Expert: Robert Cummings, P.E. - 12/15/2008

Question
I'm wanting to install a swinging (16')steel gate across my cottage lane in Newfoundland. There is no deep soil into which I can pour a footing for each end.  This gate will lie across solid rock which has only has a couple of inches of duff on top of it.  Is it better to anchor the posts with a big slab of concrete on top of the rock  or drill into the rock and fabricate a steel base for each post that would be anchored by bolts cemented into drilled holes?  How hard is it to drill into rock to place anchors?

Answer
The unsupported vertical loads for a swinging steel gate 16 ft wide are significant and would require pretty large slab of reinforced concrete to resist them.  You should definitely anchor the gate into rock.

You can accomplish this by drilling a large hole and setting the post several ft below grade in the rock, which entails a large hole.  If you want to rent a jackammer drill you may be able to get away without a lot of fabrication, but hand-drilling a 3-inch hole in hard Newfoundland rock may be difficult.  The alternative is to start by excavating to the rock surface, cleaning it, and placing a leveling course of concrete the top of which matches your driveway grade.  A 2 ft square slab would be ideal. It would be ideal if the concrete were reinforced with welded wire mesh or a hoop of #4 rebar.  Have the gate post fabricated with a heavy steel base welded to the post, (I would have stiffeners welded all around to resist the base bending moments also) and 4 holes 3/4 in. cut into the base.  It is not hard to drill through the concrete (be sure when you place the reinforcement in the concrete you position it so that you will not have to drill through it later) using an electric hammer drill that you can buy or rent.  Use a 1 in. bit and go at least 1 ft below the concrete into competent rock.  Use a professional grade 2-component injectible epoxy or a high-strength cementitious anchoring compound (like Anchor-Lok or equal) and insert the anchor bolts.  Ideally you should use 5/8 in. high-strength thread bar but that may be special-order.  The thread bar you get at home stores may suffice as onlg as it is at least 5/8 in; the steel used for those is not high-tensile.