Site class based on Rock data 2

[pelelo]

Engineers,

I performed 4 borings for a proposed bridge. All the borings were rock cores. Based on RQD, the top 9 mts are very poor to poor shales and the rest (up to 15 mt) are excellent quality shales.

I got few UCS on both the poor and the excellent quality rocks.

I was wondering if based on this data, is it possible to assign a site class?, please advise.

 

[EireChch]

Site class needs to be determined based on shear wave velocity (Vs) from ground surface to 30m. UCS is irrelevant when it comes to site class.

If you dont have Vs you can use N values. You are permitted to adopt N=100 for rock.

If you have 30m of assumed N=100, then the best site class you can give is site class C, which is N=50+. Without Vs data I wouldnt give anything better than that.

You also only have data to 24m not the 30m required. You also have to state that you are assuming that rock is continuous to 30m. It is up to the client to confirm that rock is continuous to 30m.

If you have spec'd the investigation then client may force you to do another borehole to 30m as technically you did complete your scope properly.

Or hopefully you know the local geology and area and are comfortable to make this assumption and put your stamp on it.

 

[Mccoy]

I do agree with Eirechch as above.

Also, If you can extrapolate safely to 30 m of depth, then you should also have reliable numbers for Vs of each rock, by reliable I mean citing sources.

In my experience, a weathered shale may have a 400 m/s cautious Vs, whereas a hard shale may have from 600 to 900 and maybe more.

So, applying the ASCE-SEI 7/10 20.4.1 formula for Vs harmonic average you would end up with something like Vs-ave = 526 to 666 m/s, range which lies all into the site C class.

With your data, It would not be wise to assign a B class site.

Of course, the presence or absence of specifications and the importance of the structure may guide your decision

 

[pelelo]

Thanks a lot Engineers.

I agree with EireChch's approach.