"Shear Strength of Rock Mass" for AASHTO Fig. 5.6.2A (b) 2

[civilmonk]

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)

 

[PEinc]

If you are designing a soldier beam wall with embedment in rock, you should not need to use a very high shear strength. Pick something conservative, like the shear strength of a medium clay. That way, you can say you are conservative for rock and still get an acceptable toe depth.

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.

 

[cap4000]

I have a Rock Mechanics book by Richard Goodman. If you tell me the type of rock your dealing with I can give you the S value or its Residual Shear Strength. S is determined by Mohr-Coulomb Tension Cutoff graphing of pressures. Is it Manhattan Schist in NYC?? On page 83 in this book they are 20 different types of rock listed. For Solid Granite Rock its about 8000 psi for much weaker Indiana Limestone its about 1000 psi.

 

[PEinc]

8000 psi = 1152 ksf.
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.

 

[cap4000]

PEinc

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.

 

[PEinc]

3' to 4' rock sockets are good for braced or tiedback soldier beams but, in my opinion, too shorts for a cantilevered soldier beam - unless the rock is hard and massive (unfractured), which is seldom the case.

 

[cap4000]

PEinc

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??

 

[PEinc]

Not surprising at all. Here are a few of my thoughts:
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.

 

[cap4000]

PEinc

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.

 

[cap4000]

I just tracked down thru a Caltrans 2004 Bridge Specification pdf the AASHTO diagram noting "Sm" as the Rock Shear Value?? for soldier piles in rock. I beleive AASHTO might be referring to the "axial shear side resistance" generally on the order of 20 to 40 psi. Typically the ultimate value of 200psi to 400psi is factored down with a safety factor of 10 for short stubby rock sockets with an L/D = 1.0 Can someone verify the AASHTO "Sm" rock property description?? Is it the Shear Value or the Side Shear Unit Stress?? The Shear Unit Stress value is the basis of the ASCE Carter and Kulhawy rock socket experiments.

 

[PEinc]

Toe ties are usually short, rock bolts that are attached to each soldier beam the way that tiebacks are attached. For a permanent wall, the toe ties will need corrosion protection just like the tiebacks. The toe ties are usually installed a couple feet above subgrade so that the beam does not kick out before the toe tie is installed. The toe ties should be short enough, if possible, to allow their installation with a single pass of the drill steel. Usually, the toe ties have low design loads and are grouted into rock. The toe ties are locked off with the hydraulic jack but are not necessarily "tested" like tiebacks. For temporary walls, you could install one toe tie per two soldier beams. For a permanent wall, you should probably install one toe tie per each soldier beam so that you do not need a wale. You use a thru-beam connection instead of a wale.

 

[cap4000]

PEinc

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.

 

[PEinc]

I usually use threaded bars, Gr60, 75, or 150, as required. The toe ties go back at a 45 degree angle and are usually about 10' to 12' long. Remember, the toe tie needs to be installed while you still have some embedment so that the beam does not kick out. Also, the grout is neat Portland cement with a W/C ratio of 0.5. No aggregate. I never use high strength grout.

 

[PEinc]

Have used Gr60 rebars welded to the soldier beams but the welding can be "iffy" in the field.

 

[cap4000]

The long length of your toe tie surprises me, albeit its on a 45 degree angle. You basically now have a combination system say with a 2 foot deep rocket socket plus the toe tie itslef. That works for me. The problem I guess is once you get down to subgrade you then must immediately install the toe tie. Am I missing something??

 

[Panars]

AASHTO LRFD specifications, 3rd ed. (with 2006 revisions) defines Sm as "shear strength of rock mass (ksf)" in Section 3.2. Section 10.4.6.4 describes how to calculate the rock mass shear strength using the rock mass rating system (RMR) and the Hoek Brown criteria.

 

[PEinc]

cap4000,

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.

 

[PEinc]

PS, you must get the soldier beam down to subgrade or lower. You can't leave the soldier beam up on a vertically cut rock bench unless the rock bench is several feet wide and is stable. Or, unless you install a grouted, near vertical, steel dowel which is attached to the soldier beam and extends to below subgrade. This vertical steel pin keeps the soldier beam from dropping and collapsing the rock bench. Again, the rock bench must be stable or be stabilized.

 

[cap4000]

PEinc

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.

 

[cap4000]

Panars

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.