe embedment length. I usually use as low a value of s as I can that will give me a reasonable toe depth. Most of the rock strengths will be stronger than a medium stiff or stiff clay. Use a shear strength that is representative of a medium stiff to stiff clay and you will be conservative. If hard rock is encountered, you are OK and the socket will not be too long. You can always redesign the toe if really hard rock is encountered in the field. You can shorten a soldier beam in the field a lot easier than you can lengthen one. If you encounter bad rock, you are still probably OK with your toe length. You don't need an extremely high shear strength, s, to have a reasonable toe embedment.

 

[cap4000]

PEinc

I have the SPW911 Sheet Piling Program which can be modified for Soldier Pile Design. What I have done in the past is use a typical cohesion value of say 4000 psf to 8000 psf. Of course this 911 program design is based on soils info and not rock properties. It does however come up reasonable 3ft to 5ft deep rock sockets for 10 foot to 12 foot high lagged walls. What has been your typical rock socket depths assuming a 2 foot diameter hole with a 10 foot high wall??. Thanks for the tip.

 

[PEinc]

Trash SPW911. Do hand calcs for a layerd soil condition where one of the layers is a relatively weaker soil. Then compare your results with the SPW911 results for the same soil conditions.

A wall about 10 to 12 feet high should be a cantilevered wall. For a cantilevered wall, I would use no less than a 5 foot socket in good rock or about 7 feet in fractured rock. For the stiff clay or rock, the passive pressure diagram would be rectangular, not triangular. When I am not sure if there will be rock at every soldier beam, I design embedment for both all soil and all rock, call for soldier beams long enough for soil, and give a note saying that the soldier beam embedment needs not be longer than the all soil length or can be terminated once the minimum rock socket length is drilled, if rock is encountered. For example, if the embedment length in soil is 12 feet and the embedment length in rock is 6 feet, and if I the rock is encountered 3 feet below subgrade, then the lotal embedment length would be 3' + 6' = 9' which is less than 12'.

Some of the older AASHTO manuals do not indicate that the passive pressure should be multiplied by the socket width or wider. It's amazing that thousands of highway engineers use the manuals every day and haven't questioned the absence of a width multiplier. It's also amazing that AASHTO never corrected this.

Also, in the 1994 Imterim of AASHTO's Standard Specifications for Highway Bridges, Figure 5.6.2A.a, there is a mistake in the formula for active pressure behind the soldier beam below subgrade. The Pa2 formula should be 0.5 x Ka2 x b x D etc..... Not 1.5 x Ka2 ....... For some reason, AASHTO applied the active pressure behind the beam to 3 times the soldier beam or socket width as is done for the passive pressure. I mentioned this to one of FHWA's experts. His response was that I should be using the LRFD manual. Some help! He obviously didn't give a _ _ _ _ !

 

[cap4000]

PEinc

I bought the SPW911 Program about 3 years ago for about $300.00. I know they have had financial problems not to mention the negative feedback on this website. I happen to like it for the basic calculations. Your rock sockets are deeper than I thought they would be. In the book "Elastic Solutions for Soil and Rock Mechanics" by H.G. Poulous, he does a sandstone rock socket and comes with only a 15 to 24 inch deep socket for a 50 kip shear load 2 ft up. He uses very tricky load-deflection, moment-displacement and moment-rotation formulas. He does does also indirectly use 2 times the "c" value for his rectangular pressure "statics" diagram. Does AASHTO have any idea about what your really saying? They should just issus an errata on their website and be done with it. They certainly charge enough $$ for their publications. Unreal.

 

[PEinc]

My rock sockets usually calculate to be shorter. However, I don't think it is wise to install a 3' socket. Especially if the rock is fractured. That's why I said I would not use less than ......

 

[cap4000]

Check out this amazing report at

http://www.eng.uwyo.edu/civil/publications/thesis/VuTung.pdf.

 

[BigH]

PEinc - interesting point about the width multiplier. If people would read Golder and Seychuks paper in the Pan-Am Geo Conference (Puerto Rico, I think - 1964 or 1967 or so), they discuss soldier pile design for the Toronto Subway. Old paper but good practical engineering from practical engineers.

 

[jdonville]

PEInc,

I recently attempted to derive the AASHTO formula presented in HB-17 from the original Reese equation presented in the COM624P manual. The AASHTO formula as printed does seem to omit a width multiplier. Does this same problem affect the version presented in the LRFD specifications?

Jeff

 

[PEinc]

I checked the AASHTO LRFD Bridge Design Specifications, Figure 3.11.5.6-1 and Figure 3.11.5.6-4 for cantilevered soldier beam walls. Both figures show width applied to active and passive below subgrade. They also fixed the mistake of 1.5 instead of 0.5 (or 1b instead of 3b) for the active pressure in the toe embedment for Figure 3.11.5.6-1 (formerly Figure 5.6.2A.a). It only took them 10 years!