Dowelled Ret Wall Footing, into rock?

[BurgoEng]

I have a retaining wall (about 10ft retained)that will have the footing supported directly on bedrock. I am assuming 10000psf bearing capacity. The Client now would like to reduce the width of the footing, which was initially about 8ft heel and 2 foot toe. I can reduce the width to about a 4ft heel and keep the e<L/6, but I will not meet a FS=1.5 for OT and Sliding. The idea of using dowels into the bedrock was suggested, which sounds like it'd be fine, but how do I calculate the tension load applied to the dowels, and hence size the dowels? Do I even need to keep the e<L/6 if I am going to use dowels, taking up the induced tension with the dowels?

I was thinking of just using the bearing pressure diagram for e<L/6 and saying the Tension force that would be taken up by the dowels is equal to the Compression force (area of the pressure triangle). Does that sound right? Conservative?

I also thought of using, Moment = [ C(area of pressure diagram) + T(dowels) ] x [dist between C & T] and solve for T.

 

[jdonville]

BurgoEng,

If you keep e<L/6, then OT should not be a problem.

Typically FS(overturning)=2.0 or greater needs to be satisfied.

The dowels will not experience tension - they are there to resist shear. If used, neglect the effects of sliding friction in your sliding analysis - the dowels will provide all of the sliding resistance. Work out the spacing and size of the dowels to meet FS(sliding)=1.5 or greater. Remember to allow for some reduction in dowel section ove the design life of the wall.

Jeff

 

[cvg]

it sounds like you are thinking rock anchors, not dowels? If so, you need to ask your geotech/geologist to give you a conservative number for the strength of the rock / allowable load per anchor. To be safe you should require pull testing also. However, it may be more economical to make the footing wider than to install and test anchors.

 

[BurgoEng]

jdonville,
I went back and checked my calc, and saw a bit of an error which I have now fixed which so I see now that indeed, if e<L/6, the FS will work out. Thanks. But why could I not use the dowels to resist tension, and only for shear?

cvg,
by dowels, I guess I meant rock anchors. I was meaning to have rebar grouted into the rock and extending into the footing (bent at 90deg).

In addition, I failed to mention before, I have an additional horizontal load being applied to the top of the wall because the wall is supporting the framing of a 2 story building, hence wind load is being directed to the wall via vertical bracing. The load is pretty conservative...about 1.5kip per 1ft of wall length.

 

[DRC1]

I have done this before. I used the distance of the bars to the toe as the moment arm and put all the tension into the bars. i used seperate bars to carry shear. Very conservative, but the number of bars was not great. worked okay. What you are proposing may work too. I used rebar in rock drilled holes w/grout. You can get threaded ends which would allow you to put a late on it and test with a center hole jack. I would use a couple sacrifical bars and test prior to drilling & grouting all.

 

[fattdad]

If you have hard rock, consider post-tensioning. Install rock anchors, form the wall, leave a vertical sleeve (or how ever many you really need) for the cable to run up (i.e., a continuation of the free length) and after the concrete cures (and before you place the backfill) apply the tension. By forcing more bearing pressure on the bottom of the foundation, you will then get the full benefit of the bearing surface. This can really work if you have the contractors and the scale of the job warrants it.

Good luck.

f-d

 

[BigH]

5 tsf bearing on rock is not that much - I'd almost use this on a dense granular glacial till. I don't think that I would go to the expense, if you need it at all, for post-tension; I understand the thought of adding more vertical loading due to the permanent extra tie-down force. But I would imagine that for the cost (hassle of drilling, sleeving, grouting, tensioning, etc.) that you could add an extra 500mm or 1000 on the heel. We don't know, yet, why the client wanted to make the heel smaller - to save money? - likely, but the post-tensioning is money too. Dowels are okay - but yet, again, you could simply jack-hammer a key into the rock (seems like it might be shale if you are only putting on 5 tsf) to give you sliding safety if you need it.

 

[BurgoEng]

My understanding was the contractor/client wanted to limit the amount of rock removed.

I think we have decided make the footing long enough to keep the e<L/6, but if I need more resistance for sliding and OT, then I am including dowels.

However, I just got word, that now they want to use CMU for the wall, rather than concrete. The wall actually has concrete piers for column supports integrated into it, making these piers 10' high. I am concerned about having this pier essentially isolated with the CMU as support, as opposed to completely tied into a concrete wall.