Rock Anchor Loads 2

[ldeem]

I am a structural engineer and have done some small underground projects. I have seen a lot of equipment (mostly light weight) hung from the mine roof. I have always wondered about the capacity of the anchors. The contractors and maintenance all tell me the capacity is the DAG bolt capacity (10' embeds). However it seems to me the location of the load relative to a rib and the roof span would also be a factor.

I understand the roof bolting is a way to bond the rock layers and create a beam to span the roof. In such a case it would seem the resulting beam would support a section of rock shaped like triangle. If thats true than a long roof span would be limited to the beam capacity.

I also talked to a rock bolt tech rep who suggested pull out testing. However that would seem to only apply to the rock bolt configuration tested. Roof span and location relative to the ribs.

Can anyone point me to a reference on this subject.

 

[BigInch]

http://dywidag-systems.com/uploads/...echanical-Anchors-and-Rebar-Rock-Bolts_en.pdf

 

[weab]

I work for a client on a copper mine. We use 20 mm diameter and 32 mm diameter rock bolts. I am not a geotechnical engineer but I believe that a geotechnical report is used to determine the capacity.

The bars (rebars) have service load capacities of 5 metric tons and 10 metric tons, respectively. However, the bars themselves (500 MPa yield) have a capacity of more than 3 times this value. If it was only about 1.5 times, then this could be just a normal safety factor. Therefore, this tells me that there are other things going on here. It could be the rock capacity. It could be that rock needs a much higher safety factor due to uncertainty about cracks/stability.

I have a book at my office which has a section on this subject, but I cannot currently remember the name of the book. I realize that this doesn't help you much. I'll try to post the name later.

 

[ldeem]

That would be great if you could tell me the title of the book. I have done a little more research on roof design and so far haven't found anything that specifically addresses adding concentrated loads. Most of what I can find seems to be based on historical results and then monitoring for roof falls.

 

[weab]

Here is the book. Please investigate before buying. I would hate for you to waste a bundle of money for something that is not quite what you are looking for. This may answer your general questions but not a specific question you may have.

Underground Mining Methods: Engineering Fundamentals and International Case Studies


Link

 

[RobPE]

Here is the book you need to read:

https://www.rocscience.com/hoek/corner/Support_of_Underground_Excavations_in_Hard_Rock.pdf

and it's free.

You are correct in that the span and the size of the beam or wedge dictate the ground support needs. The geotechnical design starts with of course, a geologic investigation to get measurements of joints/fractures/and faults that define the dimensions of the beam or wedge that needs to be supported. This is an analysis of the orientation of the structure relative to the orientation and dimensions of the excavation. If all joints are flat lying, then a beam analysis is appropriate for a horizontal drift. However, if you have joints dipping at 45° relative to the back, then a wedge/tetrahedron analysis is appropriate. The bolts design is then done by targeting a specific bolt capacity and factor of safety (ie 100T wedge needs 12 10T bolts to support). The type of bolt used depends on the design life of the excavation, required capacity, cost, and availability of contractors to install the bolts.

There are other empirical methods that don't consider the geologic structure (ie Excavation Support Ratio which utilizes the Q' index of rock quality and the Mathews stability method for cable bolts).

 

[ldeem]

Thanks RobPE - this is very helpful.