"Shear Strength of Rock Mass" for AASHTO Fig. 5.6.2A (b)
"Shear Strength of Rock Mass" for AASHTO Fig. 5.6.2A (b)
(OP)
In the absence of any laboratory tests, can someone guide me in the right direction for obtaining general values of the shear strength of rock mass, "s", in AASHTO Figure 5.6.2A (b). (NYC specifically)





RE: "Shear Strength of Rock Mass" for AASHTO Fig. 5.6.2A (b)
If you use a high shear strength, it may be questioned by the reviewer or you may calculate a very short toe embedment which may be OK for solid, massive rock but not OK for a fractured rock.
RE: "Shear Strength of Rock Mass" for AASHTO Fig. 5.6.2A (b)
RE: "Shear Strength of Rock Mass" for AASHTO Fig. 5.6.2A (b)
1000 psi = 144 ksf.
For soldier beam toe design, as little as 2, 3 ,or 4 ksf will give short, economical toes. You don't need 144 or 1152 ksf.
RE: "Shear Strength of Rock Mass" for AASHTO Fig. 5.6.2A (b)
I don't have the AASHTO book and do not want it even for free. But I do have many other good rock mechanics books. Personally, I use the cohesion value of 8000 psf for competent solid diabase rock here in Northern NJ. I usually get the RQD value first. I think the AASHTO "s" value with their formulas are very misleading, irrational and perhaps wrong to use for soldier pile design with out any rock confining overburrden pressure to speak of. My rock sockets are normally only 3 to 4 feet deep. The values you indicate are amazingly correct which goes to show you how strong rock can be provided its not too jointed and fissured. I have read cases where footings have settled as much as 8 inches on rock due to the unknown joint structure below the rocks apparent solid surface.
RE: "Shear Strength of Rock Mass" for AASHTO Fig. 5.6.2A (b)
RE: "Shear Strength of Rock Mass" for AASHTO Fig. 5.6.2A (b)
To put all of this into a broader perspective. I have an article with lab results from a lateral load test done with rock sockets into solid granite. Basically the concrete is 3 ft in diameter, 6ft deep into granite rock and the lateral load was applied at 17 inches up from the rocks surface. With a lateral displacement of only 0.1 inches the load read was 140,000 pounds. I doubt a typical steel H-Pile at say 8 ft on center could ever realistically resist this type of force even under the strangest layout.Yet the granite rock had no problem supporting this load!! What say you??
RE: "Shear Strength of Rock Mass" for AASHTO Fig. 5.6.2A (b)
1. I agree about the soldier beam and 140 kips.
2. 3ft. diameter is large, especially for a socket in rock. I try to keep my sockets to 24 inches maximum. The smaller, the cheaper.
3. It is rare (maybe never?) that I see a soldier beam socketed into solid granite. Usually, I will see shale or limestone, usually weathered or highly fractured.
4. If I run into hard rock at subgrade, I try to stop the beam there and use a short toe tie. This is usually cheaper than drilling hard rock, buying and fabricating a longer soldier beam, and concreting the socket.
RE: "Shear Strength of Rock Mass" for AASHTO Fig. 5.6.2A (b)
In both Bergen and Hudson County of New Jersey I run into the Palisades Diabase Rock alot. I call it "black granite".Typically I also use 2 foot diameter sockets. I really like your short tie alternative. Can you explain that short tie further. I may use some form of a diagonal grouted rock anchor next time if I run into deeper sockets. Thanks for the tip.
RE: "Shear Strength of Rock Mass" for AASHTO Fig. 5.6.2A (b)
RE: "Shear Strength of Rock Mass" for AASHTO Fig. 5.6.2A (b)
RE: "Shear Strength of Rock Mass" for AASHTO Fig. 5.6.2A (b)
Thanks for the practical advise. I normally use #8 bars 12 inches long with 2.5 inch diameter holes pinned to the rock with 10k non shrink grout.The nominal pullout value in solid rock I typically use is 30,000# in tension. I will consider doing this short tie next time to each soldier pile. I will use the thru beam conn'n on each one.
