Shallow Foundations on Rock 3

[Geotek75]

Looking for some help with shallow foundations on rock. I'm trained/experienced in the traditional areas of soil mechanics and have little background in rock mechanics. The situation involves bridge pier footings bearing directly on rock (hard sound Dolomite in this case). An existing waterway (cut in rock)will be deepened several feet adjacent to the bridge piers to allow for larger flows. I'm interested in how the foundation load is distributed in the rock (Boussinesq analysis seems appropriate),and what slope the rock can be excavated to and still be safe (a 4V:1H seems reasonable). I will probably use untensioned vertical rock dowels ("comfort" dowels) at close spacings to pin potential rock blocks together. At this point I do not have any information about joint sets, but I do know the rock is horizontally bedded. Any suggestions?

 

[oldestguy]

I'd do a slope stability analysis for reinforced "earth" and plan on inclined anchorages to provide some confinement of that critical section that would slip off into the excavation.

In my experience for un-anchored foundations like this I have kept the foundation back from the cut face, using a "rule of thumb" with a distribution of stresses within a 2 V to 1 H slope plane from toe of foundation.

You cannot depend on fractures being oriented in your favor.

As an alternative, lower the foundaton base accordingly to limit the set back.

 

[BigH]

http://www.usace.army.mil/publications/eng-manuals/em.htm

and download EM 1110-1-2908.

Check out, too, Hoek's book on Rock - you can do a google on Hoek. He has a website and the book is freely downloadable.

 

[Touma59]

I would second BigH on that and would like to propose a good soil investigation to determine the degree of fracture in the rock, confirm the horizontal bedding etc... We recently had an incident where a private developper went against our recommendation for shoring the full site and insisted on shoring only close to the buildings because the rock face was horizontally bedded, and "there was no need for an overkill". Four months later, this side (~ 40ft high) collapsed and it turned out that behind the rock facade, the layers where actually "rippled" at places with the down side portion of the ripple dipping towards the site. This would at least help you design the length of your dowels.

Tsoft;

http://www.tsoftonline.com

 

[Geotek75]

Many thanks go out to my fellow geotech's for their thoughtful replies and insight. It is clear that we are all on the "same page". My feeling is that you simply can't be too conservative in a situation like this. I totally understand that you can't count on the joint conditions being in your favor - if it can happen, it probably will, and we should plan for it accordingly! It's just helpful to get some feedback from other geotech's. Thanks again.

 

[cap4000]

I would first get the RQD values from rock coring. A great book I have is Structural Foundations on Rock by Pells. It shows a typical 2V to 1H for its load calculations relating to settlement. Another book I have uses a finite element design to actually derive the tensile stresses for a 90 degree vertical cut in rock. What is bridge pierload onto the rock?