Foundation Excavation In Rock

[Bridgeguy1]

I have a contract to build a concrete arch bridge. The thrust blocks for the bridge require excavations up to 45 feet deep. The existing roadway is 20 ft from the corner of the excavation. The material is intensely to moderately weathered metasiltstone. I am looking for a guideline to design an excavation safety plan. Specifically how does one design a rock anchor/ shotcrete face to protect the excavation. From the research I have done it seems that you start digging and than determine if the rock is stable and begin to add anchors if it is not stable. The Contract requires us to have a plan before we start digging.

 

[cvg]

weathered silt stone can be some nasty stuff. it can turn to muck when it rains. you need an expert on this

step 1 - hire a geotech to design this for you
step 2 - see step 1

 

[BigH]

Step 3 - return to Step 1 . . . (PEinc) . . . there are manuals out there for rock anchor design . . . Might want to look at a paper by Dr. Bruce "Evolution of Rock Anchor Practice Over Three Decades" . . . not for the faint-hearted

 

[PEinc]

Most geotech reports do not provide sufficient information to design support for a rock excavation. Without information on rock bedding and joints, you cannot determine if you will have a sliding rock mass. Pressures from a sliding rock mass may be greater than lateral pressure from soil. Without appropriate rock sampling and testing, you will be guessing at the forces to be resisted by the support system and/or rock bolts.
Check out the FHWA web site for geotechnical publications. Also check AASHTO and CALTRANS publications and PennDOT's Design Manual 4. There are many manuals and guide specs available for anchored walls and soil nail walls. Try to find a copy of Professional Users Handbook for Rock Bolting by B. Stillborg, 1986. It may still be available from McGraw-Hill. These publications will guide you in your design.

http://www.PeirceEngineering.com

 

[Bridgeguy1]

Thank You PE. I appreciate your time and the resources you have suggested. I have engaged a PE/CEG who was basically telling me the same thing. He did a site recon modeled the rock using stereo nets and performed a slope stability analysis but at the end of the day he said we would have to start digging and see what we have. As a PE and contractor involved in heavy civil construction for 25 years I have designed many temporary structures. I also have engaged many consultants but when i do so I want to understand what they are doing. So I will review the references you have suggested.

 

[miningman]

Depending on where you are located, you should perhaps engage the services of a diamond drill contractor. He could put down, say four holes, each about 50 metres deep quite quickly. I have never worked with " weathered metasiltstone" but if I were able to examine the core, I would feel quite capable of forecasting how much difficulty I would have excavating it for you, and speccing the necessary bolting and shotcrete. Any geotech who could not do the same, is probably incompetant