RE: "Shear Strength of Rock Mass" for AASHTO Fig. 5.6.2A (b)
RE: "Shear Strength of Rock Mass" for AASHTO Fig. 5.6.2A (b)
RE: "Shear Strength of Rock Mass" for AASHTO Fig. 5.6.2A (b)
RE: "Shear Strength of Rock Mass" for AASHTO Fig. 5.6.2A (b)
RE: "Shear Strength of Rock Mass" for AASHTO Fig. 5.6.2A (b)
The toe tie is installed BEFORE the excavation reaches subgrade. Therefore, the soldier beam still has a couple of feet of embedment in dirt while the toe tie is being installed. The dirt just above the rock is usually fairly stiff and competent.
If I have a toe tie, I don't need the rock socket. The toe tie is long because it starts a couple feet above subgrade and then goes down on a 45 degree angle into the rock. The length is usually about the length of a single drill steel or whatever the drill can handle without needing to add any more drill steel. The materials are relatively cheap and the installation is easy.
RE: "Shear Strength of Rock Mass" for AASHTO Fig. 5.6.2A (b)
RE: "Shear Strength of Rock Mass" for AASHTO Fig. 5.6.2A (b)
Got it. Thanks. The normal price for only a rock drilled 2 ft diameter hole I get quoted is around $1,800.00 each at 4 feet deep. I will seriously consider your toe tie system on my next project because it looks alot cheaper and just as effective. The only pit fall I see is the soil toe issue. The 10 foot tie is no problem.
RE: "Shear Strength of Rock Mass" for AASHTO Fig. 5.6.2A (b)
The Hoek Cell done for triaxial testing is in fact the basis of the AASHTO shear strengths. Its portable and can easily be done in the field with a small solid core of the rock. I have the Hoek typical rock shear strength values ranging from 100psi to 6000psi. AASHTO has finally explained this Sm value now with only further complications and unnecessary math. I believe PEinc once called AASHTO directly for an explanation on something just like this issue for which as I recall left him scratching his head based on their baffling response to him. Thanks for the tip.
RE: "Shear Strength of Rock Mass" for AASHTO Fig. 5.6.2A (b)
That's $1,800.00 plus the socket concrete, plus possibly extra soldier beam length, plus the extra time to drill rock, plus possibly adding another subcontractor or BIG down hole hammer and several compressors into the mix. The cost is much more than $1,800.00.
RE: "Shear Strength of Rock Mass" for AASHTO Fig. 5.6.2A (b)
You sold me on toe tie its the best bang for the buck. Except. The one big problem I see is that you can not use adjoining property to drill the tie into. I get many retaining walls just off the property lines and can tell its a nightmare in some NJ towns. Tenafly, NJ requires an agreement in writing signed by all property owners before anything can get done espically if trees are on the line and in the way.
RE: "Shear Strength of Rock Mass" for AASHTO Fig. 5.6.2A (b)
Also, if trees are along the proposed wall line, you may have trouble with branch and root interference during soldier beam installation and excavation.
RE: "Shear Strength of Rock Mass" for AASHTO Fig. 5.6.2A (b)
I normally only do cantilevered soldier pile systems. I plan to use your concept anyway. Say for 8 foot center pile spacing. In between that say at 4 feet I would go with a steel beam waler and a toe tie anchor if I see the rocket socket depth should of been deeper that I designed it for. Its a nice fall back position for me to keep in mind. Just in Case! Thanks Again.
RE: "Shear Strength of Rock Mass" for AASHTO Fig. 5.6.2A (b)
RE: "Shear Strength of Rock Mass" for AASHTO Fig. 5.6.2A (b)
I would appreciate your feedback.
RE: "Shear Strength of Rock Mass" for AASHTO Fig. 5.6.2A (b)
I would appreciate your feedback.
RE: "Shear Strength of Rock Mass" for AASHTO Fig. 5.6.2A (b)
This sounds like a new question. You probably would get better feedback if you started a new question with an appropriate subject line.