Rock Blocks and Rock Pins

[dapra69]

Hi,

I am analyzing a 35' Keystone Wall w/ 22' GeoGrids.
Due to rock in the reinforced zone, I have to provide rock anchors or pins for the lower 15' of the wall to tie the GeoGrids.

I estimated that if I place the rock acnhors/pins at 6' c/c spacing along the length of the wall to tie the GeoGrids, then I would need 3" dia concrete grouted coreholes w/ 1" dia 50 ksi steel rods inserted in the center,to provide me with 18kips pullout capacity. The embedment length into the rock would be approximately 48 inches.

I am now trying to figure out the size of a block of rock that will be influenced by this 18kip pullout anchor. I am assuming a cylindrical shape of the rock block that will be pulled out.

The rock at the site is Diabase, with varying degree of weathering, fractures and joint spacings. I need the block size which can be used as a limiting factor to verify the the feasibility of installing these anchors in the field.

Please provide your valuable input and/or any suggestions - Thanks Chikkad

 

[GeoPaveTraffic]

Rock bolts has not been a big part of my pratice, however typically, the length of the rock bolts will depend on the spacing of the rock joints and cracks. I'm not familer with Diabase rocks, but you typically want to get into the block behind the first one.

That said, are you sure you need to tie the geogrid to the rock bolts? If the rock is within the active zone of the wall, then the pressure on the wall will be less than if the rock were not there; assuming of course that the rock is stable at the angle which it is cut. If the rock is not stable, then the rock bolts would be needed to stabilize the rock likely resulting in longer bolts.

Intuitively it seems that if the rock cut is stable and the geo grid runs from the wall facing to the rock face, then the wall will be stable. Since these walls are really just gravity retaining walls, if the wall (i.e. the reinforced part) is stable and the rock face is stable then the system is stable.

 

[cap4000]

Assuming Solid Rock. I would think you could use a "cone" surface area calculation based on a 45 degree (max) pullout volume at 4 feet deep. I think you have a situation very simliar to a post installed anchor into concrete. The angle for concrete typically was 45 degrees however tests recently indicate an angle much less than 45 degrees from the horizontal. Diabase intact cohesion is an excellent property of this very hard rock. Assuming your right with 18 kips, what geogrid can handle that load?? I suggest a closer spacing to conform to a quasi grid spacing. Good Luck

 

[DRC1]

3" grout Holes with 1" diameter will hold a lot more than 18 kips. They will also be pretty expensive. Best bet is to use specialty anchors. Williams or Diwadag are the two industry leaders. Both have excellent engineering support and would be glad to reviw you application. after 20 min. on the phone you will have learned a lot about rock bolts.

 

[exploengineer]

Just use resin bolts. 6' will easily give you 18kips. you can install them with a jackleg pretty easily. Willians, SAS stressteel, and Dywidaq are your best sources.


Frank Lucca M.I.Exp.E.

http://www.terradinamica.com