KEY BRIDGE PIER FOOTING IN TO ROCK

[sponcyv]

I designed the bridge piers originally to be supported by driven H-Piles and a pile cap footing. The contractors encountered very hard rock within a foot of the proposed bottom of footing. We will be going with a spread footing rather than the pile cap footing. In the local DOT's bridge structures design manual, it states that the footing shall be keyed in to the rock if the bottom of footing elevation is within the scouring depth. I can not find any details for this and am not sure how deep the footing must be keyed in. Does anyone have any experience with this? Thanks for your help.

 

[BigH]

The big question, in my view is why you designed a pier to be founded on piles with rock so close to the underside of the pile cap?? I've actualy seen this before in a SE Asian country - . . .

 

[sponcyv]

The geotechnical exploration showed rock being deeper. They did not take borings exactly where the footing location is now.

 

[jorton]

Are you working with steep terrain? This can cause problems with boring elevations if the borings are mis-located. If not, I'm surprised there is that much variation. How far off were the borings? Do the borings indicate a sloped rock surface? You may want to drill to ensure the rock is not a large boulder.

 

[BigH]

Jorton - I've seen that kind of variation before - for a large tank at a sewage treatment plant - one boring showed rock at 20 ft or so - which was typical - but another boring some 50 ft away (towards the river) showed the rock to be in excess of 100 ft!. It can happen. It is sad when borings are not put down at the bridge pier foundations or some other means not used to estimate the bedrock depth.

 

[racookpe1978]

Sounds like your "hole" is already in place - that is, you are already "standing" on top of the actual rock. Have you checked the nearbay 4-8 feet to be sure you don't have a boulder/half-supported small rock outcropping?

Or do you really have a good solid foundation ("something hard that does not move") large enough (after the key is chipped out) to carry your bridge loads?