Depth of borehole in soil and rock 1

[ac4u2nv]

Dear all
when determining the dpeth of borehole the genreal rule is 2-3B or when stresses imposed by the strcuture equal round 20% at the corresponding depth.
however should the desk study indicate bedrock - hwo deep into the bedrock should you core and test?
1m of good quality bedorck would be sufficent for the load im anticipating on site. max 200kn.m2.

 

[miningman]

Even 5 metres of bedrock when dealing with Karst limestone will very rapidly get you into trouble. Thats the problem with rules of thumb.

 

[jdonville]

You should be working for an experienced senior engineer to answer these kinds of questions.

It depends on local experience (which your senior eng should have). Some clients have prescriptive minimum requirements for boring depths.

I would say, however, that in the absence of other information, a good starting point is on the order 10ft / 3m of rock coring BELOW your anticipated foundation elevation.

 

[BigH]

First, know your geology; local experience. As miningman indicated, karst limestone is a real bugger. Doing site investigastions in areas "well known" to us, we would use 3 m minimum to "prove" rock. Another very well known firm years ago in Canada use a rule of thumb of 1.5 m. Needless to say, there was the one time that after proving 1.5 m, they found out that it was ledge rock with soil beneath. I am sure they decided to follow their former boss and go 3 m minimum after that.

 

[geotechengineer1]

In the past I've used 3m as a minimum, but this was primarily for economic reasons (winning jobs), and the risks associated were managed with absurdly conservative parameters.

In glacial areas you can have rafted / floating chunks of bedrock above the actual bedrock surface. British engineers are always going on about Karst (you'll have to ask them about it).

At my current employer / country they like to do things properly and we drill fully cored holes to 30m for every job, sometimes 60m, even if that means 15m into rock.