retaining wall construction on a steep rock face

[theRock]

So there's a steep rock face, approximately 25 m high, that is situated directly behind a government building. Action is required as the rocks falling from the face are becoming more frequent. The slope is irregular, with large jointed blocks overhanging in places.

We've done some scaling, but as this is a long-term securing job, there will be a retaining wall built on the rock face to stop rocks falling from the upper reaches. We're hoping to build a sort of concrete base for the wall about half way up the rock face on a naturally occurring, irregular ledge. I'm trying to calculate the forces acting on the proposed wall. Can someone recommend a friction coefficient for concrete on rock (sandstone and shale)?

Any input on the proposed wall would be appreciated too.

 

[ishvaaag]

Advice you can find in

Rock engineering applications
Franklin/Dusseault
Mc Graw Hill
section 2.6 and others

We must think the ledge shelf is stable.

Never forget to allow proper draining.

If you backfill with something, angle of inner friction and friction to wall from the back fill.

If not the cohesionless rock debris give a conservative friction angle, in the end the wall stays vertical.

Knowledge of the layers is required, to decide where to anchor and in what direction. A shear-friction-like approach can be ideal in that does not rely in the base support. Considering the base support however will give lesser strength requirements for the anchors.

 

[72shortbox]

your situation sound hairy. I would lean toward anchoring the wall in using something like drilled mini piles or even simplier drill into the rock and grout in some rebar for shear resistance.

 

[derubertis]

Concrete to rock 0.4, if the rock is sound and properly cleaned prior to placing concrete.

Rockfall netting seems a better choice, cheaper and more certain of result.

 

[Sean2]

Have you considered casting concrete slabs onto the rock face, and anchoring it to the rock? From where I stand, the design of a retaining wall high above the base of the slope sounds as if there is considerable risk associated with the construction and with the completed wall.

If there is room to carry out the necessary frequent maintenance safely at the toe of the slope, the rockfall netting suggested by derubertis should be considered.

The decision on the solution actually applied must include a risk analysis of each of the possible options.

Good luck!

 

[theRock]

Thanks to everyone for valuable input. It was decided that a concrete wall would be built from the ground, the full height of the rock face. There is a road along the top of this slope and there were concerns that any further scaling would undermine the road. The wall will be anchored to the rock face with some mesh for surrounding areas and dowels for questionable rocks that will not be covered by the wall.
thanks again!