Dowelled Ret Wall Footing, into rock?
Dowelled Ret Wall Footing, into rock?
(OP)
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.
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.





RE: Dowelled Ret Wall Footing, into rock?
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
RE: Dowelled Ret Wall Footing, into rock?
RE: Dowelled Ret Wall Footing, into rock?
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.
RE: Dowelled Ret Wall Footing, into rock?
RE: Dowelled Ret Wall Footing, into rock?
Good luck.
f-d
RE: Dowelled Ret Wall Footing, into rock?
RE: Dowelled Ret Wall Footing, into rock?
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.