Depth of Borehole Investigation

[ONENGINEER]

I am working on ground investigation for a 28 floor high building on predominantly dense clayey sandy silt soils. I would like to know how deep others have gone down for boreholes. If it was a dam site, one would expect a borehole depth equal to the dam height. I do not think there would be a standard depth as it all depends on the geology, and soil properties, etc. but just to have a scope from experienced engineers. Thank you.

 

[DaveAtkins]

A building that tall will be on deep foundations. Your borings should extend to bedrock. And you should obtain rock cores as well.

DaveAtkins

 

[ONENGINEER]

Thanks DaveAtkins. The bedrock is expected to be approx. 300 ft deep overlain by clayey sandy silt materials with some sand interlayers. Just wondering what would an overall exploration depth be even if we get some high N values at a depth of 80-90 ft.

 

[geotechguy1]

There are lots of published guidelines out there although alot also depends on local practice and experience and for that you need some geotech engineers and geologists with local knowledge. For example, if there is a geological reason to expect that you might have features deeper than generic guidelines that could influence the foundation (eg. sinkholes or voids).

Eg. Polous's book on tall building geotechnical engineering references the following table of guidelines from the infamous Burt Look handbook:

Shallow - 2Bf (pad), 4Bf (strip) where Bf = footing width
Raft - 1.5B, where B = building width
Piles to rock - 3m or 3d below founding level where d = pile diameter (you will find mixed guidance on this, some people will say to go 5m into rock or more)
Floating piles or piled rafts - 1.5B below 2L/3, where B = building width, L = pile length

 

[MTNClimber]

If you don’t have anticipated foundation loads, you won’t know how deep to go. Ask the structural engineer if they have ballpark foundation loads. Once you have those, run a preliminary calc on how many and how deep your piles need to go. Then use the guidance above to start planning your exploration program.

If they can’t provide estimated loads then I would plan a few deep borings for a preliminary subsurface investigation and plan on drilling some more if you find out you need more information.

 

[ONENGINEER]

Thank you for all the valuable feedback. I assume if the building is 25m wide, for a prospective 40m pile at least one borehole should extend to 65m. I assume Burt Look has had the pile group effect in mind.

 

[geotechguy1]

Yes the 2L/3 is the group effect thing IRC.

 

[ONENGINEER]

Thanks. I know realize that Look’s suggestion maybe come from the equivalent raft idea and that the 1.5B is the influencing zone below the raft as in the attached figure (if upload). The influencing bulb with a depth of 1.5B is understandable when we have end bearing poles and for settlement calculation. The question is if this influencing bulb zone develop under a group of friction piles, too?

 

[ONENGINEER]

attachment

 

[EireChch]

MTN climber nailed it, you need to do a preliminary pile calc. The structural engineer can give you anticipated loads. We have even assumed 10kPa per floor as a rough estimate.

Ball park your calc then revise your calculation after you get the first borehole done, you can use drillers filed logs instead of logs.

 

[ONENGINEER]

Proceeding to the next stage, if the pile lengths (D) in the group are not all of the same lengths, how do they establish the equivalent raft at 2/3 D for settlement calculations. Would the D be:
- the average length of all piles,
- the length of perimeter piles or
- the longest pile?
Thank you again for your valuable advice.

 

[EireChch]

You could check shortest and longest case and pick which on is conservative. Longer piles obviously have more capacity and therefore are stiffer, this effects settlement. But tbh the best way is to do it in PIGLET. Poulous software for pile group analysis, it caters for piles of different length

 

[GeoEnvGuy]

As others have mentioned figure out your preliminary answer for a reasonably bad conditions. Then get it in writing that this investigation and design is for a 28 story building and will be told to drill more if they want a 30 story building or account for them not knowing ow big they want the building.