Bridge Abutment Cantilevered Wall Founded on Inclined Rock Surface

[sponcyv]

I'm designing a +/- 20' tall bridge abutment at a stream crossing that will be founded on an inclined rock. The abutment is a cantilevered wall that will support the bridge beams and also retain 20' of fill. I have scoured the web for design examples or guidelines and am finding nothing to hang my hat on. In my mind, the horizontal load will create an additional overturning moment at the base of the footing due to this incline. Does anyone know where I may find design examples or guidelines for this type of condition?

 

[ACtrafficengr]

Approaches I've seen include chipping away the rock to provide a level surface, possibly keyed into the rock to provide more sliding resistance, or grouting rebars into the rock, then pouring a concrete leveling pad. You should probably get a foundation recommendation report from a geotech engineer.

 

[sponcyv]

ACtrafficengr,

Thank you for your response. We have a BFI (Bridge Foundation Investigation) report. They recommend keying the footing in to the rock or anchoring the footing to the rock. I have been asked to investigate the costs between the two options. The footing will be 13.5' wide and at an incline, much rock will need to be excavated to create a flat surface. I was wanting to investigate pouring the footing atop the smooth rock surface and providing rock anchors to resist the shear and tensile forces.

I have sized the footing to be stable against overturning and bearing, so the rock anchors would be resisting direct shear. However, these calculations have been made assuming a level footing surface. I know that we will have tendency for slip between the concrete and rock surface. Also, I feel like the overturning moment is increased by the inclination of the rock. Perhaps I can use the height from the lowest rock point to the top of the wall for my overturning calculations to be conservative?

In regards to the shear rock anchors. I know that with any shear there is some quantitative component of tension albeit small in relation to the shear. I just want to make sure that I account for this. I cannot seem to find any design examples for using rebar anchored into rock at the bottom of a footing to resist sliding. I normally like to check my work against examples or guidelines, so I am really trying to find more information. If you can point me in the right direction, I'd be grateful.

Also, let's assume I just show a key way at the toe of the footing instead of rock anchors. I can get the depth from the geotech. What is a good width for constructability considerations? I would think that if using a ram hoe, it may need to be a certain width?

Thanks

 

[oldestguy]

For discussing this here, why not describe that rock as best as you can, even with a cross section? Let's suppose it has weak bedding planes or fractures parallel to that rock surface. What did rock cores (if any) show about these details? If no cores, is there exposed rock at stream bed?

 

[sponcyv]

oldestguy,

The borings showed gneiss, (hard, excellent, blue-gray, and white, slightly weathered)very near the surface (1.5' below). I have asked the geotech for recommendations on minimum depths of rock anchor embedment. They have given allowable bearing pressures, minimum key depths, and ultimate grout to rock bond strength.

 

[oldestguy]

Looks like a good rock condition, possibly in the "best" category, per this reference. Your attachment didn't read for me.

http://www.rocscience.com/hoek/references/H1998d.pdf

A rough rule of thumb for me would be the shear strength at 1/2 the unconfined compression strength, and then apply a generous safety factor (+/- 10) to allow for weak zones, if the report gives no values.

 

[sponcyv]

oldestguy,

Thank you for that article and input. This post should show the image of the cross section through the existing bridge. The existing bridge is to be replaced.

 

[graybeach]

I usually think of rock anchors on the bottom of an abutment or dam as resisting sliding through tension - as in concrete shear friction calculations.

 

[Syr111]

sponcyv:

I am designing an abutment on footing and the client is requesting that we use dowels to tie in to rock. I have been looking for an example with rebar anchored into rock but I can not find any thing. I was hoping that you can refer me to an example or a reference that can help me in my design.

Thank you

 

[sponcyv]

Syr111,

Unfortunately, I don't have anything that would be of use.