How to Pin a Foundation?

[3Fan]

I haven't had to deal with many foundations into rock here in Ohio, but now am doing some retaining wall designs in New Hampshire. Code requires the bottom of footing to be 4' below grade. The geotech engineer wants the rock overexcavated an additonal foot below bottom of footing then backfilled with compacted granular backfill "yield cushion" There is about 700' of retaining wall with a footing size of 6'. Lots of rock excavtion.

The geotech engineer says we can get away with raising the bottom of footing above the frost line by pinning it thus reducing the amount of rock excavaiton. How is pinning accomplished? Drilling and epoxying in dowels into the rock that extend into the footing?

 

[miecz]

I don't get it, but then I'm not a geotech. What's the point in excavating rock to get to the frost line? Is the rock subject to frost heave? Seems as though you should be able to bear directly on the rock, wherever it extends above the frost line. Am I missing something here?

 

[eric1037]

The IBC says that you can bear on competent rock at depth above the frost depth.

The geotech may be concerned about sliding for the retaining wall. The dowels are likely for shear. Depending on the loads, you may need rock anchors that are more beefy than just epoxy dowels.